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No, that's NOT what I'm saying. Average the output of multiple subs at any ONE seat.

Posted by Duke (D) on March 8, 2007 at 12:56:05

In Reply to: Average the frequency response at multiple seats "smooths" the bass (average enough seats and standing waves disappear!) posted by Richard BassNut Greene on March 8, 2007 at 09:47:21:

Good one on Hitler & the Farm Animals. And I will concede that the average of my weight, my age, and my IQ is roughly 100.

I hate it when people go through and quote me and then shoot me down piece by piece. Quote 'n shoot, quote 'n shoot. So my apologies for using that format, but I tried writing this another way and it was hard to keep it all straight. I'm an old fat guy - do the math.

Richard BNG wrote: "Forget averages -- at every INDIVIDUAL seat the bass frequency response is still uneven no matter how many subwoofers you use (unless every listener gets his own near field subwoofer)."

Duke replies: Still uneven, but definitely improved.

* * *

Richard BNG wrote: "The other guy, I forget his name, recommended left-right subwoofers connected out of phase, the last time I read his paper."

Duke replies: Griesinger is the guy, and he recommended left-right subs based on maximizing the interaural phase difference to maximize the psychoacoustic sense of "envelopment". The recommendation of 90 degree out-of-phase operation of one sub was to synthesize envelopment in recordings with monaural bass by maximally de-correlating the in-room low frequencies. I haven't tried it yet but plan to.

* * *

Richard BNG: "Linkwitz goes for dipole bass which reduces excitation of most room modes -- perhaps the best advice of all for those unwilling to parametrically equalize their subwoofer, or buy a batch of bass traps."

Duke: Now we're on some common ground. I like dipole bass - it tends to be very natural sounding, with excellent pitch definition. The reason for the naturalness of dipoles is not that they excite fewer room modes - you see, the initial figure-8 pattern is immediately swamped by the reflections, so the room modes get excited anyway. And if anything the more modes excited the smoother the perceived in-room frequency response (this because the ear integrates sound over roughly 1/3 octave wide "critical bands" - room modes are plentiful above the bass region, but aren't an audible concern because they're bunched up enough to average out across a 1/3 octave interval). Rather, the dipole's naturalness arises because the in-room bass is effectively de-correlated. You can think of a dipole as two monopoles displaced in phase (or time, if we only look at one frequency) rather than displaced in space. My proposal displaces the monopoles in space, which is an other technique for de-correlating the in-room bass energy.

It is rare that owners of dipole speakers are happy with the way a single sub integrates into their system. This is because the output of a single sub in as small room is well correlated, and has a very different "feel" from the dipoles' output which is highly de-correlated in-room. Dipole owners are more likely to be happy with two subwoofers than one, this based on my own observations of posts by dipole owners. I believe the reason is improved de-correlation of the bass so that there's less discrepancy between what the subwoofers are doing and what the dipoles are doing. In my opinion the next logical step is scattered multiple subs.

I use dipoles and have built diople subs, and while they can sound very good in some ways dipole subs do have disadvantages. The disdvantages include large size required to go down deep, reduced power handling at low frequencies, and an absence of the impact that characterizes a good monople system. I can go deeper with better impact and still get good smoothness and pitch definition by using scattered multiple small monopole subs.

* * *

Richard BNG: "There is no evidence from real rooms that more than one subwoofer in one location is needed for decent bass under 80Hz. (other than SPL requirements that require more than one subwoofer)."

Duke: So why do you use and advocate two subs, if only one is needed?

* * *

Richard BNG: "As soon as you scatter subwoofers beyond positions that are close to the two main speakers, you start deteriorating the intended stereo image soundstage."

Duke: That's true unless you do it the way I recommend: Steep slope crossovers, preferably below 80 Hz. If you have to use a highe crossover or shallower slope, then put the multiple subs over closer to the main speakers. So if there's problem, the solution is easy.

* * *

Richard BNG: "And the closest thing to a bass transient, the slap of a hammer on a kick drum, will not have the best possible sound quality if the two to three drivers in each channel (subwoofer driver, bass driver and mid-range driver ... or subwoofer driver and bass-mid-driver) that reproduce its sound are not located near one another."

Duke: Excellent objection, as intuitively the loss of impact seems obvious. This is the objection that kept me away from trying scattered muliple subs for a long time.

In reality, the ear doesn't hear the initial wavefront of a bass signal. In tests where a single bass frequency cycle was presented to listeners, it was difficult to even detect. It takes several cycles for the ear to detect the presence of bass energy, and several more for the ear to correctly hear the pitch. So by the time the ear is detecting bass, the outputs of all subs in the room is being integrated.

The only data I have examining whether or not there is a loss in perceived bass impact from scattering the subs (as opposed to placing them next to the main speakers) is my own listening. I don't like to cite myself as a source and then ask you to take my word on faith alone. Especially given what you've uncovered about my family's rich genetic diversity.

Cheers,

Duke

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