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Acoustical Treatments Master Thread - Page 6

post #151 of 10210
From BasementBob's web page.

Product thickness density mounting 125hz 250hz 500hz 1000hz 2000hz 4000hz NRC

703, plain 2" (51mm) on wall 3.0 pcf (48 kg/m3) 0.17 0.86 1.14 1.07 1.02 0.98 1.00

Ignoring the excess absorption of exposed edges which will not occur with a covered wall - OC703 absorbs pretty much 100% of the treble frequencies. Of course an absorption coefficient like this has insufficient accuracy to tell you how many -dB down the reflection will actually be.

I never said anything negative about Def. Tech speakers (my opinion of bipolar front speaker is irrelavent to this discussion). The point is if YOU like the expansive reverberant sound-stage that a rear facing bipolar gives you - treating the wall to absorb treble makes that sound-stage go away.

If you want to just tame that sound-stage a bit and like the bipolar sound - then use a less aggressive absorption scheme that leaves some % of that front wall facing reflective.
post #152 of 10210
Kras,

I'm not using 2" 703 and Dennis does not recommend using 2" material as it absorbs too much (but I'm sure you knew this if you have already read this thread--it's in his third post on the first page). I am using a 1" equilivant (it's not 703), and it does not absorb the entire high end frequency and even less in the mids.

Not that's it's important, but I did set up the 1" ductboard and spent a few hours listening to different music and movies with and without the 1" material on the front wall before I permanently covered it, and I do prefer it treated.

-Jason
post #153 of 10210
Well OK 1" maybe down 10% at 1KHz - but the higher frequencies are still pretty close to 100% absorption - at least with the Linacoustic numbers I see on BasementBob's page. The difference with the thinner stuff is at mids at 250-500Hz.

IS Black 1" (25mm) 1.5pcf (24kg/m3) 0.09 0.29 0.67 0.89 1.03 0.99 0.70
Linacoustic RC 1" (25mm) 0.08 0.31 0.64 0.84 0.97 1.03 0.70

So I still maintain that you like it because the bipolar has been converted to monopolar (or technically a directional front firing tweeter)- at least in treble frequences the back wall wave is being absorbed.
post #154 of 10210
Quote:


Originally posted by krasmuzik
So I still maintain that you like it because the bipolar has been converted to monopolar (or technically a directional front firing tweeter)- at least in treble frequences the back wall wave is being absorbed.

Kras,

You may be correct, but I maintain that I still like the way my Def's sound even over other monopoles....so oh well.

The point I was making was......in general, the front wall of a HT should be acoustically treated. Period!
post #155 of 10210
Kras is just making some valid technical points here. If the speakers work for you, then that's great. Enjoy. Given the HT application and desire for front wall treatment, it simply wouldn't be advisable for the next guy who's shopping around for speakers.
post #156 of 10210
One must understand the genesis of speaker designs with a significant amount of "back radiation" or "back wave" (bipolar, rear firing tweeters, etc.) Back in the two channel only days, the only way to create room filling spaciousness was to rely upon reflected sound throughout the room. On method of augmenting this was to make better (more?) use of the front wall...which also helped fill the coverage hole in the middle.

You can get excellent sound stages with treated front walls and even better sound stages with three channels. In the multi-channel world it is most often very counterproductive to use such designs...effectively using your room as a surround processor...since ambiance extraction techniques and multi-channel recording can do a much better job.

(...and, I invite anyone to come and hear this in our demo facility in Atlanta)
post #157 of 10210
FD,

> What I REALLY want to find out is if I still need to put panels behind the heavy velvet curtains. <<br />
Yes, at least for bass trapping. All rooms need substantial absorption at low frequencies. Curtains won't do that, but you can certainly put traps behind the curtains.

> I would think that since the curtains are NOT acoustically transparent that it would be a moot point. <<br />
Bass waves go right through curtains, no matter what they're made of.

--Ethan
post #158 of 10210
Jason,

> I'm curious as to why you disagree with Dennis and others about treating the front wall <<br />
I still have not gotten around to measuring the free-field response of my Mackie HR624, and I plan to do that eventually. I'm not convinced that loudspeakers radiate a significant amount of mids and highs out the rear (or that mids and highs wrap around) enough to worry about treating the front wall. Listening rooms I've been in that had absorption over the entire front wall always sounded too dead to me.

