What makes a whip crack or a bullet snap?


Clyde Caplan

Joshua Wagener, Staff Writer

The crack of a whip, the snap of a bullet, or even the boom of a fast traveling aircraft can make a sound similar to an explosion. How?

The sound associated with fast moving objects such as a whip or fighter jet involves no pyrotechnics, so how does it work? The answer is as simple as a sonic boom, but what is a sonic boom? A sonic boom occurs when any object moves faster than the speed of sound.

Cracking whip

How fast is fast?

“A sonic boom is a thunder-like noise a person on the ground hears when an aircraft or other type of aerospace vehicle flies overhead faster than the speed of sound, or ‘supersonic’.” says Yvonne Gibbs, an editor at NASA. This sound also applies to a cracking whip or snapping bullet, although the noise is not as loud. Any object that travels faster than the speed of sound will create a sonic boom. The speed of sound is Mach 1 or 767.269 mph. Which means you could get from San Francisco to New York City in just over four and a half hours. About an hour quicker than flights in subsonic airliners.

Sound waves of an aircraft at different speeds
Sound waves of an aircraft at different speeds

What does it look like?

“Air reacts like fluid to supersonic objects. As those objects travel through the air, molecules are pushed aside with great force and this forms a shock wave, much like a boat creates a wake in water. The bigger and heavier the aircraft, the more air it displaces.” says Gibbs. When a boat travels through water at slow speeds, waves appear on all sides of the boat. Much like when a pebble is thrown into the water and creates ripples. The ripples in the water can be compared to sound waves. When a boat starts to move faster through the water, the waves in front of the boat are shorter spaced than the longer spaced waves behind the boat. As the boat travels even faster the waves can not keep up and all travel behind or under the boat, creating wake. The larger the object the louder the boom, which is why aircraft create booms that are loud enough to shatter windows. Smaller objects like whips and bullets displace less air and their sonic boom sounds more like a snap.

F/A-18 Super Hornet approaching the sound barrier

How do you break the barrier?

“Because the aircraft that created the shock wave is moving faster than the speed of sound in air, the wave will appear, to trail the aircraft.” says Andy S. Rogers, a Senior Analyst at AOT, Inc. An object moving at high speeds will reach a barrier while flying at transonic speed, between 552 mph and 767 mph, where a sudden increase in aerodynamic drag occurs, as well as other undesirable effects. As the object passes through the barrier the sudden onset and release of pressure, and the combination of sound waves as they travel behind the object, create the audible boom. Any object traveling faster than the speed of sound is seen before it is heard because the sound waves travel completely behind it.

Bell X-1
Bell X-1

A great pilot.

Chuck Yeager was the first man to travel at the speed of sound in the Bell X-1 on Oct 14, 1947, flying at a speed of mach 1.06 or 813 mph. The rocket powered aircraft was carried through the air by a B-29 Superfortress, before it was dropped. Yeager then activated the rocket engine and gained speed until he broke the barrier. “Just before you break through the sound barrier, the cockpit shakes the most.” says Brig Gen Chuck Yeager USAF Ret, the pilot of the X-1. The test flight was important for the future of supersonic aircraft. Since the flight many aircraft have broken the barrier, even some larger aircraft such as the Concorde, or the B-1 Lancer. Now most modern fighter jets are supersonic.


Concept art for the future hypersonic SR-72

The future?

The sound barrier seems easy to break, all you need is some rope. However to get a human to break the sound barrier is not that easy. It took 44 years from the time the Wright Flyer took off until the Bell X-1 achieved supersonic speed. What uses for supersonic speeds will we have in the future? Cars, trains, maybe even a new supersonic passenger plane? Could we one day reach light speed? We will just have to wait and see what the future holds. For now we can admire the crack of a whip, the snap of a bullet, and the window shattering boom of a supersonic jet.