# 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? [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=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.