Echo Chamber: Room Acoustics Part 3

A couple of months back in part two of this series, I explained how room modes can wreak havoc on in-room frequency response, and how kinetic bass traps work by slowing down the air particles that carry sound waves, reducing the energy of the reflected waves that cause peaks and nulls in response. The problem with these types of traps is that while they are very effective at medium to upper frequencies, they can be ineffective at dealing with low frequency peaks and nulls, which can often be the most problematic areas in a room. The reason why is that the velocity of the particles directly at the wall (where you would normally mount the panel) is zero, increasing to maximum at a distance of ¼ of the wavelength. For upper frequencies this distance isn’t very long, which is why a broadband absorption panel that’s only 2” thick can still be very effective. The lower in the frequency range you get though, the longer the wavelength. Once that quarter distance starts to become several feet long, 2” (or even 6”) just won’t cut it anymore. You could try and treat problematic low frequencies with impractically massive kinetic energy bass traps, but a smarter approach is to use bass traps that are designed to target sound pressure, rather than velocity.

Pressure traps like GIK Acoustics’ Scopus tuned traps use a moving membrane contained within a sealed chamber, which absorbs pressure where it’s highest – directly at the wall surface. Unlike broadband absorbing bass traps, the membrane has to be tuned to a specific frequency range, and so pressure traps are best saved for last, when the rest of the room has already been treated, and a few stubborn deep bass problems remain.

Unfortunately, in addition to room modes, there is another, often overlooked factor that can also cause frequency response problems – speaker-boundary interference response, commonly referred to as SBIR. If you read the manual for a typical pair of speakers designed for use in a living room environment, the manufacturer will likely recommend that the speakers be placed at least a couple of feet from the front wall. Pulling the speakers out into the room like this rather than placing them directly against the wall greatly improves the sense of depth to the soundstage – but there’s a tradeoff, and that’s SBIR.

If you’ve ever used a Maglite flashlight, you’ll know that twisting the barrel around the front element allows you to change from a very small, focused beam to a much larger, more diffuse beam. If you imagine the sound emanating from a conventional speaker (forward facing drivers in a box) as a light beam, high frequencies coming from the tweeter are like that small, focused beam, going straight forward in the direction that the speaker is pointing. As you go lower in frequency, this “light beam” becomes wider and wider, until it eventually is omni-directional, wrapping completely around the speaker cabinet, and bouncing off of the front wall. It’s this wall bounce that causes SBIR.

Assuming your speakers are a couple of feet out into the room as the manufacturer suggests, there’s going to be a point in the frequency range where the speakers are one quarter wavelength from the wall for that particular frequency, causing the same sort of wave cancellation as an out of phase room mode problem. Unlike regular room modes, SBIR always causes a null in response at this quarter wavelength distance, never a peak. The reason why is that the particular problematic frequency travels a quarter wavelength to the front wall, bounces off, and then reflects back on itself for another quarter wavelength, putting it half a wavelength apart from the original source – or perfectly out of phase. And as I covered in part two, out of phase frequencies always cancel each other.

So what can you do about it? While mounting speakers right up against the wall might work fine in a studio environment, it’s not a great option in a home environment. Aside from just the loss in soundstage depth, many speakers are rear-ported, and very close positioning to a wall can cause the bass response from these speakers to sound lumpy and unrefined. If you have a very large room, you can pull the speakers out far enough that this quarter back and quarter forward distance occurs at a low enough frequency not to matter. For the rest of us, the best option is more bass traps.

That just about covers all of the major issues you’re likely to deal with bass frequencies in a traditional listening space, at least with regular speakers. Speakers with a di-polar radiation pattern like open-baffle designs and electrostatic or planar magnetic panels have their own problems, but that’s a story for another day. Next time in part four, I’ll cover how to treat the rest of the frequency range.