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20/9/2015 - In Praise of Subwoofers

Subwoofers have universal acceptance as part of home theatre systems. Their acceptance in the more purist stereo setups is not so universal. In fact it's unusual in the more upmarket systems, where larger three-way floor standing speakers are more the rule. Those speakers are expected to do it all, and usually require a substantial amplifier too.

There has been a tendency to write subs off as not providing a true transition through the frequency range from speaker to subwoofer, or for them to be relegated to duty in mid-fi systems, so-called Lifestyle systems, and those where very small speakers are required by the customer, often for cosmetic purposes.

I'm not one of those "addicted to bass" head-bangers. I listen to a lot of acoustic jazz, and the combination of instruments always sounds better with a sub going. I love the sound of a big double bass, which can plumb the depths to around 33Hz. A large piano might reach down as far as 28Hz. Bass Guitar 31Hz. Tuba 37Hz.

OK, you could say, there are plenty of larger speakers that do go down to 40Hz. Maybe, but look closely at the specs and you'll see that this is usually measured at the -3dB point, and in some cases -6dB. For example, I've seen ones like the B&W CM8/S2 ($2999) quoted as going down to 48Hz at -3dB, but to 43Hz at -6dB. So there are losses in impact on the way down the scale. Also, speakers are measured by the manufacturers in more ideal condition than the speaker might encounter in the real world, in your house or apartment. If you're lucky, your room will be bass-friendly, not bass-squelching. But we have to face the fact that a lot of floor standing three-ways are starting to run out of puff below 50Hz.

So, even with floor standing three-ways, adding a subwoofer can help. But in the case of smaller two-way speakers I regard the subwoofer as essential. A lot of these are struggling to produce much below 60Hz. Yes, I know that a certain amount of bass is "implied" or filled in by our adaptive brains. But any listening test with and without the added subwoofer will show you how much you're missing. And that's not the end of it - it's not just about those extra Hz, but about the overall perceived performance of your speakers and your amplifier.

Having a dedicated amp in the sub makes a lot of difference. It sounds like you're suddenly using a better main amp, and the speakers seems to be less stressed - probably because they are! You get a bigger, more accurate sound without having to push your main speakers as hard. The transformation is quite arresting. The whole frequency range sounds better, not just the bass.

As a dabbler in home building, I've seized on the following approach, and am really enjoying the results. I choose the midrange driver for its articulation, and tend to go for larger ones, like 6"/15cm or 8"/20cm. These can be so-called "full range" drivers, but I don't worry too much about how much bass response they have, because I'm going to use the sub for that. It's good if they have some, however, as it avoids the "hole in the middle" effect you can get when the main speaker is too small. I'm looking mainly for a nice, natural sound. Then I try various tweeters to see which one complements the bigger one, and what capacitor trims it appropriately.

Last, I adjust the subwoofer to suit the speaker, trying various crossover settings, volume, and phasing. All this sounds bleeding obvious to anyone who's a regular fiddler with speakers. The difference is that I assume the existence of the subwoofer right from the beginning.

Here's the cruncher. I reckon if you spend (at new speaker prices) $900 on the two-way speakers and another $900 on the subwoofer, and choose well, you'll end up with a speaker system that outperforms good floor standing ones at up to $3000, and probably higher. Select the "satellites", the two-way bookshelf or stand mount speakers for midrange and treble quality, not for bass extension. The price differential is remarkably better with my own creations.

After tinkering with home-built speakers for some time, I can say that I've enjoyed the process, enjoyed the results, and am always on the lookout for good secondhand subwoofers at $100-150 or less. If these DIY speakers are to find a good home - somewhere, sometime - they'll all need a sub to accompany them. It might sound like a lot of trouble at first glance. Getting $3000 worth of performance for a tiny fraction of that is certainly satisfying, however.

Footnote: I recall that Ralph Waters went on after he sold Richter to do subwoofers under the SubSonic brand. He also made a large floor standing speaker that was available in two forms: one which was the usual passive design, and another which had a powered bass section, effectively adding a powered sub but not requiring the separate box. It was a very good speaker, retailing I think for around $4k the pair. I sold one pair just before we wound up our shop in Canberra, but would have promoted them more if we had stayed on.

What Makes A Good Subwoofer?

I bought my first powered subwoofer back in the 1980s (before I became involved in the hifi industry), reckoning that it would add a bit of bass to what were pretty detailed stereo speakers. It was a Yamaha NS W1 and it did just that. I wasn't just after pounding bass lines from pop or rock music, which it could do, up to a point. What I really enjoyed at that time was the sound of orchestral bass drum (in Berlioz's Symphonie Fantastique for example) or the double basses in any string or symphony orchestra. When I played the Sheffield Lab Drum record one morning, it sounded to the wife as if I'd spirited a drum kit into the lounge room while she slept, giving her a moment or two of concern until I appeared at the door while the "drummer" kept playing.

