1. Chapter 12
Sound

2. 12.1 Sound Waves All sound waves are produced by a vibrating source.
Sound waves are spherical.

3. Some terms
Point source: emitting source is small compared to the distance between the source and the listener.
Wave front: surface where the oscillations are the same.
Rays: perpendicular to wave front, traveling direction.
At large radius (far from a point source):
spherical wavefront planar wavefront

4. Characteristics of Sound Waves
Sound is a longitudinal wave
Each cycle of a sound wave includes 1 condensation and 1 rarefaction

5. Characteristics Con’t.
Sonic spectrum – range of frequency for longitudinal waves
~audible range: 20 – 20,000 Hz
infrasonic – below 20 Hz
ultrasonic – above 20,000 Hz

6. Frequency determines pitch
The effect of frequency (determined by source) is interpreted as pitch (determined by listener)
Pitch: how high or low a sound is perceived to be.
In general,
high pitch = high frequency
lower pitch = lower frequency

7. Speed of sound depends on the medium.
Sound waves travel through solid, liquid, gas
Speed determined by medium, see table (refer to pg. 407)
sound travels 15 times faster in steel 4 times faster in water than in air).
The speed also varies with temperature (most noticeable in gases).

9. The speed of sound in air
given by:
331.4 m/s m/s (C)
average speed = 340 m/s

10. The Doppler Effect
Doppler Effect: shift in frequency due to relative motion between the source & observer

12. v= speed of sound in medium
vd = speed of detector
vs = speed of source
f = frequency of source
f′ = Doppler shifted frequency

13. If motion is toward, use upper sign
If motion is away, use lower sign
(in both numerator and denominator)