How Many Subwoofers to Use and Where to Put Them

April 2, 2013

 
Anthony Grimani (agrimani@pmiltd.com) is president of Performance Media Industries, with offices in Novato and San Anselmo, CA.
We need to stop reinventing the wheel where bass is concerned. I’ve met two industry veterans recently who’ve espoused very antiquated ideas as if they were new revelations of “purist” audio. We abandoned these ideas more than 10 years ago because they don’t produce smooth, tight bass for the whole audience. There’s a better way to do it.

We don’t use full-range speakers to play bass anymore. We don’t have separate subwoofers for each main channel or the LFE. That’s because in the worst-case scenario, that approach can lead to absolutely no bass in the main seating area of a home cinema because of inherent standing wave resonance issues.

Let’s review. Subwoofers showed up in the early 1980s before home theaters. Companies like Velodyne, M&K, or Kinergetics Research made them to extend the response of Hi-Fi speakers from 40 Hz down to 20 Hz and below. Later, companies like Triad, Bose, and M&K realized that speakers could be smaller and more attractive when paired with subwoofers. The early ’90s brought on multi-channel home theater systems, like THX, that used a common subwoofer for all channels. By the 2000s, we realized how important it was to place those subwoofers correctly to minimize bass standing waves. Now, thanks to the excellent and gamechanging work of Todd Welti and Allan Devantier at Harman International, we know how many subwoofers to use and where to put them for smooth response and high output.

The short and sweet version is that you need at least four identical subwoofers. Eight, 12, 16, are OK, too, but you need to have a multiple of four. Why? They go in the four corners of the room, and each corner needs equal drive capacity. It’s true that the midpoint layout typically produces the smoothest bass response by cancelling standing waves, but that layout isn’t very efficient on sound pressure levels, and it can be difficult to reach the prodigious output levels required by movies (115dB SPL peak). Moving the subwoofers to the four corners couples them to the room for higher output while still negating most standing waves. It’s a good compromise.

 
Thanks to the game-changing work of Todd Welti and Allan Devantier at Harman International, we know how many subwoofers to use and where to put them for smooth response and high output in a home theater. The short version is that you need at least four identical subwoofers. Pictured are the Niles SW6.5 and the SW8.
The subwoofers are fed from the same output of the surround processor. That feed should contain summed bass (typically below 80 Hz) from all the main channels, plus the LFE channel low-pass filtered at between 80 Hz and 120 Hz. It has been confirmed many times that frequencies below 80 Hz are not localizable, and many people cannot tell directionality as high as 120 Hz.

Now is the part where I interject my experience from the hundreds of rooms I’ve commissioned. Hooking up the subwoofers requires a manually controlled one-input, four-output audio processing box that provides separate delay, gain, and digital parametric EQ (multiple filters) for each subwoofer. Products that can perform this function are readily available from pro audio companies like BSS, Ashly Audio, Symetrix, DBX, and Peavey. All of these use high-quality digital conversion and audio processing, so sound quality is not an issue.

Begin calibration by flattening out the overall response of the subwoofer group. Make the same adjustments (delays, levels, and EQ) to each subwoofer. It doesn’t have to be perfect, just a decent starting point. But do more than just listen; employ a digital FFT analyzer or something similar with a long window. Take many measurements across the audience. A tool like LASSIE from Seagrave Instruments may be helpful. LASSIE uses a system of microphones and LEDs located throughout the room to provide visual feedback of the response. I find that, with bass response issues, you need as many tools as possible to help visualize the results.

Next, play a polarity pulse. (Pulse generators are available from the likes of Gold Line, or on test discs like the PMI 5.1 Audio Toolkit.) Try delaying the back pair or front pair of subwoofers together, 1 ms at a time. Listen for the pulse to become richer and fuller. As it does, measure the response across the audience to see if it improves, or the overall SPL gets louder. There may be 6 dB of in-band gain achieved just by delaying groups’ subwoofers. However, this may counteract standing wave cancellation. Ensure that the frequency response hasn’t deteriorated.

All that’s left is to tweak the individual delays, gains, and EQ for each sub to coax the best possible response and SPL out of the system. Expect all this to take at least 3-4 hours.

It is theoretically possible to measure and predict the correct settings for each subwoofer without all the manual trial and error. It requires a true impulse response and some heavy know-how in the math department. For now, the manual method works just fine.

Chase Walton contributed to this column.

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