The invisible speaker market is well established, but what about invisible subwoofers? Stephen Dawson looks at one such offering from market leader Sonance.

It’s all well and good installing high quality, virtually invisible loudspeakers in your walls and ceilings, but what do you do about the bass?

Subwoofers are big lumps of things that can easily spoil the décor of any room. Unless, that is, you can somehow reduce it to appearing as just another 175mm diameter perforated grille on the wall.

The Equipment

That, incredibly, is what the Sonance BPS8 subwoofer promises: strong deep bass with effectively no physical presence. Of course, you cannot produce bass from nothing. The subwoofer is in fact a good-sized box – 235mm by 313mm by 610mm – but it is designed to be hidden in a ceiling (or, I’d guess, under the floor).

It is a bandpass design, which means that the 203mm driver is entirely contained within the enclosure, which is sealed apart from the port through which the sound is emitted. A flexible tube is attached to the port. This can bend through 90 degrees. Its end is clamped into a collar which is installed in the wall.

There are a choice of collars and grilles. The kind of default ‘finish’ consists of a 4” collar and round perforated grille. You can instead go for 6” round, or change the look to 4” square or 6” square.

The driver has two voice coils and is provided with two sets of binding posts, one set for each of the voice coils, because it is a passive subwoofer (i.e. it has no amplifier built in). The installation instructions make clear that if provided with the mono signal, it should not be wired in parallel (i.e. to both sets of inputs). So typically you will be using one of the voice coils.

Of course Sonance has available a suitable amplifier to drive the subwoofer. The Sonamp A150 is specifically designed for the purpose. A mono unit, it is rated at 90W output into 4Ω. It has a fixed 150Hz low pass filter built in. That is, it cannot be repurposed as a full range amplifier. It also has an additional variable low pass filter which can be adjusted to between 40Hz and 150Hz, plus a level control. A bypass switch can cut both the level control and the variable low pass filter out of the circuit. There’s also a phase reversal switch, plus you can choose to have the unit switch on and off automatically, or do it manually.

Around the back is a single pair of binding post outputs for the subwoofer. There are stereo line level and speaker level inputs. And stereo line level and speaker level outputs as well. The speaker’s ones are spring clips. The outputs have a gentle (6dB per octave) high pass filter set to 150Hz and 200Hz respectively. These reduce the bass load on any speakers further down the line.

Setting Up

I confess that I wasn’t quite prepared to do a proper installation of the subwoofer, since that would involve cutting holes in my office. Instead I put the subwoofer in the same corner I use for all subwoofers, which has proven over the years to lead to a smooth and powerful performance.

I had planned on using the ‘Bypass’ mode of the amp, cutting out its own built in attenuator and the variable low pass filter, leaving just the fixed low pass filter in place; however, this also left the system’s level way too high for the Yamaha receiver’s (and I’d guess, most other brand’s) speaker adjustment range.

With the subwoofer turned down to -10dB and the other speakers to +10dB, it was still far too loud. I’d guess that at least another 10dB would have to come off its level for proper balance. So out went the ‘Bypass’ mode. I put the variable filter at the highest setting so it would have the least effect and the level control at 10 o’clock, which gave a good balance with all the receiver’s level controls centred.

Performance

The Sonance amp and subwoofer combo were a fine pairing when it came to listening. The bass delivery was clean, as powerful as required, and while not reaching down to dinosaur footsteps territory, delivered enough truly deep stuff to be completely and thoroughly satisfying. It slammed with explosions, and rumbled with the aftermath, yet ground out a pipe organ bass pedal and delivered a tight, clean performance with the kick drum.

I for one would live very happily with this subwoofer performance.

Bandpass subwoofers, as the name suggests, allow a well-defined band of frequencies to pass through their outlet, so to speak. With this subwoofer that band proved to be 39Hz to 85Hz. Above 85Hz the output fell away very sharply to be down by 48dB by 190Hz (i.e. in one octave). That was with the variable filter in the amp bypassed, so that the only electronic filters were the fixed 12dB per octave one at 150Hz, and the receiver’s filter, which was set for 200Hz. So most of the filtering was apparently acoustic.

Consequently it would be wise for your main installation speakers to provide competent coverage down to 80Hz or so, and to set the crossover frequency in your receiver to 80Hz or 90Hz if possible.

At the bottom end things were more gentle, with good output a bit lower in level down to 30Hz. By 20Hz the output was 18dB down.

I had some difficulty measuring the Sonance Sonamp A150 amplifier. In the end, I managed to draw 62.5W into 8Ω from it before clipping. This test was conducted only with some difficulty.

After setting up my test rig, I found myself unable to get any signal at all out of the unit. I was perplexed. I pulled out another amplifier to make sure my rig was working properly, which it was. I read the manual with some care and discovered that it recommended changing the speaker output fuse to a higher value (from 1.6A to 2.0A) if it was intended to drive two subwoofers.

Fuse? It has been years since I’ve used an amplifier with a speaker fuse. These days all but the cheapest gear has electronic protection of the output. Many is the time I’ve overdriven an amplifier during output testing, in which case the unit would either switch itself into an ‘overload’ condition (it won’t produce any sound until switched off and switched on again) or drop back to a low output state.

I checked the fuse and, yes, it had blown. The next day I purchased a packet of five. When I popped in the new fuse, the amp worked again. I plugged it into my rig, and confirmed it was producing the 50Hz sine wave, then turned it up and after a couple of minutes the fuse went again. New fuse and I repeated, but went through the process carefully to ensure I didn’t drive the unit much into clipping, and didn’t leave it just below clipping for very long. That was the result that translates to 62.5W output.

As I mentioned, the fuse is 1.6A. A naïve application of Ohm’s law and the power formula would suggest that the fuse would allow the amp to produce around 20.5W into 8Ω, and only 15.4W into 6Ω. Fortunately it doesn’t blow very fast when overdriven a bit, so presumably you can get peaks up around the 90W that’s mentioned. During my measurement the 1.6A fuse managed to survive, briefly, 2.8A. At the rated 90W into 4Ω it would have to cope with 4.7A, which would be a challenge, even if the fuse were replaced with a 2.0A one.

The amp clearly did the job in the listening tests, driving the subwoofer to impressive levels. But I find the primitive output protection rather disappointing and wonder whether there might be considerably better value for money elsewhere. Remember, if used in a home theatre context you don’t actually need the filter. The LFE output on your home theatre receiver will manage all that for you.

Conclusion

Those reservations aside, the clever design of an invisible subwoofer makes the Sonance BPS8 subwoofer and Sonamp A150 amplifier well worth considering for your invisible high-end home theatre system.