I drive in my car at v_L= 18 m/s down a road. Two motorcycles are also driving d
ID: 1512543 • Letter: I
Question
I drive in my car at v_L= 18 m/s down a road. Two motorcycles are also driving down the road, both at 20 m/s. When one motorcycle is ahead of me, and one behind, I hear a beat frequency. The motorcycles' engines both produce a frequency of 400 Hz. The speed of sound is 344 m/s. What frequency do I hear from the motorcycle behind me? What frequency do I hear from the motorcycle ahead of me? What beat frequency do I hear? When a motorcycle is 3 m away, the intensity of the sound from the engine is 0.6 W/m^2. What is the sound intensity level (in decibels) of a single motorcycle engine at 3 m? If both motorcycles are 3 m away, what is the sound intensity level (in decibels) of the sound from both of their engines combined?Explanation / Answer
a)
from Doppler effect
f1 = (V-VL)*f/(V-VA)
f1 = (344-8)*400/(344-20) = 414.8 Hz
(b)
f2 = (v + Vl)*f/(V + VB)
f2 = (344+8)*400/(344+20) = 386.8 Hz
(c)
beat frequency f = f1 - f2 = 28 Hz
(d)
dB = 10*log(I/Io)
dB = 10*log(0.6/10^-12) = 117.8 dB
(e)
I = 2I = 0.12
dB = 10*log(0.12/10^-12) = 110.8 dB
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