Multiple Small Subs - Geddes Approach

[Wayne Parham]

Originally posted by inertial
I understand your point but agreed only partial.
If you are familiar with room's simulators ( yes you have said this previously) you sure know that floor is not the only issue.
There are six walls, you can virtually add one at time and see the difference in your listening position. A caos!
When you have added all the six walls, the influence of the only floor
is not so rilevant .
I am referring at european rooms, not very large, very solid walls and
listen triangle about 2 metres.
In this scenario, i always see peaks and dips in the midbass zone and basic measurement semms comfirmate this. Problems in the 80-300Hz zone. ( of course there are big problem also below 80 Hz)
It is easy listenable with normally a lack of punch, no body, no weight , very bad sound .
This is a very typicall performance in our "small" european rooms ( to say 20-25 square meters), not sure how much different can be in yours americans rooms .
I know simulators makes big simplifications respect real conditions, but something is interesting to my eyes .
So the question remain IMHO, who takes care of the 80-300Hz?

Yes, you're right. The solid boundaries of European homes are much more problematic than the framed drywall construction common in North America. Many European flats have concrete and solid plaster, very little damping. You might add false walls that act as dampers.

• Panel dampers

[Wayne Parham]

Originally posted by badman
Oh, and while the vertical dimension spread is valuable to combat floor bounce when the subs are close to the speakers, in the multisub setups we're discussing it's going to be spread via the lateral/depth variations fairly effectively.

Only if the low-pass of the subs is high enough. But that can cause a problem with localization if they're too far from the mains. The subs closest to the mains can be low-passed higher, further ones should be low-passed lower.

[inertial]

Yes Wayne, I think panel dampers is sure a possibility, maybe the best, for sure very impegnative

Cheers,
Paolo

[markus76]

The problem with very early floor (and ceiling!) reflections is that they not only create a cancellation at let say 500 Hz (1 ms delay) and reinforcement at 1000 Hz but also at multiples of these frequencies. Does placement really help? I think only absorption helps.

Best, Markus

[Wayne Parham]

I'm talking about floor bounce in the 100Hz to 200Hz range. I see it pretty frequently in mini-monitors on stands and towers. Its frequency is dependent on height, and changes slightly with respect to listener position because it is caused by the path length difference between the direct sound and the floor reflection. You never see it in vertical line arrays, because of the dense interference in the vertical plane. I have also found 2.5-ways and low-crossed mids that overlap with woofers mitigate the notch. Same with a sub that is used fairly high, overlapped with the mains. Again, the problem that can come up is localization, if the sound sources are too far apart.

[markus76]

How does a real world situation look like that produces a peak (?) at 100/200 Hz?

[Wayne Parham]

It's not a peak, it's a notch. Pretty much anytime you have a point source loudspeaker on stands, that's what you get.

[markus76]

You're right. If your're 2 m from the speaker and 1 m from the floor then you'll have a cancellation at 94 Hz and a reinforcement at 188 Hz. For 3 m distance it would be 66 Hz and 132 Hz.

[Pan]

Originally posted by soongsc

I'll take two.


The main issue with multiple subs is that you excite multiple room modes. This translates to the muddiness that Cal is referring to because now the low frequency energy cannot decay fast enough.

Two subs placed on opposing walls will cancel the first standing wave that will be there when any of the two subs are operated alone. Using three or four subs on the right locations will cancel many of the standing waves leading to smoother in room response of the speaker system.

Just talking about subs and placing in general seems to simplify the issue. As a personal preference, I would want the mains to go down as low as the subs. This is necessary to preseverve the initial tranient impact before the room modes are excited.

Viewed in isolation a pair of main speakers (decently designed)will have less groupdelay than a sub+sat combo. However again, using several subs will decrease the level of the standing waves in the room which totally swamps the fine GD performance of the main speaker used alone. So what I'm saying is a main speaker crossed to several subs will likely sound much better than a main speaker in a non treated room. In a treated room the differences get smaller but will still be there. But as you seem to imply, various kinds of 2.5 way systems where the mains and "subs" work together works fine as well. Of course, depending on the "x-over" there's only so much you have to play with regarding placement of the added "fill-in" drivers.

Then if the overal feel is not satisfactory, subs would be used at specific frequencies that are the most annoying, which means taylored frequency response depending on location. The "listening area" would be measred to identify the problematic frequencies, and the subs to fix the problem would be located at appropriate reflection points with delayed inverted phase of appropriate amplitude. The purpose would be to allow the subs to be used as absorbers to minimize reflection waves, and thus the room modes would not be excited. Care should be taken to limit the upper frequency as low as possible with sharp cutoffs for remote subs.

Sure, active basabsorbers can be used either fed by a mic or placed at the wall opposite to the main speaker wall and DSP controlled to "make the wall dissapear" but that will be more expensive and that money and those active correction speakers can be used on more subs instead that will increase output and decrease nonlinear distortion and thermal compression.

But when room modes are excited, the the following drum rumble cannot be localized.

That's true (or rather the focus and localization suffers) and multiple subs, bassabsorbers, DSP and dipoles are different ways of dealing with this


/Peter

[Pan]

Originally posted by markus76


The preference to have huge level variations (±15dB) at low frequencies?
Multiple subs do nothing to the low frequency reverberation time, that's true. Only active (not available as a product) or passive absorption can help. But what multiple subs do is to smooth the frequency response in a way it can't be done with any other method (I know of). That IS the number one goal for low frequency reproduction.

Multiple subs decrease the decay time of the room resonances. It's not strictly correct to talk about reverberation time in the modal range of small rooms but when the frequency response is improved (flattened) the decay time and thereby transient response is improved as well. AFAIK standing waves are more or less minimum-phase phenomena.

My feeling is that you can have an excellent result with only two subs if you're completely free with their placement and have a lot of time (days). But that's virtually never the case. So 3 subs seems to be the best recommendation that will yield best results in a vast variety of domestic listening rooms.

Three is still better than two and four is even better. There are more than a few that ended up on four as the most sensible compromise betweeen cost, performance and "practicallity".


/Peter