# Frequency and Intensity Level of Sound Waves

(b) The frequency of sound waves received by a detector when both source and detector move
relative to the air is:
f=f0[u(+ or -)vd/u(+ or -)vs] . 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? 
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=f0[u(+ or -)vd/u(+ or -)vs]

## The Attempt at a Solution

For part (i):
I took u=344ms-1 (speed of sound)
vd=-0.05c (going in opposite direction of train 1)
vs=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.