I guess my main objection to treating the entire front wall is that it puts all of the absorption in one place, rather than spread evenly around the room. Also, many people wrongly believe that if they treat the entire front of their room that's all they need. The thin materials often used for this absorb only down to the midrange, and that's just not adequate.

--Ethan
post #159 of 10210
Quote:


Originally posted by Ethan Winer
I still have not gotten around to measuring the free-field response of my Mackie HR624, and I plan to do that eventually. I'm not convinced that loudspeakers radiate a significant amount of mids and highs out the rear (or that mids and highs wrap around) enough to worry about treating the front wall. Listening rooms I've been in that had absorption over the entire front wall always sounded too dead to me.
--Ethan

I see a conundrum here, Ethan. If there is minimal direct radiation of mids and highs to the front wall, why does treating it make the room too dead? Of course, it may be that anyone who covers the entire front wall has also put too much treatment elsewhere.

Kal
post #160 of 10210
Kal,

Sorry, I was still editing/adding while you were replying.

> If there is minimal direct radiation of mids and highs to the front wall, why does treating it make the room too dead? Of course, it may be that anyone who covers the entire front wall has also put too much treatment elsewhere. <<br />
The only reason to need mid/high frequency absorption anywhere is to handle early reflections. Everything else is for more general ambience and echo control, and to my way of thinking that absorption should be spread around evenly.

--Ethan
post #161 of 10210
OK. So are you saying that concentrating it on the front wall deadens more than spreading it around? What I am really asking is why the front wall is any more or less effective a site for mid-, hi- absorbtion if its proximity to the speakers is not an issue?

Kal
post #162 of 10210
Kal,

> So are you saying that concentrating it on the front wall deadens more than spreading it around? What I am really asking is why the front wall is any more or less effective a site for mid-, hi- absorbtion if its proximity to the speakers is not an issue? <<br />
The front wall is no less effective than any other surface if the goal is to reduce ambience generally. But treating the entire front wall and leaving the entire rest of the room reflective gives an unbalanced sound. You get obvious echoes off the rear wall, floor to ceiling flutter echo if there's no carpet, and also flutter echo between the side walls where they're not treated.

All of this depends entirely on the room in question! There are tiny rooms and huge rooms, so that too is a factor. I'm speaking in general terms about smallish rooms like you find in most homes.

--Ethan
post #163 of 10210
Kal:
In the overall scheme of things, deadens more or less is less a function of placement than it is a matter of the total amount of absorption (and characteristics) in the space. In small rooms, placement of absorption becomes significantly important. Now with respect to your specific question above, the answer becomes "well that depends". While you elminated the proximity issue to the front wall, we have other factors to consider...for example distance to side and rear walls as we manage not only room reverberation times but path differences and their related effects. Typically, however, as long as you're NOT using full range speakers, you'll find the L/C/R in the 3' range from the front wall...then considering the other issues, the front wall becomes a very effective place for absorption.
post #164 of 10210
Quote:


You get obvious echoes off the rear wall, floor to ceiling flutter echo if there's no carpet, and also flutter echo between the side walls where they're not treated

You may have an overly reverberant space; but, flutter/slap echoes will not be the issue (unless you have a speaker on the floor or ceiling, this cannot happen).
post #165 of 10210
Dennis how far is your business from Buckhead? I will be in Atlanta late January early February....

Edit for not reading before posting:
What happens Dennis in the case that your L/R are full range?
post #166 of 10210
Quote:


The front wall is no less effective than any other surface if the goal is to reduce ambience generally. But treating the entire front wall and leaving the entire rest of the room reflective gives an unbalanced sound. You get obvious echoes off the rear wall, floor to ceiling flutter echo if there's no carpet, and also flutter echo between the side walls where they're not treated.

All of this depends entirely on the room in question! There are tiny rooms and huge rooms, so that too is a factor. I'm speaking in general terms about smallish rooms like you find in most homes.

