Question

What is the intensity of a sound with an intensity of 10^-7 W/m^2?
A. 50dB
B. 20dB
C. 20dB
D. 40dB

Part A cyclist is traveling at a constant velocity of 5.00m/s. The cyclist approaches a stationary musician playing a note frequency 281Hz. The air is still and the speed of sound is 340m/s. What is the frequency of the note that the cyclist hears?
A. 290Hz
B. 283Hz
C. 277Hz
D. 285Hz
Part B. A cyclist is traveling at a constant velocity of 5.00m/s. The cyclist approaches a stationary musician playing a note frequency 281Hz. The air is still and the speed of sound is 340m/s. What is the frequency of the note that cyclist hears after he passes the musician?
A. 270Hz
B. 277Hz
C. 274Hz
D. 284Hz

Answer 1

The sound intensity level (dB) = \[10*\log_{10} (I/I_{0})\]

I =\[10^{-7} W/m^{2}\]\[I_{0}=10^{-12} W/m^{2}\]

Sound intensity =\[10*\log_{10}(10^{-7}/10^{-12} )=10*\log_{10} 10^{5}=10*5=50dB\]

Answer = option (A)

Part A

This is Doppler's Effect, you approaching the source, the frequency increases

\[f^{'}=f*(v+v_{0})/v\]

= 281*(340+5)/340=285Hz

The answer is option (D)

PartB

This is Doppler's Effect, you moving away from the source, the frequency decreases

\[f^{'}=f*(v-v_{0})/v\]

= 281*(340-5)/340=277Hz

The answer is option (B)