PsychoAcoustics: A Practical Guide to How Our Minds Distort Sound

You can’t believe everything you hear. No, we’re not talking about rumors, gossips or myths. We’re talking about the music you hear. We can perceive sounds as loud even at low volumes, hear the illusion of space and depth with tricks of delay, and even our eyes can fool us into hearing things we aren’t actually hearing. This brief practical guide will help you understand the ways in which our ears play tricks on our brains.

Perception of Loudness and Power

Loud music is great. Hearing damage is just the price you have pay for the vitality and great energy of live, loud music, right? Thankfully, that’s not the case. Loudness is perception-based and is not the same thing as volume.

Human ears are biased toward mid-range frequencies almost as if they’re tuned to the frequencies of human speech. They tend to hear mids much more easily at low volumes. This non-linear perception of frequencies is often described as the Fletcher-Munson curve.

Human ears are biased toward mid-range frequencies almost as if they’re tuned to the frequencies of human speech. They tend to hear mids much more easily at low volumes.

To get an idea of just how mids-biased your ears are, try this: In a quiet room, try listening to a recorded solo piano at a very low volume, then gradually turn it up (the broad frequency range of solo piano makes for a good example with a simple arrangement, but you can do this with any recording). You’ll notice that the lower and higher frequencies in the recording become more prominent as the volume increases, and that the ear’s natural mid-range boost is less apparent. The music sounds more full as it gets louder, simply because the high and low frequencies start to be heard more effectively.

In a practical sense, this mid-range bias of the human ear means that it’s entirely possible to create the perception of loudness, fullness, and even power by attenuating the mid frequencies and/or by increasing the low and high frequencies of an instrument, a studio mix, or a live soundboard without losing anything but the audience’s earplugs and hearing damage. Just listen to the attenuated mid-range of Nirvana’s “Smells Like Teen Spirit,” or the latest dance club banger at low volumes, and you’ll hear how volume and loudness/power aren’t mutually exclusive.

Our brains are also tricked in our perception of long vs. short sounds when it comes to loudness and power. We tend to think longer sounds are louder than shorter sounds of the same volume. Producers use this bias all the time, especially on drums. If you use a compressor to raise the volume of the decay of a drum hit, the drum will sound louder without actually increasing the volume of the initial occurrence of the sound. It will also sound more powerful because it doesn’t decay as fast.

Conversely, since the ear also has a natural reflex to shut down slightly in order to protect itself from very high-volume sounds, if you artificially shut down a sound after the initial transient, also known as the initial occurrence of that sound, you can mimic the ear’s protective shutdown, fooling your listener into believing that the sound is much louder than it actually is. The latter is a psychoacoustic trick that movie sound designers use on explosions all the time. My bet is that it could also be used to accentuate something like a bass drop in dance music.

Spatial Perception

In 1949, Helmut Haas was studying how humans perceive the relationship between a sound’s source and its early reflections in a room. He determined that if a sound’s early reflections occurred less than 35 milliseconds after the source of the sound and were no greater than 10 db louder than the source, then the sound and its early reflections would be perceived as one sound. This came to be known as the Haas Effect.

Setting a delay within the described parameters will essentially create a copy of the affected audio with a slight phase difference from the original. It will give a little more spatial presence without diminishing the ear’s ability to locate the source of the sound in a directional sense.

A useful way to understand the Haas Effect is to imagine being in a small club. You hear the band in front of you, but in fact you’re also hearing the sound of the band bouncing off of the wall next to you. Since the bounced sound isn’t travelling in a straight line to your ear, it arrives at your ear slightly later than the sound coming directly from the band. This thickens the sound, but doesn’t confuse your ear into thinking the band is anywhere other than right in front of you. The Haas effect can be a handy tool when placing instruments in a mix (live or recorded), but make sure you listen to a recorded mix in mono from time to time if you’re working with this type of delay, because some nasty comb filtering can happen if you’re not careful.

Our auditory spatial perception is strongly affected by how we learned to understand sound in our environment as our ears developed. Whereas boosted high and low frequencies will make something appear louder, closer and more powerful, attenuated highs, and to a lesser extent lows, will do the opposite. Our brain especially likes to turn the perception of attenuated highs into a spatial thing. Since high frequencies are fast and weak, and are easily slowed by the Earth’s atmosphere, they’re harder to hear the farther we get from a sound source. Therefore, because of our ear’s “common sense,” attenuated highs will not only make something sound quieter, but also farther away.

What We See Affects What We Hear

A couple of years ago when it was revealed that some band toured with a wall of empty, speakerless Marshall 4x12 cabinets as stage props, a bunch of people cried foul and it went viral on social media. I didn’t hear anybody complain about Tom Waits displaying an array of disconnected loudspeaker horns behind his band on his Glitter And Doom tour, however. Whichever way you tend to opine about stage props in music, it’s interesting to think about in terms of the McGurk Effect.

This illusion causes what we see to influence what we hear. In 1976, Scottish psychologist Harry McGurk brought to light this interesting phenomenon. Imagine seeing a video of someone saying ‘ba’ over and over. Now imagine the same person mouthing ‘fa’ while the audio continues to repeat the ‘ba’ sound. If you’re looking at the video, you will start hearing ‘fa,’ even though the audio is still, in fact, ‘ba’. If you search Youtube for the McGurk Effect, you’ll see a list of videos that will demonstrate this phenomenon quite effectively. Does this transfer to music performance and stage setups? I’m not aware of any studies proving either way, but I find it an interesting conversation starter. It’s a strange phenomenon that we truly do hear what we see.

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