What Happens When an Aircraft Breaks the Sound Barrier?

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 aloud 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.

jet aircraft breaks the sound barrier
A jet fighter breaks the sound barrier. The head wave can be seen clearly. Photo: Wikipedia

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?

police car siren doppler efffect
Compressed sound waves produce louder sound, but elongated waves produce softer sound.

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 by an outside observer; the driver hears the same pitch of sound at all times. This phenomenon is called the Doppler effect, which is named after its discoverer.

What are ultrasonic waves?

Ultrasonic waves helps the doctors to see the outlines of the baby
A small device sends out ultrasonic waves inside the uterus of a pregnant woman. This helps the doctors to see the outlines of the baby. Photo: Thinkstock

Ultrasonic waves are those sound waves that are inaudible to our ears because they have a very high frequency. These energy – rich waves are used in many ways such as cleaning dental braces or spectacle glasses. Ultrasonic waves excite the iquid to vibrate so fast that it removes the dirt. Ultrasonic waves are also reflected from the inner organs of our body; thus, an echo is produced. This echo can be recorded and displayed by medical devices. In this way, doctors can safely monitor the development of a foetus inside the mother’s womb.

What is an echo?

If you call out your name in the direction of a vertical, bare rocky wall, you will hear it again after some time. An echo comes because the sound waves are reflected back from the wall. When sound is produced in enclosed spaces, such as churches and gymnasiums, multiple reflections build up and blend together. This phenomenon is known as reverberation.

How to bats orient themselves with ultrasounds?

bat emits ultrasonic waves that are reflected back by a prey
A bat emits ultrasonic waves that are reflected back by a prey. The brain observes the position of the prey from the time difference. Photo: CK12

Bats cannot see very well. To orient themselves during their night flights, they emit ultrasounds. We cannot hear these sounds. The ultrasonic waves falling on obstacles or preys get ‘reflected’ back and reach the sensitive ears of bats. Thus, they come to know what all is present around them and where their prey is. This concept is also used in shipping to determine the depth of the sea using a device called sonar. It comprises a sound transmitter, a receiver, and a clock. The time difference between sending and receiving sound waves is measured, which is used to calculate the depth of the sea.

Did you know that…

Hard materials conduct sound better than soft materials? For this reason soft materials are good sound absorbers.

Soldiers sometimes die of the bang after a grenade explodes near them? Their air sacs can burst by the propagation of explosive sound.

Astronauts can talk in space even without radio? They only need to press their helmets together; their helmets transmit sound.