Psychoacoustics Q&As

My classic simple example is the squeak/rattle on the bike you're riding - you spend ages greasing up the front because you're convinced its there, turns out its behind you.

This is a good example of jumping into the topic of localization - the concept of perceiving a sound in space. Firstly, high pitched sounds are just harder to localize (like trying to find a cricket). Secondly, this taps into our normal ways of trying to localize a sound.

Let's start with a simple situation: a sound is immediately to your right.

The first thing we do is called interaural timing differences (ITDs). This is the fact that our ears are in different places in space and sound travels at a relatively fixed speed (at least it isn't going to change speed in the distance it reaches one ear to the other). So the sound reaches the right ear first and then the left ear second. Think of this as triangulating where sound is coming from.

The other strategy we use is interaural level differences (ILDs). This is due to sound losing power as it travels a greater distance (you can also think of it as the inverse: the closer you are to a sound, the higher intensity it is). So sounds reaching that left ear will be softer.

But all sound isn't made equal and there's this thing called the head in the way between the ears. Higher frequency sounds are more likely to reflect off of the head (making level differences more substantial) and lower frequencies are more likely to wrap around the head with little drop in intensity (so timing differences are key here).

This then circles back to the bike. The squeak is on the vertical plane. Our ears are in the same space on the vertical axis, so we do not have these timing or level differences to make use of since the signal is reaching the ears more or less identically. There are potentially "pinna cues" (reflections caused by the ear on the side of the head) that can help, but its pretty tricky. Owls actually have asymmetrical ears on the vertical plane, which helps in vertical localization.

Let's talk about the anatomy of the ear before we dip our toes into perception.

We talked previously about how sound is basically 3 dimensional - there's frequency, intensity, and timing. These are the physical properties of sound. Our ear and auditory system is setup to receive these properties and it breaks down into neuronal impulses that the brain figures out.

Let's follow each parameter as it goes through the auditory system and we'll start with timing. Physical sound is the push and pull of air particles and the air pushes and pulls the ear drum, moves the bones behind the ear drum, and that motion is conveyed to the cochlea. The cochlea is what translates physical motion to neuronal impulses. Our auditory system does maintain this physical property because if we measure a test where we measure structures of the ear (called a cochlear microphonic) we see the phase relationship is maintained. But we generally do not have a strong sensitivity to phase. That is if we play a tone that is starting with a push versus a sound that starts with a pull a person is not going to perceive this sound differently.

Now for intensity, this is how strongly the air particles are moving to and fro. This then displaces the ear drum, the bones, and the opening to the cochlea further. Now we have to delve into the cochlea. The cochlea is fluid filled and has several membranes, but the key one is the basilar membrane. This is the one that moves with sound. This membrane has cells embedded in it (the famous hair cells) that then deflect against another membrane and that triggers the neural response. The cochlea with the magnitude of the basilar membrane more cells are activated. There's also stepped encoding of intensity within the auditory system, so some neurons are responsible for soft sounds, some for medium, etc. So intensity is coded within our auditory system. Ultimately, we perceive this as loudness.

I saved frequency last because it's a little strange. If you record a sound, you'll get a waveform, which is just intensity changes over time. Fourier was a mathematician/physicist who developed an idea that a complex waveform could then be described by a series of pure tones/sine waves. So if you're playing a sawtooth wave, you can think of it as a collection of sine waves that produce this sound. Doing a Fourier transform will give you a spectrum, which gives you intensity and frequency (no timing, since it is averages over a period of time). Also sounds of different frequencies will have different properties, like some sounds will resonate with different materials/objects based on their frequency (think rubbing the rim of a glass to produce a sound).

Going back to the cochlea and that basilar membrane. The basilar membrane is narrow at one end and wide at the other. This then causes different parts of the basilar membrane to resonate based on the frequency content of sound. So the ear does breakdown the frequency component of a sound and stimulates different parts of the cochlea based on the sound. This is referred to as tonotopic organization of the cochlea, so that the cochlea will code frequency information and that information is preserved as it goes up the auditory system!

Lots of anatomy today. To me, its one of those things where the more you learn about it, its crazier that it even works. And I usually spend ~3 weeks teaching those couple paragraphs, so....it's a bit simplified for this purpose. It goes much deeper and I can answer questions about that too!