Download Determining the Wavelength of Sound

Chapter 15 Sound
We as humans can hear a wide variety of sounds such as a giant truck driving by to the writing of a pen. The few
sounds we cannot hear are the sounds produced by bats. Their frequencies of producing sound are much higher
than ours, which is why we can't hear them.
Frequency is how fast an object vibrates and sound vibrates in waves. The vibrations are measured in Hertz. So
one Hertz (1 Hz) is equivalent to one vibration in a second. Humans can hear from 20 Hz to 20 kHz. Anything
higher is called ultrasound or ultrasonic sound. The reason that bats use ultrasound is because it has such a high
frequency and it has a low diffraction or it bends less. They use this sound to do a couple of things like to catch
their prey and also just to get around. The method of doing such tasks is called echolocation. They make a sound
and wait for it to bounce back to hear it. If they hear it come faster in a particular area than the rest of the sounds
then they know that something is near. The frequencies of bats are different in many books. In one source it says
that the frequency is 120 kHz, while in another it says 100 kHz. The truth is it ranges because when the bat makes
a sound it isn't of the same frequency all the time.
Sound and music are important components of the human experience. Primitive people made sounds not only with
their voices, but also with drums, rattles, and whistles. Stringed instruments are at least 3000 years old. Some animals use sound for survival with frequencies too high for humans to hear. Bats, in particular, hunt flying insects by
emitting pulses of very high-frequency sound. They learn how far away the insect is, how large it is, where it is, and
what its relative velocity is. Further, bats that hunt in forests must distinguish the insect from the vegetation. In
addition, since bats live in large colonies, they must separate the echoes of their sounds from the sounds of
hundreds of other bats. The methods they use are truly amazing, and are based on a few physical principles of
sound.
15.1 PROPERTIES OF SOUND
Sound is a longitudinal wave. A longitudinal wave consists of alternate areas of high pressure called compressions,
and low pressure called rarefactions. It can be characterized by velocity, frequency, wavelength, and amplitude just
as any other periodic wave. It shares with other waves the properties of reflection, refraction, and interference.
Living things use sound to obtain information about the surroundings and to communicate with others by means of
speech and music.
Sound Waves
Sound is produced by the compression and rarefaction of matter. Sound waves move through air because a
vibrating source produces regular variations in air pressure. The air molecules collide, transmitting the pressure
oscillations away from the source of the sound. The pressure of the air varies or oscillates about an average value,
the mean air pressure, Figure 15-a. The frequency of the wave is the number of oscillations in pressure each
second. Sound is a longitudinal wave because the motion of the air molecules is parallel to the direction of motion
of the wave.
FIGURE 15- a. Graphic representation of the change in pressure over time in a sound wave. Dark areas
indicate high pressure, light areas, low pressure.
************
http://www.kettering.edu/~drussell/Demos/waves/wavemotion.html students use computers to look at different sound
waves.
http://www.youtube.com/watch?v=-d9A2oq1N38&feature=related sonic booms – fighter jets – concord – bull whip
http://www.sky-flash.com/boom.htm more sonic booms
The velocity of the sound wave in air depends on the temperature of the air. Sound waves move through air at sea
level at a velocity of 343.6 m/s at room temperature (20°C) ( 331.5 m/s at 00C) (increase .607 m/s /C0) . Sound can
also travel through liquids and solids. In general, the velocity of sound is greater in solids and liquids than in gases.
Sound cannot travel through a vacuum because there are no particles to move and collide.
Cell phone in Bell Jar – Demo
Sound waves share the general properties of other waves. Hard objects, such as the walls of a room, can reflect
sound waves. Reflected sound waves are called echoes. The time required for an echo to return to the source of
the sound can be used to find the distance between the source and reflector.
Bats use this principle, as do some cameras, and ships employing sonar. Sound waves can also be diffracted,
spreading outward after passing through narrow openings. Two sound waves can interfere, causing "dead spots" at
nodes where little sound can be heard.
http://www.youtube.com/watch?v=2xr9P4MSl80&feature=related manmade echos
http://www.youtube.com/watch?v=b wLr19qyO_I =1# boiler up
The wavelength of a sound wave is the distance between adjacent regions of maximum pressure. The frequency
and wavelength of a wave are related to the velocity of the wave by the equation
v = f.
Example Problem 15-1
Determining the Wavelength of Sound
A sound wave has a frequency of 261.6 Hz. What is the wavelength of this sound traveling in air at 343 m/s?