--Ethan

Ethan:

Then can you offer a suggestion on a good starting point for this room for treatment? I was planning on entire front and 42" of side all the way around. Floor=carpet, ceiling=tile, soffit=drywall.

http://www.mlec.net/scott/HTBigRoom1.gif
http://www.mlec.net/scott/HT3DNorth.jpg
Thanks,
Scott
The more I listen the less I know
post #167 of 10210
I am finishing out our basement and am putting a theater/ media room down stairs. Due to family consideration I cannot dedicate the space to just a theater. I was thinking about using a curtain on a track as a means of "creating" a back wall when i want to watch a movie. The benefit of course would be that when the theater is not in use the curtain could be pulled back, opening the whole space. The question is what would the effects be of the curtain on the acoustics of the room? Thanks for your help!!

Ron
post #168 of 10210
The curtain is a great idea for light control, but it will absorb a lot of HF and do nothing about the bass. Bass traps or thicker panels elsewhere behind the curtains can help with the bass absorption.
post #169 of 10210
Quote:


Dennis how far is your business from Buckhead? I will be in Atlanta late January early February....

About 11 miles north.

Quote:


Edit for not reading before posting:
What happens Dennis in the case that your L/R are full range?

As you move a speaker closer to the wall the SBIR notch frequency increases. With non-full range + a subwoofer, you can move the sub toward a boundary to raise the notch frequency above 80 Hz. You move your mains out from the wall (about 3') to move the notch below 80 hz. With full range, your speakers need to be on the order of 15 feet from any boundary. Further, the locations required for your main speakers to achieve the best imaging will not be the best location for LF to achieve smooth bass response in the room.

Quote:


but it will absorb a lot of HF and do nothing about the bass. Bass traps or thicker panels elsewhere behind the curtains can help with the bass absorption.

But you also have a much larger room from a bass point of view so you must deal with modal response, smooth bass reponse, and amplifier power based upon the larger space.
post #170 of 10210
I've noticed a lot of abbreviations used in the Home Theater section of AVS, is there a sticky or a Faq somewhere that has the definitions?

HH
post #171 of 10210
Since Dennis brought up the SBIR (Speaker-Boundary Interference Response), some of you may be wondering how to measure it. You can do this with a program like ETF.

The idea is based on the fact that SBIR is a short-time effect. It happens within the first tens of milliseconds of the sound, where 1 millisecond = about 1 foot of sound travel. So if you take the frequency response using a short impulse-response time window (20 milliseconds or so), you will see what is going on during the time period when the direct sound from the speaker interferes with its reflection from a nearby wall.

On the other hand, if you look at a long-term frequency response (time window of a hundred or more milliseconds) it will span several wall reflections back and forth through the room. You now see the effects of room modes added to the SBIR, since the modes need time to build up over several wall reflections. Get it?

Now, there is one gotcha. With a small time window, you automatically get less detail in the frequency domain. It is a fundamental trade-off in the signal processing world. So if you set the time window to 20 milliseconds for example, you won't see any frequencies below 50 Hz. ETF will show you a smooth response above this, but there is still limited low frequency information, and you will not see a lot of detail. Nevertheless, you still should be able to spot any dip corresponding to the 1/4 wave cancellation which Dennis described.

Apologies to those for whom this discussion was too technical, but it seemed like a good opportunity to explain more about the "mysterious" SBIR.

- Terry
post #172 of 10210
Quote:


Originally posted by Dennis Erskine
As you move a speaker closer to the wall the SBIR notch frequency increases. With non-full range + a subwoofer, you can move the sub toward a boundary to raise the notch frequency above 80 Hz. You move your mains out from the wall (about 3') to move the notch below 80 hz. With full range, your speakers need to be on the order of 15 feet from any boundary. Further, the locations required for your main speakers to achieve the best imaging will not be the best location for LF to achieve smooth bass response in the room.

Quote:


Originally posted by Terry Montlick

Since Dennis brought up the SBIR (Speaker-Boundary Interference Response), some of you may be wondering how to measure it. You can do this with a program like ETF.

The idea is based on the fact that SBIR is a short-time effect. It happens within the first tens of milliseconds of the sound, where 1 millisecond = about 1 foot of sound travel. So if you take the frequency response using a short impulse-response time window (20 milliseconds or so), you will see what is going on during the time period when the direct sound from the speaker interferes with its reflection from a nearby wall.

On the other hand, if you look at a long-term frequency response (time window of a hundred or more milliseconds) it will span several wall reflections back and forth through the room. You now see the effects of room modes added to the SBIR, since the modes need time to build up over several wall reflections. Get it?

