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Last update: 2024-10-30

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:

• Electromagnetic: a disturbance in the electromagnetic field. Human eyes can see a range of those waves, this range is known as visible light
• Gravitational: their existence was predicted some time ago, now is confirmed and we're still experimenting with them. We don't know much about them so far!
• Mechanical waves: waves that propagate through matter like molecules and atoms. Sound is a vibration of matter, so it's a mechanical wave

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:

• Rotating: the vibration is both perpendicular and in the same wave's propagation direction so it runs in circles (like ocean waves near coast):
  

  Wikimedia Commons image by user Kraaiennest

  
  
• Transverse: the vibration is perpendicular to wave's propagation direction (like ocean waves in deep sea or electromagnetic waves/light):
  

  Wikimedia Commons image by user Christophe Dang Ngoc Chan (cdang)

  
  
• Longitudinal: they vibrate in the same direction their wave propagates. Sound vibrates in the same direction it propagates, so it's a longitudinal wave.
  

  Wikimedia Commons image by user Christophe Dang Ngoc Chan (cdang)

  
  

Sound propagation

This is a good representation of sound propagation:

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!

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

• Amplitude: this is the depth of the disturbance. The higher the amplitude, the higher the pressure changes in the medium - and also the louder the sound. We also have some names for lower and higher pressure zones, they're called rarefaction and compression respectively. This is cool to know but not quite important. In this example you can see how pressure changes move in the Longitudinal Wave zone.
• Frequency: this is the number of full oscillations per second. Humans can typically hear sounds in a frequency range between 20 and 20,000 oscillations per second but when we grow older or suffer any hearing damage, this range can be greatly reduced. The unity of frequency is s^-1 but in sound we usually use the alias Hertz, expressed as Hz. We perceive lower frequencies as bass tones and higher frequencies as treble tones.
• Equilibrium positions: once again we can choose an equilibrium point so we can see the particles vibrating around their equilibrium positions.

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.

Attenuation

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. Power loss is not the same for all frequencies. Higher frequency waves will lose power faster, providing a natural low-pass filtering effect that gives more high frequency detail to sounds near us and less detail to sounds far from us.

Reflection

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.

Reverberation

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:

• Early reflections: they're the first reflections that we receive give us an idea of the space within the room and our position against the emitter one. They come in the first milliseconds, up to 80-100ms after the direct signal.
• Reverberant field or late reverb: it's the diffuse scrambling of early reflections that keep bouncing until they lose all energy.

Absorption

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

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've got 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 absorption.

Interference

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:

• Constructive interference: the resulting wave has a bigger amplitude.
  

  MikeRun, CC BY-SA 3.0, via Wikimedia Commons

• Destructive interference: the resulting wave has a smaller amplitude.
  

  MikeRun, CC BY-SA 3.0, via Wikimedia Commons

  
  

  When two waves have opposite phases and the same amplitude the resulting wave has a 0 amplitude. This is called phase cancellation.

Doppler Effect

The Doppler Effect is a change in the frequency of the sound that depends on the relative speed between the sound emitter and the listener. When the emitter moves towards the listener, the sound will be compressed due to the waves stretching. When the emitter moves towards the listener, the sound will be compressed due to the waves expanding. This will affect the pitch of the sound, shifting to a higher pitch if the emitter gets closer to the listener (in the sample image, listening the emitter from right axis) and to a lower pitch if the emitter gets far (listening the emitter from the left axis).

Conclusion

If we want to recreate a realistic accoustic behavior to create an inmersive experience, we first need to understand how sound behaves in real life. Depending on the case, we may want to focus more on some aspects or others.