Given: frequency,
Unknown: wavelength, 
f = 261.6 Hz velocity, v = 343 m/s
Basic equation: v = f
Solution: v = f, so  = v/f = (343 m/s)
(261.6 Hz) = 1.31 m
Do practice problems 15-1a
The Doppler Shift
Have you ever noticed the pitch of an ambulance, fire, or police siren as the vehicle sped past you? The frequency
is higher when the vehicle is moving toward you, then suddenly drops to a lower pitch as the source moves away.
This effect is called the Doppler shift, and is shown in Figure 15-3. The sound source is moving to the right with
velocity vs. The waves it emits spread in circles centered on the location of the source at the time it produced the
wave. The frequency of the sound source does not change, but when the source is moving toward the sound
detector, O, on Figure 15-3b, more waves are crowded into the space between them. The wavelength is shortened
to 1. Because the velocity is not changed, the frequency of the detected sound is greater.
When the source is moving away from the detector, O2 on Figure 15-3b, the wavelength is lengthened to 2, and
the detected frequency is lower. A Doppler shift also occurs if the detector is moving and the source is stationary.
The Doppler shift occurs in all wave motion, both mechanical and electromagnetic. It has many applications. Radar
detectors use the Doppler shift to measure the speed of baseballs and automobiles. Astronomers use the Doppler
shift of light from distant galaxies to measure their speed and infer their distance. Physicians can detect the velocity
of the moving heart wall in a fetus by means of the Doppler shift in ultrasound. A bat uses the Doppler shift to
detect and catch flying insects. When an insect is flying faster than the bat, the reflected frequency is lower, but
when the bat is catching up to the insect, the reflected frequency is higher.
http://www.kettering.edu/~drussell/Demos/doppler/doppler.html - have students look at site
http://www.youtube.com/watch?v=-t63xYSgmKE just kidding on the devil – Doppler shift – slight – red low to blue high
http://www.youtube.com/watch?v=a3RfULw7aAY&feature=PlayList&p=4E7C0D40FAE4369A&playnext=1&index=20 car
horn – If it does not work do it in the car
http://www.youtube.com/watch?v=D1tajOObltQ&feature=related siren
Pitch and Loudness
The physical characteristics of sound waves are measured by frequency, wavelength, and amplitude. In humans,
sound is detected by the ear and interpreted by the brain. Sound characteristics are defined in terms describing
what we perceive. Pitch is essentially the frequency of the wave. Loudness depends on the amplitude of the
pressure variation wave.
Mersenne (1588-1648) and Hooke (1635-1703) first connected pitch with the frequency of vibration. Pitch can also
be given the name of a note on a musical scale. Musical scales are based on the work of Pythagoras, a Greek
mathematician who lived in the sixth century B.C. He noted that when two strings had lengths in the ratio of small
whole numbers, for example 2:1, 3:2, or 4:3, pleasing sounds resulted when the strings were plucked together. Two
notes with frequencies related by the ratio 2:1 are said to differ by an octave. For example, if a note has a
frequency of 440 Hz, a note an octave higher has a frequency of 880 Hz. A note one octave lower has a frequency
of 220 Hz. It is important to recognize that it is the ratio of two frequencies, not the size of the interval between
them, which determines the musical interval.
On a piano
In other common musical intervals, the frequencies have ratios of small whole numbers. For example, the notes in
an interval called a "major third" have a ratio of frequencies of 5:4. A typical major third is the notes C and E. The
note middle C has a frequency of 262 Hz, so E has a frequency (5/4)(262 Hz) = 327 Hz. In the same way, notes in
a "fourth" (C and F) have a frequency ratio of 4:3 and those in a "fifth" (C and G) have a ratio of 3:2.
The human ear is extremely sensitive to the variations in air pressure in sound. It can detect wave amplitudes of
less than one billionth of an atmosphere (1 atm = 1.01e 5 N/m2 so one billionth would be 2 e -5 N/m2). At the other
end of the audible range, the pressure variations that cause pain are one million times greater, (20 N/m 2). Notice
that this is still less than one one-thousandths of an atmosphere.
Because of this wide range in pressure variation, sound pressures are measured by a quantity called sound level.
Sound level is measured in decibels (dB). The level depends on the ratio of the pressure of a given sound wave
to the pressure in the most faintly heard sound, Such an amplitude has a sound level of zero decibels (0 dB).
