Download Sound – waves: Characteristics of sound waves, Reflection of sound

Sound – waves:
Sound:
Characteristics of sound waves, Reflection
of sound waves and echo, Velocity of sound
in different media, Human ear, Ultrasonics,
Infrasonics, Applications
Ear: parts and function,
SONAR, Audible range, Unit of Sound level
Decibel.
Sound: normal, noise and musical sound.
Characteristics of musical sound, Types of
musical instruments with illustrations (in
brief), Mode of vibration in stretched string,
Resonance, Tuning fork
Physics: Level III
Sound:
Concept: Ear: parts and function
The living animals understand world around them through five senses. There are five
sensing organs, one corresponding to each sense. We ‘listen’ sounds coming to us with the
help of ‘auditory system’. There are many steps in the process of hearing. Ear is one of the
major organ in this auditory system.
Ear helps to understand and interpret the sound waves. Sound is appreciated in terms of
intensity, pitch, timbre i.e. quality and the direction from which it comes.
Other important parts of the ear are ‘receptors of equilibrium’. They help us to maintain the
‘balance of the body’.
Parts of the ear:
There are three parts of the ear. (I) External ear (II) Middle ear (III) Inner ear.
I: External ear:
External ear consists of pinna, auditory canal and the ear drum (tympanic membrane).
Pinna: Pinna is also called as auricle. It is the visible part of the ear. It has a typical shape
and it is present in mammals. All vertebrates have pair of ears on both sides of head. The
typical shape of pinna helps to identify mammals.
Pinna is cartilaginous thick folded flap. It gathers the sound waves falling on it and
channels them to auditory canal.
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Auditory canal: It is 2 to 3 cm long tubular structure. The outer part of the canal has thick
skin and inner part of the canal which lies on the bone of the skull has thinner skin. Only
the thicker skin (cerumen) has hairs. There are sebaceous glands also in it. Sebaceous
glands secrete earwax. The hairy part and wax prevent minute bodies like dust to enter in
inner part of ear.
Ear drum or tympanic membrane lies at the end of the auditory canal.
Ear drum or tympanic membrane: It is a thin membrane stretched at the end of the
auditory canal. The diameter of the membrane is 6 to 7mm. It separates external ear and
middle ear.
This membrane vibrates according to sound waves incident on it.
Middle ear
The middle ear is an air-filled cavity behind the ear drum (tympanic membrane). It consists
of three ear bones or ossicles. These bones are i) malleus (or hammer), ii) incus (or anvil),
and iii) stapes (or stirrup). The Eustachian tube also opens in middle ear.
The malleus has a long handle attached to eardrum. The incus is the bridge between the
malleus and stapes. The stapes is the smallest bone in the human body. The three bones are
arranged so that movement of the eardrum is transferred to stapes through malleus and
incus. The area of the stapes is approximately 20 times less than eardrum. Therefore
pressure exerted by the sound waves on stapes is approximately 20 times of pressure on the
ear drum. The lever action of the ossicles also enhances the pressure on the stapes. Stapes
is attached to a membrane called as the oval window. The oval window membrane
separates air filled middle ear and fluid filled inner ear. When the stapes footplate pushes
on the oval window, it causes movement of fluid within the cochlea (a major constituent of
the inner ear).
Though middle ear in humans and other land animals is filled with air, it is not in direct
contact with the atmosphere outside the body. The Eustachian tube connects the middle ear
to the back of the nasopharynx. The Eustachian tube is normally closed. It opens during
swallowing and yawning. The air pressure in the middle ear equalizes with pressure of the
eardrum, i.e. atmospheric pressure during this time. If the pressure is not equalized then
there is possibility of developing intense pain, hearing impairment and vertigo.
Inner ear:
The inner ear consists of bony labyrinth and it is filled with fluid. There are two main parts
of the inner ear.
i)
ii)
Cochlea: this is dedicated to hearing.
Vestibule and semicircular canals: this is dedicated to balance.
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The inner ear is encased in the hardest bone of the body i.e. skull. The cavities in this
region are fluid-filled. Cochlea consists of three fluid filled spaces: the scala tympani, the
scala vestibule and the scala media. When sound strikes the ear drum, the movement is
transferred to the footplate of the stapes. Stapes passes the vibrations into one of its fluidfilled ducts through the oval window of cochlea. The vibrations move through the fluid to
receptor cells of the organ of corti. It contains hair receptor cells. The tiny hair differ in
length. Shorter hair respond for high frequencies and longer hair respond for low
frequencies. They generate corresponding electrical signals. These signals stimulate the
spiral ganglion, which sends information through the auditory portion of the eighth cranizal
nerve to the brain. The sensation of listening takes place in brain.
The semicircular canals: There are three fluid filled semicircular canals, existing in three
mutually perpendicular planes. One end of each semicircular canal is enlarged and is
connected to ampula. The cristae in semicircular ducts are the sense organs of dynamic
equilibrium.
Dynamic equilibrium means maintenance of the balance of the body when body is moving.
Otoliths are the particles of CaCO3. They are present in the endolymph of semicircular
canals. As the body moves, the fluid in the canals moves and otoliths strike the hair of the
sensory cells of semicircular canals. These hair generate the electric signals and supply it to
brain. The brain then maintains the balance.
When we spin, the liquid in the canals also spins. When we stop ourselves suddenly, the
liquid inside continues to spin for some time. Therefore we feel dizzy or still moving after
being suddenly stopped. We may lose balance if do not hold some fixed thing.
We note here that hair cells in canals are also the receptor cells involved in balance. These
hair cells and the hair cells of the auditory systems in cochlea are not identical. Vestibule
consists of two sacs called utricle and saccule. These sacs are interconnected by small duct.
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Movement of body stimulates vestibular hair cells. The signals generated are connected to
brain, and they are concerned with static equilibrium.
Note: We have seen that sound waves are longitudinal waves. Human ear can here sounds
of frequency range 20 Hz to 20000 Hz. The normal sounds heard with greatest clarity are in
the range 1000 to 4000 Hz.
The intensity of the sound waves are measured in units called as ‘decibels’.
