Question

The frequency of a stationary siren is 1500 Hz when measured by a stationary observer. If an observer moves away from the siren at mach number .85 what is the frequency he hears? (assume room temperature so that velocity of the air is 340m/s)

I know that I would use f'=f (v - v_o)/(v + v_s)
But what I don't get is the mach number .85

Isn't mach number .85 = v_s/v?

How would I set up this problem?
Should it look like this:
=(1500Hz) (1-0)/(340m/s+.85)
=4.4Hz

Is this correct?

Answer 1

I'm not familiar with that particular term, but mach number .85 could possibly mean 0.85 times the speed of sound.

Answer 2

f'=f(v-vo)/v+vs