You will hear a beat frequency of 6 Hz as the two tuning forks oscillate in and out of phase with each other at a rate of 6 times per second.

1. Ch 15
Sound Waves

2. Sound Wave Properties
 Sound waves are longitudinal waves consisting
of alternate areas of high pressure
compressions and low pressure rarefactions
 The frequency of a sound wave is the # of
oscillations in pressure each second
 The wavelength of a sound wave is the distance
between adjacent regions of maximum pressure

3. Velocity of a Sound Wave
 The velocity of a sound wave is 343 m/s at
20 degrees Celsius. The velocity depends
on the temperature and increases 0.6 m/s
for each Celsius degree increase in
temperature.
 Sound waves generally travel faster in
solids and liquids

4. How fast will sound travel at 30
degrees Celsius?
 Answer :
 30 – 20 = 10
 10 x 0.6 m/s = 6 m/s
 So, 343 + 6 = 349 m/s

5. How fast will sound travel at -5
degrees Celsius?
 Answer :
 -5 – 20 = -25
 -25 x 0.6 = -15
 So, 343 -15 = 328 m/s

6. Why does the siren sound different when it is
coming toward you than when it is going away?
 This is called the
doppler shift – the
apparent change in
frequency of a sound
wave caused by the
motion of either the
source or the detector

7. Doppler Equation
 fd = fs (v + vd)
(v - vs)
 fd = detector frequency
 fs = source frequency
 v = wave velocity
 vd = detector velocity
 vs = source velocity
 vd and vs are positive if they are moving towards
one another and negative if they are moving
away from one another

8. A police car traveling at 30 m/s towards us sounds
the siren at 300 Hz. What frequency do we hear if
we are standing still?
Answer:
300 (343 + 0)/(343 – 30) =
300 (343/313) =
328.75 Hz

9. A police car traveling at 30 m/s towards us sounds
the siren at 300 Hz. What frequency do we hear if
we are running away at 5 m/s?
Answer:
300 (343 + -5)/(343 – 30) =
300 (338/313) =
323.96 Hz

10. A police car traveling at 30 m/s towards us sounds
the siren at 300 Hz. What frequency do we hear if
we are running toward the car at 5 m/s?
Answer:
300 (343 + 5)/(343 – 30) =
300 (348/313) =
333.55 Hz

11. A police car traveling at 30 m/s away from us
sounds the siren at 300 Hz. What frequency do
we hear if we are running toward the car at 5 m/s?
Answer:
300 (343 + 5)/(343 – - 30) =
300 (348/373) =
279.89 Hz

12. More Sound Properties
 Sound is produced by vibrating objects
 The pitch is the frequency of the wave
 Loudness depends on amplitude
 An octave is two notes whose frequency
differ by a ratio of 2:1
 Sound level is measured in decibels (dB)
 For every 20 dB increase the sound is ten
times louder.

13. How much louder is 100 db than 60 db?
 Answer
 100 – 60 = 40
 40/20 = 2
 10^2 = 100 x louder

14. Sound Definitions
 Fundamental – lowest resonating
frequency (1st harmonic)
 Harmonics – whole # multiples of the
fundamental frequency

15. Open-pipe vs closed-pipe
resonators

16. Closed-pipe Resonators
 Closed at one end and open at the other
 The open end is a pressure node and the
closed end is a pressure antinode
 The first resonating frequency is at one-quarter
wavelength
 It resonates at odd-quarter wavelengths and
picks up only the odd harmonics
 Spacing between resonances is one-half a
wavelength

17. Open-pipe Resonators
 Open at both ends
 The ends are pressure nodes
 The first resonating frequency is at one-
half wavelength
 It resonates at all half wavelengths and
picks up all the harmonics
 Spacing between resonances is one-half a
wavelength

18. Beat
 The beat is the oscillation of the wave
amplitude produced by two waves of
slightly different frequencies
 The beat frequency is the difference
between the two frequencies

19. A 256 Hz tuning fork and a 262 Hz tuning fork are
struck simultaneously. What do you hear?
 Answer:
 262 – 256 =
 6 Hz