Multiple Small Subs - Geddes Approach

[Pan]

Originally posted by gedlee
This is a classic example where EQ is necessary, to bring down this mode that nothing else can deal with.

Maybe more subs would deal with it? Placed relatively close to the walls that likely are about 5-6 meter apart?


/Peter

[Rybaudio]

Originally posted by john k...
Mathematically it means the system is stable and causal.

That's not quite true and it's not because of some trivial detail- it misses the whole idea of why minimum phase is defined the way it is.

Consider the set of all linear, time-invariant, causal, and stable systems. The minimum phase systems are the invertible members of this set. To be clear, "invertible" means has an inverse in this set.

In terms of the properties of the system, a LTI system is minimum phase if and only if it is causal and stable and has a causal and stable inverse.

The point is that what a minimum phase system does to the signal is reversible by a realizable (LTI, causal, stable) filter.

Here are a couple counterexample to your assertion:

- an allpass filter is causal and stable but does not have a causal inverse
- a highpass filter is causal and stable but does not have a stable inverse (pole at 0)

[gedlee]

Originally posted by markus76


By what else should modes be dictated other than the room?? CABS or DBA or whatever you would like to call it works perfectly (when the room is rectangular and has sufficient rigid walls).

Best, Markus

Markus

It turns out the the lowest mode of any room is independent of shape and depends only on the floor plan (or largest surface actually if the room is very tall). Thus calculate the lowest mode for a rectangle of the same floor space and you know what the frequency will be for any shape. The mode shape differs of course, but not the frequency. An interest result from my thesis, but it can also be proven in other ways, like the Rayleigh Coefficient.

[gedlee]

Originally posted by Rybaudio


That's not quite true and it's not because of some trivial detail- it misses the whole idea of why minimum phase is defined the way it is.

Agreed - I think John left off an important conditional requirement - a realizable inverse. Probably typing too fast.

[john k...]

Originally posted by Rybaudio


That's not quite true and it's not because of some trivial detail- it misses the whole idea of why minimum phase is defined the way it is.

Consider the set of all linear, time-invariant, causal, and stable systems. The minimum phase systems are the invertible members of this set. To be clear, "invertible" means has an inverse in this set.

Yes, I should have mentioned that. I was trying to express it without saying poles and zeros and mess up. It happens. Causal and stable require poles in the left half of the s plane. Having a stable inverse requires the zeros also be in the left half plane. Stable, causal with stable, causal inverse is MP. So I would just add that MP implies not just invertible, but a stable, causal inverse.

Thanks for picking up my error.

[john k...]

Originally posted by inertial



Thanks Dr Kreskovsky,

My math is worst of my english!
So a LTI system can be MP or not. But if a system is MP , is it automatically LTI ?

MP is usually discussed in the context of LTI system.

If the system is LTI, we can pass from time domain to amplitude domain. If we correct the amplitude automatically we correct also time, am I right?

Yes.

- a listening room, you have showned it is not MP. Is it at least LTI?

We should be able to consider it LTI.

When we apply digital room correction ( for one point) are we correcting only the amplitude or also the time?
Perdone my confused exposition

Paolo [/B]

Only amplitude is for sure, assuming the correction is MP. But it doesn't have to be MP eq.

[inertial]

Originally posted by john k...

We should be able to consider it LTI.

This is important IMO, I have to reflect why you do not have answered " absolutely yes" ........

Originally posted by john k...
Only amplitude is for sure, assuming the correction is MP. But it doesn't have to be MP eq.

Ok, if we use FIR filters we can correct both amplitude and time ( in one point ), is it this how" Digital Room Correction" works? And can we assume the procedure is mathematically rigorous-correct?

Many thanks,
Paolo

[john k...]

Originally posted by inertial

This is important IMO, I have to reflect why you do not have answered " absolutely yes" ........

Because I'm not a structural engineer and I can not assume that the room structure and other damping behaves linearly. Most likely for audio the assumtion of the room being LTI is a good one.

Ok, if we use FIR filters we can correct both amplitude and time ( in one point ), is it this how" Digital Room Correction" works? And can we assume the procedure is mathematically rigorous-correct?

Many thanks,
Paolo

In theory FIR filters can unwrap the phase. But this results in a non causal filter, and I don't know off hand if these will always be stable. Lack of causality can be corrected by introducing a sufficient delay to shift the impulse to the right on the time axis and then truncating the remaing negative time part of the impulse. I can not speak for the various room corections algorithms. Many are based on MP eq. Those can be based on IIR or FIR approaches. If they correct phase then I would assume they are predominatly FIR.

[Wayne Parham]

Originally posted by gedlee
This was not the case in my room, there was a strong mode at about 30 Hz, but nothing made any difference, not phase or location (within what was possible) etc.

Could it be that this was actually the result of an adjacent chamber such as a crawlspace?

Most homes where I live have framed drywall structures over solid concrete foundations. Those aren't too bad because the walls and ceiling are lossy enough to provide some damping. There are a few though that have hardwood floors over a crawlspace, and in those, the crawlspace forms an acoustic chamber. So there are actually two resonant spaces that are tightly coupled, one the room and the other, the crawlspace.

Those are tough to deal with. There are things you can do to modify the resonance and damp the adjacent space, but I've never really been satisfied with any listening room that has a hardwood floor over a crawlspace.

[inertial]

Originally posted by john k...


Because I'm not a structural engineer and I can not assume that the room structure and other damping behaves linearly. Most likely for audio the assumtion of the room being LTI is a good one.



In theory FIR filters can unwrap the phase. But this results in a non causal filter, and I don't know off hand if these will always be stable. Lack of causality can be corrected by introducing a sufficient delay to shift the impulse to the right on the time axis and then truncating the remaing negative time part of the impulse. I can not speak for the various room corections algorithms. Many are based on MP eq. Those can be based on IIR or FIR approaches. If they correct phase then I would assume they are predominatly FIR.

Thanks Dr. Krekovsky,
I appreciate a lot your answers, now I have much more clear ideas in my head. This forum is really a great place to learn!

Cheers,
Paolo