1. Chapter 15 Sound What is Sound? a pressure disturbance
Mechanical longitudinal wave
moves away from the source with velocities that are characteristic of the medium
Examples of the speed of sound
Air m/s
Water m/s
Iron m/s

2. How is sound detected?
Sound detectors convert sound energy into another form of energy
The human ear is a sound detector.
It receives pressure waves and converts them into electrical impulses which are then sent to the brain for interpretation.
What are the Frequency Ranges?
0-20 Hz infrasonic
20-20,000 Hz audio
20,000 Hz ultrasonic

3. Physical Properties of Sound
1. Wavelength- distance between compressions or rarefactions
2. Frequency- # of waves/s
V=f
3. Amplitude- measure of the variation in pressure along a wave

4. Perceiving Sound Properties
1. Pitch- highness or lowness of a sound wave
- depends of the frequency of vibration
2. Loudness- sound intensity as sensed by the ear and interpreted by the brain
- depends on the pressure wave’s amplitude
Measured on a logarithmic scale called the Sound Levelmeasured in decibels

5. The Doppler Effect
A shift in frequency caused by the relative motion of the source and/or the listener
Example: As an ambulance nears the pitch of the siren gets higher. As it passes and moves away the pitch gets lower

6. Doppler Effect Fd= fs(v-vd) (v-vs) V= velocity of sound wave
Vd= velocity of the detector
Vs= velocity of the source
fs= frequency of source wave
fd= frequency received by the detector
Remember to define your coordinate system so that the positive direction is from the source to the detector

7. Example Problem 1 page 409
A trumpet player sounds C above middle C(524 Hz) while traveling in a convertible at 24.6 m/s. If the car is coming toward you, what frequency would you hear? Assume that the temperature is 20° C.
What if the car is moving away from you at the same speed?

8. Practice problem # p. 409
A sound source plays middle C(262 Hz). How fast would the source have to go to raise the pitch to C sharp(271 Hz)? Use 343 m/s as the speed of sound.

9. The Physics of Music
Resonance- the frequency at which sound waves selectively absorb energy
Is different for each type of instrument( brass, reed, string)
Let’s look at stringed instruments
Sound is produced by a vibrating object.
Human voice is produced by vibrations of the vocal chords
In brass instruments, the performer lips vibrate.
Reed instruments have a reed that vibrate
Flutes have an opening in the pipe that when air is force across the opening sets the column of air in the flute into vibration
Guitars have strings which are set into vibration

10. Resonance on Strings -has 2 nodes and an antinode in the center
-the first mode of vibration is ½ of a wavelength longFundamental or the 1st Harmonic
2nd Harmonic- 1 wavelength
3rd Harmonic- 1 ½ wavelength
Since a stringed instrument is clamped on both ends, it will have a node and antinode when it vibrates
Draw on board: 1st, 2nd, and 3rd harmonic

11. Cont. The length(L) of the fundamental is ½ wavelength L=1/2  So =2L
Look at the equation:
V=f  if we substitute in 2L we get
V=f2L

12. Cont. 2nd Harmonic would be 1 wavelength So L=
3rd Harmonic would be 1 ½ wavelength
L=3/2  or =2/3 L

13. Beats 2 waves close in frequency but out of step
- The frequency of the beat is the magnitude of difference between the frequencies of the two waves
Fbeat= |fA - fB |