Deafness or hearing discrepancies:
The hearing loss is mainly because of two reasons
i)
Nerve loss or Sensorineural loss and ii) conduction loss.
i)
ii)
Sensorineural loss: This is due to damage of sensory nerves in cochlea or damage
of auditory nerve. Electrical signals are not generated if nerves in cochlea are
damaged.
Conduction loss: If cochlea is in good condition then signals generated are not
conducted to brain by the damaged nerve.
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Concept: Tuning fork, Modes of vibration in stretched string, Resonance.
The tuning fork was invented in 1711 by British musician John Shore, He was leading trumpeter
of England of his times.
Tuning forks are acoustic resonators in the form of fork with two prongs. They are U-shaped
and made from elastic metal. Usually they are made from steel. They are struck by hard rubber
and set in to vibrations. They vibrate at a specific constant frequency (pitch). The single constant
frequency with which it vibrates is called as ‘pure musical tone’. We need to wait for a moment
to allow some high overtones to die out. (Very little energy goes in producing higher overtones
but they die very early). The frequency that a tuning fork generates depends on the length of the
two prongs.
The shape is typically fork type, it has a reason. The upper ends of the two prongs vibrate with
maximum capacity. There is node (a point of no displacement) at the base of each prong. The
handle is small, and we can hold the fork without damping the vibration. Handle allows
transmitting the vibrations to the resonator. Resonator may be a table top or predesigned
resonating box. The sound is too low in absence of resonator because the two prongs vibrate out
of phase and they cancel the effect considerably.
The main use of the tuning forks is to generate a standard frequency. This is used to tune other
musical instruments.
The modes of vibration of the stretched string can be seen well by Melde’s Experiment.
Melde’s Experiment:
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The arrangement of the Melde’s experiment appears to be very simple. Normally an electrical
vibrator is used. One end of the long thread is connected to the vibrator while the other end
passes over the pulley to the weight and hanger. A mains driven vibrator vibrates with the a. c.
mains frequency 50 Hz. It excites one end of the string. The load is adjusted until steady loops
are clearly seen. Count number of loops and note down the load in the pan or load attached to the
hanger.
We can now change the load and adjust the number of loops.
We can change the orientation of the vibrator w. r. to length of the thread. The vibrator in line of
the length will vibrate perpendicular to the length of the string, and string will produce some
number of loops for given tension (load). If the vibrator is kept perpendicular to the length of the
thread, then it will vibrate along the length of the thread. The disturbance to the point of contact
is too and fro. This disturbance is in perpendicular direction of the previous one. Hence the
number of loops formed in two cases are different.
This experiment demonstrates the modes of vibration of the stretched string and also it can be
used to verify the laws of the stretched string. This experiment is used to test the relationship
between the tension, mass per unit length, frequency, and wavelength.
Note: A vibrator can be designed which is driven by frequency generator. In this case a suitable
load is applied to the string and frequency of the frequency generator can be gradually adjusted
so that we get stationary loops on the string.
Velocity of a transverse wave along a stretched string
The nature of the wave formed on a stretched string is very near to the sine wave. LPQM are the
points near the crest of the wave. Tangents drawn at P and Q intersect at point A. Point A is very
near to crest, and it can be approximated as a ‘crest’ itself. In magnified diagram below, point A
appears much above the crest.
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Let LAM represent a portion of a stretched string in which a transverse wave is travelling
towards the right, with a velocity V. Let PQ represent a small element of this portion of the
string. PQ is so small that apex of the curve and A is assumed to coincide. It is in the form of an
arc with its center of curvature at O. Let
= 2θ. Let the tension in the string be T at P or Q
and it is understood that it is constant everywhere in the string. At a given point the tension is
along the tangent. The tangents representing the tensions at P and Q meet at A. Join AO. AO
represents the radius of the circular arc PQ, which is denoted by r.
If m is the mass per unit length of the string, then
Mass of length of the arc PQ = m . PQ = m x r x 2θ
The components of the tensions at P and Q along the radius will add up, while components
perpendicular to it will cancel out. Therefore, the resultant tension in the element PQ is 2T sin θ
acting along AO and this provides the necessary centripetal force, making the particles of the
string trace a circular path.
But the centripetal force = mass of the arc PQ x
= m x r x 2θ x
= 2mv2θ
2mv2θ = 2T sin θ
v2 =
Since θ is small
1
v2 =
v =√
--------------------------------- A
Frequency of vibration of a stretched string
The fundamental mode of vibration of a stretched string is shown in the figure. It has two nodes
at the ends and an antinode in the middle. If L is the length of the vibrating segment between the
two nodes, then
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or λ = 2L
L=
But v = νλ
or ν =
λ
 ν=
Substituting for v from equation A, we get
ν=
√
Laws of transverse vibrations of stretched strings
There are three basic laws of vibrating string (which can be written from the above equation).
Law of length
"For a given string under constant tension, the frequency of vibration is inversely proportional to
the length of the string”.
i.e.
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Law of tension
"For a given string of constant length, the frequency of vibration is directly proportional to the
square root of the tension”.
√
i.e. ν
Law of mass
"For a string of constant length and under a constant tension, the frequency of vibration is
inversely proportional to the square root of its mass per unit length”.
√
i.e. ν =
Note: The diameter of the string is a direct measurable quantity and density is the constant of the
material. We can write above formula in terms of diameter of the string and density of the
material of the string. We proceed as follows.
Let M be the total mass and L is the length of the vibrating string, then
m=
=
=
=A
Here area A is the cross sectional area of the string.
If d is the diameter of the wire, then
A=
m=
Substituting in equation for frequency
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ν=
√
ν=
√
The laws of frequency of the vibrating string may be stated in terms of diameter and density as
below.
Law of diameter
“The frequency a string of a given material and length, under constant tension is inversely
proportional to its diameter”.
i.e.
Law of density
"The frequency a string of a given length and diameter, under constant tension is inversely
proportional to the square root of the density of the material of the string”.
i.e.
√
Resonance
Resonance was first recognized by Galileo Galilei with his investigations of pendulums and
musical strings in 1602. Resonance is a wide phenomenon that has evidences in all branches of
science and in nature. We will try to learn resonance mainly w. r. to its meaning in physics.
