Psychoacoustics 101: The science of what we hear

Illustration: Jocelyn Tsaih

Psychoacoustics lies at the center of all things audio — it’s the science of how we hear.

We often take our own senses for granted. The sight of a rainbow, the scent of fresh laundry, the taste of your favorite food — we often get so caught up in these experiences that we don’t stop and think about what’s going on behind the scenes. When it comes to sound, our bodies have developed some pretty amazing mechanisms to help make sense of it all. In fact, a lot of the technology you use on a daily basis was developed with psychoacoustics in mind, from MP3s to phone calls.

The microphones in our heads

Studying the human perception of sound has led to major advancements in both audio technology and other fields of science. This is because psychoacoustics is so interdisciplinary — it merges areas like acoustics and engineering with biology, psychology, computer science, and more. But where does it all start? Enter the ear.

Ears basically act as the microphones of the human body. They analyze incoming sound waves and send that information directly to the brain. Here’s an overview of the major parts of the human ear and the role they play in our perception of sound:

  • Pinna (or auricle): This is the part you can actually see. The pinna helps “catch” incoming sound waves and directs them into the ear canal.
  • Eardrum: This thin membrane at the end of the ear canal acts as a transducer that transforms energy from one form into another. Incoming sound waves exist as changes in air pressure, which cause small bones attached to the eardrum, called ossicles, to start moving.
  • Cochlea: This snail-shaped membrane is filled with fluid and receives amplified vibrations straight from the ossicles. A complex network of tiny hairs reacts to different vibrations based on frequency (pitch), which in turn creates a set of electrical pulses that travels along the auditory nerve and into our brain.

It’s truly amazing how our ears act just like the technology we’ve created: transducers, amplifiers, and even frequency analyzers! But, as with any piece of hardware, even our ears have their limits. For example, some sounds are way too high-pitched, or soft, or even too fast for us to actually perceive. Much of psychoacoustics is about understanding these limits.

Perceptual audio coding

We humans can only hear sound if it lies somewhere between 20 Hz and 20 kHz, and our sensitivity to each of these frequencies isn’t the same all around. If you follow an equal loudness contour graph, you’ll notice that we’re actually way more sensitive to high and mid frequencies than we are to low bass frequencies, which is important to consider when it comes to mixing audio, tuning a sound system, designing headphones, and more.

One way that we’ve taken full advantage of these discoveries is through perceptual audio coding. Ever wonder why MP3 and AAC files are so much smaller in size than WAVs or AIFFs? It’s because a lot of the original data has been removed in a process called lossy compression, and psychoacoustics helps us decide what content in a waveform can be deleted without affecting the perceived quality of the audio. Since we can only hear a certain set of frequencies, any frequencies outside of that range can be immediately removed. The same goes for any sounds that we can’t actually hear because the amplitude isn’t high enough, since they would just be perceived as silence. Then there’s the concept of masking, in which the presence of one sound affects our perception of another. Sounds events that happen too quickly, frequencies that are too close together, or loud noises drowning out softer ones are all examples of masking in which an algorithm can remove the data that we wouldn’t have heard anyway.

The result of all this compression is a file that can be up to ten times smaller in size than the original version, while sounding almost the same. Similar methods are often applied in other situations, like transmitting audio over video or telephone calls. Wonder why that hold music sounds especially bad? It may be because the audio is being passed through an encoder that was developed specifically for human speech.

The mysteries of sound and psychoacoustics

While psychoacoustics presents some very clear benefits to the worlds of computer science and audio engineering, it also influences fields like psychology and neuroscience. There’s no denying that different rhythms, tempos, musical scales, and sonic textures make us feel a certain way. Our own personal reaction is often shared by others as well. This concept that sound can affect a person’s mood and / or brain activity creates new opportunities for research and therapy. And sometimes, the ripple effects of psychoacoustics come full circle; the low-quality artifacts we hear as a result of audio being over-compressed (similar to a bitcrusher) has ultimately become its own recognizable aesthetic that’s often applied on purpose in music production. At the end of the day, all that really matters is if we like what we’re hearing.

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September 24, 2019

Matteo Malinverno Matteo Malinverno is a New York-based music producer currently working on the Content team at Splice.