Science and Engineering

Can You Hear a Delay?

Voices and other sounds take time to move through the air. However, the time it takes for sound to cross a room is so short that you never notice a delay between hearing a voice and seeing the movement of the speaker's lips.

But if you were in a wind tunnel, might the speed of sound be different? And if so, would you be able to notice the delay between voice and lips?

Sound is a compression wave. As a wave passes, tiny differences in air pressure compress the air for a short time. The speed of a sound wave is set by how responsive the air is to these pressure differences.

A representation of a sound wave traveling through air

A representation of a sound wave traveling through air

The speed of sound in air is \(\SI[per-mode=symbol]{343}{\meter\per\second}\) under ordinary circumstances, and can be a little faster or slower depending on temperature and pressure. Sound waves in another medium, like water or railroad track, travel at a completely different speed that depends on the material properties of the medium at rest.

Thinking about moving sounds the same way we think about moving objects can lead to misconceptions. Suppose you're on a bike pedaling at a constant speed while your friend watches from the side of the path. If you throw an apple out in front of you, who will see the apple moving at a higher speed?

Your friend does, because the speed of your bike adds to the speed of your throw.

A sound wave's speed, on the other hand, is set by the medium. If you call out "Apple!" instead of throwing one, your friend would measure the sound of your voice moving at the same speed no matter how fast you're moving.

Today's Problem

You and your friend are both riding your bikes at the same speed, \(\SI[per-mode=symbol]{13}{\meter\per\second},\) separated by a distance of \(\SI{3.43}{\meter}.\)

You call out a warning about an apple in the road. How long does it take before the sound wave reaches your friend's ear?


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