Multiple Small Subs - Geddes Approach

[mat02ah]

Originally posted by gedlee
Markus - not an easy thing to do.
So the subs need to be "blended" into the existing sound field created by the mains.

Ok. No HP for the mains (the Summas represent a ~80Hz, 2nd order HP, right?).

This raises the next question: If the mains are 'full-range' (e.g. down to 40Hz), will the four free parameters for each sub (placement, level, LP cut-off frequency and phase) be enough to "blend" the subs into the existing sound field or will a parametric EQ be needed (for the subs and/or the mains)?

Thomas

[gedlee]

There are three Summas and each has a different HP point. The Nathans, the smallest, are about 80 Hz.

I have found just level and phase to be pretty effective (location is fixed by practicalities), but a paramteric band of EQ or two has also been very handy. If you find you need more than two bands then the setup is probably not right as I have never needed more than two.

[youngho]

Originally posted by markus76

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

Sorry, I am not able. It seems that you are not willing to accept my statement "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."

If that is the case, please look at Toole's book, page 224, figure 13.13a. The dotted line shows the corner subwoofer. The solid line shows the subwoofer positioned midway across the front wall, which is the node for the first-order width mode, which as you can see is no longer activated. Output at this frequency is not zero (which is what I assume you meant when you wrote "When you place a sub at a null, that particular mode is "cancelled". But that means that we will hear nothing at all"), but it is indeed much less than at the frequencies reinforced by modes. This level is what I think of as the baseline output level, although I am sure I am mangling the terminology, so please just bear with me. Adding the second subwoofer in 13.13b boosts the baseline output level, maybe around +6dB? Adding two more still in 13.13.c boosts the baseline output level further still, maybe another +6dB? Figure 13.13d shows the quarter-room placement addressing the even-order modes by placing the subwoofers in the nodes for these. Again, there is still output, but it's much less than at the height mode frequency.

If the problem is instead a conceptual one, I believe that basically all you have to do is think of a 2-dimensional wave superposition model of a source transmitting energy at a non-modal frequency. If I understand correctly, energy falls off as a function of distance but increases as one approaches a boundary.

[mat02ah]

Earl,

Originally posted by gedlee
There are three Summas and each has a different HP point. The Nathans, the smallest, are about 80 Hz.

ok, seems I confused the names.

Originally posted by gedlee
I have found just level and phase to be pretty effective (location is fixed by practicalities), but a paramteric band of EQ or two has also been very handy. If you find you need more than two bands then the setup is probably not right as I have never needed more than two.

Are you talking about one or two EQ bands per subwoofer or two EQ bands applied to the LF channel (same EQ for all subs)?

And (sorry ...) another question:

If the mains were 'full-range' but I wanted to gain some max. SPL below 100Hz, I'd have to use a HP filter. Am I correct to assume that you would suggest to use a 'gentle' HP filter like 80-100Hz/2nd order or even 1st order? I am thinking about a multi-sub arrangement for Klein+Hummel O300 speakers. While they go down to 40Hz, their max. SPL at 40Hz (one 8" woofer in a small closed box, according to K+H 85dB SPL @ 40Hz, 1m, 3%THD, free field) is of course limited ...

Thanks

Thomas

[Russell Dawkins]

I was thinking of the same arrangement. I will shortly be taking delivery of a K+H 0300/0800 system and, though I was tempted to take a chance with multiple subs, I decided to start with the stock arrangement since it is reputed to blend so well.

I do intend to use this set up as a basis for comparison while I experiment at my leisure with alternates to the 0800 and also the 0300.

I was exceedingly impressed with the accuracy of the tonality of the 0300, but think that I may eventually prefer more dynamic capability. The Abbey or Nathan are likely contenders.

For my purposes, though, the combination of performance, cost and size made the K+H a very persuasive prospect, especially considering their utility as portable field monitors - 250 x 380 x 290 mm and 14 kg is small, considering it includes internal tri-amplification!

In fact I couldn't think of any alternatives, so I bought them!

[alexclaber]

I came very close to taking this approach myself, ordering some Minotaur subs from BK Electronics. However I ended up cancelling my order after suffering their completely unresponsive customer service and mythical 7-10 day lead time.

Alex

[john k...]

Originally posted by gedlee
John



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.

I pulled the text form my web site. It is not the complete discussion. In my analysis both Kn and Psi are complex. The imaginary part of Kn defines the amplitude of the modal resonance. I also never said that the eigen functions are zero anywhere. I simply said that the position of both the listener and source are equally important. What works well for one listening position may not work well for another. And if the eigen function is zero at either point then the contribution to the SPL form that mode would be zero (at all frequencies). That does not mean the SPL will be zero at the modal frequency. The part of the text in bold should have make that clear.

I simply off the software for those who care to play around with it. There are two sections, the upper section allows the user to look at a single monopole, dipole or cardioid woofer. The lower section allows the user to look at a room with up to 4 monopole woofers. Damping can be changed as well. It’s a simply model of wall admittance and I am sure you would criticize it. But it is sufficient for the demonstrative nature of the software.



It's a nice little "What if" tool that lets the user have a little fun. If you can have a little fun with this you’re taking it way too seriously.

[gedlee]

Originally posted by mat02ah

Are you talking about one or two EQ bands per subwoofer or two EQ bands applied to the LF channel (same EQ for all subs)?

And (sorry ...) another question:

If the mains were 'full-range' but I wanted to gain some max. SPL below 100Hz, I'd have to use a HP filter. Am I correct to assume that you would suggest to use a 'gentle' HP filter like 80-100Hz/2nd order or even 1st order? I am thinking about a multi-sub arrangement for Klein+Hummel O300 speakers. While they go down to 40Hz, their max. SPL at 40Hz (one 8" woofer in a small closed box, according to K+H 85dB SPL @ 40Hz, 1m, 3%THD, free field) is of course limited ...

Thanks

Thomas

I think that EQ for each sub would be very difficult to setup, I've never done it that way. I just do the best that I can with the levels and phase and then there is usually some peaking left which I attack with the EQ on all channels equally.

As to you K&H question, 85 dB @ 40 Hz max is not even usable. A HP filter may make this more reasonable. But the question isn't about using multiple subs, but how to correct the K&H.

[markus76]

[mat02ah]

Originally posted by gedlee
As to you K&H question, 85 dB @ 40 Hz max is not even usable.

"Not even usable" for what purpose? Home Cinema without additional subs? Yes. Music? No.

Originally posted by gedlee
But the question isn't about using multiple subs, but how to correct the K&H.

What do you want to correct? This is a small speaker built for near-field applications, and certainly not a bad one.

Thanks, but this is leading nowhere.

Regards
Thomas