Transcript Sound Waves

An Intro to Waves
Hz are units of:
A. amplitude
C. period
B. frequency
D. wavelength
In a sound wave, rarefactions are areas of:
A. high amplitude
B. low amplitude
C. high pressure
D. low pressure
An Intro to Waves
Hz are units of:
A. amplitude
C. period
B. frequency
D. wavelength
In a sound wave, rarefactions are areas of:
A. high amplitude
B. low amplitude
C. high pressure
D. low pressure
An Intro to Waves
Hz are units of:
A. amplitude
C. period
B. frequency
D. wavelength
In a sound wave, rarefactions are areas of:
A. high amplitude
B. low amplitude
C. high pressure
D. low pressure
Sound Speed and Frequency:
Learning Goal
The student will be able to explain the nature
between the speed of sound in various media
and the particle nature of the media and explain
selected natural phenomena (echolocation,
infrasound/ultrasound) with reference to the
characteristics and properties of waves (E3.5,
E3.6).
Sound Speed and
Frequency
3U Physics
The Speed of a Wave
As demonstrated by the slinky waves, the speed at
which a wave propagates is dependent on the
properties of the medium.
Sound Waves
Remember that sound is a
pressure wave. A
vibration (e.g. of a
tuning fork) bumps air
particles, which bump
the air particles next to
them, etc.
Sound in Air
Sound travels faster in warmer air (in which the
particles are faster-moving):
v  332 ms  (0.6
m
s
o
C
)T
Sound in Air: Example
Example: You see a flash of lightning from a
storm 1.4 km away and hear the crack of
lightning 4.0 s later. What is the temperature of
the air?
Sound in Air: Example
Example: You see a flash of lightning from a
storm 1.4 km away and hear the crack of
lightning 4.0 s later. What is the temperature of
the air? First , find the speed of sound in air:
v?
d  1400 m
t  4.0 s
d
v
t
1400 m
v
4.0 s
v  350 ms
Sound in Air: Example
Example: You see a flash of lightning from a
storm 1.4 km away and hear the crack of
lightning 4.0 s later. What is the temperature of
the air? First , find the speed of sound in air:
v?
d  1400 m
t  4.0 s
d
v
t
1400 m
v
4.0 s
v  350 ms
Sound in Air: Example
Example: You see a flash of lightning from a
storm 1.4 km away and hear the crack of
lightning 4.0 s later. What is the temperature of
the air? First , find the speed of sound in air:
v?
d  1400 m
t  4.0 s
d
v
t
1400 m
v
4.0 s
v  350 ms
Sound in Air: Example
Example: You see a flash of lightning from a
storm 1.4 km away and hear the crack of
lightning 4.0 s later. What is the temperature of
the air? Then find the temperature :
T ?
m
m
v

332


s
v  332 ms   0.6 o s T  T 
m
C


0.6 o s
C
350 ms  332 ms
0
T

30
C
m
0.6 o s
C
Sound in Air: Example
Example: You see a flash of lightning from a
storm 1.4 km away and hear the crack of
lightning 4.0 s later. What is the temperature of
the air? Then find the temperature :
T ?
m
m
v

332


s
s
v  332 ms   0.6 o T  T 
m
C


0.6 o s
C
350 ms  332 ms
0
T

30
C
m
0.6 o s
C
Sound in Air: Example
Example: You see a flash of lightning from a
storm 1.4 km away and hear the crack of
lightning 4.0 s later. What is the temperature of
the air? Then find the temperature :
T ?
m
m
v

332


s
s
v  332 ms   0.6 o T  T 
m
C


0.6 o s
C
350 ms  332 ms
0
T

30
C
m
0.6 o s
C
Sound in Liquids and Solids
And sound will also travel faster in denser materials
(liquids and solids).
E.g. the speed of sound in water is typically 1500 m/s.
Sound in Liquids and Solids
And sound will also travel faster in denser materials
(liquids and solids).
E.g. the speed of sound in water is typically 1500 m/s.
A swimmer 1500 m away from a loud sound would
hear it the sound 1 s later through the water or
more than 4 s later through the air.
Echolocation: Example
Example: A dolphin sends out a pulse at a
frequency of 100 000 Hz and hears an echo
back from an object 2.4 s later. How far away is
the object?
Echolocation: Example
Example: A dolphin sends out a pulse at a
frequency of 100 000 Hz and hears an echo
back from an object 2.4 s later. How far away is
the object?
d  ?
v  1500 ms
t  12
. s
d
v
t
 d  vt
d  1500 ms 12
. s
d  1800 m or 18
. km
Note that the frequency doesn' t matter !
Echolocation: Example
Example: A dolphin sends out a pulse at a
frequency of 100 000 Hz and hears an echo
back from an object 2.4 s later. How far away is
the object?
d  ?
v  1500 ms
t  12
. s
d
v
t
 d  vt
d  1500 ms 12
. s
d  1800 m or 18
. km
Note that the frequency doesn' t matter !
Echolocation: Example
Example: A dolphin sends out a pulse at a
d
d  ?
v
t
000
vfrequency
 1500 ms of 100
 
d Hz
vt and hears an ec
back from an object 2.4 s later.
How far
m
t  12
. s
d  1500 s 12
. s
the object?
d  1800 m or 18
. km
Note that the frequency doesn' t matter !
Frequency
The frequency of any sound wave is always the
frequency of the source and does not change or
affect the speed of the wave as it propagates.
Frequency of a wave is how many cycles/peaks
pass a point in a second
Frequency
The frequency of any sound wave is always the
frequency of the source and does not change or
affect the speed of the wave as it propagates.
(Changing the speed will change the wavelength,
not the frequency.)
Wavelength

Distance between two similar points in
successive identical cycles in a wave
The Wave Equation

How are frequency, wavelength and velocity of a
wave connected?
V=d/t, we can substitute λ for d, so V= λ/t and
t is the period of a wave, T therefore
V= λ/T

V= λf


Frequency
Humans can hear sounds between about 20 and
20 000 Hz (though most humans lose the ability
to hear very high frequencies by the time they
are teenagers).
Most human speech is between 200 and 8000 Hz,
and the ear is most sensitive to frequencies
between 1000 and 3500 Hz.
Ultrasound
Sound above 20 000 Hz
is called ultrasound.
A common use of
ultrasound is range
finding under water:
i.e. echolocation or
sonar.
Ultrasound
Diagnostic medical ultrasonograms work on the same
principle.
Infrasound
Sound below 20 Hz is called
infrasound, which may be
produced by events such as
earthquakes.
Some animals such as elephants
use infrasound for
communication, even over
long distance.
Infrasound
While humans cannot consciously
detect infrasound, they may still be
sensitive to the vibrations:
In particular, infrasound is known
evoke feelings of awe and fear, and
frequencies close of 18 Hz, the
resonant frequency of the eye, may
cause optical illusions of “ghosts.”
Infrasound may be responsible for
many alleged “hauntings.”
Frequency and Pitch
Within the audible range,
frequency of a sound wave is
commonly referred to as the
pitch of the sound.
 A high-frequency sound
wave has a high pitch.
 A low-frequency sound wave
has a low pitch.
Frequency and Noise
A sound wave of a single frequency will be heard
as a distinct tone or note.
Irregular sound waves will be heard as noise.
More Practice
Sound Speed Quick Lab
Homework: Sound Speed and Frequency