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relative to the air is:

f=f

_{0}[u(+ or -)v

_{d}/u(+ or -)v

_{s}] . Two trains are approaching each other at a level

crossing and blow their whistles at a frequency of 500 Hz. Train 1 is travelling at 20% of the

speed of sound while train 2 is travelling at 5% of the speed of sound.

i) What is the perceived frequency of the whistle sounded by train 1 when heard by an

observer on train 2? [3]

ii) The sound intensity levels of the two trains are 88 dB and 85 dB respectively. What is

the combined sound intensity level of the two trains?

## Homework Equations

f=f

_{0}[u(+ or -)v

_{d}/u(+ or -)v

_{s}]

## The Attempt at a Solution

For part (i):

I took u=344ms

^{-1}(speed of sound)

v

_{d}=-0.05c (going in opposite direction of train 1)

v

_{s}=0.2c

since it is going towards train 1 i made the sign on the top of the equation a plus and since the source is moving towards the detector on train 2 I made the bottom sign a minus. Using this I received and answer of f=125Hz for the perceived frequency. Is this correct?

For the second part do I simply add the intensity levels together and divide by two to get an average since I am unsure on this part. Thanks in advance.