Physics of Sound
Sound Waves
Energy, do not have mass
Sound wave = changes in pressure caused
by vibrating object
Sound needs a medium to “vibrate”
Usually air, but could be anything
Speed of sound depends upon the medium
Air = 1130 ft/sec
Water = 5000
ft/sec
Measuring sound waves
Sound waves are longitudinal waves
Vibrating object compresses the air around it.
Pushes air away leaving an area of low pressure
Vibrating object then compresses more air to create a
Measuring methods
Cycle
A single push and pull of the vibrating object
One are of compression followed by one area of
rarefaction
An initial increase in atmospheric pressure from the
norm, followed by a drop below the norm and then a
return to normal
Mathematically displayed by a sine curve
Pressure on Y axis
Measuring methods
Period (T) and Frequency (f)
Period - The time it takes to create one cycle
Frequency - The number of cycles in one second
1
f
T
Measuring methods
Example
It takes ¼ sec to create one cycle. What is
the sound wave’s frequency?
1
4 cycles per second
.25
Measuring methods
Human hearing range
Low range between 15 to 30 Hz (cycles per
second)
With enough power lower than 15 Hz can be felt, but
not heard as “sound”
High range varies with age and gender
Women - up to 20 kHz
Men – between 15 to 18 kHz
High frequency range will lower with exposure to high
Human sound range in
frequency
//www.szynalski.com/tone-generator/
Measuring methods
Frequency will determine pitch
High frequency = high pitch
Low frequency = low pitch
Tuning
Traditional orchestra would tune First Chair
Violin A first.
Remaining instruments would tune relative to that
A above middle C was tuned to about 420 Hz
As halls grew larger it was found to be desirable to
tune sharper
1939 A was established to be 440 Hz
Corresponds to the 49
thkey on a full size piano
Tuning is not a science. The relative frequency
Sound
Common
Formulas
v
f
λ
=
wave length
v = velocity of sound in medium
f = frequency
1
f
T
Measuring methods
Frequency
Pitch, tone
Amplitude
How high the pressure goes above and below
normal atmospheric pressure
Constructive Interference
Resonance
When the natural shape or structure of an
object corresponds to the frequency of a
sound
This amplifies the wave
Examples are tuning forks of similar length or
a musical instrument designed so that it’s
Scientists have detected sound
waves with frequencies as low as
13 Hz being produced by
elephants. Assuming a speed of
sound of 350 m/s, determine the
wavelength of these sounds waves.
v
f
λ=
𝟑𝟓𝟎
Sound
Common
Formulas
v
f
λ
=
wave length
v = velocity of sound in medium
f = frequency
1
f
T
A deep sea ocean vessel uses SONAR to detect the ocean's bottom. Sound
waves are emitted from the surface of the ocean and travel through the water
at 1450 m/s. The ocean bottom is 1630 m below the surface. Determine the
amount of time that passes before the sound waves are reflected back to the
surface and the frequency if the wavelength is 2m.
v
f
Indoor pests such as mice and other rodents are sensitive to ultrasonic sound waves (sound waves above the human range of frequency detection). Some companies have produced (allegedly) rodent repellant devices that emit ultrasonic waves with frequencies of approximately 45 kHz. Assuming a speed of sound of 344 m/s,
Mama G used to be the leader of South's Pep Club. As she started in on a
round of Wash Them Down the River, her voice would ring out at an
attention-drawing 855 Hz. Assuming a speed of sound in the Titan Dome of 355 m/s,
determine the wavelength of the sound waves produced by Mama G.
Determine the decibel rating of the following sound sources and their estimated sound intensities.
a. Science office at 5 PM on a weeknight: I = ~1 x 10-9 W/m2 b. South's student library after school: I = ~1 x 10-6 W/m2 c. Period 7 at the beginning of class: I = ~1 x 10-4 W/m2
d. Titan Dome on a Friday night during basketball season: I = 8.1 x 10-3 W/m2 e. Fall Out Boy concert - front row: I = 7.4 x 10-2 W/m2
dB = 10 • log ( I / 1.0 x 10
-12
W/m
2
)
Mama G used to be the leader of South's Pep Club. As she started in on a round
of Wash Them Down the River, her voice would ring out at an attention-drawing 855 Hz. Assuming a speed of sound in the Titan Dome of 355 m/s, determine the wavelength of the sound waves produced by Mama G.
On a recent PE-sponsored adventure education program, students went hiking at Devil's Head State Park. At one point, Jeremy let out a holler which reflected off a nearby rocky cliff and was detected as an echo 1.80 seconds later. Determine the distance to the
rocky cliffs. Assume a speed of sound of 344 m/s.
The intensity of sound waves decreases as the distance from the source of sound increases. The relationship between intensity (I) and distance (d) is an inverse square relationship which follows the equation I = P/(4•π•R2) where P is the power
Velocity of sound = 340 m/s
Speed of light = 300,000,000 m/s
Velocity = ( frequency) x (wavelength)
T = 1/f
______________________________________________________________________________ ____
1) Find the wavelength of FM radio station 92.3 MHz. 2) Find the period of this wave.
3) If the frequency of the above wave is tripled, what will happen to its period?
4) A hammer sends out a sound wave at 340 m/s. The sound wave bounces off a distant wall and returns in 1.4 s, how far away is the wall?
5) A tuning fork vibrates back and forth 200 times a second. Find the frequency of the tuning fork. Find the period. If the wavelength of the tuning fork is 1.8 m, find the velocity of the wave.
6) How long does it take sound to travel a mile?
7) A bat flying in a cave emits a sound and receives its echo 0.3 s later. Determine the distance to the cave wall.
•What is the source of all sound? Explain.
•What does the following diagram represent? Explain. Determine the speed of this
wave?
•Find the frequency of the following graph. (ms = milliseconds) What is the amplitude?
•What is the wavelength of a radio signal broadcast at 102.5 MHz? What is the period? •A sound wave is sent out and bounces off a large building and returns 2.8 s later.
Determine the distance of the building. •Determine the beat frequency of the
frequencies 100 Hz and 104 Hz. Explain the concept of a beat frequency.
•An ambulance sits stationary on the side of the road blaring its siren at a frequency of 300 Hz. You ride your bike towards the ambulance at a velocity of 10 m/s. What frequency do you hear? Why?
•How much more intense is a 80 dB sound that a 50 dB sound?
•What is resonance? Explain. How can a
