Why do we hear a strange sound when we cover our ear

This question also applies to the sound heard when we place such hollow objects such as a sea shell or even a cupped hand. There are various explanations given. One theory is that the sound heard is from echoing of the blood flowing through the arteries in the ear.

This theory can be disproved by the fact that the sound should increase after exercising, where blood flow is increased. But it remains the same.

Another theory is that, it is due to the air flowing in and out of the hollow object. Again this is not, since the sound stops in a soundproof room even though there is air. So the likely explanation is the amplification of the ambient noise around us.

The ambient noise resonates within the cylindrical object such as a tea cup, sea shell, or even our cupped hand.

This explains the fact that we do not hear such a sound in a soundproof room and that the intensity of sound does not increase after exercise. The frequency of sound changes depending on the size and shape of the object, as a result of difference in the resonance.