Question

1/

a/ How fast should a sound source move away from you if the frequency you perceive is equal to half the actual frequency?

b/ The speed of sound in the air is 350 m / s. What would you hear if this sound source approached you?

Answer 1

a.)

Using doppler's effect:

frequency received by observer will be, when Source is moving away from observer:

f1 = f0*(V - Vo)/(V + Vs)

here, f0 = actual frequency

f1 = perceive frequency = f0/2 (given)

V = Speed of Sound = 350 m/sec

Vs = speed of source = ?? m/sec

Vo = speed of observer = 0

So,

f0/2 = f0*(350 - 0)/(350 + Vs)

350 + Vs = 350*2

Vs = 350*2 - 350

Vs = 350 m/s = speed of sound source

b.)

here,

frequency received by observer will be, when Source is moving towards observer:

f1 = f0*(V + Vo)/(V - Vs)

here, f0 = actual frequency

f1 = perceive frequency by observer = ??

V = Speed of Sound = 350 m/sec

Vs = speed of source = 350 m/sec (from above part)

Vo = speed of observer = 0

So,

f1 = f0*(350 + 0)/(350 - 350)

f1 = f0*350/0 =

So if the same sound approached you with same speed, then frequency will become very very high ( Hz), since human ear can only hear sound between 20 Hz and 20000 Hz, So you will not be able to hear this sound if it approached you.

Let me know if you've any query.