Decibels
The decibel (dB) is used to measure sound level, but it is also widely used in electronics, signals and
communication. The dB is a logarithmic unit used to describe a ratio. The ratio may be power, sound pressure,
voltage or intensity or several other things. The decibel is defined as one tenth of a bel where one bel represents a
difference in level between two intensities I1, I0 where one is ten times greater than the other. What is a logarithm?)
For instance, suppose we have two loudspeakers, the first playing a sound with power P1, and another playing a
louder version of the same sound with power P2, but everything else (how far away, frequency) kept the same.
The difference in decibels between the two is defined to be
10 log (P2/P1) dB
where the log is to base 10.
If the second produces twice as much power than the first, the difference in dB is
10 log (P2/P1) (2/1) = 10 log 2 = 3 dB.
If the second had 10 times the power of the first, the difference in dB would be
10 log (P2/P1)(10/1) = 10 log 10 = 10 dB.
If the second had a million times the power of the first, the difference in dB would be
10 log (P2/P1) = 10 log 1000000 = 60 dB.
This example shows one feature of decibel scales that is useful in discussing sound: they can describe very big
ratios using numbers of modest size. But note that the decibel describes a ratio: so far we have not said what
power either of the speakers radiates, only the ratio of powers. (Note also the factor 10 in the definition, which puts
the 'deci' in decibel).
Sound pressure, sound level and dB. Sound is usually measured with microphones and they respond
(approximately) proportionally to the sound pressure, p. Now the power in a sound wave, all else equal, goes as the
square of the pressure. (Similarly, electrical power in a resistor goes as the square of the voltage.) The log of the
square of x is just 2 log x, so this introduces a factor of 2 when we convert to decibels for pressures. The
difference in sound pressure level between two sounds with p1 and p2 is therefore:
20 log (p2/p1) =
20 log (p2/p1) 10 x increase = 20 dB
20 log (p2/p1) 100000 x increase = 100 dB
20 log (p2/p1) 100 x increase = 40 dB
20 log (p2/p1) 1000000 x increase = 120 dB
20 log (p2/p1) 1000 x increase = 60 dB
20 log (p2/p1) 10000000 x increase = 140 dB
20 log (p2/p1) 10000 x increase = 80 dB
20 log (p2/p1) 100000000 x increase = 160 dB
Threshold of hearing 0 dB
Rustling leaves
Motorcycle (30 feet)
20 dB Food blender (3 feet)
88 dB
90 dB
Quiet whisper (3 feet) 30 dB Subway (inside)
94 dB
Quiet home
40 dB Diesel truck (30 feet)
100 dB
Quiet street
50 dB Power mower (3 feet)
107 dB
Normal conversation 60 dB Pneumatic riveter (3 feet)
115 dB
Inside car
117 dB
70 dB Chainsaw (3 feet)
Loud singing (3 feet) 75 dB Amplified Rock and Roll (6 feet) 120 dB
Automobile (25 feet)
80 dB Jet plane (100 feet)
130 dB
10^(n/20) if comparing sound wave pressure increase in dB –
One sound is 10dB another sound is 40dB – what is increase in sound wave pressure – 40 -10 = 30dB
10^(30/20) = 31.6 times
Loudness, as perceived by the human ear, is not directly proportional to the pressure variations in a sound wave.
Further, the ear's sensitivity depends on both sound frequency and sound level. Perception is also different for pure
tones than it is for mixtures of tones. Most people find that a 10-dB increase in sound level is heard as being about
twice as loud.
Must read decibel paper
15-2 The sound of Music
In the middle of the nineteenth century, a German physicist and an English physicist, Hermann Helmholtz and Lord
Rayleigh, studied how the human voice as well as musical instruments produces sounds, and how the human ear
detects these sounds. In the twentieth century, scientists and engineers have developed electronics that permit not
only detailed study of sound, but the creation of electronic musical instruments and recording devices that allow us
to have music whenever and wherever we wish.
Sources of Sound
A vibrating object produces sound. The vibrations of the object create molecular motions and pressure oscillations
in the air. A loudspeaker has a diaphragm, or cone, that is made to vibrate by electrical currents. The cone creates
the sound waves. Musical instruments such as gongs or cymbals and the surface of a drum are other examples of
vibrating sources of sound.
The human voice is the result of vibrations of the vocal cords, two membranes located in the throat. Air from the
lungs rushing through the throat starts the vocal cords vibrating. The frequency of vibration is controlled by the
muscular tension placed on the cords.