In physics, resonance is the tendency of a system to oscillate with greater amplitude at some
frequencies than at others. These are known as the system's resonant frequencies. At these
frequencies, even small periodic driving forces can produce large amplitude oscillations, because
the system stores vibrational energy.
Each system has a natural frequency. If a system is made to oscillate with some periodic driving
force then such system is called as the system under external force. When the frequency of
external driving force is equal to natural frequency of the system, there is maximum transfer of
energy from external force to the system. Therefore the system acquires large amplitude. This is
called as the resonance.
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A child moving on a swing is the classic example of resonance. All that we discussed above
applies to this situation. Kids intuitively or by experience understand the period and give same
periodic push. This makes the swing to oscillate with larger amplitude and derives more pleasure
from it. This is because, the energy absorbed by the swing is maximized when the pushes are 'in
phase' with the swing's oscillations. When the pushes are not in phase, some of the swing's
energy is actually extracted by the opposing force of the pushes. In such case the swing does not
rise and kid gets tired.
We note here that, there are some losses of energy in each cycle. It is called as damping. When
damping is small, the system continues to oscillate for long time. In the context of resonance, the
system will vibrate with maximum amplitude even when driving force of same frequency is
small (for small damping).
Resonance occurs when a system is able to store energy between two or more different storage
modes (such as kinetic energy and potential energy in the case of a pendulum). Some systems
have more than one degree of freedom. They can vibrate in more than one direction. Such
systems have multiple, distinct, resonant frequencies.
Resonance phenomena occurs with all types of vibrations or waves: there is mechanical
resonance, acoustic resonance, electromagnetic resonance, nuclear magnetic resonance (NMR),
electron spin resonance (ESR). Resonant systems can be used to generate vibrations of a specific
frequency (e.g. musical instruments), or pick out specific frequencies from a complex vibration
containing many frequencies (e.g. filters).

These are some examples of resonance:
Mechanical and acoustic resonance


Acoustic resonances of musical instruments and human vocal cords.
The shattering of windowpanes when an airplane passes above.
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
Shattering sound produced in bus at a particular speed.
Electrical resonance

Electrical resonance of tuned circuits in radios and TVs that allow individual stations to be
picked up.
Optical resonance

Creation of coherent light by optical resonance in a laser cavity.
Atomic, particle, and molecular resonance

Material resonances in atomic scale are the basis of several spectroscopic techniques that are
used in condensed matter physics and in medical diagnosis.
o Nuclear Magnetic Resonance
o Electron Spin Resonance.
Most of the sounds that we hear are resonant vibrations produced by that object. E.g. when metal
or glass is struck, main sound heard is resonant vibration of the object. Other frequencies are
produced but they are not listened much because their amplitudes are small and the vibrations of
higher frequencies die early.
Resonators
The natural objects are three dimensional. They experience resonance due to vibrations
inside them and hence they are called resonators. Such objects are as organ pipes, vibrating
strings, quartz crystals, microwave cavities, laser rods etc. The vibrations inside them travel
as waves. They travel with an approximately constant velocity, which is characteristic of the
medium. These waves bounce back and forth between the sides of the resonator. If the
distance between the sides is ‘d’, the length of a round trip is ‘2d’. Resonance occurs when
initial phase of a sinusoidal wave and the phase of the wave after a round trip are equal, so
the waves will vibrate with maximum amplitude. Therefore the condition for resonance in a
resonator is that the round trip distance ‘2d’, should be equal to an integer number of
wavelengths λ of the wave:
2d = N λ
where N = 1, 2, 3………
If the velocity of a wave is ‘v’, then the frequency is f = ⁄ so the resonant frequencies
are:
f= ⁄
where N = 1, 2,3………
The lowest frequency for N = 1 is called as the fundamental frequency. All other
resonant frequencies of resonators corresponding to higher values of N are called
normal modes or higher harmonics. They are equally spaced multiples of the
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fundamental frequency. These multiples are also called as the overtones. There may
be several overtones present in the vibrations.
Examples of resonance
Mechanical and acoustic resonance
Mechanical resonance is the tendency of a mechanical system to absorb more
energy when the frequency of oscillations matches the system's natural frequency of
vibration than it does at other frequencies. It may cause violent swaying motions of
the atoms and even disastrous failure in improperly constructed structures including
bridges, buildings, trains, and aircraft. While designing objects, engineers must
ensure the mechanical resonant frequencies of the component parts do not match
driving vibrational frequencies of motors or other oscillating parts, so as to avoid
resonance disaster.
Many clocks keep time by mechanical resonance in a balance wheel, pendulum, or
quartz crystal.
Acoustic resonance is a branch of mechanical resonance that is concerned with the
mechanical vibrations across the frequency range of human hearing, or audible
range. For humans, hearing is normally limited to frequencies between about 20 Hz
and 20,000 Hz (20 kHz).
Acoustic resonance is an important consideration for instrument builders, as most
acoustic instruments use resonators, such as the strings and body of a violin, the
length of tube in a flute, and the shape of, and tension on, a drum membrane.
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Atomic, particle, and molecular resonance
Main articles: Nuclear magnetic resonance and Resonance (particle)
21.2 T NMR Magnet at HWB-NMR, Birmingham, UK. In its strong field, the proton
resonance is at 900MHz.
Nuclear magnetic resonance (NMR) is the name given to a physical resonance
phenomenon involving the observation of specific quantum mechanical magnetic
properties of an atomic nucleus in the presence of an applied, external magnetic
field. Many scientific techniques exploit NMR phenomena to study molecular
physics, crystals and non-crystalline materials through NMR spectroscopy. NMR is
also routinely used in advanced medical imaging techniques, such as in magnetic
resonance imaging (MRI).
All nuclei containing odd numbers of nucleons have an intrinsic magnetic moment
and angular momentum. A key feature of NMR is that the resonant frequency of a
particular substance is directly proportional to the strength of the applied magnetic
field. It is this feature that is exploited in imaging techniques. When a sample is
placed in a non-uniform magnetic field then the resonant frequencies of the sample's
nuclei depend on where in the field they are located. Therefore, the particle can be
located quite precisely by its resonant frequency.
