The Basics of Acoustic Treatment
If you are anywhere like the Rapallo guys (and we know many of you areJ), you will have regular drooling bouts over some of the really, really nice AV gear that is on sale at Rapallo. But the cost of it aside, none of it will leave you satisfied if you haven’t had a close look at room acoustics.
Now, room acoustics is a rather broad topic that encompasses a wide range of issues and solutions . As a matter of fact, it’s quite a science in its own right. Bart has a bit of a passion for the science of waves, how they behave and how they can be tamed. It started off with some papers on acoustics during his engineering training, but he became rather fascinated by it when building his music/recording studio and later on his home theatre.
‘Complex’ doesn’t begin to describe it and it tends to scare off many audiophile and movie lover. But we’ll make an attempt to take you through the muck without leaving you with a headache at the end.
We’ve dedicated many blogs on room set-up, positioning of speakers, the benefits of dual subwoofers, EQ-systems, etc. Considering the vastness of the topic that is acoustics, we will merely refer you back to these blogs. But what about acoustic room treatment?
So here goes.
What is acoustic treatment?
Room acoustics is the broad term that describes how sound waves interact with a room.
Each room, and all the objects in it, will react differently to different frequencies of sound. Every speaker will sound different in different rooms.
Some of the factors that will influence the acoustic quality of your room:
- The shape of your room
- The material your room walls are made of.
- The positioning of your listening position.
- The furniture and furnishings in your room.
It makes total sense to optimise these factors first. And while we are very aware that the ideal situation is not of this world, you do want to get this is as close to it as you reasonably can.
Speaker placement is another variable that can make or break the sound quality of your set-up. So speaker placement is the other thing to optimise before looking at acoustic room treatment.
But when all is said and done on these topics, a certain level of acoustic room treatment is on the list of many audiophile and AV lover. So what’s the big deal with acoustic room treatment and why would you need it in the first place? We have a few factors at play, all of the boiling down to physics. No need to start transpiring, we’ll keep it to the absolute basics.
Room modes
So, first of all, when you consider that a room is basically a cabinet with your speakers inside it, that room is going to have a few dominant tones because of the shape and size of the room. Think of “tone” as “sonic signature” or “resonant frequency.” Those dominant tones are called room modes and an untreated room will influence what you hear by making some frequencies much louder than they should be, and other frequencies much softer than they should be. This is why music mixed in an untreated room will sound completely different when you listen to it in another room. So one of the goal of acoustic treatments is to absorb or diffuse the room tones / modes to such an extent that you can clearly hear what’s coming out of the monitors and the way it’s meant to be heard.
Early Reflection
But there is also the issue of early reflection.
Early reflection points occur in all enclosed spaces no matter the size. Here’s what happens: sound coming directly from the loudspeakers will arrive at the listener’s ears, which is all as intended. However, these same direct signals also bounce off the walls, ceiling and floors and arrive at our listening position with a time delay and mix with the direct signal. These bounced off signals are called early or first reflections because listening tests have shown that these reflections are received within milliseconds of the direct sound, and they are perceived as part of the original. The result is irregular spikes in response and smearing of dynamics.
When you read acoustic treatment recommendations from certain sources, they will recommend a whole array of absorbers and diffusers in order to ‘remove’ the room mode or any form of reflection and/or reverb. We think that this is something to be careful with. Not only will you spend a whole lot of money, ‘more’ is also not necessarily ‘better’. As with anything in life, it is a fine balance between treating something to the point where it improves or the point where you overdo it and create a very clinical sound.
Low frequencies and Bass Traps
Generally speaking, you will encounter acoustic problems with mid- and upper-frequencies; but in a small room, low end frequencies are the bigger challenge. The reason the bass response is usually more uneven than the higher end, especially in smaller rooms, is due to bass frequencies being longer and more powerful than treble frequencies. Don’t worry about this statement too much if it’s not making too much sense (Physics remember). The thing that matters is that because of that, there’s more of a chance of an inaccurate low end response.
Tackling the room modes we discussed earlier; low frequencies will build along every hard boundary surface (walls), but they will intensify in corners where two hard boundary surfaces meet. That means starting in the corners with bass traps is a good place to start. The reason for this is because this is usually the place where you’ll get the most bang for your buck as far as frequency response and decay time goes. Get the front corners… and the back corners.
Early reflection part 2
We mentioned it just earlier the issue of ‘early reflection’.
There are two ways of dealing with early reflection points. One of the is absorbing them, or you can deal with them by diffusing them. As for the difference with diffusion; while when absorbing you ‘trap’, when you diffuse, you ‘scatter’. Rough surfaces disperse audio better, while smooth surfaces (think glass, smooth walls, stone floors) create more echoes and reflections. Remember the egg cartons against the wall to treat room acoustics when we were children? The ingenious egg cartons simply act as a diffusor.
Absorption and diffusion can correct the same problem, just via different methods. Put simply, trapping will prevent sound waves from traveling around too much. Diffusion also helps with this, but the way it works is that for example, when a powerful sound wave hits a diffusor, it scatters the wave which makes it less intense and therefore less likely to interfere with the original signal.
While it may be tempting to think that killing all reflective sound is a good idea, a room with all-absorptive surfaces is going to create a ‘dead room’ and it’s not going to sound very good.
The cheapest acoustic treatment solution for early reflection are acoustic panels made of 4” insulation material (like Pink bats, insulation wool, etc.), put in a timber frame if you choose to (there is some debate around this, but that’s how we do it) and cover with a breathing material. Leave a little bit of room between the insulation material and the wall. We recommended 4” insulation, because this will also help you with bass trapping.
Obviously, there are ready made options as well from companies like Primacoustic.
Determining early reflection points
So how do you determine first early reflection points?
Sit at your listening position as you normally would when listening or mixing. Starting with the left wall, have a friend hold a mirror up against the wall next at your speaker height, then move toward the back of the room. When you can see the reflection of the left speaker in the mirror, mark that spot. That’s your first reflection point. Continue moving down the wall, and when you can see the right speaker, mark that spot. That is your other reflection point. Switch walls and repeat. If you’re dealing with multiple seating locations, repeat this process for each seat.
While corner bass traps and acoustic treatment for early reflection are an excellent starting point in our opinion, some sources will tell you to also add some acoustic panels to the front wall.
To top it off…
But that’s not where it stops. Due to the reflection of low end frequencies creating nulls and peaks at the listening position, they also recommend bass traps on the back wall. Yes, we did the corners bass traps earlier, but bass is also susceptible to reflection (in very much the same way as we explained for the early reflection) and these back wall bass traps are meant for dealing with that as well as some additional modal room treatment.
And finally, they suggest installing acoustic panels above your head in the seating position to form a ‘cloud’.
And then…you can always add some diffusors.
Are you thinking what we are thinking? For most of us, this is a hobby, right? Clearly, there is a lot more to be said on this topic. But for now, we’ll leave it at that.
Source: Gik Acoustics, Acoustic Research, Auralex, Primacoustic