Adventures In Audio

What can we learn about room acoustics from this image?


Auralex, a noted manufacturer of acoustic treatment suitable for the home studio, offers their Roominator starter kit to "immediately improve the acoustics of any small to medium-size room". So what can we learn?

What can we learn about room acoustics from this image?

Charles Darwin. He didn't invent evolution - evolution invented evolution. But Darwin described it well...

"Any small random change in an individual of a species that makes that individual more likely to survive to reproduce will be to the benefit of the species as a whole."

Well he didn't exactly say those words, but I think I've caught the gist.

So, to put this into a recording context, any small improvement that you can make to your studio or your process will be to the benefit of your future successful career as a writer, musician and producer.

I left out the random part because you don't want to be making just any old changes but changes that are carefully considered to help propel you along your desired path.

So, to say this more directly, you don't need a professionally-designed and built studio, created by one of the world's top acoustic consultants, to make better recordings...

You just need to make small improvements.

Yes big improvements would be better, but any small improvements are very well worth having, particularly if they are easy to apply so you can spend more time on recording, and cheap so you can invest your money in your career.

And so with studio acoustic treatment. Many so-called experts will tell you that it's a waste of time unless you spend pro dollars and go the all-or-nothing route. And because you don't have pro dollars, it's better to settle for nothing.

Well Charles Darwin's pet Galapagos Islands giant tortoise would know better.

Small improvements

I've heard over and over, many times over and over, in work sent in by Audio Masterclass students, that small improvements in acoustic treatment - and by small I also mean easy and cheap - can make a fantastic difference to the quality of their recordings. This is particularly noticeable in speech which is very sensitive to the quality of the acoustic, and everything else in the recording process. The same will apply to mixing.

Easy and cheap improvements can be made by bringing lots of soft materials into the recording or mixing area - small rooms in particular benefit from this. These soft materials can be curtains, clothing, carpet or bedding - I can tell you there's an awful lot of acoustic treatment in a mattress.

It's also good to have irregular surfaces to diffuse reflections. An ideal room will have a combination of absorption and diffusion. For cheap diffusion I've often liked bookshelves. You can stock them with the leftovers from a jumble sale - the organizers will be happy for you to take them off their hands for a modest contribution to their cause. Or choose weighty tomes that will convince people how intelligent you are.

Let's stick with absorption for now because it's my feeling that in small to medium-size rooms it is much more important than diffusion.

What can we see?

So let's turn to the image. Here it is again (click to enlarge), with some letters that I've superimposed so I can make my points more easily...

Auralex roominator with identifiers

This is the Roominator starter kit from Auralex, and the pic tells a lot about how acoustic treatment should be done to get the most bang for the buck.

Clearly this is a mixing setup so for today let's leave acoustic treatment for recording off the table.

Now let me say that this is a starter kit. It's the answer to the question, "What's the least I can do to make a decent improvement to my acoustics?". Of course you can do more. But this really will get you started.

Firstly let's consider those loudspeakers...

Although it is easy to imagine that sound comes out of the front of the loudspeaker and from no other of its six surfaces, sound comes out from all around the cabinet. In fact there is a school of thought that if you make the cabinet really thick and heavy then what comes out from the top, bottom and sides will sound dreadful in comparison to a lighter cabinet that has been carefully designed to minimize resonances, even if the level of the cabinet radiation is higher.

And there's diffraction too where sound bends around corners.

A lightbulb moment

But forget about the loudspeaker. Replace it with a tiny torch (flashlight) bulb. Let's start on the left. And the walls and ceiling are all mirrors.

So sitting in your producer's chair, in your most attentive posture, where do you see the reflections of the bulb?

You'll see a reflection in the wall in front of you. Exactly where absorber A is in the image.

Look to your left. You'll see a reflection of the bulb at the site of absorber B.

Look up. You'll see a reflection of the bulb where absorber C would be.

These absorbers are placed exactly where light will reflect from the bulb to your eyes as you sit in the producer's chair.

Now replace the bulb with the loudspeaker. Take away the mirrors and leave hard surfaces. Don't put the absorbers in yet.

Sound will reflect from the front wall, side wall and ceiling in exactly the same way as light. We call reflection from a hard surface specular reflection and, if you want to spend some time on the web, try looking up angle of incidence equals angle of reflection. Or just remember it from your school science. (Actually I'd like to say that the angle of reflection equals the angle of incidence, because incidence comes first and reflection follows, but that's just me.)

So let's put in the absorbers. As in the image, they should be placed exactly where sound will reflect directly to your ears. That's where they will do the most good, and that's where they are placed in this image.

By the way, notice that the absorbers are ridged. Research has shown that a simple rectangular tile performs better than a tile with material cut away to form ridges or cones. This seems to me to be a fashion sparked off by the huge foam wedges used in anechoic chambers, but that's on a far larger scale of things.

The thicker the absorber is then the more effective it will be down to lower frequencies. Ideally the thickness of the absorption should be a quarter of the longest wavelength (which is the lowest frequency) of the sound you're trying to absorb, but anything over around a tenth of that wavelength will do useful work for you. Or you can space the absorber away from the wall by that distance to catch the fastest-moving air molecules.

Standing waves

The other absorbers you will see in the image are positioned in the corners of the room. You will often see statements like, "Sound builds up in the corners" as justification for placing acoustic treatment here.

Here's a more detailed way to look at it...

Standing waves are resonances in the room where sound bounces back and forth between parallel surfaces (and via more complex routes) such that the incident and reflected waves are in phase (i.e. reinforce each other).

This causes regions of high and low pressure that do not move. If you set up an oscillator at the right frequency and create a standing wave purposely, you can walk across the room and hear the high points and low points in the wave.

Always, right on the surfaces are regions where the pressure of the sound wave is as high as it gets. There may be other high-pressure regions between the surfaces, but the surfaces are - guess what - the best and most convenient places to put the absorption. (Bear in mind my points on thickness and spacing above.)

And the corner between two surfaces - well you get two absorptions for the price of one, of the standing waves between two pairs of parallel surfaces.

The trihedral corners are in theory even better because they catch all three pairs of parallel surfaces in a cuboid-shaped room, but Auralex has made what in my mind is a good compromise by placing the absorption around head height, sitting or standing.

By the way, porous absorption is not the best way to reduce standing waves. You would need tuned panel or membrane absorbers for that. But then you need to know acoustic theory or hire an acoustic consultant, and have absorbers constructed to the requirements of your room. We're back in pro dollar territory again.


So, the Roominator. I think Auralex has done a good piece of work here, both in providing the kit to buy, and letting us see the diagram showing how it should be set up. If you don't buy the actual product and use soft materials from around the house or sourced cheaply, then you now know where to place them.

You can learn more about Auralex's Roominator kits here...

Thursday February 13, 2020

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David Mellor

David Mellor

David Mellor is CEO and Course Director of Audio Masterclass. David has designed courses in audio education and training since 1986 and is the publisher and principal writer of Adventures In Audio.

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