Now, there is one gotcha. With a small time window, you automatically get less detail in the frequency domain. It is a fundamental trade-off in the signal processing world. So if you set the time window to 20 milliseconds for example, you won't see any frequencies below 50 Hz. ETF will show you a smooth response above this, but there is still limited low frequency information, and you will not see a lot of detail. Nevertheless, you still should be able to spot any dip corresponding to the 1/4 wave cancellation which Dennis described.

Apologies to those for whom this discussion was too technical, but it seemed like a good opportunity to explain more about the "mysterious" SBIR.

- Terry

Hi Dennis and Terry,

Thanks for the great information on a subject that, as Terry notes, is rather mysterious to the layperson.

I have been attempting to use EFT to aid in selecting my crossover points. I've been using a long time window and comparing the logarithmic frequency responses of various pairs of crossover points for my mains and LFE subwoofer. Based on this long time window it appears to me that selecting 60 hz for my mains and sub yielded the smoothest frequency response. However, based on your information it would appear that I am missing the short time effects of Speaker Boundary Interference. Should I be attempting this with a 20 ms time window? I realize I won't see the lowest bass response, but I assume the object would be to see how smooth I can get the responses at the crossover points?

Due to practical trade-offs of accommodating a moderately sized non-perforated screen, my floor standing main speakers are located less than the ideal distance from the side walls. The manufacturer's instructions recommend maintaining more than three feet from the side walls to the sides of the speaker cabinets. The sides of my speakers are 1'4 from the side walls or 2' from the walls to the center of the bass driver. (Surprisingly they state that the back of the speaker cabinet are permitted to be as close as 1-1/2 from the front wall. The back of my speakers are about 5 from the front wall.)

I have 1 of fiberglass on the front and side walls. Given that I can't change the horizontal orientation of the mains, is there anything I can do to mitigate SBIR effects?

Would merely setting the crossovers higher tend to overcome these effects?

In addition, would filling the space between the speakers and the side walls with rolls of my left-over 1 fiberglass be helpful? If so, since my overall reverberation time measures fairly low, would it be wise to cover the rolls with paper or some other reflective covering to avoid further reductions in the higher frequency reverberation times?

In order to center my main's tweeters vertically on the screen, I have placed them on concrete blocks. However, that puts the centers of the bass drivers at the same distance to the stage floor as they are from the side walls. Do these common dimensions aggravate SBIR effects?

Thanks in advance.

Larry
post #173 of 10210
Quote:


Originally posted by Dennis Erskine
As you move a speaker closer to the wall the SBIR notch frequency increases. With non-full range + a subwoofer, you can move the sub toward a boundary to raise the notch frequency above 80 Hz. You move your mains out from the wall (about 3') to move the notch below 80 hz. With full range, your speakers need to be on the order of 15 feet from any boundary. Further, the locations required for your main speakers to achieve the best imaging will not be the best location for LF to achieve smooth bass response in the room.

In my room I have full-range towers and am forced to put them approx 14" from the sidewalls. I also measure using ETF a null at ~220Hz. This to my thinking is SBIR (1130/(220*4) = ~15"). Where would wall treatment go to correct this - directly to the sides of the speakers, or more towards where the first reflection points are?

I did try heavily trapping the side walls beside the speakers with 2" fiberglass at one point, and it made the speakers sound dead. However, I may have used too much of it. I just checked my speaker specs - the woofer crossover is at 250Hz. I may try just treating the wall down low where the woofer is.

From a general point of view, do you recommend non-full range + sub even for 2-channel audio?

Thanks,

Andy K.
post #174 of 10210
Quote:


Originally posted by kromkamp
In my room I have full-range towers and am forced to put them approx 14" from the sidewalls. I also measure using ETF a null at ~220Hz. This to my thinking is SBIR (1130/(220*4) = ~15").

Hi Andy,

What did you set your time window for your measurement?

Thanks.

Larry
post #175 of 10210
Larry -
The idea with the crossover frequency and SBIR is to arrange sub and main speaker so that neither generates an audible 1/4 wavelength frequency. Just a simple calculation based on speaker distance to the wall is necessary.