Electron paramagnetic resonance, otherwise known as Electron Spin Resonance
(ESR) is a spectroscopic technique similar to NMR, but uses unpaired electrons in
place of nucleons. Materials for which this can be applied are much more limited
since the material needs to both have an unpaired spin and be paramagnetic. ESR
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technology is used in various branches of science, such as chemistry and physics, for
the detection and identification of free radicals and paramagnetic centers such as F
centers. EPR is a sensitive, specific method for studying both radicals formed in
chemical reactions and the reactions themselves.
Medical and biological applications of EPR also exist. Although radicals are very
reactive and so do not normally occur in high concentrations in biology, special
reagents have been developed to spin-label molecules of interest. These reagents are
particularly useful in biological systems. Specially-designed nonreactive radical
molecules can attach to specific sites in a biological cell, and EPR spectra can then
give information on the environment of these so-called spin-label or spin-probes. It is
possible to detect cancer or tumor like structures in some parts of the body by using
this technique.
Characteristics of musical sound
When we strike a table by hand or hammer, it creates sound. When two metallic
utensils strike each other, then also sound is created. Sound also is created when
violin is bowed, flute, saxophone, shehnai is blown, or when drum or tabala is
played. There is difference in sounds in first two types and created by other
instruments. The sound created by these instruments is called as ‘musical sound’.
The sequence of notes that pleases us, sometimes touches heart and sometimes puts
you at higher level of abstract pleasure can be termed as ‘music’. Music creates
different feelings in different people.
Musical sound: A periodic sound of specific frequency that gives sweet feeling to
ears and gives pleasure of listening is also termed as ‘musical sound’. E.g. sound
created by flute or sitar.
Noise: The sound which is not periodic and not of specific frequency, which is not
pleasant to ear is termed as noise. If there are groups of people talking on relevant
subjects seriously but not related to each other may also be felt as the ‘noise’. The
sound which is necessarily not bad but if is unwanted and creating hindrance to a
situation is also termed as ‘noise’.
Music is vocal and also instrumental. Songs are sung for personal pleasure as well as
for group pleasure in celebrations. Folk songs and folk dances of farmers, fishermen
during harvesting are examples of group celebrations. Indian classical music has
long tradition and philosophical base. Classical vocal music is based upon ‘sets of
rules’ (they are known as ‘Ragas’). There are traditional houses (gharanas) which
possess their specific style and which follow their own traditional set of rules. People
keep on learning, reciting, practicing, making new additions and they become
masters. Hence it is all science; still rules of material science like replicability cannot
be applied as they are. The quality of sound of the singer, his/ her practice,
experience and emotional involvement create the great quality of music. The learned
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listener when gets emotionally involved may resonate with singer and then
experiences supernatural pleasure.
We come across music and musical notes everywhere in nature. Birds are known and
identified by their musical notes like Kokil is known for ‘Pancham’ and peacock is
known for ‘kekarav’ or ‘sa’ note. You can listen music when wind blows in the
jungles of bamboos, flowing brooks, different seasonal calls of birds and so on.
There are some singing birds also.
Characteristics of musical sound:
There are only four basic characteristics of musical sound namely pitch, loudness or softness,
quality or timbre and duration of sound. We will see their meaning one by one.
Pitch and Frequency
We have seen that sound wave is produced in a medium by a vibrating object. The vibrating
object is the source of the disturbance in a medium. The vibrating
object could be the vocal cords of a person, the vibrating string and
sound board of a guitar or violin, the vibrating tuning fork etc. The
particles of the medium through which the sound moves vibrate back
and forth at a given frequency. The frequency of a wave refers to how many times the particles
of the medium vibrate when a wave passes through the medium.
If the particles move 500 times back and forth in a second then the frequency of the sound is 500
Hz. The unit of the frequency is Hertz.
1 Hertz = 1 vibration/second
As a sound wave moves through a medium, each particle of the medium vibrates at the same
frequency.
The frequency of a sound wave also refers to the number of compressions or rarefactions that
pass a given point per unit of time. The typical output provided by such a detector is a pressuretime plot as shown below.
Since a pressure-time plot shows the fluctuations in pressure over time, the period of the sound
wave can be found by measuring the time between successive high pressure points
(corresponding to the compressions) or the time between successive low pressure points
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(corresponding to the rarefactions). Frequency is simply the reciprocal of the period. The
diagram below shows two pressure-time plots, one corresponding to a high frequency and the
other to a low frequency.
The human ear is capable of detecting sound waves with a wide range of frequencies, ranging
between approximately 20 Hz to 20 000 Hz. Any sound with a frequency below the audible
range of hearing (i.e., less than 20 Hz) is known as an infrasound and any sound with a
frequency above the audible range of hearing (i.e., more than 20 000 Hz) is known as an
ultrasound. Dogs can detect frequencies in a range 50 Hz to 45 000 Hz. Cats can detect
frequencies from 45 Hz to 85 000 Hz. Bats, being nocturnal creature, must rely on sound
echolocation for navigation and hunting. Bats can detect frequencies as high as 120 000 Hz.
Dolphins can detect frequencies as high as 200 000 Hz. Elephant possesses the unusual ability to
detect infrasound, having an audible range from approximately 5 Hz to approximately 10 000
Hz.
The sensation of a frequency is commonly referred as the pitch of a sound. A high pitch sound
corresponds to a high frequency sound wave and a low pitch sound corresponds to a low
frequency sound wave. People, who have been musically trained, are capable of detecting a
difference in frequency between two separate sounds that is as little as 2 Hz.
Loudness:
Sound loudness is a subjective term describing the strength of the ear's perception of a sound. It
is closely related to sound intensity but it is not identical to intensity. The sound intensity is also
related to the ear's sensitivity to the particular frequencies contained in the sound. It must also be
considered that the ear's response to increasing sound intensity has a logarithmic relationship
(power of ten). This is why the decibel scale is used to measure sound intensity. A widely used
"rule of thumb" for the loudness of a particular sound is that “the sound must be increased in
intensity by a factor of ten for the sound to be perceived as twice as loud”. A common way of
stating it is that it takes 10 violins to sound twice as loud as one violin.
