Chapter 12 Sound

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

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

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

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

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

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).

The speed of sound in air given by: 331.4 m/s + 0.6 m/s (C) average speed = 340 m/s

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

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

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