If you have correctly set up a subwoofer it shouldn't be all that obvious. It should just sound as if your speakers are genuinely "full range" and can deliver whatever is on the recording. The term subwoofer means it handles the bass below the comfort zone of whatever woofers are in your main speakers, large or small.

These days there are so many satellite/subwoofer speaker system on the market that the term is often a bit fudged. What you're getting is a satellite/woofer system, since the little speakers have no woofer, often enough no midrange worth boasting about, so the job of midrange and bass is left to the so-called subwoofer. A chunk of your frequency range thereby becomes mono, and a lot of these systems sound like they're playing low-grade MP3 even when they are playing uncompressed sources. To be properly effective, a sat/sub system should have two-way satellite speakers or a very capable full-range driver in each satellite. But I digress.

The development of subwoofers accelerated in the 1990s as the demand for more and deeper bass ramped up along with the demand for more home theatre systems. We used to assess subwoofer responses with such things as the Clear And Present Danger movie's opening scenes, which include some good low frequency as the swells hit the ship's hull.

Things have gone on developing over the past fifteen years or more since we did that, and the current range of subwoofers on the market is nothing short of astonishing, with some costing up to five or six thousand dollars, and others way more. The 6000w Velodyne 1812 flagship model is in the region of $20k.

Ralph Waters will follow on with a more technical run down on what makes a good subwoofer - one day, he's a bit busy with his new business at present! He's designed quite a few under the Richter and SubSonic brands - but here's my summary, for what it's worth.

The cabinet should be rigid and non-resonant, as with any speaker. There are ported designs, and sealed designs. Ported designs have an efficiency dividend, which means less power is required, but they also have "port-noise" complaint! Sealed enclosures will give a truer and tighter result but need more power.

The size of the driver ultimately governs how much air is moved, so in most cases a good big one will beat a good little one. This is why most ranges start at 8'/200mm and go up to 15"/375mm or even 18"/450mm. the rigidity of the cone is important, but lightness is also desirable as it helps it to move quickly.

Amplification these days has gone up to 1000w and beyond, usually class D for efficiency's sake and lower heat production, and this makes it possible to produce some pretty brutal, piston-like subwoofers. But brute force has to be well controlled.

The electronic suite included in the box has also evolved remarkably over the past 20 years. To begin with, some feedback analysis was used to apply the brakes when the woofer was being driven into distortion. A bit like the ABS braking system, it stepped in and eased off to avoid obvious distortions in extreme moments.

The amplifier has to start the cone's movement as quickly as possible, but in order to avoid excessive blurring by "overhang" or delayed and unwanted movements, the cone is also stopped quickly by the better amplifier-control sections.

Later, as digital processing became more part of the makeup, it became possible to do more adjustments in the digital domain, and the actual response in-room could be graphed on a screen using an attached microphone then EQ applied to flatten the response curve.

There are brands like Velodyne and Sunfire who specialize in subwoofers, and put a lot of effort into making them work really well. As a broad rule of thumb, a good subwoofer will usually cost $1000 upwards, and have a 10"/250mm driver or larger. Some will use two drivers, which can be good while maintaining a smaller size - such as those with two side-mounted drivers. The B&W PV1D at $1899 is an example, but Sunfire also make a number of dual-driver models. Both of these companies have recognized that a sealed subwoofer is essentially a pressure vessel, hence the shape of the PV1D, and the sturdy construction of Sunfire's little Atmos model. I'm reminded of the 1654 Magdeburg experiment of Otto von Guericke, where two teams of horses could not overcome the atmospheric pressure holding two hemispheres together. That gives me an excuse to insert a link to the full size example of this fabulous engraving!

Downward-firing drivers work well in most situations, but are not favoured for those times when the sub is hidden inside cabinetry. A forward-firing driver will usually be better in those cases.

Phasing is adjustable for the reason that if the bass drivers in your main speakers are moving forward while the subwoofer's is moving back, there can be cancellation. Basic phasing is a simple switch between 0 and 180 degrees. The correct setting is whichever sounds more solid. Some subwoofers will have more elaborate phase settings, perhaps four options.

How To Set Up Your Subwoofer Properly

"A subwoofer is a very important weapon in an audiophile’s armoury in the pursuit of perfect sound. Despite what their specifications might say, and their manufacturers might claim, very few hi-fi loudspeakers—even the largest floor-standing models—can reproduce the deepest musical frequencies at appropriate volume levels.

To do this requires a properly-tuned subwoofer: one whose volume, crossover frequency and phase controls have been set in such a way that the subwoofer’s output integrates perfectly with that of the main stereo or front/main loudspeakers…wherein lies the problem. Very few people—even experts—are capable of doing this by ear and, until recently, the measuring equipment required to correctly tune a subwoofer was prohibitively expensive. Now, thanks to the processing power found inside any Smartphone, it is possible to accurately calibrate a subwoofer with a Smartphone running a $10 app and a low-cost CD with appropriate test tones."

Read more at Australian HiFi.