Take the distance of the speaker or subwoofer to the wall in feet, and divide 282.5 by this distance. (282.5 = 1130 feet/sec, the speed of sound at room temperature, * 1/4 wavelength). The result is the frequency in Hz that would generate 1/4 wave cancellation.

This frequency calculated for your subwoofer, should be above the crossover point. This frequency calculated for the mains should be below the crossover point. Then, both sources completely avoid the problem.

Andy -
Wall treatment should be centered around the first reflection points, but it also needs to cover a broad area since the wavelength is relatively large. Without doing the math, I would guess that dimensions of 1/2 wavelength, or around 30", would be pretty safe.

One idea would be using a narrow tuned bass absorber for this specific frequency. That way, it will not affect the overall sound that you like. A 220 Hz tuned bass absorber (either Helmholtz or limp acoustic mass panel) shouldn't require a great deal of physical depth.

- Terry
post #176 of 10210
Quote:


Originally posted by Terry Montlick
Larry -
The idea with the crossover frequency and SBIR is to arrange sub and main speaker so that neither generates an audible 1/4 wavelength frequency. Just a simple calculation based on speaker distance to the wall is necessary.

Take the distance of the speaker or subwoofer to the wall in feet, and divide 282.5 by this distance. (282.5 = 1130 feet/sec, the speed of sound at room temperature, * 1/4 wavelength). The result is the frequency in Hz that would generate 1/4 wave cancellation.

This frequency calculated for your subwoofer, should be above the crossover point. This frequency calculated for the mains should be below the crossover point. Then, both sources completely avoid the problem.


Hi Terry,

Thanks for the response.

When measuring the distance from the speaker or subwoofer to the wall, are we measuring from the center of the bass drivers, or the side of the cabinet nearest the wall?

Thanks.

Larry
post #177 of 10210
Quote:


Originally posted by LarryChanin

When measuring the distance from the speaker or subwoofer to the wall, are we measuring from the center of the bass drivers, or the side of the cabinet nearest the wall?

Center of the bass drivers, mostly. There will be diffraction at the edges of the cabinet which will create secondary sound sources, but these will be lower in volume.

However, in the case of a side wall, you need to consider the time DIFFERENCE between direct and reflected sound, since the sound is not reflecting at right angles as it would be (more or less) from the front wall. Use the tape measure technique that Dennis suggested to determine this difference.

CORRECTION: When using the measured difference method, use 1/2 the wavelength, not 1/4 the wavelength. This means dividing 565 (2*282.5) by this distance difference.

- Terry
post #178 of 10210
Is there is general forumla or theory on what % of sq/ft needs to be covered with (1") acoutic treatment to achieve correct RT60? Or is this just WAY too dependent on too many moving variables? Basically I am looking to see if it is generally accepted that treating the front wall and ear level down to floor on the side walls in most HT's is appropiate does that equate to ~30-35%? More? Less? Any general rules that would be helpful in getting close (or in the ballpark) without measuring?

-Jason
post #179 of 10210
You can use some of the better spreadsheets and calculate it based on all of the materials in the room. It's far from exact but will get you in the ballpark.

How much you cover and in what depends so much on furnishings, concrete or wood subfloor, wall construction, # of people, etc. that it's awfully hard to give even a ballpark number. Also, the 'correct' RT60 is dependent on what you want to do with the room. The correct number is different for music listening than it is for HT.
post #180 of 10210
Quote:


Originally posted by Terry Montlick
Center of the bass drivers, mostly. There will be diffraction at the edges of the cabinet which will create secondary sound sources, but these will be lower in volume.

However, in the case of a side wall, you need to consider the time DIFFERENCE between direct and reflected sound, since the sound is not reflecting at right angles as it would be (more or less) from the front wall. Use the tape measure technique that Dennis suggested to determine this difference.

CORRECTION: When using the measured difference method, use 1/2 the wavelength, not 1/4 the wavelength. This means dividing 565 (2*282.5) by this distance difference.

- Terry

Hi Terry,

Thanks.

Just to be sure I know what you mean by Dennis' tape measure technique I've attached a crude diagram showing the speaker, listener and walls. The object is to measure the direct path sound (red line) and the reflected path of sound (black lines) and then use the difference in your half wave calculation. Correct?

Thanks.

Larry
LL
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