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Although this rule is widely used, it must be emphasized that it is an approximate general
statement based upon a great deal of investigation of average human hearing but it is not to be
taken as a hard and fast rule.
Why is it that doubling the sound intensity does not produce double loudness? The most logical
reason is as follows. Nerve cells have maximum rates at which they can fire, and it appears that
doubling the sound energy to the sensitive inner ear does not double the strength of the nerve
signal to the brain. It requires ten times increase in intensity to double the signal from the inner
ear, and the sound heard will be felt of double loudness.
One difficulty with this "rule of thumb" for loudness is that it is applicable only to adding
loudness for identical sounds. If a second sound is widely enough separated in frequency to be
outside the critical band of the first, then this rule does not apply at all. Backus reports that this
critical band is about 90 Hz wide for sounds below 200 Hz and increases to about 900 Hz for
frequencies around 5000 Hertz.
Intensity of sound is the amount of sound energy incident on unit area per unit time.
One joule per second is one watt. 1W= 1J/s. The minimum sound that can be heard by human ear
has a level of the order 10-16 W/cm2 and maximum sound level that a human ear can sustain
without damage is of the order of 10-6 W/cm2. These levels of audibility are for 1000 Hz. These
minimum and maximum levels of audibility depend on the frequency. 1000 Hz is considered as
the standard and further scale is defined according to it.
The minimum level of audibility is also called as the threshold of audibility. 10 times of the
threshold of audibility is called as 1 Bell. 1 Bell is a large unit hence sound level is measured in
decibell (dB). 1Bell = 10 dB.
Let I0 be the threshold of audibility and I is the intensity to be measured. Then
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I (dB) = 10 log (I/I0).
This is also called as the Sound Intensity Level (SIL).
If I = 10000 I0 ,
(I/I0) = 10000 = 104; then
I (dB) = 10 log (104) = 10 X 4 = 40 dB.
Therefore 10000 times of the threshold level is identified as 40 dB sound level on dB scale.
Maximum intensity of sound for which human ear can sustain without damage, is 10-2 W/cm2.
In terms of dB scale Maximum Intensity = 10 log (10-2/10-16) = 10 log (1014) = 10 X 14 =140dB.
Combination of SIL values:
When two SIL values are to be combined, first the intensities corresponding to these values are
added and then the SIL value of the resultant is found out.
Let us take a concrete example of what does this mean.
Let there be two sources of sound of SIL 100 dB each. When they are sounded together, what is
the SIL of the final sound?
Let I1 be the intensity of the first source and I2 be the intensity of the second source. When they
are sounded together, let I be the resultant intensity. Then,
I = I1 + I2
In decibel scale, I1 = I2 =100 dB.
Now 100 = 10 log (I1/I0)
log (I1/I0) = 100/10 = 10
(I1/I0) = 1010
I1 = I0 1010
I = I1 + I2 = I0 1010 + I0 1010 = 2 I0 1010
(I/I0) = 2 X1010
SIL of I (dB) = 10 log ( 2 X 1010) = 10 [log 2 + log 1010] = 10 [0.3010 + 10] = 10 [10.3010]
SIL of I (dB) = 103.010 dB = 103 dB.
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When the two sound sources of 100 dB each are sounded together, the total SIL level is 103 dB
and not 200 dB.
Quality of sound i.e. Timbre:
Sound "quality" or "timbre" describes those characteristics of sound which allow the ear to
distinguish sounds which have the same pitch and loudness. Timbre is then a general term for the
distinguishable characteristics of a tone. Timbre is mainly determined by the harmonic content of
a sound. It is reported that, it takes duration of about 60 ms to recognize the timbre of a tone. The
change in the quality of sound for mid and higher harmonics can be notable if the change in the
sound level is at least 4 dB. The change in the quality of sound due to lower harmonics is notable
if the change in the level is at least 10 dB.
The harmonic content: Each musical note created by any instrument contains mainly the
fundamental frequency. Maximum of the sound energy is associated with the fundamental
frequency. Some harmonics are also produced along with the fundamental frequency. Some
definite percentage of energy goes in harmonics. It depends on the instrument. There are some
higher harmonics as well as lower harmonics. The proportion of these harmonics, their number
and their relative intensities actually decide the quality of sound. Some musical sound sources
have overtones which are not harmonics of the fundamental. These overtones also affect the
quality of sound. It requires great study to develop musical ear so that it identifies minute
changes in quality of sound for constant pitch and loudness.
Types of Musical Instruments:
Musical Instrument: It is an instrument on which a composition played by a knowledgeable
person known as musician generates melody.
It is either played separately or along with vocal singer to increase the lyrical effect of the
song.
There are four main types of musical instruments. They are as follows: 1) Instruments with
stretched strings 2) Instruments with stretched membrane 3) Instruments with vibrating
reeds 4) Instruments with vibrating air column.
Instruments with stretched strings:
There are three subclasses in instruments with stretched strings. We will see some details of
the instruments later. They are as follows:

Instruments in which the stretched wires are plucked with the fingers. Many a times,
musician wears a cap on a finger. It is either made up of metal or plastic. Tanpura
(tambora), Sitar, Sarod, Veena and Guitar are some instruments in this class.
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Tanpura (tambora):
A classical vocal performer tunes ‘his’/ her Tanpura before he / she starts singing. Tanpura
provides the basic ‘sur’ (note).
Kaddu is one of the main parts of the Tanpura. It is hemispherical in shape and works as sound
box (resonator). It is made from specific variety of plumkins. It is shown in the side diagram.
They are grown for this specific purpose, they are dried, carved from inside and given a required
shape. Then neck (dandi) is connected at one end of the kaddu. The neck or dandi of the tanpura
is made from long wooden block. It is carved and made hollow. It is flat on upper side and round
from below. Males sing with low frequency compaired to female. The wavelength of male voice
is longer than female voice. Hence the length of male tanpura is more than tanpura used by
females.
