[Question 1]. You are listening to the sound produced by two sources. Source #1

Question

[Question 1].
You are listening to the sound produced by two sources. Source #1 is 5.0 m away from you and creates a
sound intensity level of 50 dB at your present location. Source #2 is 3.0 m away from you and creates a
sound intensity level of 30 dB at your present location. Find the total intensity level at your location due
to both sources. What is the sound power of each source? Assume the sources generate uniform
spherical sound waves.

[Question 2]. A police radar gun emits microwaves at a frequency f of 24 GHz. The radar is aimed at an approaching
car and the frequency of the reflected wave is measured to be 120 Hz higher than that of the emitted
wave. What is the speed of the car?Refer to Principles of physics 4th Edition by Serway textbook for the physical constants.

Explanation / Answer

1)Intensity at location=10Log(I/10^-12)

I1=10^-7 W/m^2

I2=10^-9W/m^2

Total intensity=1.01*10^-7W/m^2

Intensity level=10log((1.01*10^-7)/10^-12)=50.043dB

P1=10^-7*4pi*5^2=3.14*10^-5W

P2=10^-9*4pi*3^2=1.13*10^-7W

2)Let the speed of car be v.

Apparent frequency at the location of car=(c+v)f/c

Apparent frequency of reflected wave=(c+v)f/(c-v)

Difference=f*((c+v)/(c-v)-1)=f*2v/c=120

24*10^9*2v/(3*10^8)=120

v=0.75m/s

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