In brass instruments, such as the trumpet, trombone, and tuba, the lips of the performer vibrate. Reed instruments,
like the clarinet, saxophone, and oboe, have a thin wooden strip, or reed, that vibrates as a result of air blown
across it. In a flute, recorder, organ pipe, or whistle, air is blown across an opening in a pipe. Air moving past the
mouthpiece edge sets the column of air in the instrument into vibration.
In stringed instruments, such as the piano, guitar, and violin, a wire or string is set into vibration. In the piano, the
wire is struck; in the guitar, it is plucked. In the violin, the friction of the bow pulls the string aside. The string is
attached to a sounding board that vibrates with the string. The vibrations of the sounding board cause the pressure
oscillations in the air that we hear as sound. Electric guitars use electronic devices to detect and amplify the
vibrations of the strings.
Resonance
If you have ever used just the mouthpiece of a brass or reed instrument, you know that the vibration of your lips or
the reed alone does not make a sound with any particular pitch. The long tube that makes up the instrument must
be attached if music is to result. When the instrument is played, the air within this tube vibrates at the same frequency, or in resonance, with a particular vibration of the lips or reed. Remember that resonance increases the
amplitude of a vibration by repeatedly applying a small external force at the same natural frequency. Changing the
length of the resonating column of vibrating air varies the pitch of an instrument. The length of the air column
determines the resonant frequencies of the vibrating air. The mouthpiece creates a mixture of different frequencies.
The resonating air column acting on the vibrating lips or reed amplifies a single note.
A tuning fork above a hollow tube can provide resonance in an air column. The tube is placed in water so that the
bottom end of the tube is below the water surface. A resonating tube with one end closed is called a closed-pipe
resonator. The length of the air column is adjusted by changing the height of the tube above the water. The tuning
fork is struck with a rubber hammer. When the length of the air column is varied, the sound heard will alternately
become louder and softer. The sound is loud when the air column is in resonance with the tuning fork. The air
column has intensified the sound of the tuning fork.
The vibrating tuning fork produces alternate high and low pressure variations. This sound wave moves down the air
column. When the wave hits the water surface, it is reflected back up to the tuning fork, Figure 15-9. If the high
pressure reflected wave reaches the tuning fork when the fork has produced high pressure, then the leaving and
returning waves reinforce each other. A standing wave is produced.
A standing wave has pressure nodes and antinodes. At the nodes, the pressure is the mean atmospheric pressure.
At the anti-nodes, the pressure is at its maximum or minimum value. Two anti-nodes (or two nodes) are separated
by one-half wavelength. A pressure wave is reflected from a hard surface without inversion, like a rope wave is
reflected from an open end. Thus, there is a pressure antinode at the water surface. A pressure wave is reflected
inverted from the open end. That is, if a region of high pressure reaches the open end, a region of low pressure will
be reflected. This is similar to the reflection of a rope wave off a fixed end. Thus, the open end of the pipe has a
pressure node.
The shortest column of air (closed pipe) that can have an antinode at the bottom and a node at the top is one-fourth
wavelength long. As the air column is lengthened, additional resonances are found. Thus columns /4, 3/4, 5/4,
7/4, and so on, will all be in resonance with the tuning fork.
In practice, the first resonance length is slightly longer than one-fourth wavelength. This is because the pressure
variations do not drop to zero exactly at the end of the pipe. Actually, the node is approximately 1.2 pipe diameters
beyond the end. Each additional resonance length, however, is spaced by exactly one-half wavelength.
Measurement of the spacing between resonances can be used to find the velocity of sound in air.
Have you ever put your ear to a large seashell and heard a low frequency sound? The shell acts like a closed-pipe
resonator. The source of the sound is the soft background noise that occurs almost everywhere. This sound
contains almost all frequencies the ear can hear. The shell increases the intensity of sounds with frequency equal
to the resonant frequency of the shell. The result is the almost pure tone you hear.
The small size of the vibrating string of a guitar or other stringed instrument cannot cause much vibration in the air.
The instrument uses resonance to increase the sound wave amplitude. Vibration of the string causes the sounding
box to vibrate. The large size of the box produces a sound of greater intensity in the air around it.
Example Problem 15-2a
Measuring Sound Velocity
A tuning fork with a frequency of 392 Hz is found to cause resonances in an air column at 21.0 cm and 65.3 cm.
The air temperature is 27.0°C. Find the velocity of sound in air at that temperature.
