A/V Veteran

I was midway through a long delayed reorganization, riffling through piles of as yet to be filed home- and work-related stuff. As anyone can tell you, when you start to clear out documents and publications you haven’t seen in years you’re constantly tempted to stop and re-read much of it, which inevitably brings the entire process to a screeching halt.

Rather than keep entire audio-video magazines beyond about three years (the growth of the Internet is making even that a dubious practice), I tend to tear out articles that might be of value in the future. That was how I came across an article I hadn’t seen in 22 years. Titled “Subwoofer Secrets” (the inspiration for the name of this blog) and penned by the late Tom Nousaine, it had been published in the January 1995 issue of Stereo Review, the latter a distant godfather of Sound & Vision.

Tom had often been the bane of high-end audio magazines, including our sister publication Stereophile for what he saw as the excesses of subjective, high-end audio journalism. But he did know his stuff and, in particular, subwoofers. He once wrote another article (I must have it around here somewhere!!) about a custom subwoofer he had installed in his home. In the corner of his listening space he had cut a large, rectangular hole, build a rectangular shaft extending down from the opening into the basement, and installed several subwoofer drivers on the outsides sides of the shaft. Their front radiation went up into his listening room and their backs opened into the basement, the latter functioning as an infinite baffle. The result must have delighted his neighbors.

In the Subwoofer Secrets article he discussed the mid ‘90’s current “truths,” as he saw them, about subwoofers and subwoofer placement. Some of his ideas remain controversial today, others were controversial even then. But he backed them up with measurements. What he lacked, as practically everyone did at the time, were computer simulations that make this process easier and, perhaps, more widely useful. But even modern programs are most comfortable simulating basic rectangular and square rooms, with no openings and solid walls. The real spaces Tom tested included at least two oddities, one of them L-shaped and the other an open loft. Such rooms are difficult to model, even with today’s computing power.

It isn’t possible to provide here the curves he published. My copy is black and white and the curves were in color; I can’t tell which are which! But here’s a summary of his conclusions, together with a few of my comments.

Is bass possible in small spaces? At the time the article was written some folks argued that a sound could be usefully reproduced only if its wavelength was no longer than the longest dimension of the room in which it was playing. As Tom, notes, however, headphones and car audio pretty much debunked this idea.

In fact, below a certain frequency the room can actually reinforce the bass output, a phenomenon known as room gain. As the room gets smaller the room gain increases, and as it gets larger the gain decreases. That’s why it’s so difficult to achieve powerful, pants-flapping bass in a large room with small subwoofers; the subwoofer has to do more of the work.

Do we need stereo bass? As in the photo above?! Tom said no, and he was correct, at least at frequencies below the crossover to the subwoofer. Most often the latter is around 80Hz. He wasn’t guessing here, but rather confirmed this with a number of experiments. Listeners could not distinguish stereo subs from mono subs below this frequency.

We still hear audiophiles promote stereo subs, but even today the deep bass on most recordings is mono. When you hear the tympani on the left, it’s the frequencies above the crossover point, reproduced by the main speakers, that provide the directional cues. Positioning stereo subs next to the left and right speakers also throws away one of the main advantages of using one or more subwoofers; the best position for them is rarely the same location as that offering the best frequency balance and imaging from the main loudspeakers.

There’s one caveat here that Tom doesn’t mention. Subwoofers generate more distortion than do speakers operating at higher frequencies, particularly, when they’re driven at the high levels common in home theater. If their distortion is high enough, it can generate frequencies well above the crossover point. For example, the first harmonic of an 80Hz signal is 160Hz, and 160Hz might well be localizable. But even at 10% distortion (not unheard of in subwoofers at those high levels), this shouldn’t be a significant issue for most users with average size rooms and well-designed subwoofers.

What is the best crossover frequency? As noted above, Tom’s experiments confirmed that the now (but not then) common crossover frequency of 80Hz eliminated the ability to localize the subwoofer, at least when the entire system is operating (with the sub driven alone, it could be localized down to lower frequencies). But if the crossover slope is kept steep enough (a steep roll off is common in the subwoofer low-pass filters of today’s AVRs and pre-pros), even slightly higher points might be OK.

I might add here that in at least one setup my system produced a flatter in-room response with a crossover frequency of 110Hz. That minimized a room-induced peak in the response from the main speakers centered between 80Hz and 120Hz. The subwoofer was flatter in this range, so letting it do the honors up to the higher frequency helped to flatten the response. But discovering this would have been a long, trial and error listening process without measuring tools,

Where should I place my subwoofer? Tom’s experiments lead him to a definite conclusion: in a corner as long as there’s no door or other opening nearby. Which corner was best depended on the room. His measurements appeared to confirm this.

This is where modern opinions differ. Corner placement tends to energize all of the room’s bass modes. If these modes are reasonably well distributed (something the article doesn’t go into), corner placement might well be a good option. But if the modes are clumped around isolated frequencies, putting the subwoofer in a corner could result in boomy bass.

Tom also discouraged the use of multiple subwoofers. If you do have two, he recommended putting them both in the same corner for maximum output! This is where he differed the most from more modern opinion on the subject. Extensive experimentation at Harman has shown that the proper use of multiple subwoofers, in different locations, can significantly smooth out the bass, particularly for multiple listening positions. (The lack of a discussion about multiple listening positions is an obvious omission in the article, perhaps understandable in an era when no one gave much thought to the subject.) I’ll admit to being a long-time user of a single sub myself, but that’s mainly because speaker reviews are generally done with a single sub from the manufacturer. For most users, the main obstacles to using multiple subs are space and cost.

There’s no mention of room optimization in the article. Such processing, particularly now that affordable versions built into AVRs and pre-pros, didn’t exist for the average consumer in 1995. But such systems should be used only after the best subwoofer position or positions have been found; it’s always best to use no more electronic room correction than needed.