(a) In a quiet room, put thumbs in your ears firmly and listen. You hear a low pulsating sound. Why ?
(b) Why such low pulsating sounds are not normally heard ?

Explanation

(a) On account of contracting and relaxing of the muscle fibres in our hands and arms, a sound of about `25 Hz` is produced.
(b) Althrough these sounds are in the audible range, these are not heard because the human ear is relatively insensitive to law frequency sounds.