Given: f = 392 Hz I2 = 65.3 cm /1 = 21.0cm
Unknown: v
Basic equation: v = f
Solution: Spacing of resonances,  / = /2 - /1 = (65.3 — 21.0) cm = 44.3 cm.
Thus, ½ = l, or  = 2l = 2(44.3 cm) = 88.6 cm.
Now, v =f = (0.886 m)(392 Hz) = 347 m/s.
Do practice problems 15-2a (1-4)
An open-pipe resonator, a resonating tube with both ends open, will also resonate with a sound source. There are
pressure nodes near each of the ends, and at least one antinode between. There is also some sound transmission
at the open ends of the tube. We hear the transmitted sound. The remainder is reflected to form a standing wave.
The minimum length of a resonating open pipe is one-half wavelength. If open and closed pipes of the same length
are used as resonators, the wavelength of the resonant sound for the open pipe will be half as long. Therefore, the
frequency will be twice as high for the open pipe as for the closed pipe. The resonances in the open pipe are
spaced by half wavelengths just as in closed pipes. Have you ever shouted into a long tunnel or underpass? The
booming sound you hear is the tube acting as a resonator. Many musical instruments are also open-pipe resonators. Some examples are the saxophone and the flute.
http://library.thinkquest.org/11315/instrum.htm types of instruments
Do 15-2b pp
Detection of Sound
Sound detectors convert sound energy—kinetic energy of the air molecules— into another form of energy. In a
sound detector, a diaphragm vibrates at the frequency of the sound wave. The vibration of the diaphragm is then
converted into another form of energy. A microphone is an electronic device that converts sound energy into
electrical energy. It is discussed in Chapter 25.
The ear is an amazing sound detector. Not only can it detect sound waves over a very wide range of frequencies, it
is also sensitive to an enormous range of wave amplitudes. In addition, human hearing can distinguish many
different qualities of sound. The ear is a complex detector that requires knowledge of both physics and biology to
understand. The interpretation of sounds by the brain is even more complex and not totally understood.
The ear. Figure 15-12, is divided into three parts; the outer, middle, and inner ear. The outer ear consists of the
fleshy, visible part of the ear called the pinna, which collects sound; the auditory canal; and the eardrum. Sound
waves cause vibrations in the eardrum. The middle ear consists of three tiny bones in an air-filled space in the skull
(hammer, anvil, and stirrup). The bones transmit the vibrations of the eardrum to the oval window on the inner ear.
The inner ear is filled with a watery liquid. Sound vibrations are transmitted through the liquid into sensitive portions
of the spiral-shaped cochlea. In the cochlea, tiny hair cells are vibrated by the waves. Vibrations of these cells
stimulate nerve cells that lead to the brain, producing the sensation of sound.
The ear is not equally sensitive to all frequencies. Most people cannot hear sounds with frequencies below 20 Hz or
above 16 000 Hz. In general, people are most sensitive to sounds with frequencies between 1000 Hz and 5000 Hz.
Older people are less sensitive to frequencies above 10 000 Hz than are young people. By age 70, most people
can hear nothing above 8000 Hz. This loss affects the ability to understand speech.
http://longevity.about.com/od/healthyagingandlongevity/a/hearing_loss.htm
Exposure to loud sounds, either noise or music, has been shown to cause the ear to lose its sensitivity, especially
to high frequencies. The longer a person is exposed to loud sounds, the greater the effect. A person can recover
from a short-term exposure in a period of hours, but the effects of long-term exposure can last for days or weeks.
Long exposure to 100 dB or greater sound levels can produce permanent damage. Many rock performers have
suffered serious hearing loss, some as much as 40%. What do Bill Clinton and Pete Townsend of the Who have in
common? Yes they both smoked pot, although Pete inhaled more, and we are not sure which had more affairs. But
we do know both of them have hearing loss due to excessive exposure to loud music. Loud noise can damage the
tiny hairs in the cochlea and lead to hearing loss. Generally, this type of hearing loss is reversible (except in some
cases of a sudden, very loud noise, such as an explosion).
However, over time, repeated exposure to loud noise can cause permanent damage and hearing loss.
This condition is known as noise-induced hearing loss.
Cotton earplugs reduce the sound level only by about 10 dB. Special ear inserts can provide a 25-dB reduction.
