Avoiding Wimpy Theater Sound with a Proper Subs and Bass Setup

Anthony Grimani (agrimani@pmiltd.com) is president of Performance Media Industries, with offices in Novato and San Anselmo, California.

I was chatting with an integrator at CEDIA who reads this column, and we went through various topics until the conversation ultimately turned to bass. I could tell he was a little miffed about something, so I asked him what was up.

It turns out he had followed the advice that I, among others, have espoused on good bass performance. He had constructed a friendly room with resilient walls, positioned the seats away from peaks and nulls, arranged the subwoofers in a cross pattern at the midpoints of each wall, and even dialed in some EQ. He was ultimately disappointed with the way it sounded; the frequency response was smooth and even across the room, but it lacked the power, extension, and impact he was expecting.

Curious, I asked him to describe more about the setup. Everything was fine until he got to the subwoofers. He had selected four active 12- inch in-walls with small cabinets and remote rack-mount amps. They were chosen for footprint (or lack thereof) and cost–about the same as one large 18-inch. I stopped him at that point, because I knew what the problem was. He simply didn’t have enough horsepower to produce the thunderous bass levels he wanted. What’s more, the active limiting circuits in the subwoofers were kicking in to taper off the level increase and reduce the frequency extension. As he turned up the volume to get more impact, the rest of the system would get louder, but the bass simply wouldn’t. That’s a perfect recipe for wimpy sound.

The Four-Sub Layout

The practice of distributing subwoofers to four locations in cross patterns in the room is now well established, following intense and thorough research by the likes of Todd Welti and Allan Devantier. This layout can help neutralize the room’s natural standing-wave resonances. The benefits are smooth and consistent bass throughout the seating area while the liability can be less room gain, and therefore less sensation of loudness. Sometimes, however, the distributed solution actually rids the room of a giant dip in the middle, so bass levels can in fact go up.

What Bass-Intensive Movies Require

We all know that movie soundtracks are bass hogs–several of Marvel’s recent superhero movies come to mind. But I wonder if you know exactly how demanding those soundtracks can be? Let’s start with the dedicated LFE “0.1” channel. It alone can demand 115 decibels peak level from the subwoofers, typically over a maximum range of 20 to 120 Hz (20 to 80 Hz is more common for home theaters, as the bass management low-pass filter pulls out 80 to 120 Hz.) Assuming you’re using crossovers on all the main channels, the subwoofers can theoretically see higher levels up to 120 decibels if you have a bit of bass lift in the room EQ target curve.

So how many subwoofers are we talking about? Well, let’s take a typical THX-certified subwoofer that will play 105 decibels at the seating position in a 3,000 cubic foot room. If you add another one, you’ll get about four dB of additional gain (varies depending on summing conditions). To go from 109 dB to 113 dB, you’ll need two more subwoofers. The jump to 117 dB at the seating position will require four more subwoofers. That’s a grand total of eight THX subwoofers!

Sound mixes don’t have peak bass levels in all channels simultaneously. Even if they did, people will rarely kick the volume up to reference level. Every decibel below that is a decibel that the subwoofers will never see. That’s why it’s OK to go with a few smaller subwoofers. I usually tell folks to consider about 110 to 112 dB a reasonable target. That’s still four 105-dB-capable subwoofers, but you need at least that many to position them for maximum resonance neutralization. You won’t get quite as good a sum spreading them out as you would with them stacked next to each other, but four is still going to get you to around 110 dB in most cases at the main seat in a 3,000 cubic foot room.

Dealing with Limiters

There are still these pesky limiters. Most manufacturers build them in so users don’t disintegrate the voice coils by accident. Limiters are actually good things when properly implemented. They keep the subwoofer from producing high levels of distortion, wind, and port noise when stressed. Limiters can confuse you, though, when you’re trying to measure the maximum subwoofer output level.

For this exercise, I like to use a couple of chapters from the PMI/Gold Line 5.1 Audio Toolkit DVD, one which produces 115 dB SPL at the listening position, and another that is a mix of high-level bass noise, tones, and actual program material all mixed together. A non-limited subwoofer will usually bottom out with a host of pops, thumps, and snorts, allowing you to easily measure the max SPL with a good meter. With a limited subwoofer, you shouldn’t get all those noises. You will simply reach a point where the SPL doesn’t go up in. Take your measurement at that point and, if you’re below the desired 110 dB SPL, you’ll just have to try other positioning options, or get some more subwoofers!

Chase Walton contributed to this column.