Question

In each of the following diagrams, an object that emits sound is pictured. The sound waves are represented by the black circles.
Which picture best represents a situation where a person to the right of the object will hear a higher-pitched sound because the object is moving towards the person?

[Assume, medium is stationary ; Source and Observer are collinear]

• A.
• B.
• C.
• D.

Answer 1

The correct option is D

If a point source of sound is stationary, then sound waves propagate in all directions in the form of spherical wavefronts with speed $v$ and wavelength $\lambda$ in the medium.

When the source starts moving towards right when producing waves, in period of one oscillation, the source emits one wavelength, but in this duration, the source also move forward by a distance. Thus, the effective wavelength of the emitted sound in stationary air medium is slightly compressed by this distance.

So, a high pitched sound is heard when the object is moving towards the observer (person). This situation is represented by the picture in option (d).