The front of the sound wave

Professional Definition

In wave physics, to precisely determine how energy moves through a medium, we use the geometric concept.

The front of the sound wave – a continuous surface that is the geometric location of points in the medium to which the sound wave reaches at the same time.

In other words, it is a collection of all points in space that begin to vibrate at the same moment or are in the same phase of vibration (e.g., at the point of maximum pressure).

Acoustics in Simple Words

Imagine that you are clapping your hands. The sound originates from one point (between your hands) and immediately travels away from it in every direction at a speed of about 340 meters per second.

If we could freeze time a fraction of a second after the clap and see the sound, it would look like a soap bubble growing around your hands.

• The surface of this bubble is exactly the front of the sound wave.

• All the air molecules on this surface "receive" information about the sound at the same moment.

The shape of the wavefront tells us a lot about the sound source and the distance from it:

1. Point wave: If the source is point-like (e.g., the mentioned clap), the wavefront is a sphere (a bubble).

2. Flat wave: If we are standing very far from the source, the segment of this huge sphere that reaches us seems almost flat – like a wall. It's like the Earth appears flat when we stand on it, even though it's actually a sphere.

In practice, the wavefront determines the direction in which the sound travels – the energy of the wave always moves perpendicular to its front.

Summary

The front of the sound wave is the "first line of defense" of the propagating sound. It is a virtual surface connecting points that the wave has reached at the same moment. Its shape (spherical, cylindrical, or flat) allows acousticians to determine the type of sound source and predict how it will interact with the environment.