Multiple Small Subs - Geddes Approach

[gedlee]

Originally posted by Wayne Parham


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

My room is sound-proofed and virtually sealed. Its definately the lowest mode of the room. What you say can be true, but the coupling is usually not that high and the effect is not as pronounced as what I measured.

[gedlee]

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.

John

I think that it is safe to assume that the room structure is linear and time invariant. This of course is not so true of the loudspeakers, which are in most cases are neither linear nor time invariant. To what extent this is true is going to be highly dependent on the loudspeaker. I think that the time invariance may be the bigger complication than linear in room EQ systems. At what sound level and driver temperature is the EQ done? If the system has considerable thermal variances then the complications could be quite serious. This is why I attempt to do systems with high thermal stability and crossover designs that are as insensitive to the driver impedances as possible. An electronic EQ that assumes LTI for a system which is not could be a serious problem. The pro guys know this and develop very complex means of tracking thermal changes in the EQ. In Hi-Fi this factor seems to be ignored for the most part. In a small speaker the thermal effects can be quit substantial.

[Wayne Parham]

Originally posted by gedlee
My room is sound-proofed and virtually sealed. Its definately the lowest mode of the room. What you say can be true, but the coupling is usually not that high and the effect is not as pronounced as what I measured.

Maybe you have a different sort of construction than what I'm referring to. What I'm talking about are homes that have a crawlspace underneath for access to pipes and what not. The room and the crawlspace are tightly coupled because they share a boundary that is free to vibrate - the floor.

In the homes I've been in that had crawlspaces, the acoustics was terrible as a direct result of the crawlspace. The chamber under the floor is the worst case I can think of because the ground is obviously rigid and the four side walls are too, usually brick or rock in compression from the weight of the home. No damping materials inside the crawlspace. Modal resonance inside the crawlspace is huge, worse than a basement or bathroom. And the listening room shares one surface with it - the floor. The floor acts like a vibrating "membrane" with the crawlspace chamber underneath it. Both chambers are coupled by the floor because they share it.

[gedlee]

A crawl space like you suggest will in general cause a loss of energy in the room not a peak. Its basically a tuned absober where the floor is the membrane. The coupling between the space below the floor and the main room is not what I would call high. For the coupling to be "high" the floor would have to be very flimsy - it would have to flex with just walking on it. This doen't seem typical to me, but crawl spaces are not at all common here.

[WithTarragon]

"....
I think that it is safe to assume that the room structure is linear and time invariant.

...."
Although the effect might be small, I have wondered about the time invariance.

This is my thinking: A speaker on one wall, the opposing wall 20 ft (about 20 msec) away and a microphone in the middle (10 msec away). Let's ignore other boundaries to keep it simple. I will also ignore issues of measuring frequency resolution when analyzing with short time windows.

At the microphone, for about 20 msec, you will be measuring the Freq Response of the speaker. However after 20 msec the rear reflection will now interfere with the ongoing output of the speaker. You now have a delay-and-add filter (comb filter) on top of the speaker's response.

So would the speaker and the room as a system still be considered time invariant? Yes, I have not forgotten that I am conveniently ignoring that the speaker by itself is not time invariant.

[john k...]

Originally posted by WithTarragon
"....


So would the speaker and the room as a system still be considered time invariant? Yes, I have not forgotten that I am conveniently ignoring that the speaker by itself is not time invariant.

Yes. Time invariant refers to the steady state response.

[Wayne Parham]

Originally posted by gedlee
A crawl space like you suggest will in general cause a loss of energy in the room not a peak. Its basically a tuned absober where the floor is the membrane. The coupling between the space below the floor and the main room is not what I would call high. For the coupling to be "high" the floor would have to be very flimsy - it would have to flex with just walking on it. This doen't seem typical to me, but crawl spaces are not at all common here.

They do flex just by walking on them. Not like a diving board, certainly, but still enough to matter. They're real trouble for the people that own them, if good sound is a goal.

[inertial]

Originally posted by gedlee



John

I think that it is safe to assume that the room structure is linear and time invariant. This of course is not so true of the loudspeakers, which are in most cases are neither linear nor time invariant. To what extent this is true is going to be highly dependent on the loudspeaker. I think that the time invariance may be the bigger complication than linear in room EQ systems. At what sound level and driver temperature is the EQ done? If the system has considerable thermal variances then the complications could be quite serious. This is why I attempt to do systems with high thermal stability and crossover designs that are as insensitive to the driver impedances as possible. An electronic EQ that assumes LTI for a system which is not could be a serious problem. The pro guys know this and develop very complex means of tracking thermal changes in the EQ. In Hi-Fi this factor seems to be ignored for the most part. In a small speaker the thermal effects can be quit substantial.

Dr. Geddes,

Can we assume that "good" hi-fi speakers are LTI at least for "small signals"? ( to say up to 10w peak ?).

Just today have had a short listen to a pair of Revel Ultima Salon.
IMO they can play loud for a normal audiophile listener...

Cheers,
Paolo

[gedlee]

Originally posted by inertial


Dr. Geddes,

Can we assume that "good" hi-fi speakers are LTI at least for "small signals"? ( to say up to 10w peak ?).

Cheers,
Paolo

Yes, for low level signals this would likely be true. But note that the thermal aspects go up linearly while the SPL and perception goes up as the log. Thus, it doesn't take much of an increase in level to cause a problem with the thermal LTI.

[inertial]

Originally posted by gedlee



Yes, for low level signals this would likely be true. But note that the thermal aspects go up linearly while the SPL and perception goes up as the log. Thus, it doesn't take much of an increase in level to cause a problem with the thermal LTI.

Of course.

Thanks Dr. Geddes

Cheers,
Paolo