Question

a sound with a constant frequency is produced by the siren on top of a firehouse. Compared to the frequency produced by the siren,the frequency observed by a firefighter approaching the fire house is

Answer 1

There's an effect called The Doppler Effect, that's the "up and down" sound.
That frequency is 1.8mm and 115 per minute. 

(Hope this helped.)

Answer 2

Answer: The apparent frequency heard by a firefighter,

$n^{'}= n(1+\frac{v_{o} }{v} )$

Explanation:

Let source is stationary and produces sound of frequency n while speed of sound is $v$ and wavelength is λ, firefighter is coming towards the siren with speed $v_{o}$

The firefighter receive  $\frac{v_{o} }{\lambda}$ therefore the apparent frequency received by firefighter will be,

$n^{'}= n+\frac{v_{o} }{\lambda}$

here $\lambda=\frac{v}{n}$

therefore $n'=(n+\frac{nv_{o} }{v} )$

⇒ $n'=n(1+\frac{v_{o} }{v} )$

⇒$n' > n$

Therefore the observer will listen sound of high frequency.

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