Room mode. How to control? - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 7 Old 08-31-11, 01:37 AM Thread Starter
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Room mode. How to control?

Hello, experts! I would like to tell one interesting story. At 2000 year Iíve carried SPL measurements in the room. Speaker was placed at the corner of the room (height about 3í) and measurement points were at the same height in different places inside. Soon Iíve discovered, that LF field at 106 Hz is very inhomogenius (due to height of the room 10í and length 20'). When Iíve placed paper tube (1Ē thick and 3í long) behind the speaker on the table, IĎve sensed strong vibration of it at a little incline (gap about 1/10Ē) of the tube. And LF field has established homogenius for this frequency and even at any height! When I continued to change incline I observed very big fluctuations in positive and negative SPL changes (10-30 dB) at the measurement points. I mean, that there is Helmholz effect, but have found nothing in all the tutorials of this my fenomena. Could anybody give me an explanation? Itís interesting, that matchfire slopes outside down of tube, when it was placed at tube ďgapĒ.
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post #2 of 7 Old 09-04-11, 07:54 AM
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Re: Room mode. How to control?

First of all, If your seating position does not put your ears at 3' high, then whatever measurements you've taken really don't matter much. The only place that matters in terms of the end goal is what it measures where you listen.

Again, same thing with the tube on a table behind the speaker. If you're not listening there, it really doesn't matter. Bass building up in a corner is a well known issue. Bass builds up at ANY boundary. Corners are more efficient since they're at the end of 2 boundaries.

You're not going to control modes in a room. You do your best to avoid sitting in particularly strong ones, located speakers to try to compensate by counteracting peaks by placing speakers where they will be in nulls (and the other way around), etc. Once that is done, you can address some additional problems via the use of absorption to minimize constructive and destructive interactions.

The Helmholz effect you're referring to doesn't appear to have much to do with the rest of what you're discussing. Unless I'm not understanding the rest correctly.


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post #3 of 7 Old 09-05-11, 03:10 AM Thread Starter
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Re: Room mode. How to control?

Thank you, Bryan for discussion. Afraid, I not describe the process I observed more significantly. Shortly, I observed brief connection between dimension of tube gap and SPL in the point of measurement, as it shown on this picture: (sorry, I don't know how to paste a simple picture)

During the experiments Iíve observed very strong SPL changes at the FIXED points of measurements (more, than +-20 dB). When gap height was 1/10Ē (about 2.5 mm) all the SPL values were equals at any point of the room.
Iíve repeated these experiments in other rooms and saw the same sensibility for other modal frequencies (and tube longitude), but Iím not succeeded in explanation of it. Iíve used this method in some works with good results, but I mean, thatís the time to explain the physics with your help?
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post #4 of 7 Old 09-05-11, 07:50 AM
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Re: Room mode. How to control?

I'm not sure exactly but what I think you're describing is a Helmholz resonator. That would be effective at one frequency far more than at any other (they're tuned in a very narrow nature). Helmholz's can be very effective at minimizing a very narrow notch type of problem in a room.

Just understand that you still require some broadband control so you're addressing the entire spectrum properly in the time domain.

By the way, now that you have 5 posts, you should be able to post pictures in your thread.

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post #5 of 7 Old 09-05-11, 10:38 AM Thread Starter
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Re: Room mode. How to control?

Maybe I’m not so advanced in Helmholz resonator theory, but I’ve heard, that absorbing capacity of such a resonators (and Q-factor) depends on some quantity of absorbing material inside of resonator’s body. When I carried these experiments, I tested frequencies closer to resonant mode. It was sufficiently wide range, something about +-5 Hz. Probably (it’s just my opinion… ah, sorry, not only mine yet!) you are right: it’s important example of Helmholz resonator use for rooms with big mode problems in narrow band. It’s strange, that nobody investigated this use before… and I can’t understand the effect of SPL equalization. You know, I remember somebody told me about Connected Resonances Theory, but can not get any info about this. And it will be very interesting to read something about dependence between gap form and efficiency of H.resonators use.
Anyway, thanks a lot for discussion, Bryan.
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post #6 of 7 Old 09-05-11, 11:51 AM
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Re: Room mode. How to control?

Helmholz can be very useful certainly. Just understand that on a per unit basis, there are usually more of them required to accomplish the same thing throughout the room.

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post #7 of 7 Old 09-06-11, 09:00 AM Thread Starter
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Re: Room mode. How to control?

My method to project control stereo or HT room is as follows (for rectlinear rooms): 1. Choise of acoustically selfagreed dimensions (not like Bold's or IBU) such a manner, that there will be 1 or 2 resonances (it may be peaks or userwise). Obviously for HT rooms I pay attention to avoid absence of low bass modal frequencies, because the result of it - weak bass, and it leads to use of powerful sub or even subs; 2. Use of tube resonators (called "BassCleaner") to "damp" (or get up) SPL peaks. It's interesting, that if you have, for instance, 1-st order mode 30 Hz, that means 60, 90, e.t.c., but BC builded for 60 Hz, therefore, you'll get damping of 60, 90, e.t.c. Hz too, as a rule. But in real situations forms of rooms are more complicated, than simple rectangular. Eigenvalues calculations for such a forms is very very hard problem and efforts to do it is often useless.
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