Sound in Space
Have you ever entered a large building such as a church or a hall (for example the York Guildhall) and noticed that the sound of your voice bounces around the room? The sound that you or anything else produces reflects off solid reflective surfaces such as walls, floors, and ceilings.
Sound does not travel in straight lines like a laser, but spreads out like ripples on a pond. However, this analogy models two dimensions, sound acts more like a sphere depending on directionality of a source.
These ripples from a source get fainter the further away from the source they are. This is why further away objects appear quieter than closer objects.
There are three primary components of how sound propagates through an environment:
Reflection acts like a mirror for sound, whatever sound comes into contact with the mirror is reflected off of the surface. It bounces off the surface at the same angle it came into contact with the mirror.
Reflection takes incoming pressure energy and shoots it back off the surface at the same angle which it entered.
When a sound, such as your voice, bounces off a rough surface, for example a stone wall, it’s split into different parts which reflect in different directions. This is called “Diffusion” – it’s a like a broken mirror scattering light.
Diffusion is where the incoming signal is split and dispersed throughout the environment.
Absorption acts like a sponge with water, it soaks up the sound. When sound enters a surface, it is converted into heat.
Absorption takes the incoming sound energy and converts it to heat and dissipates it to the environment.
Listening for Acoustics
Our auditory perception can quite easily help us recognise what kind of building we are in, by listening to the reflections, diffusion, and absorption in the space. A combination of all three of these occur in almost every interaction with a material, so we are able to orient ourselves in these spaces using these spatial cues.
Being able to virtualise acoustics can help someone preview what audio might sound like in a given space. This virtualisation of acoustics is known as auralisations. An auralisation is a method of imparting the spatial information of a space onto a signal to listen to a virtual version of the space. To assess an acoustic, it is helpful to create auralisations to listen to how different material will sound in a certain space. Here we can listen to two sources, an operatic singer and a set of synthesised drums. They are first played dry or anechoic (without echo/acoustic), then with the acoustic of the Jack Lyons Concert Hall at the University of York, finally an experimental acoustic using a metal slinky toy.
Singing Source
Drums Source
A tool named Acoustic Atlas allows you to move around the globe, injecting your own microphone into them. This allows you to hear how different spaces sound vastly different from one another.
Capturing Spatial Audio
Auralisations are often created using a snapshot of the acoustic environment, which we call an “impulse response”. Much like an echo, we shoot an ‘impulse’ (a very short signal) into the room and record the room’s ‘response’.
There are a number of other methods for capturing an Impulse Response, such as using an Exponential Sine Sweep. Here a single sine wave tone is played into the room and swept up the frequency spectrum covering all frequencies of human hearing. This is recorded and then later processed to form the Impulse Response.