I did a little work on SBIR - Home Theater Forum and Systems - HomeTheaterShack.com

Old 12-30-10, 06:31 PM Thread Starter
Senior Shackster
Dan

Join Date: Oct 2009
Location: Mountain View, CA
Posts: 502
I did a little work on SBIR

It would be great if Bryan or others in the know would check it:

The Front wall, ipsilateral side wall, and floor will all cause big issues. The contralateral wall, ceiling, and possibly even the rear wall could also come into play in a small room I imagine. As the distance gets further, the less problem it will cause d/t the inverse square law.

The equation for the front wall interference:
Cancellation notch frequency = [(344m/s) divided by (4 times the distance from the speaker to the wall behind it in meters)]
picture for reference:

This equation for other significant boundaries:
notch frequency = [(344m/s) / 2(the distance of speaker to the listening position via the reflecting surface - the direct path distance from the speaker to the listening position)]

This picture may help with visualization:

This is from my blog here: http://dtmblabber.blogspot.com/

I think this can be useful for people to figure out where the dips are coming from.

Thanks,

Dan
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Old 01-01-11, 12:11 PM
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Shawn

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Re: I did a little work on SBIR

In addition...if you have a speaker equidistant from numerous boundaries, the notch will be greatly enhanced. So, based on your understanding of the mathematics of the "notch", what would you conclude as the optimum frequency for main/sub crossover? Could you deal with an of the notches by adjusting that crossover? This is really a great thread discussion since it can lead down a path of greater understanding for why things are often specified the way they are.

Architectual Acoustics & Cinema Designer
HAA Level III/THX Video Certified Calibrator
Audyssey Professional Installer/Technician
SierraMikeBravo is offline
Old 01-01-11, 01:52 PM
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Bill

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Re: I did a little work on SBIR

Something else worth mentioning is that you have described how to calculate the lowest frequency at which you should observe a null. One should see peaks at even multiples of your notch frequency, and nulls at odd multiples.

Bill
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Old 01-01-11, 03:47 PM Thread Starter
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Dan

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Re: I did a little work on SBIR

I'm glad some people decided to step in here. This is an opportunity for a great thread. If you can move the speaker far away from the front wall, you can kick that first notch below the crossover to your sub. Say you were able to move it 2 meters away the notch would be at 43 Hz. Unfortunately that's not really practical for most people. However, I did build a broadband absorber for the front wall 1ft deep and placed my monitors fairly close to it. I was able to get significant absorption from it down to 55 Hz or so.

Smaller waves are easier to absorb--takes less material. I'm rebuilding that room right now, but I'm going to try and push the monitors even closer to the wall and the absorber to raise the frequency of that first notch so that it should be easier to absorb. Just a theory right now. We'll see how that works out.

The problem with getting closer to the front wall is stronger excitation of room modes. My method there will be to cross over to a sub still plus have some heavy absorption on the rear wall. Who knows how well that will work out.

Another important--and actually the most important IMO here-- is the frequency dependent integration time and also the duration of the signal on the recording.

Quote:
gedlee wrote: View Post
I think that a strong case could be made that we hear LF ONLY in the steady state. It is well know that the ear has an integration time of about 10-20 ms. over which all sound arrivals are integrated into a single event. This corresponds to a period of about 100 Hz, meaning that a 100 Hz signal is basicaly not even recognized by or hearing until more than antire period has ellapsed. How is it then that we could "perceive" transients of these LF signals?

I only ever look at steady state signals at LF because I am convinced that this is all that we can perceive.

Above 500 Hz the situation is quite different and in fact changes 180 degrees - transients and <10ms impulses are the most important.
I think this partly accounts for why prolonged bass output has a tendency to be described as muddy, but mid and treble as spacious. This may also relate to Dr. Toole's Central Paradox's features and why the small ripples above the modal region seen in a steady stated FR are not heard as such. One thing is for sure--you can definitely hear those comb filter beats when doing a slow FR sweep with REW. They are far more pronounced with dipoles. Strangely I can't say I've heard them with music on dipoles. That's why I was hesitant to get into them. I've done it in the past on other forums and the arguments just got ugly and personal. It's strange when people take audio arguments to a personal level, but anyway.

To me, if you get the speaker close enough to the front wall with some very effective absorption covering the front wall, it should be able to remove a lot of SBIR issues.

