Take a pair of reasonably priced floorstanding speakers from a mainstream manufacturer: let’s use the B&W 804 D4 as an example.
It offers a bandwidth ±3dB of 24Hz to 28kHz and a sensitivity of 89dB, at a cost of €16,000 a pair.
Now let’s compare that to the B&W 801 D4. That speaker is considerably larger and delivers a claimed bandwidth of 15Hz to 28kHz ±3dB with a sensitivity of 90dB. In terms of raw numbers, that’s extending the bass from 24Hz down to 15Hz and adding 1dB of sensitivity. And the price of that exercise? An extra €36,000! Sure, I know that there’s other stuff going on too, but you get the point. Adding bass extension to an existing speaker design/concept ain’t cheap. Why would anybody assume that putting that bass in a separate box will reduce the cost of the exercise by a factor of ten? And let’s be honest – €3,600 is a pretty generous budget for a sub in the AV world. Then you have to consider that that cost also has to include a bunch of extra electronics and an amplifier. The sums simply do not add up – at least not if quality counts.
Does that mean that you should ignore subs for serious music reproduction? No. It just means that there’s more than one way to build a subwoofer and more than one way to use it. If you want to add effective bass to a high-quality music system, you just have to obey a different set of rules to those that apply to the world of home theatre. Those rules are simple – even if their implications tend to be expensive. But then, as I’ve already pointed out, generating low-frequencies from a separate system isn’t necessarily going to be cheaper than building them into the main speakers.
Perhaps the most contentious rule of these audio specific is also the most important: high quality subs should use passive enclosures. That doesn’t mean that you don’t drive them from an active crossover. What it does mean is that the amplifier isn’t built into the cabinet of the sub-woofer, where it is exposed to extreme mechanical vibration. More importantly, it means that you can (and should) use exactly the same amplifier to drive the sub(s) as you are using to drive the rest of the system. If the biggest single challenge facing any loudspeaker designer is integrating the various drivers in a multi-way design, putting a driver in a different cabinet, with no fixed physical relationship to the rest of the speaker and then driving it with a different amplifier tips the prospect of seamless integration from difficult to beyond unlikely.
The second rule is a direct result of the first. You’ll need to drive the sub(s) from a separate, active crossover – an analogue crossover that should aspire to match the quality of the line-stage or pre-amplifier in your system. Given the cost and scarcity of high-performance line-stages in general, that’s a far from straightforward proposition. Digital crossovers are of course plentiful – but plunge you straight back into issues of quality and latency. Don’t go there.
The third rule is more by way of an observation. Two subs are always better than one and in an ideal world, that’s how they’d always be used. However, given that in many cases, subs are added to audio systems precisely because a lack of space precludes large speakers, don’t rule out using a single sub if that’s all that can be accommodated. Done properly, the benefits will still be substantial and you can always add a second, identical sub if or when circumstances allow.
