In some of my speakers, I'm using a vented-box bass tuning approach that I think might be deserving of its own name and acronym. Sort of like EBS (extended bass shelf) tuning, which is where the bass rolls off gently at first starting fairly high up, then levels off (shelves) above the tuning frequency, and then of course plummets below the tuning frequency.
Let me say up front that I'm not actually doing something new (though it is a bit off the beaten path) - what's new is the descriptive name and accompanying acronym.
The concept behind Room Gain Complementary tuning is this: To have the speaker's natural rolloff rate be approximately the inverse of the anticipated boost from room gain. If we assume typical room gain to be 3 dB per ocatve below 100 Hz, then we'd want the woofer to be rolling off at 3 dB per octave below about 100Hz or so. RGC tuning isn't appropriate for every application - for example, I wouldn't use it for a stand-mounted monitor that's going to be placed well out into the room.
I think one of the main reasons many subwoofer systems sound slow and disconnected from the rest of the presentation is the result of room gain overboosting their output. If you have a sub that's flat to 25 Hz anechoic, after room gain it will be roughly +6 dB at 25 Hz, and that's too much for natural-sounding bass reproduction. In fact, the myth that smaller subs are "faster" may well arise from their typically rolling off higher up, so that they suffer less from undesirable deep bass overboost due to room gain.
In Earl Geddes' brief study of multisub systems published on his website, we can readily see the effects of room gain. This study probably assumes a more effectively-sealed room than most of us have, so the room gain here is probably more than we can normally expect, but I think it's wortwhile to see how it plays out in the modelled response:
http://www.gedlee.com/downloads/sub%20study%20.pdf [edit: this link appears to be dead now; I think Earl has removed the paper from his website.]
Let me give an example of a commercial speaker with RGC tuning: The Guru Pro Audio QM10 loudspeaker. This is the cute little up-against-the-wall Swedish mini-monitor that supposedly goes down to 30 Hz. When used correctly (including correct stand height), I believe the claim. Here's a link where you can see the anechoic frequency response that results from its RCG-style tuning:
http://www.soundstagenetwork.com/measurements/speakers/guru_proaudio_qm10/The Aerial Acoustics model 10T looks to me like it has an RGC bass characteristic from 60 Hz down to 20 Hz:
http://www.stereophile.com/floorloudspeakers/466/index4.htmlBy way of contrast, here's the anechoic response of a speaker that does not use RGC tuning. This speaker's bass characteristic would be the better choice for a stand-mount placed well out into the room:
http://www.soundstagenetwork.com/measurements/speakers/monitor_audio_pl100/RGC tuning is used in vented versions of the Swarm multisub system, and my other current speakers can be RGC-tuned with suitably long ports. In fact, AudioCircle member Berndt has his Jazz Modules RGC-tuned. In practice, RGC tuning is sort of like EBS tuning only with a bit smaller box so that there is no shelf at the very bottom end. Too much ultradeep bass energy can result in a subjectively heavy and slow bass response, which I seek to avoid.
I would guess that the amazing bass performance of Ed Schilling's The Horn is largely due to the room-gain-complementary bass rolloff of his corner-loaded backhorns. Because corner loading provides more gain than up-against-the-wall placement, his anechoic target response is probably steeper than 3 dB per octave rolloff.
Wayne Parham's PiSpeakers are also tuned taking room gain into account, so there's another specialty manufacturer using it. In particular his corner-loaded 7 Pi has outstanding bass extension and tonal balance (it combines RGC-style tuning with exceptional radiation pattern control and an excellent crossover).
Audio Note designs its speakers for corner placement, and presumably tunes them with their version of RGC tuning. At any rate, there's another design whose bass performance is way beyond what you'd expect from a speaker that size, provided they are used as recommended.
I suspect that the oft-cited preference for low-Q sealed boxes over typical vented boxes arises in part from the low-Q sealed box's rolloff rate (approximately 6 dB per octave) being a reasonable first approximation of the inverse of room gain. If the room were removed and we were listening outdoors (with no room gain), a flat-tuned vented box would almost certainly be preferred over the low-Q sealed box. I think that, with an appropriately designed and tuned vented box, it's possible to improve on the room-friendliness of even a low-Q sealed box in a cost-effective manner.
Obviously the concept I'm using is nothing new, but perhaps by giving it a name and an acronym awareness will increase and it will be taken advantage of by more DIYers (and maybe more manufacturers).
Duke
