How does sound behave indoors?

How sound waves travel in a room

Sounds are inevitable.

When you have a person, a fan or a coffee machine, you can’t avoid the sound they are making. Indoor spaces will always have sources of sound.

The only thing we can do about it is control them.

But how do we control them?

As we’ve seen in our previous lesson, sound travels as sound waves. On the wild outdoors, sound waves travel in a straight line, because they don’t have surfaces that will reflect them.

Inside, it's a different case: you usually have at least four surfaces that can reflect sound waves.

But you can address that reflection.

A wave can bounce off obstacles, move around them or change directions when it hits them.

Sound waves behave in different ways when they meet different obstacles. The behaviours they have depend on the surfaces they meet: they can be absorbed, diffused, diffracted, reflected, refracted and transmitted. Don’t worry, it sound complicated, but it’s easy. We’ll look into each one of them.


Absorption occurs when a sound wave is absorbed by the material.

Soft fabrics, foams and other porous materials transform sound waves into heat energy.

How much energy is absorbed depends on the thickness and the nature of the material.

When you have no absorption, sound will reflect.


Reflection occurs when a sound wave touches a surface and is completely reflected.

Buildings with cement walls, which is non-absorvent, are reflective

Overall, flat surfaces, such as stone, are really reflective.

Reflections are what causes reverberations.

However, If a surface is round or curvy, it will diffuse the sound waves.


Diffusion happens when a sound wave meets a non-flat surface, like a pillar.

When waves touch these surfaces,they’re spread out in various directions.

This is useful in auditoriums, so that sound can be carried evenly to the back of the room.

It’s important you do not confuse diffusion with diffraction.


Diffraction is what happens when sound passes through an opening or bends around a surface.

When you’re talking with a open-door in a room, sound can spread through the open-door and spread in another room.
This is important for privacy of speech, along with sound transmission.


Not all materials absorb sound completely.
Imagine that you are talking with someone through a glass window.
You might not hear them completely, but you can hear them a little.
Transmission happens when a sound travels through a surface and is still heard on the other side of that surface.
Some materials have higher transmission than others.
When a sound is transmitted, it can also refract.


Refraction occurs when a wave bends as it travels from one material to another.
When a wave is refracted, it changes directions and the speed at which it travels.
This can condition the even distribution of sound waves, creating shadow zones.
These are zones where sound can’t be heard even if you can see its source.