Last update: 2023-11-13

Basic Sound Physics

If you want to work with sound, you should know some basic sound physics so you understand what you're doing. This is a small introduction to provide all the basic concepts you need to know to work with sound. Sound is a small, simple concept that grows up to become a huge field of study. So let's start from the beginning and try to answer this question first:

What is sound?

We could define sound with just three words: sound is a mechanical wave with longitudinal propagation. Let's explore those three terms to try to fully understand that.

What is a wave?

A wave is a disturbance, usually described as a vibration, that propagates through space and time.

Depending on the nature of that disturbance, there are different types of waves. Our definition says that sound is a mechanical wave, so...

What is a mechanical wave?

There are different types of waves we can find in nature:

You probably still know some mechanical waves, like ocean waves! They are mechanical too, but their propagation is not longitudinal like sound.

What is a longitudinal wave?

Depending on the direction of their propagation, waves can be:

Sound propagation

This is a good representation of sound propagation:

Southampton University image, unknown author

In this animation we can see how each of these particles, either atoms or molecules, move back and forth while the wave travels through, propagating this disturbance on to its neighbours.

A remarkable fact you must understand is that particles do NOT move; they just vibrate around an equilibrium point. The disturbance, being the change in the medium pressure, is what moves through them.

There are three red particles in this example, so you can follow their movement easier.

Playing with waves

The best way of learning is playing! We can play a little bit with this a little bit using this small app. We can learn some new wave features in this example. Use Play/Pause to start/stop the simulation and play with the parameters!

GeoGebra interactive app by user Tom Walsh

Take a look at the sliders we have, this will allow us understand the following features:

Wave behavior

All waves exhibit a common behavior and sound isn't an exception. Let's take a look at some wave behaviors we need to understand about sound.


Waves of different frequencies lose power throughout their propagation. This overall volume loss is calculated through the inverse square low. This means that, doubling the distance from the source will decrease the sound pressure by half. This power loss is not the same for all waves. Higher frequency will loss power faster, providing a natural low-pass filtering effect.


The reflection of sound is similar to the reflection of light. When a sound wave hits a medium change boundary, like a wall, part of that wave is reflected back to the original medium, bouncing in angle. Single reflections are also commonly known as echoes.


Pure reflection isn't quite common in the real world, it requires a single obstacle in an open environment. However, in indoor environments, we can hear lots of reflections that bounce between all walls and obstacles in the current room. This set of reflections that usually is called reverberation or reverb and is different for each environment and is also different depending on the source position within the indoor environment. We can split the reverb in two parts:


The sound absorption concept is also something we can commonly experience in the real world. When the sound enters an obstacle, like a wall, part of the sound will reflect within it, so the sound that is transmited to the other side has less power than the original one in certain frequencies.


Diffraction is the ability of waves to bend around objects, like walls. You can probably notice that in a closed room with an open door. Any sound coming in from outside will be mostly heard near the door even if its source position isn't there. This is important when talking about sound obstruction which is the case when the direct path of the sound from its source has way less power than alternative diffracted paths. On the other side, we also have the occlusion case, where all paths from a sound source to the listener are blocked, thus the sound is heard coming from the original emitting point, but filtered by absortion.


When sound waves from different sources meet in a given point, they interact with each other to produce a new wave. The new wave simply is the arithmetic sum of all the different waves. This is called interference. The same thing happens when we mix different sounds in an audio editor. Depending on the results of that sum, which depends on the involved frequencies and their phase, we can have two types of interference: