#### A source of sound is moving with constant velocity of 20 m/s emitting a note of frequency 1000 Hz. The ratio of frequencies observed by a stationary observer while the source is approaching him and after it crosses him will be (speed of sound is 340 m/s) Option 1) 9:8 Option 2) 8:9 Option 3) 1:1 Option 4) 9:10

As we learnt in

Frequency of sound when observer is stationary and source is moving towards observer -

- wherein

speed of sound

speed of source

original frequency

apparent frequency

When source is approaching, frequency heard will be

Frequency heard when it crosses the observer:

Ratio of frequencies =

Option 1)

9:8

This option is correct.

Option 2)

8:9

This option is incorrect.

Option 3)

1:1

This option is incorrect.

Option 4)

9:10

This option is incorrect.