Tanpuras which are mostly used possess four strings. The middle two strings possess same
gauge, material and tension. So they vibrate with same frequency. They are called as ‘jod’ or
‘twin’. These two and other two strings start from bottom of the kaddu, pass over bridge and end
in to wooden screw called ‘khunti’. The bridge is made from the ivory and it is has some
curvature. There exits ‘jawari’ between the string and the bridge. Tensions of these strings are
adjusted so that the strings are tuned to ‘sa’ of different
saptakas.
Guitar: Guitar is originally spanish musical instrument. It has
metallic strings starting from the studs. They run over the
bridge. They are connected to screws which can be tightened to
adjust the tension. The strings are plucked by the plastic piece.
The sound box of guitar is wooden box. There is a big whole at centre that allows vibration to
spread in surrounding air. The proportion of the hormonics depends on the dimension of the
sound box, and quality of the wood and other materials used.
Sitar: Sitar is approximately one meter in length and its dandi is 8 to 10 cm in width. Sitar
originally had three strings. Later, sitars are developed with seven strings, each corrospoonding
to one note (one ‘sur’; so seven strings corrosponding to seven notes i.e. saptasur). It has
basicaly a hemispherical sound box (resonator) made from specific variety of plumkins. It is
named as ‘kaddu’ in the following diagram. They are grown for this specific purpose, they are
dried, carved and given a required shape. Then neck (dandi) is connected at one end of the
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kaddu. The neck or dandi of the sitar is made from long wooden block. It is flat from sides,
round from below and hollow inside.
The strings start from the bottom end of the kaddu, supported by the bridge and run along the
dandi (neck). Each string is finally connected to a screw (khunti), mostly wooden. There are
tuning beeds on lower side of bridge and ‘jawari’near the bridge on Kaddu. Jawari plays a
typical role in improving the quality of sound in ‘sitar’. There are seventeen (Ninteen in some
cases pardas (‘frets’ as named in the lower diagram). They are movable in some instruments and
immovable in some instruments.
Normally sitarist wares a metallic ring with pointed apex. This pointed apex is used to pluck the
wires. Sitar has to be played by a knowledgeable sitarist to create beauty of music and melody.
Veena: Veena is the instrument known from vedic literature. It is also known as the instrument
of Saraswati. Veena also has a ‘kaddu’ as a resonator. Normally it has seven metal strings
stretched along the long wooden box. The strings are connected to wooden screws called as
‘khunti’. The string are plucked to produce the musical sound. Rudraveena, vichitraveen and
Saraswativeena are most popular forms of the Veena.
Instruments in which stretched strings are struck by small hammer:
Santoor and Piano are the instruments of this class.
Santoor: Santoor possesses a wooden box, it functions as a resonator. There are fifteen to
eighteen pairs of bridges on two sides of this box. Wires of proper diameter are stretched over
these bridges with proper tensions. There are four wires per note. Two wooden sticks, six to ten
inches in length are bent at one end and there is ring at the other end. The rings are inserted in
fingers and other bent ends of the sticks are gently hammered on the wires to generate a sound
note. These sticks are called as ‘Kalam’s. The proportion and amplitudes of the harmonics
depends on how the wires are struck. Pandit Shivkumar Sharma & his father have made Santoor
a popular musical instrument.
Piano: Piano is a big voluminous musical instrument, we generally see it either in orchestra or in
movies. The wires of specific diameters are fitted by giving appropriate tensions. The vibrations
of the wires are transferred to the sound board through a bridge. Sound board vibrates the air in
the instrument. The size of the sound board is large in the piano; hence its sound also is large.
Piano is played by pressing the strips on the front board. Strip board is big in size but its nature is
like the board of harmonium. There is small hammer connected to every strip. As the strip is
pressed, the hammer strikes the wire and generates a sound note. Sometimes the hammer strikes
on group of wires. When the strip is left to original position, the hammer also gets lifted. A small
soft pad touches the wire and stops the vibrations i.e. stops sound produced by the wire.
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There are three pedals to piano. These pedals are pressed by feet to
control the sound of the piano.
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
Instruments in which the wires are bowed either by rod or by small bow. Violin,
Sarangi and Dilruba are the instruments of this class.
Violin: Violin is totally made from wood. Dry and seasoned wood is used so that it does
not change its shape. The wooden planks are given specific curves and shapes by cutting,
carving and chiseling. The dense wood with small pores produces thick sound and soft
wood with low pore density produces soft sound. This is because their elasticities are
different. The back of violin is made from dense wood of mepal or pear and front plank
of the violin is made from soft wood of Swiss pine or fur. So the sound is neither thick
nor soft, but it can be a controlled combination of both. The front and back of violin are
connected by side planks of mepal from outside and pine from inside.
Different parts of violin are shown in following diagram. A thick strip of pine attached to
front side supports the wires of the violin. This strip is known as the base bar, it also
functions as a wave carrier. F or S type letters are carved through and through the top
plank on front side. These are sound holes through which sound waves come out of inner
cavity and spread in nearby space. Bridge is at the center of these sound holes. There are
four strings in violin. These strings start from the tailpiece and are stretched over the
bridge. There is piece of mepal wood at the other end of the base bar having wooden
screws to which these strings are attached.
The bridge is curved. Hence the rod has to change its angle to bow different strings. The
bowing rod is made from ‘parnavak’ wood from Brazil. This wood is elastic so that the
rod can bend and it is supple so that it does not break by overusing. Previously 200 to 250
hairs of horse tail used to be tied to the bow. Resin is applied to the hairs so they become
rough and provide sufficient friction so that strings are properly bowed. These days,
nylon fibers are used instead of horse tail hair.
The string is bowed and finger is pressed at a proper place to create node at that point. A
note of sound is created because the string vibrates due to pulling outwards by the bow
and inwards due to inherent elasticity. The amplitude and quality of sound depends on
pressure by the bow on the string, the velocity of the bow, the time for which the string is
pulled. All these factors matter in generating the harmonics and so the quality of sound.
We note here that Albert Einstein and Sir C. V. Raman, the great physicists of all the time
were great violinists also.
