Lexicon Psychoacoustics

How much of what we hear is our own perception of it? Dr Frank Stevens, lecturer in audio and music at the University of York, chats with Daniel Dylan Wray about the science of sound


Noun: the branch of psychology concerned with the perception of sound and its physiological effects.

For listeners, music is a weapon. Use it to conjure powerful memories, form new ones; as transport to subliminal worlds or to connect to other people. Sometimes we can feel its heart gently thudding in our chests and, at its best, flow right through our entire bodies.

Magical, some might say. But what we rarely confront is why it feels this way. What is the science behind sound, and what it stirs within us? Psychoacoustics goes some way to explaining this. This science explores the relationship between the physical existence of sound, how we receive it and, critically, how we perceive it. Psychoacoustics also plays an imperative role in informing science engineers and designers developing headphones and speakers to their highest possible quality.

So, where do we begin to understand what this means for music and for ourselves? Dr Frank Stevens, Associate Lecturer in audio and music technology at the Department of Electronic Engineering at the University of York explains.

A better listening environment is one that allows you to hear what you want and to hear that more strongly than you would otherwise.

In layman’s terms, what is psychoacoustics?

It is the study of how we perceive sound, right from how we understand going from a vibration in the air to something we understand as a sound that has already happened.

What’s the difference between acoustics and psychoacoustics?

​​Acoustics is about the science of sound and how sound waves behave. For instance, say you have a balloon popping, acoustics is the study of how that sound wave moves through the air, how it interacts with buildings and how it’s absorbed.

Psychoacoustics is the process that then happens physiologically and psychologically. So, both in terms of the actual physical structure of our auditory system, but also in terms of our perception – that we recognise that is the sound of a balloon popping in a room.

Is it using science to explore our subjective response to music?

That’s a part of it. Psychoacoustics is quite a deep and broad discipline, and there are aspects of it that you could say are objective. But as soon as you start looking at perception, then it does become a subjective thing.

We can listen to a basic audio signal and explain what is going on in our auditory system but once you start to listen to tracks or albums then our subjective responses get in the way. For example, my favourite LP is Red by King Crimson and I prefer listening to it on my knackered old vinyl than on the high-quality blu-ray.

What might be a good example of how psychoacoustic theories work?

One thing everyone will have heard is MP3 files. You can use MP3 files as an example of different psychoacoustic theories because that’s how the MP3 codec was developed, to essentially exploit the human auditory system and perception of sound in order to reduce file size.

In MP3 encoding you’re throwing away some information that you don’t need so that when someone listens to it, most people can’t, in most scenarios, hear any difference. But in a proper listening environment, and you know what you’re listening out for, it’s really good to hear these things. Generally speaking, you will hear a loss in high-frequency detail and overall dynamics. With increased compression, you will tend to lose the bottom end too.

This website has a fairly good WAV versus MP3 comparison, and also gives you the difference between the two so you can hear what is thrown out – referred to here as the ‘null test’. You can imagine how these differences are not going to be clear over cheap headphones or a small Bluetooth speaker.

How about noise-cancelling headphones in this context?

You could see that as a definite example. It’s also an example of acoustic theory, digital signal processing and psychoacoustics all coming together. You think: what is the motivation for noise cancelling headphones? Well, obviously to remove noise but also to create a better listening environment. And a better listening environment is one that allows you to hear what you want and to hear that more strongly than you would otherwise.

The auditory system is crucial but most people are visual creatures. Part of psychoacoustics is also about thinking: what is the context of these sounds?

And how does it factor into the creative process of making music?

If we’re talking about music production, you might be trying to make something that sounds good. So, whether producers do it knowing the theory, or just knowing what sounds good to them, that’s using psychoacoustics in order to make something sound better.

There might be other sound designers, whether it’s other musicians with a different aim or, say foley artists, where they are trying to unsettle or trying to create a mood. Again, that is using psychoacoustic understanding of how we perceive sounds in order to get a particular effect. The Haas effect is worth looking at, as an example. Also, this video on the sampling practices of J Dilla has some good examples of how the ear can be fooled because of what we are expecting to hear.

Foley is an interesting example because that deliberately tries to alter perceptions around sound, doesn’t it?

It’s ironic that when you’re studying psychoacoustics to focus solely on the auditory system. The auditory system is crucial but most people are visual creatures. Part of psychoacoustics is also about thinking: what is the context of these sounds?

With foley that might – on its own – not sound convincing. If it’s married to the correct image, then it sounds convincing. An example is if you have footage of someone swinging a baseball bat and hitting the ball but use the sound of an axe hitting a piece of wood. If you listen you may think that it doesn’t sound right. But put them together and your brain stitches it up and tells you that that’s the right sound. So for most people, how that psychoacoustic experience relates to vision is quite important.

Can you recommend any listening exercises to practise on a home stereo system with psychoacoustics in mind?

There’s the Shepard tone, which is a tone that sounds like it is constantly increasing/decreasing in pitch. This has some uses in music, referred to as ‘barber pole flanging’ where it sounds like a flanging effect is going on forever. One example of something is the drum sound here on a My Bloody Valentine song. There is also the extra weird Risset rhythm, which is the percussive equivalent of the Shepard tone, where it sounds like it is speeding up constantly.

There is this Karlheinz Stockhausen composition Kontakte, which is an example around the continuum of tones and beats, where once a tone passes below a threshold we stop perceiving tone and start to hear beats. You can hear it really clearly starting at 17:20 where it decreases in pitch until you hear the individual pulses.

If you are a glutton for punishment there are some entertaining audio illusions making use of MIDI encoding to cause you to ‘hear’ a voice were there isn’t one.

Daniel Dylan Wray is a freelance music and culture writer based in Sheffield.

He writes for publications such as The Guardian, VICE, Pitchfork, Uncut, Bandcamp, Huck, The Quietus, Loud & Quiet and several others.

Lexicon of Sound