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SOUND
a range of compression wave
frequencies to which the
human ear is sensitive
The audio spectrum
extends from approximately
20 Hz to 20,000 Hz.
Range of Some Common Sounds
Intensity Range for Some Common Sounds
Sounds are produced by vibrating matter.
1. reeds
3. membranes
2. strings
4. air columns
Sound is a mechanical wave (longitudinal).
It will not travel through a vacuum.
Sounds possess the
characteristics
and properties that
are common to all
waves.
Just like all longitudinal (compression)
waves, sound waves possess a
velocity, frequency, wavelength,
phase, period, and amplitude.
Sound waves also reflect, refract,
diffract, and interfere.
The velocity of sound in air depends
on the air temperature. The speed of
sound in dry air is 331.5 m/s at 0 ºC.
This speed
increases
with temperature:
about 0.6 m/s
º
for every 1 C
increase in
temperature.
Sound Probs.
1.
2.
3.
4.
5.
What is the speed of sound in air if the
temperature is 306 K?
Find the f of a sound wave moving in air at room
temp (20 C) with a wavelength of .667 m.
What are the smallest and largest wavelengths
that the human ear can detect at 20 C?
If you clap your hands and hear the echo from a
distant wall .20 s later, how far away is the wall?
What is the frequency of sound in air at 20 C
having a wavelength equal to the diameter of a
38 cm woofer loud speaker? Of a 7.6 cm
tweeter?
Sound generally travels fastest
in solids and slowest in gases,
but there are some exceptions.
Medium Velocity (m/s)
Medium Velocity (m/s)
Air 330
Carbon dioxide 260
Helium 930
Hydrogen 1270
Oxygen 320
Water 1460
Sea water 1520
Mercury 1450
Glass 5500
Granite 5950
Lead 1230
Pine wood 3320
Copper 3800
Aluminium 5100
The human ear relates
amplitude to
loudness
and
frequency to
pitch.
Listen to various sound frequencies here
and mixtures of sound waves here.
Click here to make your own sound waves.
You should hear that frequency
relates to pitch and amplitude relates
to loudness (for a given frequency).
Sound waves refract.
Click here to view a simulation
of the refraction of sound waves.
* Fact *
All objects have a natural
frequency of vibration.
Resonance - the inducing
of vibrations of a natural
rate by a vibrating source
having the same frequency
A resonant air column is
simply a standing longitudinal
wave system, much like
standing waves on a string.
closed-pipe resonator
tube in which one end is open
and the other end is closed
open-pipe resonator
tube in which both ends
are open
A closed pipe resonates when the length
of the air column is approximately
an odd number of quarter
wavelengths.
l = {(1,3,5,7,…)/4} * l
With a slight correction for tube diameter,
we find that the resonant wavelength of a
closed pipe is given by the formula:
l = 4 (l + 0.4d),
where l is the wavelength of sound,
l is the length of the closed pipe,
and d is the diameter of the pipe.
An open pipe resonates when the length
of the air column is approximately
an even number of quarter
wavelengths long.
l = {(2,4,6,8,…)/4} * l
With a slight correction for tube diameter,
we find that the resonant wavelength of an
open pipe is given by the formula:
l = 2 (l + 0.8d),
where l is the wavelength of sound,
l is the length of the closed pipe,
and d is the diameter of the pipe.
 Oscilliscope video (saved in TAMU file)
 http://scientist.wetpaint.com/page/'The+S
peed+of+Sound+in+AIR+using+RESON
ANCE'+Lab?t=anon
Sound and strings
 Velocity depends on tension and mass per unit
length of the string.
V=√(FT/μ)
FT=Force of tension in string
μ=mass/unit length of string
A piano string is 1.10 m long and has a mass of 9.00
g. How much tension must the string be under if it
is to vibrate at a fundamental frequency of 131 Hz?
What are the frequencies of the first four
harmonics?
Click here to see a simulation of
standing waves in a resonant tube
(closed and open).
Learn more about resonance here.
Why aren’t there “black keys”
between every two “white keys”
on a piano keyboard?
Can you look at this
chart of notes and
frequencies for the
“white keys” and
decide where
“black keys” should
be placed?
Note
Frequency
(Hz)
A
220
B
247
C
261.5
D
293.5
E
329.5
F
349
G
392
A
440
B
494
C
523
D
587
E
659
F
698
G
784
Now look at a graph of those values.
Does this graph help you decide?
Frequencies
800
700
600
500
400
300
200
A
B
C
D
E
F
G
A
B
C
D
E
F
G
Frequencies
800
750
700
650
600
550
500
450
400
350
300
250
200
A Bb B
C C# D Eb E
F
F# G Ab A Bb B
C C# D Eb
E
F
F# G
Note
Frequency
(Hz)
A
220
B
247
C
261.5
D
293.5
E
329.5
F
349
G
392
A
440
B
494
C
523
D
587
E
659
F
698
G
784
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