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Dilruba:
Dilruba possesses four strings. There is a quadrangular wooden box at lower end. It is
covered by stretched leather. It is called as ‘tumba’. A hollow long box stem is attached
to tumba. There are metal pardas and tarafas on this stem. The stem looks somewhat like
sitar stem. Strings are bowed by the rod and pressed by fingers on paradas. A node is
created where finger is pressed and corresponding sound note is created.
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Sarangi:
Sarangi is prepared by creating cavity in the wooden brock. There are four main strings
starting from the lower end, run over the bridge and finally connected to wooden screws
‘khunti’. Tension is adjusted by turning the khunti. There are more than fifteen twenty
taraf strings below four main wires. Each string has a separate khunti. The main strings
are bowed by the rod and pressed while bowing. A node is created below the pressed
finger and a note of sound is generated. The taraf strings vibrate due to resonance when
main strings are bowed. The sound reverberates in cavity and produces characteristic
musical sound.
Instruments with stretched membrane:
In this class of instruments there is vibrating membrane. These membranes were made of
leather in traditional instruments. Leather membranes are still essential in some
traditional instruments like tabala, dholaki, dholak, pakhawaj, mridangam etc. Now a
days, there are polythene membranes in some new instruments like drums, bongos etc.
Polythene membranes have replaced leather in the instruments like dhol, tasha etc.
The membranes are stretched. They are put in to vibrations by hammering them by
fingers and base of palm. Some instruments are struck by wooden sticks. Vibrating
membrane imparts its vibrations to the air in contact and they travel with characteristic
velocity in the surrounding space. The quality of sound always depends on the number
and proportions of the harmonics in the sound. This depends on dimensions of the cavity,
material used for making the instruments, and how the membrane is vibrated by the
player.
Tabala:
Tabala is a set of two instruments as shown. One is sleek and has wooden body, it is
called as tabala. The other is metallic, appears double in size is called as dagga.
Dagga is generally made from copper, brass or german silver. The shape of dagga is
typical, it is tapered towards bottom. It is 10 to 12 inches in height; the diameter of the
top is about 9 inches. Leather is stretched on this top; it is called as ‘pudi’. The circular
layer of ink or ‘shai’ is applied to pudi. It is off centered. The area of the leather around
ink (shai) is called as ‘lav’ or ‘maidan’. An annular strip of width half inch is pasted on
the circular leather, it is called as ‘goth’ or ‘chat’. There is leather woven circular rim
called as ‘gajara’, it is mounted on the rim. The leather strip is woven through the ‘gajara’
along the periphery. This leather strip is stretched and tension is given to the leather
membrane.
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Tabala is narrower and smaller than dagga. Dense wood like ‘shisam or khair’ is used for
making tabala. A solid block is carved and the cavity is generated in the block. One end
is naturally closed and leather is stretched at the other end. The ink or shai of the tabala is
centered. There are cylindrical wooden pieces movable in leather strip, called as the
‘gatthe’. They are pushed up or down to adjust the tension. A small hammer is skillfully
used to move these gatthe and the ‘gajara’ also to adjust the tension.
Shai or ink is most important part in tabala. Shai is prepared from very fine powder of
iron, coal and ‘saras’. Saras is the natural adhesive. The ratio of the densities of shai and
leather is 1.768:1. This gives best expected results from tabala. Sur and taal (rhidam) are
most important in classical vocal. Sur is provided by tambora and taal (rhidam) is
provided by ‘tabala’.
In northern Hindustani classical music tambora and tabala are main instruments without
which singer even does not start singing. In south indian music mridangam and ghatam
are the main instruments used for taal.
Mridangam and Pakhawaj:
Mridangam and Pakhawaj are taal (rhidam ) instruments. Mridangam is mainly used by
south Indian Music and Pakhawaj is used for tall in Kathak dancing. Pakhawaj is also
used in prayer songs ( Bhajan) in temples.
The body of both instruments is wooden. It is carved in a single log of wood. They are
cylindrical and tapered on both sides as shown. The diameters of two ends are different.
Both ends are covered by stretched leather. The tension in mridangam is by the leather
strap and gatthe. The tension in pakhawaj is by screws. Both of these instruments can be
played only after long study and practice i.e. by pandits and ustads.
We note here that tabala is more versatile than these two taal instruments. Tabala can be
used in place of mridangam or pakhawaj but the converse is not true.
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.
Dholaki:
Dholki looks very similar to Pakhawaj but it is smaller in size. The working principle, the
materials from which it is made, the process of making is almost similar to that of
Pakhawaj. It is mainly used as a ‘taal’ in the traditional ‘lavani’ in Marathi tamasha.
(Tamasha is traditional folk of Marathi culture.)
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Dafali, tasha, Dholak:
These taal instruments are used in different folk types. There are many instruments which
work on above said principles. They are used with very specific instruments: Choughada
with ‘Shehnai’, Sanbal with tuntuna , Dimadi with ghati and so on.
Instruments with vibrating reeds:
A thin strip of wood or plastic vibrates when blown by air under pressure. Generally such
strip is fixed at one end and other end is free. The wavelength of the fundamental mode is four
times the length of the strip. It produces fundamental mode as well as harmonics depending on
the velocity of the air by which it is blown. Shehnai, harmonium, organ, mouthorgan, accordion,
saxophone etc. are some musical instruments in this class.
Harmonium:
In harmonium sound is produced by air being blown through sets of reeds. The air is usually
supplied by bellows operated by the foot or hand.
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Harmonium consists of banks of brass reeds. Reed is a metal strip that vibrates when air flows
over it. Harmonium has bellows (a pumping apparatus), stops for drones (some models feature a
stop that causes a form of vibrato), and a keyboard. The harmonium's timbre is actually produced
in a critically different way. Instead of the bellows causing a direct flow of air over the reeds, an
external feeder bellows inflate an internal reservoir bellows inside the harmonium, from which
air escapes to vibrate the reeds.
