Jets that fly faster than sound compress the air at the front. This forms a head wave. After a certain point of time, the pressure of the air gets discharged like an explosion. As a result, a loud bang takes place and we hear a loud sound. However, ultrasonic sounds are not audible to us. Bats and other animals use ultrasonic waves for orientation. Sound always propagates in the form of waves. Because of this reason we notice a change in the pitch of the siren of the vehicle passing us.
How do we perceive sound?
Sound is produced when the chords of a guitar are set in vibration. This vibratory movement then propagates in the form of waves, like the waves that arise when a stone falls in water. Waves can travel only in a medium such as water or air, but the vacuum is completely silent. Sound waves compress the air. Sometimes sound compresses the air so strongly that it causes pain in the eardrum because of the high pressure of the waves. Very deep sounds propagate into the ground, and our body feels vibrations. This is why, the deaf can also perceive sound and dance to it.
Why does a police car siren change pitch when it passes us?
As the police car approaches us, the pitch increases, and when it moves away from us, the pitch decreases. When the car approaches us, the sound waves of the siren are 'compressed' more towards us, resulting in higher pitch. When the car is moving away, the sound waves get stretched; as a result sound fades away. This effect can be noticed only try an outside observer; the driver hears the same pitch of sound at all times. This phenomenon is called the Doppler effect, which is name after its discoverer,
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