Acoustics

• Learn the different ways sound travels.

• Understand the basics of room acoustics, Room Impulse Response (RIR), and Acoustic Impulse Response (IR).

• Learn methods of recording room impulse response.

• Understand reverberation time as a property of room acoustics.

• Discover an application of the impulse response: auralisations.

There are three primary components of how sound travels through an environment:

Have you ever noticed as you enter a building, your ears can gather information about the space you are in from the way things sound? Your ears listen to the reflections, diffusion, and absorption of sounds that’s happening in the space. A combination of all three of these components occurs in almost every space that you enter.

A theatre where actors are performing a show will usually have a carpet on the floor. This is because it can absorb the harsher parts of sound and help create an environment that is suitable for listening to dialogue.  Alternatively, in a cave, the stone walls reflect sound, so we perceive the space as reverberant or 'echoey'.

Room acoustics is a study of understanding how sound behaves in different spaces, and Room Impulse Response (RIR) is a tool we can use to do this.

Acoustic Impulse Response (IR) can be considered as the ‘acoustic fingerprint’ of a space. IR describes how a space responds to sound within it.

Before we explore impulse response further, we need to think about how sound travels in space. There are three stages: direct sound, early reflections and reverberation.

Let's say we have a sound source or 'bang' in a room…

Below is a diagram that uses historically informed 3D geometry and acoustic simulation to generate room impulse responses that capture how sound would have propagated, reflected, and reverberated in St Mary’s Abbey (found in York’s Museum Gardens) before its destruction.

And here is an animation of how the sound waves would travel in this space!

An impulse is a short but loud signal that is used to excite a room. It allows us to listen to how the room responds to the impulse, and tell us about the acoustic characteristics of the space.

There are two main ways to record IRs: the transient method and the exponential sine sweep method. 

1) The transient method: using natural physical acoustics, such as hand claps, balloon pops, or a starter pistol.

2) The Exponential Sine Sweep (ESS): uses sine waves sweeping logarithmically through frequencies of interest (normally within the human hearing range i.e. 20 Hz - 20 kHz). This allows the signal to be spread over a longer length of time instead of a short moment.

ESS can be played back over a loudspeaker in a room of interest, and recorded using a microphone. After we obtain the recording, an audio process called convolution can be used to achieve the room impulse response (RIR). 

Apart from listening to resulting IRs, we can also do further IR analysis to understand the acoustic character of the space, such as reverberation time.

Auralisation is the virtualisation of acoustics. Just as a 3D render allows us to preview a building's design, auralisation allows us to preview how a specific space will sound before we ever step foot inside.

The primary purpose of auralisation is to test the suitability of a room for its intended use. For example, if we are designing a lecture hall, we need to know if the acoustics support clear, intelligible speech. By creating an auralisation, we can 'pre-listen' to a lecture in that virtual space and determine if the room’s reflections will make the speaker easy to understand.

To achieve this, we use the Room Impulse Response as the acoustic blueprint, which contains all the data about how sound bounces off the walls, ceiling, and floor. For a precise simulation, we must use an anechoic recording as our sample sound. These recordings are captured in specialised environments like the anechoic chamber at the University of York (pictured below).

An anechoic chamber is designed with deep foam wedges that absorb nearly 100% of sound reflections.

By using an anechoic source, we ensure that the final auralisation is untainted. When you listen to the result, you are hearing only how the chosen virtual space responds to the audio, providing insight into the  room's acoustic performance.

Listen to the recording below of singing in an anechoic chamber. Notice how crisp the sound is: that’s because almost all of the sound reflections are being absorbed by the walls. 

Using the RIR of the space, we can now place this singer in various locations across the world. Listen to the audio below to hear the RIR, and then the auralisation of the singing in that space.

Congratulations! You have reached the end of this module 🎉

You have now learned the basics of acoustics. You have learned how sound travels in a room and how a room impulse response (RIR) acts as an acoustic fingerprint of a space, which describes how sound responds within it. You have also learned how to record impulse responses and how they provide important acoustic properties of a space, such as reverberation time (RT60). Finally, you have explored how impulse responses can be used for auralisation, allowing us to virtualise how audio would sound in a given space.

Bonus: how would burping sound like in slinky toy?