Vidyadhar Oke has developed a 22-shruti harmonium, which can play the 22 Indian shrutis
(microtones) in an octave, as required in Indian classical music. The fundamental tone (Shadja)
and the fifth (Pancham) are fixed, but the other ten notes have two microtones each, one higher
and one lower. The higher microtone is selected by pulling out a knob below the key. In this
way, the 22-shruti harmonium can be tuned for any particular raga by simply pulling out knobs
wherever a higher shruti is required.
The free reed of the harmonium is riveted from a metal frame and is subjected to airflow. Air is
pumped from the bellows through the reservoir. It pushes the reeds and brings them to resonating
mode of oscillation to produce sound. Reeds require some threshold pressure to vibrate
normally.
Accordion is very similar to harmonium having different orientation. Key board, bellows and
stops are in a horizontal line. It is hanged from the neck and hold near the chest. It is bellowed by
one hand and played by the other. This instrument is light and can be played even when musician
is standing or moving. It is popular instruments in orchestras and parties.
Shehnai:
The shehnai (shahnai, shenai, sanai or mangal vadya) is an instrument played by blowing air.
This tube-like instrument gradually broadens towards the lower end. There is metallic flare bell,
horn like structure at the end. It usually has six to nine holes. It employs double reed set at a
time. Each instrument has two sets of double reeds, making it a quadruple reed instrument. Using
suitable reeds, by controlling the breath, various tunes can be played on it. The tunes played by
the shehnai are typically sweet to listen; its double reed structure is one of the reasons.
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Its sound is thought to create and maintaining a sense of auspiciousness and sanctity. Therefore,
it is widely used during marriages, processions, and in temples of West India. It is also played in
concerts. The South Indian instrument nadaswaram is very similar of shehnai.
Ustad (Master) Bismillah Khan was a well-known shehnai player.
Saxophone:
Saxophones are usually made of brass and played with a single-reed mouthpiece similar to that
of the clarinet. The saxophone was invented by the Belgian instrument maker Adolphe Sax in
1846. It was originally designed for military bands.
While proving very popular in military band music, the saxophone is most commonly associated
with jazz and classical music.
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The saxophones are most commonly made from thin brass tubes and sometimes plated with
silver, gold, or nickel. The smaller instruments are straight while the larger ones possess U bends
on lower side. The bell of the instrument is pointed almost directly upward and slightly forward.
The mouth piece containing reed is bent 900, as seen in the diagram. It possesses 20 and 23 tone
holes of varying sizes, including two very small 'speaker' holes. These holes are covered by keys
(also known as pad cups), containing soft leather pads, which are closed to produce an airtight
seal. Normally some of the holes stand open and others are closed. The keys are controlled by a
player and it is very similar to the flute or clarinet.
The characteristic typical sound of saxophone bridges the gap between the vocal and orchestra.
This brings completeness to the orchestra.
Mouth Organ:
The mouth organ, is a free reed wind instrument. The body of the organ is wooden. The
instrument has two rows of rectangular chambers. Each chamber contains at least one reed made
up of brass or bronze. The reed is fixed at one end and its other end is free. The free end vibrates
and creates sound when wind is blown over it. It is played by blowing air into it or drawing air
out by placing lips over individual holes (reed chambers) or multiple holes. The pressure caused
by blowing or drawing air into the reed chambers causes a reed or multiple reeds to vibrate
creating sound.
Reeds are pre-tuned to individual tones, and each tone is determined according to the size of
reed. Longer reeds make deep, low sounds and short reeds make higher-pitched sounds.
It is used primarily in American folk music, jazz, country, and rock and roll.
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Instruments with vibrating air column:
Air is blown to produce musical note in this class of instruments. The length of the air
column can be varied by closing and opening holes in some instruments. Bansuri, flute, shankha
etc. are the instruments in this class.
Flute and Bansuri:
A selection of flutes from around the world

The flute is a musical instrument that produces its sound from the flow of air across an opening.
Flute does not possess reeds.
Flutes are the earliest known musical instruments from 35,000 to 40,000 years. They were
found in the Swabian Alb region of Germany. These flutes demonstrate that a developed
musical tradition existed from the earliest period of modern human presence in Europe.
The Indian bamboo flute
A Carnatic eight-holed bamboo flute
An eight-holed classical Indian bamboo flute mainly used for Carnatic music.
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The meaning of the word bans means bamboo. Sur and taal are the soul of classical music. The
word ‘bansuri’ is composed from its meaning. In Indian classical music, flute (pawa or algooj)
and bansuri are made from a special bamboo. Bamboo flute has six holes and it is blown by
holding the other end in lips. It is like a pipe closed at one end. Bansuri is played by blowing it
through the hole at one end on the side. So both ends of the Bansuri are open. The fundamental
note of the flute and bansuri depends on the length of the instrument and ‘weight of sound note’
depends upon the diameter of the instrument. The quality of the sound depends on the type of the
bamboo and process by which the instrument is made. The best known bamboos for bansuris are
from Braj and Vrundavan on the banks of Yamuna river in north India while in south India best
bamboos are from ‘Nagarcoil’. The Indian flute and bansuri are simplest instruments because it
has neither keys nor reeds. It has just holes which are properly carved.
The bamboo flute is an important instrument in Indian classical music, and developed
independently of the Western flute. The Hindu God Krishna is traditionally considered a master
of the bamboo flute.
Pannalal Ghosh, a legendary Indian flutist, was the first to transform a tiny folk instrument to a
bamboo flute (32 inches long with seven finger holes) suitable for playing traditional Indian
classical music, and also to bring to it the stature of other classical music instruments. The extra
hole permitted madhyam to be played, which facilitates the meends (like M N, P M and M D) in
several traditional ragas.
Pandit Raghunath Prasanna, Pt. Bhola nath Prasanna, Pt. Hari Prasad Chaurasia, Pt. Rajendra
Prasanna globally known for their melodious music.
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Temple car carving of Krishna playing flute, suchindram, Tamil Nadu, India
References:
I)
II)
III)
IV)
CBSE text book (Xth standard) published by NCERT.
ICSC text books (Xth standard).
Wikipedia (the ear), musical instruments .
Vadyanmadhil Vidnyan: Dr Varsha Joshi (Neelkanth Prakashan, Pune)
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