Acoustic Absorption Panels – Part I. Intro
14 August 2020
A few months ago I built some acoustic absorption panels for my new home recording studio. Not only did this DIY project help keep me company during self-isolation (confused readers of the future are encouraged to look up this thing called ‘covid-19’), but the little bundles of joy actually drastically improved the acoustics of the space as well. It only felt right to share my assembly and installation process, as well as a few fancy-pants graphs to demonstrate the effect of the panels in real terms. This will be a three-part series, with this week’s post focusing on the sound wave theory behind the build. I will try to be as detailed as possible in my explanations without getting overly technical, so even without a background in audio you will hopefully be able to walk away with:
1) the confidence required to make it seem like you know what you’re doing; and
2) at least enough technical lingo to impress your mom.
Before we embark on our epic arts & crafts project, it would be good to understand why we’re doing this in the first place. Let’s start by imagining a simple scenario where nothing exists but you, the listener, at the centre of the universe (naturally), and a single speaker, some distance away, pumping out your favourite tune. With no surfaces to reflect off of, the sound can only reach you along one, direct path. Pure bliss: the speaker’s vibrating cone pushing uninterrupted sound waves straight into your ear holes.
Now, not that you care or anything, but that favourite tune of yours is actually emanating from the speaker in all directions, no matter how much it looks like it’s facing just you. This means that if we were to introduce some surface, say, a wall or a table, anywhere in the vicinity, there would be at least one other path for the sound to reach your ear holes. (In case you’re taking notes, I should point out that ear holes is not an official acoustics term.) Long story short, the sound that reflects off the surface would naturally take longer to reach you than the direct sound, which would result in a short delay between the two sounds.
The Haas Effect & Comb Filtering
What’s important to note is that the delay between the two sounds will not necessarily be perceptible to you. If the reflected sound reaches you within about 40ms of the direct sound, your ears will not even perceive the two sounds as separate—a psychoacoustic effect known as the ‘Haas effect’. Instead of a clear slap-back echo like in a cave—or, for the more socialized creatures out there, an indoor swimming pool—the delay would only ‘colour’ the sound a bit. The ‘colouration’ in this case refers to the way two overlapping sine waves can reinforce or destroy one another, just like the waves of an ocean, or an indoor swi—no wait, that analogy doesn’t work anymore. Anyway, since the reflected sound is built from the exact same combination of sine waves as the original sound, some sine waves will be 180 degrees out of phase and cancel each other out, whereas others will coincide perfectly and add together. Which frequencies will add and which will subtract is entirely dependent on the delay time, because the delay has to coincide with half of the wavelength for the frequency in question. This effect is known as ‘comb filtering’: a series of boosts and cuts in frequencies a regular interval apart, resulting in what resembles a comb across the frequency spectrum.
Normally this isn’t the end of the world—after all, we’re used to hearing coloured variations of the sounds around us all the time. Once we enter a studio environment, however, objective mixing decisions cannot be made if the perceived sound is too dissimilar from the sound actually being worked on. That’s like painting a picture while looking at it through a layer of frosted glass, or with the lights dimmed low.
Why not just boost and cut the frequencies in the other direction, then? It’s true: if we go back to our stripped down single-speaker/single-surface scenario, reversing the comb filter effect would be entirely possible. It would just be a matter of mapping out which frequencies are affected and then compensating for them at the speaker’s output: a simple, no-nonsense solution. The only thing to keep in mind would be that the distance between the listener’s ears and the speaker is what determines where those boosts and cuts occur, so the comb would only flatten out in one place in the room. Hopefully you can overlook the mild inconvenience of never, ever being allowed to move. Or even turning your head, for that matter, as that would move your ears, which would change their distances from the speaker, which would change the frequencies of our comb. But before you start looking into a neck-brace, let’s also remember the real world studio is built of multiple surfaces, all at different angles and distances from the speaker. Oh, and there’s usually more than one speaker.
All in all, it must be accepted that reflections in a studio will not ever be fully eradicated. And should not, either. Even putting comb filtering issues aside, a completely dead room (i.e. one devoid of any reflections) is not a good representation of what the listener will eventually hear. Since the chaotic, real world we live in constantly adds reflections to the sounds around us, we should try to at least partially emulate that environment in the studio.
This is where acoustic panels come in. Since sound waves lose energy as they travel through the air (as per the ‘inverse square law’), the waves that have the shortest distance to travel will have the greatest effect on the final sound. By strategically placing absorbing panels at these ‘first reflection’ points (i.e. where the waves would need to bounce off once and once only in order to reach the listener), the most obscuring delays can be reduced, allowing for more emphasis on the original, direct sound. Reduce being the key word here, though. By absorbing and/or diffracting some of the reflected sound waves, we can attenuate and/or disperse just enough reflections in order to be able to make objective decisions, but hopefully without going overboard and deadening the room too much. How much of the room exactly should be treated is a point of contention between audio engineers, but the generally accepted figure is about 30% of the total surface area.
To Diffuse or to Absorb
In a studio environment, there are many ways to help keep these and other auditory gremlins at bay, with room treatment materials largely falling into two categories: the absorbers and the diffusors. Diffusors are highly-irregular surfaces used to disperse reflections randomly, with the intention to spread the reverberation equally throughout the room. While any well-treated room contains a mix of diffusors and absorbers, I believe absorbers are the logical place to start.
First and foremost, the absorption material used in these panels is effective at dealing with low-mid frequencies, a part of the spectrum notoriously tricky to control using other forms of acoustic treatment, but which happens to also be integral to achieving a proper mix. It should be noted that while lows and low-mids like to congregate in corners of rooms, and are therefore a job better suited for bass traps (a special type of absorber), bass traps don’t do much else, and so all things considered, aren’t the best all-round solution to treating a room when nothing else is yet in place.
The acoustic absorption panels we’ll be building are light, easy to mount, and affordable, seeing as they only consist of standard construction materials and a bit of upholstery. At $20 per panel, it’s hard to justify the big brand equivalents at 20 times the price, especially when they don’t offer anything more in terms of build quality or acoustic properties. On the whole, I believe homemade acoustic absorption panels are the way to go, and definitely the place to start when building a new recording studio space. So without further ado, let’s get building! Next week, that is.