Multiple Small Subs - Geddes Approach

[mat02ah]

Originally posted by gedlee

With "mains to 40" and "subs below that" you won't get any multi-sub benifits at all. There would be no point in doing it.

... which leads exactly to my question: Will "mains to 40" and subs (positioned as suggested by your method, adjustable level, LP filter and phase) below ~100Hz work?

I'm wondering about this because with full-range mains you already have two "subs" in the system, but you can't move them and there is no level, filter or phase you can adjust.

In other words: If my main speakers are full-range, should I add a HP filter (say 80Hz, 2nd order) to the mains for a multi-sub arrangement or not?

Regards,
Thomas

[janneman]

Originally posted by Pan



A a short transient signal that will excite resonances.

In responce to Janneman:



/Peter

I think we're straying from the issue. My response was to "messing around with subwoofers will destroy localisation because it will mess up lf transients". My point is that localisation depends on the mid/hf parts of the lf signal, and those signal parts are routed to the mid/hf drivers. Therefore, messing with subwoofers will not mess up localization.

It is beyond me how any knowlegable person here should suggest to low-pass a transient and use that as an argument in this discussion. Lowpassing a transient essentially gets rid of it, of course.
Anyways, unless my original argument is addressed, I don't think I can add anything to it.

I am learning a lot from the rest of this thgread, though.

Jan Didden

[soongsc]

Originally posted by janneman


...
My point is that localisation depends on the mid/hf parts of the lf signal, and those signal parts are routed to the mid/hf drivers. Therefore, messing with subwoofers will not mess up localization.
...

It really depends on what musical instrument you are listening to.

[mayhem13]

Originally posted by mat02ah


... which leads exactly to my question: Will "mains to 40" and subs (positioned as suggested by your method, adjustable level, LP filter and phase) below ~100Hz work?

I'm wondering about this because with full-range mains you already have two "subs" in the system, but you can't move them and there is no level, filter or phase you can adjust.

In other words: If my main speakers are full-range, should I add a HP filter (say 80Hz, 2nd order) to the mains for a multi-sub arrangement or not?

Regards,
Thomas

I'm a little unclear on this myself. I guess it would depend on the inherent Low freq roll off of the mains. It almost sounds like sealed mains is a given for proper integration into a Geddes system, not a bad thing IMO. No need for the HP, just a natural overlap of the subs low pass and the natural roll off of the mains ??

[markus76]

Originally posted by youngho
If you place a subwoofer at the node for a mode and have it play the frequency for that mode, you will hear that frequency, but the mode will not be activated. The relative level will be low because it's not reinforced by standing waves, but you will still hear it.

Is someone here able (and willing) to simulate a 2-dimensional wave superposition model of a source transmitting energy at a modes null?

Best, Markus

[gedlee]

Markus - not an easy thing to do.

The rest - modal effects are significant well up to 150 Hz. in most rooms. As long as there are modal effects we need multiple sources to "tame" them. The mains are some of those sources and yes they do work better I have found as closed box (which is why thats all I sell anymore.) But its also correct that the mains LF sources are seldom if ever in desirable locations and never phased or leveled (as pointed out above). So while they might help at the very LFs they are not really all that effective. So the subs need to be "blended" into the existing sound field created by the mains. This is why I suggest overlaping and not HPing the mains. A smooth blend smoothes out the response and the main retain the imgae cues for propoer localization of the instruments. Done properly one never localizes on the subs only the mains as they should.

In short, no I never HP the mains.

[john k...]

Looking again at the equation for pressure we can make several observations.



First, recognize that the pressure at any frequency, as determined by the value of k = w/c, is the sum
of contributions from all modes. Second, the term Kn squared - k squared in the denominator controls,
in part, the magnitude of the contribution of each mode to the pressure at a given frequency. When Kn
is close to k the frequency is very close to a resonant mode and a large peak in the response may
occur. When Kn is much greater than k the mode contributes little to the pressure at that frequency.
The form of the denominator indicates that each mode behaves as a 2nd order low pass filter with cut
off frequency defined by the real part of Kn and Q defined by the imaginary part.

Examining the numerator we see that the contribution from each mode is dependent on the product of
the magnitude of the eigenfunction (Psi) for each mode at r and ro. This tells us that the placement of the
source is no more or less important than the placement of the listening position. If the eigenfunction
value is zero at the listening position then it makes no difference whether the mode is excited by the
source or not. It will not contribute to the sound pressure at the listening position.

Room response simulation tool

[markus76]

Originally posted by gedlee
Markus - not an easy thing to do.

But it would definitely help in showing what could be heard at different locations in such an idealized situation:

[Pan]

Markus,

the phase would be identical on right vs. left in your pic.

edit: also a two dimensional model has limited value since in reality we use min. two sources in a 3-D space for stereophony.

Plus, don't forget all the virtual sources due to the "mirror effect" of the surrounding surfaces in a room.


/Peter

[gedlee]

John

No need to argue again about HP, LP or BP other than to repeat that this all depends on how one defines the source. It can be any one of the three depending on this definition.

Also notice that the contribution for any mode, no matter how high in frequency, is never zero unless it is at a node. But, I repeat, kn is in general complex as are the modes and as such in a real room there are no nodes where the contribution is exactly zero.

The situation is not so simple that broad reaching generalizations can be made. Put a source at any spot in the room and the listener at any other spot and play any fequency. It will always be heard - its never identically zero. It can be very small, but never zero. Its not uncommon to see deep notches in the response with a very low response sometimes only a fraction of 1% of the neighboring frequencies, but its never zero. When talking about these very small values of excitation the obscure aspects of the problem like the complex modes and the evanescent wave are all on this order of importance and have to be considered.

That said, its virtually meaningless to get into this kind of detail or discussion as it has no bearing on the real problem at hand.