Specifically designed earmuffs and inserts can reduce the level by up to 45 dB. Another source of hearing loss is
the result of listening to loud music on headphones or earbuds from stereos or ipods. The wearer may be unaware
just how high the sound level is. Some doctors have said that an earphone is "like the nozzle of a fire hose stuck
down the ear canal." http://sportsmedicine.about.com/od/tipsandtricks/a/iPod_safety.htm
The Quality of Sound
Musical instruments sound very different from one another, even when playing the same note. This is true because
most sounds are made up of a number of frequencies. The quality of a sound depends on the relative intensities of
these frequencies. In physical terms, it depends on the spectrum of the sound. In musical terms, sound quality is
called timbre (TOM bur) or sometimes "Tam bur in US."
When two waves of the same frequency arrive at the ear or another sound detector, the detector senses the sum of
the amplitudes of the waves. If the waves are of slightly different frequencies, the sum of the two waves has an
amplitude that oscillates in intensity. A listener hears a pulsing variation in loudness. This oscillation of wave
amplitude is called a beat, Figure 15-14. The frequency of the beat is the difference in the frequencies of the two
waves. Musical instruments in an orchestra are often tuned by sounding them against a standard note, and then
adjusting them until the beat disappears. Piano tuners also use this technique.
FIGURE 15-14. Beats occur as a result of the superposition of two sound waves of slightly different
frequencies.
Example Problem 15-2c
Beats
A 442-Hz tuning fork and a 444-Hz tuning fork are struck simultaneously. What beat frequency will be produced?
Given: f1 = 442 Hz f2 = 444 Hz
Unknown: beat frequency
Basic equation: beat frequency, f = (f2 = 444 Hz) – (f1 = 442 Hz) = 2 Hz
Do pp 15-2c
The human ear can detect beat frequencies as high as 7 Hz. When two waves differ by more than 7 Hz, the ear
detects a complex wave. If the resulting sound is unpleasant, the result is called a dissonance. If the sound is
pleasant, the result is a consonance, or a chord. As discovered by Pythagoras, consonances occur when the wave
frequencies have ratios that are small whole numbers. Figure 15-15 shows the waves that result when sound
waves have frequencies with ratios of 2:1 (octave), 3:2 (fifth), 4:3 (fourth), and 5:4 (major third).
As discussed earlier, open and closed pipes resonate at more than one frequency. As a result, musical instruments
using pipe resonators produce sounds that contain more than one resonant frequency. The lowest frequency
making up the sound is called the fundamental. Waves of frequencies that are whole-number multiples of the
fundamental are called harmonics. The fundamental is also called the first harmonic. Usually the intensity of the
higher harmonics is less than the intensity of the fundamental.
FIGURE 15-15. Time graphs showing the superposition of two waves having the ratios of 2:1, 3:2, 4:3, and
5:4.
An open pipe resonates when the length is an integral number of half wavelengths, /2, 2/2. 3/2,.... Thus, the
frequencies produced by an open-pipe instrument with fundamental frequency f are f, 2f, 3f, .... The second
harmonic, 2f, is one octave above the fundamental. The third harmonic, 3f, is an octave and a fifth (a musical
twelfth) above the fundamental. Many familiar musical instruments are open-pipe resonators. Brass instruments,
flutes, oboes, and saxophones are some examples. Closed-pipe resonators, like the clarinet, have only the odd
harmonics.
Sound can be transmitted through the air or changed into electrical energy and back into sound by a public address
system. The air transmits different frequencies with varying efficiencies that can lead to distortion of the original
sound. An electrical system can also distort the sound quality. A high-fidelity system is carefully designed to
transmit all frequencies with equal efficiency. A system that has a response within 3dB between 20 and 20 000 Hz
is considered to be very good.
On the other hand, it is sometimes useful to transmit only certain frequencies. Telephone systems transmit only
frequencies between 300 and 3000Hz, where most information in spoken language exists. Words can be
understood even when the high and low frequencies are missing. The distortion of musical sounds can also
produce effects that are interesting and even desired by musicians.
Noise consists of a large number of frequencies with no particular relationship. If all frequencies are present in
equal amplitudes, the result is white noise. White noise has been found to have a relaxing effect, and as a result
has been used by dentists to help their patients relax.
The human voice uses the throat and mouth cavity as a resonator. The number of harmonics present, and thus the
quality of the tone, depends on the shape of the resonator. Closing the throat, moving the tongue, and closing the
teeth change the shape of the resonant cavity. Even nasal cavities, or sinuses, can affect the sound quality. The
complex sound waves produced when the vowels a (as in salt) and u (as in suit) are greatly affected due to these
factors.