Acoustic treatments can often be an afterthought, or worse still forgotten all together. Anthony Grimani explains why treatment matters and why.

If you’re involved with home cinema, you’ve probably heard about acoustical tuning and treatment. I’m a big proponent of it – partly because it’s my business, but mostly because it’s vitally important to sound quality. Unfortunately, I continue to see even high-end rooms that are poorly treated – or not at all. Hopefully, most of you are already up to speed on acoustical tuning products. For those who aren’t, this won’t substitute for an acoustics textbook, but it will get you started.

You Gotta Have It
A small portion of the sound from speakers goes directly to the listener; the rest bounces off other surfaces in the room.

It’s easy to figure out why that’s bad. Those reflections cause frequency and phase errors that wreck clarity, spectral balance and spatial imaging. This is especially true of speakers with poor off-axis frequency response. Even with flagship electronics and speakers you have to manage reflected sound energy to get the right balance of direct versus reflected sound at the seats.

It’s not optional, it’s not an upgrade and it’s not something you can fix with DSP. Everyone from AES members to record producers to residential integrators understands this.

Get Covered (But Not Too Much)
Sophisticated equations and computer modelling can predict exactly how much acoustical treatment a room needs. You can even tweak parameters like room construction and contents.

When you’ve seen as many prediction results as I have, you can boil it down to a simple rule of thumb: cover 15-20% of the room’s surfaces with absorbers that deaden sound reflections and another 15-20% with diffusers that scatter them.

Diffusers come in two flavours, and you need both: 2D scatter sound out in a plane and 3D scatter in a hemisphere. Bass absorption is important, too, but that’s a topic in itself.

Are You Thick?
Thick is exactly what you want when it comes to acoustical treatments. The most common error I see is to blanket everything with 2.5 cm absorptive material. This does nothing but suck out all the high frequency energy. It fails to address more serious acoustical concerns at mid and low frequencies.

In fact, the room can actually sound worse than if you did nothing; 10cm deep absorption and 10-15cm diffusion is the starting point for high-quality sound. You can get away with a small number of thinner treatments as part of a larger design, but always think thicker, not thinner. The treatments can hide behind a fabric wall anyway, so depth is not an aesthetic issue.

Mix It Up
Historically, there have been differences of opinion on the best way to distribute absorption and diffusion. Many practices that were advocated years ago are being revised in light of new research. Without going into great detail, a general balance of absorption and diffusion around the room is the objective. You don’t want one type of treatment in one area exclusively.

Here are a few more practical guidelines:
•    Spread absorption evenly throughout the room. I like using 60x120cm vertical strips of thick mineral wool.
•    Be a little absorber-heavy near the middle of the back wall to catch the front-back energy from the front speakers.
•    Favour 2D diffusion on the front portion of the side walls.
•    Go with 3D diffusion toward the back of the side walls and on the outer portions of the back wall.
•    The ceiling should be more absorptive up front; the back should be more diffuse. You want to kill reflections from the front speakers but encourage the surrounds to bounce around and increase envelopment.
•    Remember that the front wall is going to see mostly back wave from the speakers, which is low frequency. Thinner treatments do nothing in that range, so mid-bass absorbers are a good call for the front wall.
•    Opposing hard surfaces – including the faces of some diffusive/reflective acoustical treatments – can generate undesirable slap echoes. If you have a diffuser on one side of the room, put an absorber on the other.
•    Assuming that your bass absorbers actually reduce deep bass energy, it is best to place them in corners where energy is highest. Making the riser a large bass absorber (constructed from perforated metal and filled with insulation) is a clever tactic.
•    If you’re firing speakers through an acoustically-transparent screen, absorb the front wall to catch slap-back off the screen. The same applies to a baffle wall.

Benefit and Profit
In addition to the technical benefits, acoustical tuning products represent a great value. The per-unit cost is low, the margin is high and the product is easy to install and never requires maintenance.

Most acoustical treatment orders represent about 10% of the total budget. That’s $3,000 for a $30,000 room or $30,000 for a $300,000 room, so we’re not talking about pocket change.

Those are the basics. You may be thinking, “This doesn’t sound too hard. I can do the math, build this stuff and do it all myself for less money.”

Don’t.

After you’re neck and nose deep in glass or mineral fibres, sniffing glue, trying to cut odd shapes for diffusers and you still don’t know where to put anything, you’re going to wish for an alternative. And there is one. Check out some of the ready-made offerings that are pre-engineered for various applications and ready-to-hang.

For example, take a look at Wavetrain. To be fair, though, I was involved in the creation of some of their products!