Download introducing sound waves

Kristela Milan
III - Galileo
Sound surrounds us all the time. From the chirping of
birds when we wake up to the croaking of frogs at
night. All the sounds that we hear have one thing in
common. Every sound is produced by vibrations of an
object. When an object vibrates, it makes the
surrounding air vibrate. The vibrations in the air travel
outward in all directions from the object. When the
vibrations enter our ears, the brain interprets them as
sounds. Although many of the sounds we hear travel
through the air, sound can move through any material.
Acoustics is the science of sound and of its effects on
people.
If you drop a pebble into a still pond,
you will see a series of waves that
travel outward from the point where the
pebble struck the surface. Sound also
travels in waves as it moves through
the air or some other medium
(substance). The waves are produced
by a vibrating object.
As a vibrating object moves outward, it compresses
the surrounding medium producing a region of
compression called condensation. As the vibrating
object then moves inward, the medium expands into
the space formerly occupied by the object. This
region of expansion is called rarefaction. Sound
waves consist of these condensations and
rarefactions.
Sound waves must travel through a medium. Thus,
sound is absent in outer space, which contains no
material for a vibrating object to compress and
expand.
Sound is a longitudinal wave because the vibrations
are in the same direction as the motion and speed of
the wave. This is most easily pictured if you imagine a
slinky stretched slightly across a tabletop.
Sound is created by the alternating
compression and expansion of air.
Sound travels at a speed of 331 m/s at
0°C. The speed increases as the
temperature of the air increases, at a rate
of 0.6 m/s per Celsius degree. If
something moves faster than sound, it is
said to be supersonic. If it travels at the
speed of sound, it travels, Mach 1; three
times the speed of sound is called Mach
3.
Frequency of sound waves refers to
the number of condensations or
rarefactions produced by a vibrating
object each second.
Pitch is the degree of highness or
lowness of a sound as perceived by a
listener.
The more rapidly an object vibrates, the
higher will be the frequency. Scientists
use a unit called hertz to measure
frequency. One hertz equals one cycle
(vibration) per second. As the
frequency of sound waves increases,
the wavelength decreases.
A higher-frequency sound has a
higher pitch. Frequencies of
sound waves that humans can
hear range from about 2020,000 hertz or cycles per
second. The loudness of the
sound is related to the wave’s
energy or intensity.
1. Wavelength is the distance between any point on one
wave and the corresponding point on the next one.
2. Infrasound is the sound with frequencies below the
range of human hearing which is 20-20,000 hertz.
3. Ultrasound is the sound with frequencies above the
range of human hearing which is 20-20,000 hertz.
4. Resonance Frequency is approximately the frequency
at which an object would vibrate naturally if disturbed in
some way.
High-pitched
sounds
have
higher
frequencies than low-pitched sounds.
Musical instruments can produce a wide
range of pitches. For example a trumpet has
valves that can shorten or lengthen the
vibrating column of air inside the instrument.
A short column produces a high-frequency,
high-pitched sound. A long column results in
a note of low-frequency and low-pitch.
Intensity of a sound is related to the amount of
energy flowing in the sound waves. Intensity
depends on the amplitude of the vibrations
producing the waves. Amplitude is the distance
that a vibrating object moves from its position of
rest as it vibrates. The larger the amplitude of
vibration is, the more intense will be the sound.
Decibel is the unit used to measure the intensity
level of a sound. A 3,000 hertz tone of zero
decibels is the weakest sound that the normal
human ear can hear.
The loudness of a sound refers to how strong the
sound seems to us when it strikes our ears. A highfrequency or low-frequency sound does not seem as
loud as a sound of the same intensity in the middle of
the frequency range because our ears have low
sensitivity to sounds near the upper and lower limits of
the range of frequencies that we can hear.
Take note: The loudness of a sound decreases as the
distance increases between a person and the source
of the sound.
Sound Quality, also called timbre, is a
characteristic of
musical
sounds.
Quality
distinguishes between sounds of the same frequency
and intensity produced by different musical
instruments.
Almost every musical sound consists of a
combination of the actual note sounded and a
number of higher tones related to it. The actual note
played is the fundamental. The higher tones are
overtones. The number and strength of the
overtones help determine the characteristic sound
quality of a musical instrument.
