akg_ca:
"Ideally, you want two subwoofers so that you have true stereo separation down deep into the bass. Stereo subs can also help to lessen room interaction problems by providing two discrete sources of bass information."
Hello akg_ca,
Your statement cited above is not accurate. Two subs will not and cannot provide true stereo separation down deep into the bass. The information below, however, is accurate and useful to know:
Virtually all humans are unable to localize deep bass frequency soundwaves that are below about 100 Hz. This means we cannot determine where the sound of bass tones are coming from that are below about 100 Hz. But we're very good at localizing higher frequency soundwaves in the remainder of the audible spectrum, from about 100 to 20,000 Hz.
This is the reason there's no such thing as 'true stereo' deep bass and why the bass is summed to mono on frequencies below 100 Hz on all vinyl and cd recordings. If you doubt this, try to find a single vinyl lp record or cd on which the bass is not summed to mono and has discrete L+R bass content recorded. This means it's pointless to configure subs in a stereo configuration with one located by the left main speaker and one by the right. A better approach is to simply operate all subs in mono mode.
However, thanks to psychoacoustics and our remarkable brains, it is possible to create the perception of stereo bass in our systems. Here's how it works:
Whether you use 2, 3 or 4 subs, run them in mono and optimize the bass at your listening seat. The bass below 100 Hz won't be able to be localized but there are bass harmonics or overtones of the deep bass fundamental frequency that extend into higher frequencies that are reproduced by the main stereo speakers and can be localized. Our brains are able to associate the fundamental deep bass frequency reproduced by the subs, that are not able to be localized, with the deep bass's higher harmonic frequencies, that extend well beyond 100 Hz, which are reproduced by the main speakers that are able to be localized. This psychoacoustic association allows us to localize the deep bass in the soundstage, for example the kick drum is located in the rear center and the upright bass is located in the front to the left, which would not be otherwise possible without this psychoacoustic association our brain's are capable of.
Other factors I think are important to integrating subs seamlessly with the main speakers are the subs volume levels and the crossover frequency setting. For best results, the precise level of these settings will vary by system, the main speakers and the subs used.
My main speakers are a pair of 6'x2' Magnepan 2.7QR 3-way planar-magnetic dipoles that are very detailed and smooth but have limited bass extension (down to only 35 Hz) and limited bass impact and dynamics. To provide the missing bass extension and impact, I use an Audio Kinesis 4-sub Debra distributed bass array (DBA) system that is very similar to their Swarm system, consisting of four relatively small passive subs with 10" aluminum long-throw drivers powered by a separate class AB 1K watt amp/control unit.
My goal was to keep the perception of the four dynamic subs as low as possible, have them only become active when the source material required it and to blend in with my main speakers as seamlessly as possible to attain a cohesive unity to the overall sound quality. I've achieved the best performance and integration results thus far by running my mains full-range with the subs' volume level set at about 45% and a cutoff frequency of 40Hz. I believe using multiple subs with smaller and quicker 10" drivers, rather than larger but slower12-15" drivers, allowed the bass to better match the accurate, agile, detailed and smooth sound qualities that the Magnepan main speakers possess, resulting in a seamless blending or integration between the subs and mains.
I agree that many systems will achieve significant improvements in overall sound quality by utilizing the high frequency pass thru connections for the L+R main speakers on most subs and the Swarm/Debra's amp/ control unit. This is because the main speakers and their amps are relieved of the need to reproduce deep bass frequencies (usually with a cutoff of about 80 Hz) which usually results in improved midrange and treble performance from the main speakers and amps combination. I tried this method but didn't notice significant improvements in the midrange and treble performance of my Magnepans and D-Sonic mono-block amps combination. My thought is that, since these class D mono-blocks are capable of outputting 1,200 watts into my 4 ohm main speakers, the relief on the power requirements gained from the amps and speakers not being operated full-range didn't result in the normally significant sound improvements in mid-range and treble response attainable with less powerful amps and different main speakers.
However, I realize every system is unique and suggest trying out both configurations, running the mains full-range and with restricted low frequency output, to determine which you like best.
Tim
