Sound
Chapter 13
Sounds from Saturn
https://files.oakland.edu/users/blswartz/web/Images/saturn.jpg
But wait…

“But Denzmore, I thought sound couldn’t travel
through space”

Brief explanation:

Molecules transmit sound waves and although interstellar
space isn’t very dense it isn’t a complete vacuum

Light waves don’t need a medium and we can sometimes
“convert” them to sound waves

http://www.space.com/scienceastronomy/mystery_monday_
030922.html
More eerie space sounds for you

http://www-pw.physics.uiowa.edu/spaceaudio/

http://www.nasa.gov/vision/universe/feature
s/halloween_sounds.html
What is a sound wave?

Sound waves are longitudinal
(compressional) waves produced by
vibrating objects

Air molecules move parallel to the wave motion
Longitudinal (Compressional) Wave
Crests: Regions of High Density because
The coils are compressed
Troughs: Areas of Low Density because
The coils are stretched
Longitudinal (Compressional) Wave

Compression: The region of the longitudinal
wave where the density is highest

Rarefaction: The region of the longitudinal
wave where the density is lowest
Audible Sound Waves

For humans, audible sound waves have
frequencies between 20 and 20,000 Hz

Infrasonic Waves: Frequencies below 20 Hz

Ultrasonic Waves: Frequencies above 20,000 Hz
Uses for Ultrasonic Waves

Ultrasonic waves have short wavelengths
and consequently are easily reflected off
small objects

That’s how ultrasounds are produced!
Frequency and Pitch

Pitch: How high or low we perceive sound to be

The frequency of a sound wave determines the
pitch


High frequency = High Pitch
Low frequency= Low Pitch
http://phet.colorado.edu/simulations/sims.php?sim=Sound
The Speed of Sound

The speed of sound depends on the
medium and the temperature of the
medium

Solid- Fastest speed of sound
Gas- Slowest speed of sound


Higher temps mean the speed is higher
Speed of Sound in Various Media(p.482)
Shape of a sound wave

Sound waves travel away from a vibrating
source in all three dimensions

Therefore sound waves are spherical waves
http://wpcontent.answers.com/wikipedia/commons/1/12/Spherical_wave2.gif
Spherical Waves

Wave front: Center of
compression

Ray: Lines
Perpendicular to the
wave fronts that indicate
the direction of motion
What is The Doppler Effect?

http://www.colorado.edu/physics/2000/appl
ets/doppler2.html
Explaining the Doppler Effect

Relative motion between a source and an
observer creates a change in frequency

Since pitch depends on frequency, relative
motion creates a change in pitch
Illustration of the Doppler Effect
Explaining the Illustration

The car is moving toward person A.
Although the frequency of the horn is
staying the same, since the car is moving
toward person A, the wave fronts “bunch
together”. The wavelength decreases, the
frequency increases and that increases the
pitch.
Doppler Effect for Light
M33: Which way is it rotating?
Sound Intensity (p.487)

As sound waves travel, energy is
transferred from one air molecule to the
next.

The rate at which energy is transferred
through a unit of area is called INTENSITY
Equation for Intensity (p. 487)

Intensity of a spherical wave:
Power
P
Intensity 

2
4 (distance from source)
4r 2
Intensity (measured in Watts/m2) decreases as
distance from source increases
Sample Problem p. 488 #4

How much power is radiated as sound from
a band whose intensity is 1.6 x 10-3 W/m2
at a distance of 15 m?
Solve the problem



Given:
I= 1.6 x 10-3 W/m2
r= 15 m
P  I 4r  (1.6 x10
2
3
P
I
2
4r
W
2
)(
4

(
15
m
)
)  4.52 Watts
2
m
Intensity and Frequency (p.489)

Intensity and frequency determine which sounds
are audible.

Sounds below 50 Hz and above 12,000 Hz need
to be relatively intense to be heard
Graph on p. 489 of your book
Decibel Level (p. 490)

Decibel Level (measured in dB; also known
as relative intensity): Found by relating the
intensity of a given sound to the intensity at
the threshold of hearing.


A difference in 10 dB means the sound is approx
twice as large.
i.e. a 30 dB sound is twice as large as a 20 dB
sound
Intensity to Decibel Level p. 490

When the intensity of a
sound is multiplied by
10, the decibel level
increases by 10
Sample Problem (not in book)

When the decibel level of a crowded room
rises from 20 dB to 40 dB, how much
louder does the noise seem? If the original
intensity of the noise was 1.0 x 10-10 W/m2,
what is the new intensity?
Resonance (pp. 491 – 492)

Resonance: the tendency of a system to oscillate
at maximum amplitude at certain frequencies,
known as the system's resonance frequencies

In other words, resonance occurs when the
frequency of a force applied to a system matches
the natural frequency of the vibration of the
system
Tacoma Narrows Bridge