Loudness and intensity can be confusing
because loudness is a measurement based
on the sensation caused in a human by
sound intensity. Because the human ear is
sensitive over a very large range of intensity,
loudness is defined using a logarithmic
scale. A sound wave that is twice as loud
has roughly ten times the intensity.
β (in dB) = 10 log ( I /Io)
(Eqn. 12.1)
Where:
β – is the loudness measured in decibels (dB)
I – is the intensity of the sound wave in watt/m²
Io – is the constant equal to 1 x 10−12 watt/m²
- represents the most faintly heard sound by
humans and is defined as the loudness
of 0 dB.
Scientists use a unit called the decibel to
measure the intensity level of a sound. A
3,000 hertz of zero decibels marks the
threshold of audibility – the weakest
sound that the normal ear can hear. A
sound intensity level of 140 decibels is the
threshold of pain. A whisper amounts to
about 20 decibels. Ordinary conversation
has an intensity level of about 60 decibels.
Loud rock music can produce up to 120
decibels.
The sound in a particular place has an intensity of 2.0 x
10−7 W/m². What is the loudness of that sound? If the
intensity is tripled, what is its corresponding loudness?
Given:
Sound intensity
I
2.0 x 10−7 W/m²
Find:
A. Loudness (β) at the current intensity
B.
Loudness (β) if the current is tripled
Solution:
A. Loudness (β) at the current intensity
β (in dB)
= 10 log ( I /Io)
2.0
x
10−7 W/m²
= 10 log ( 1 x 10−12 W/m² )
= 10 log (2.0 x 105 )
= 10 (5.3) = 53 dB
B. Loudness (β) if the current is tripled
Sound Intensity = 3 (2.0 x 10−7 W/m²)
= 6.0 x 10−7 W/m²
β (in dB)
= 10 log ( I /Io)
6.0 x 10−7W/m²
= 10 log (
)
−12
1 x 10
W/m²
= 10 log (6.0 x 105 )
= 10 (5.8) = 58 dB
Phon is a unit often used to
measure the loudness level of
tones. The loudness level in
phons of any tone is the intensity
level in decibels of a 1,000 hertz
tone that seems equally loud.
The speed of sound depends on
the medium through which the
sound waves travel. The properties
of a medium that determine the
speed of sound are density and
compressibility.
DENSITY is the amount of material in a unit
volume of a substance.
COMPRESSIBILITY measures how easily a
substance can be crushed into smaller
volume.
Note: The denser a medium is and the more
compressible it is, the slower the speed of
the sound is.
In general, liquids and solids are denser
than air. But they are also far less
compressible. Therefore, sound travels
faster through liquids and solids than it does
through air. Compared with its speed
through air, sound travels about 4 times
faster through water and about 15 times
faster through steel. The speed of sound
through air is commonly measured at sea
level at 59°F. The speed of sound, which is
340 m/s increases as the air temperature
rises.
Jet airplanes sometimes fly at supersonic speeds. A plane flying
faster than the speed of sound creates shockwaves, strong
pressure disturbances that build up around the aircraft. People on
the ground hear a loud noise, known as a sonic boom, when the
shock waves from the plane sweep over them.
Medium
Speed in ft/s
Speed in m/s
Air at 59 °F
1,116
340
Aluminum
16,000
5,000
Brick
11,980
3,650
Distilled Water at
77°F
4,908
1,496
Glass
14,900
4,540
Seawater at 77°F
5,023
1,531
Steel
17,100
5,200
Wood (sample)
13,480
4,110
Table 12.1 Speed of Sound in various media
The speed of sound v is related to the frequency f and
wavelength λ of the sound wave by the following formula:
v=fλ
(Eqn. 12.2)
You could also assume sound to be traveling constant
across a medium, thus you can apply the following
velocity equation:
d = vt
(Eqn. 12.3)
Where d is the distance of the sound heard, t is the time
before you heard the sound and v is the speed of sound.
Example 12.2 Gunshot heard
Using binoculars, you saw someone fired a gun from a
distance. You heard the sound 2 seconds later. How far
away are you from the person? Use the speed of sound
340 m/s.
Given:
time before sound is heard (t)
2s
Find: Distance from the person who fired the gun (d)
Solution:
Using Eqn. 12.3, d = vt = 340 m/s (2 s) = 680 meters
away.