Psychoacoustics and the interpretation of sound

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Psychoacoustics describes how your brain interprets audio stimuli. Anthony Grimani explains how to use what you hear to grow your business.

You may have heard the word ‘psychoacoustics’ and thought it described acousticians gone mad.

In reality it’s a fascinating field that explores the way in which our ear/brain system actually perceives sound – as opposed to plain old acoustics, which is just a linear view of sound propagation.

It turns out that knowing something about psychoacoustics can actually be beneficial to your business

So, why is it important? If you’ve done much measurement analysis of sound systems, you know that they don’t always sound like what they look like on the response chart of a computer screen. Imagine how maddening it would be to measure 10kg, but have it feel like 7kg in some rooms and 13kg in others. Well, that’s pretty much what psychoacoustics does to sound.

Our ear/brain system listens for a lot of cues, including how the direct and reflected sounds interact with each other, and the relative times and directions of different reflections. For that reason, you can’t simply measure with a traditional Real Time Analyser (RTA) and trust implicitly that the results will correlate to what you perceive sonically. Further, the automated EQ systems found in many of today’s mid and high-end products can’t be trusted, because they have no way to compensate for psychoacoustic subtleties in our hearing.

To get a result that is closer to what we hear, you have to be a little more precise and methodical in your measurement method. Instead of an RTA, use something like a Room EQ Wizard (REW). It has an impulse response mode with a time tracking filter. But you also need an audible reference point for your ears.

If you listen to pink noise on a system long enough, it starts to sound right, even when it’s not. A great trick to re-calibrate your ears is to listen to pink noise through a set of Etymotic-brand ER4S in-ear reference transducers. Go back and forth between them and the system you’re analysing and you’ll be able to quickly identify spectral errors.

So, what are some other areas where psychoacoustics comes into play?

 

Reflection decay time and properties

The amount of time sound hangs in the room has a big effect on clarity, soundstage and immersion. Our hearing is sensitive to decay times that are either too long or too short. A large body of work from about 20 years ago points to 0.3 seconds as a good target for decay time in the home cinema room sizes we use.

You can calculate it using this equation: Target time = 0.3(Cube root(V/100)), where V is the room volume in cubic metres.

Once you know the target reflection decay time, you can calculate the amount of absorption materials you need by using one of the Sabine, Eyring or Arau-Puchades predictive equations.

On average, coverage of only 20% of the wall surfaces will get you pretty much right on target. Remember to use absorptive materials that are at least 5cm thick (preferably 10cm thick) so as to control reflections down to below 500Hz.

Once you know how much absorption to use, use a proper blend of absorption and diffusion to achieve the most realistic effect in tailoring the decay time. Don’t stack one type in one part of the room. Interleave absorption and diffusion evenly.

Scattering the sound is essential as it ‘smoothes out’ the hard boundary reflections of the room and creates the sensation that the room is larger than it actually is. Use 2D diffusion (horizontal scattering only) toward the front of the room and 3D (horizontal and vertical scattering) toward the back of the room and on the ceiling and you’ll get glorious concert-hall type soundstaging. Warning: it is hard to accurately measure all this with conventional audio analysers because these times are very short and you need an omnidirectional sound source to energise the room.

Ceiling speakers for whole-house audio

Our hearing system is designed to protect us from danger. As a result, when we hear something important, our impulse is to turn our heads and look at it.

If you put speakers in the ceiling, everyone has to constantly fight the urge to look up. It may be a subtle annoyance, but it will lead to a general feeling of unrest.

If you must hide speakers in the ceiling, place them forward of the listening position. Or you can use directional ones that are aimed at walls so that you get a wash from a lower plane. As an alternative, try placing hidden speakers low in the room for a more ‘ambient’ effect.

Is it stereo?

For two-channel stereo to work right, you need to be pretty much equidistant from two speakers that form a 45º angle to your listening position.

In whole-house sound systems, there are rarely any conditions that allow this. As you move closer to one of the two, psychoacoustics dictate that you will locate mainly the sound coming from the closer one, and it only takes a difference of 60cm. It may just be better to produce good monophonic sound out of those arrays of speakers and forget about trying to fire ‘single-point stereo’ by placing two tweeters side by side… it just doesn’t help!

Equalise! Equalise! Equalise!

No matter how good a speaker and room are, when you add them together there will be frequency and phase anomalies in the response that your advanced ear/brain system will be able to hear. You will always need to tailor the spectral response in each room for it to sound right.

A lot of folks get this for home cinema but then forget about whole-house systems. But, the odd places where those speakers often get placed makes EQ even more important. Remember to use both advanced analysis (REW) and listening tests to set the system voicing.

Background noise

It’s more than just the housekeeper with a vacuum. I’m talking about things in and around the room (HVAC, plumbing, traffic noise, mechanical equipment) that produce constant or recurring sound that subtly raises the noise floor.

Not only are these sounds annoying, but they also force you to raise the volume to maintain proper dynamic range. This stresses both the speakers and your ears.

Fortunately, there are numerous products designed to control noise and a good acoustician will design room boundaries that keep the majority of noise out. Lowering the noise floor can be the simplest way to increase the perceived sound quality of your system.

Overall, if you follow these basic psychoacoustic-driven guidelines in your design work, you will get better results… and happier clients.

 

Chase Walton contributed to this column.

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Reference: Connected Home