For the second bass issues, I was thinking something like a broadband absorber coffee table.

Thoughts gents?

Thanks again,

Dan
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Old 01-01-11, 04:57 PM
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Bill

Join Date: Sep 2009
Location: Austin, TX
Posts: 676
Re: I did a little work on SBIR

I've not yet installed any treatments, so my suggestions may be limited.

I did calculate the path length differences to all the single reflection points in my room. I then compared these to the time/distances to peaks visible in the envelope (ETC) graph of the impulse curve, viewed as %FS. It was encouraging to find peaks at the predicted delays from the initial direct sound -- that at least confirms the calculations are correct, and the magnitude of the ETC peaks gives some help in prioritizing the problems to solve.

Quote:
DanTheMan wrote: View Post
... To me, if you get the speaker close enough to the front wall with some very effective absorption covering the front wall, it should be able to remove a lot of SBIR issues.
...
I did try something like your idea here. I moved my front speakers closer to the front wall to raise the frequency of the first reflections. Putting one of them in the corner did greatly reduce the reflection off the near side wall -- the downside was that it now does a wonderful job energizing the tangential mode.

Bill
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Old 01-02-11, 08:08 AM Thread Starter
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Dan

Join Date: Oct 2009
Location: Mountain View, CA
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Re: I did a little work on SBIR

Cool experiment Bill. Hopefully I'll get some time here soon to do something similar. I'll post results when I do. What all do you want to see? I was truly just thinking of FR, but if I find other things of interest or you have a suggestion, I'll post.

I sometimes wonder if the spaciousness and perceived dynamics of dipoles doesn't have something to do with the comb filtering...... Maybe I am hearing it but my brain doesn't know how to make sense of it. Maybe that's partly responsible for the tonality issues I've heard. I've attributed it to midrange bloom in the polar response previously and I'm sure it's a large part of the issue.

Just thinking (somewhat) out loud,

Dan
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Old 01-02-11, 10:35 AM
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Bill

Join Date: Sep 2009
Location: Austin, TX
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Re: I did a little work on SBIR

Hi Dan,

I've not read yet Toole's books or papers yet, so I can't comment directly on that. I did find, when I began taking full range measures after buying a calibrated mic, that if I placed the mic in exactly the position the Audyssey mic was in when it set distances, I saw significant comb filtering at the high end when measuring the two front speakers together. It had me quite confused for a while until I recognized what was happening. After I figured it out, I gave up on measuring the two front channels together except at the very end of a session as a sanity check. So I don't remember hearing the comb filter effects. Measuring the fronts separately makes it easier to interpret the ETC graphs and to correlate frequency response anomalies with reflection distances.

Bill
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Old 01-02-11, 08:18 PM Thread Starter
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Dan

Join Date: Oct 2009
Location: Mountain View, CA
Posts: 502
Re: I did a little work on SBIR

Measure a solo dipole. You'll hear the comb. Too bad so little effort has been put into studying the psychoacoustics of dipoles.

Dan
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Old 01-03-11, 06:36 PM
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Jim

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Re: I did a little work on SBIR

Quote:
DanTheMan wrote: View Post
To me, if you get the speaker close enough to the front wall with some very effective absorption covering the front wall, it should be able to remove a lot of SBIR issues.
This seems consistent with what Tim Hall stated on page 38 of the manual for my AR-9s.

Quote:
DanTheMan wrote: View Post
I did build a broadband absorber for the front wall 1ft deep and placed my monitors fairly close to it. I was able to get significant absorption from it down to 55 Hz or so.
So in my case would you go with more than 2"?
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Old 01-03-11, 09:02 PM Thread Starter
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Dan

Join Date: Oct 2009
Location: Mountain View, CA
Posts: 502
Re: I did a little work on SBIR

Yea, I personally would go with a lot more that 2". 4" would be a bare minimum. If your speakers are close, you may notice a substantial improvement. I honestly can't say b/c I haven't tried it.

I couldn't find where AR was doing what I'm proposing..... did I miss it? They do handle SBIR in a clever fashion. It's amazing how much has been known in the industry and how much us rather serious hobbyists don't.

Thanks for that info,

Dan

Last edited by DanTheMan; 01-04-11 at 10:41 PM.
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