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Please help me stop the back wall BOOM! - Page 2

post #31 of 96
Thread Starter 
Quote:
Originally Posted by GIK Acoustics View Post

This I can agree with. There's always the possibility to do great things with positioning, testing, treating, etc yourself at home but trying to get great bass over three rows is difficult regardless of the budget, regardless of the designer, if the room isn't entirely purpose built and designed for it in the first place. Treatment certainly helps, but is of course one of the many tools necessary to get the space worked out well enough.

I hate to think what 99.9% of cinemas must be like then with 50 or more rows:cool:
post #32 of 96
Quote:
Originally Posted by RapalloAV View Post

I hate to think what 99.9% of cinemas must be like then with 50 or more rows:cool:
That's chalk and cheese...commercial cinemas and home theaters are entirely different acoustically!
post #33 of 96
Quote:
Originally Posted by RapalloAV View Post

This Modex plate looks great, do you think it might improve my back wall?
Have you used this product before?
Yes it will improve things and yes I've used it before. I have four of the type 1s in my 12x16 demo theater...as well as four subs and a bunch of eq...
post #34 of 96
Thread Starter 
Quote:
Originally Posted by Nyal Mellor View Post

Yes it will improve things and yes I've used it before. I have four of the type 1s in my 12x16 demo theater...as well as four subs and a bunch of eq...

I've done some searching on it and there seems to be a lot of controversy over the price, thousands of dollars for a strip!
They seem to think its just a money making product......
Its also probably going to be very expensive to ship to NZ, we don't have it here.
post #35 of 96
Quote:
Originally Posted by RapalloAV View Post

Quote:
Originally Posted by Nyal Mellor View Post

Yes it will improve things and yes I've used it before. I have four of the type 1s in my 12x16 demo theater...as well as four subs and a bunch of eq...

I've done some searching on it and there seems to be a lot of controversy over the price, thousands of dollars for a strip!
They seem to think its just a money making product......
Its also probably going to be very expensive to ship to NZ, we don't have it here.

Green glue and thin wood panels may be an alternative.

http://www.greengluecompany.com/benefit/how-green-glue-works/effects-of-damping

At these low frequencies you get more involved with the architecture of the room's walls.
post #36 of 96
Quote:
Originally Posted by arnyk View Post

Green glue and thin wood panels may be an alternative.

http://www.greengluecompany.com/benefit/how-green-glue-works/effects-of-damping

At these low frequencies you get more involved with the architecture of the room's walls.

Have you had any experience or know of any papers published on using this kind of approach for absorbing the low bass?

Me and at least one other acoustical designer I know have built bass traps using a similar principle (a heavy, 'dead' layer) but also using isolation clips. We designed these based on unpublished test data from Kinetics that shows how using isolation clips increases the absorption of the wall system in the low bass. It works but there is no published data on it that I know of.
post #37 of 96
Beautiful room, so congrats on that regard.

The back row being so, so close to the rear wall is just what it is... a tough situation to address regardless of the tools utilized. Also, you're being offered advice by all the right individuals, so all good there.

From the layman's/enthusiast's DIY perspective, I'd approach this as is being done above; with EQ but with massive and focused LF treatment too. In any space, with the mains/subs flush mounted and up front, this places increased acoustic demands on the importance of the physical characteristics of the rear boundary. Immediately, a brief examining of the rear wall surface, I can see locations under-utilized for treatment. That said, every possible square inch of that rear boundary, including the doors, could and should be utilized for some proper bass trapping.

I realize this is an issue, but how motivated are you to address it? Most draw the line, and deal with issues, however many can not simply move on ...
Is there anyway to move the rear seating forward to accommodate more space for rear wall treatment?
Is there any way to re-locate, or add additional subwoofer sources to the rear wall area?
Are you familiar with the DBA (double bass array) concept, and how it eliminates many of the negative effects associated to the rear wall?

1.) The rear wall is essentially untreated
2.) The row 3 seating is too close to the untreated rear wall
3.) The two flush mounted subs up front exacerbate the issue

Which elements are at play here,.. wrt changes?


thanks
post #38 of 96
Quote:
Originally Posted by Nyal Mellor View Post

Quote:
Originally Posted by arnyk View Post

Green glue and thin wood panels may be an alternative.

http://www.greengluecompany.com/benefit/how-green-glue-works/effects-of-damping

At these low frequencies you get more involved with the architecture of the room's walls.

Have you had any experience or know of any papers published on using this kind of approach for absorbing the low bass?

Just an anecdote:

http://www.avsforum.com/t/1136101/my-bass-story-ii#post_16201573

Others probably exist, but this is the one that I found quickly.
Quote:
Me and at least one other acoustical designer I know have built bass traps using a similar principle (a heavy, 'dead' layer) but also using isolation clips. We designed these based on unpublished test data from Kinetics that shows how using isolation clips increases the absorption of the wall system in the low bass. It works but there is no published data on it that I know of.

Yes, isolation clips suspending a panel can provide the benefit we seek, but as you say predicting what a given configuration of them will do does not seem to be a well-known or well-supported activity.
post #39 of 96
Quote:
Originally Posted by RapalloAV View Post

I can make padded panels that fit into the existing three timber frames I already have, they are only 2" thick though. This could be extended to a max only of 4", anything thicker than this will interfere with the backs of the seats and an air vent. Some here say you need at least 12" for this to work, will 4" do anything or not?

Traps four inches thick will be twice as effective as traps two inches thick. In your case the foam is sculpted, so its average depth is more like one inch. A 4-inch thick trap with a membrane will work even better. That still won't do much at 30-40 Hz, but you'll benefit greatly from absorption at higher bass frequencies too. As Nyal said, RPG's Modex trap will work well. They're very expensive, but it seems that's not an issue. In all honesty, you should just hire Nyal to help you. It will be money very well spent.
Quote:
your room has so many cream / white colours in it, have you ever thought of painting it and changing the colours of all those traps to black or really dark colour to improve your blacks on the screen?

I like the light colors, and they disappear completely when the lights are down because the room is wide enough, and high enough, to avoid reflected light from the screen. Further, I have 55 bass traps, diffusers, and other panels, that were installed over several years. The cost and effort to remove them all, paint the room, and buy them all again in black would be enormous. I'm totally satisfied with what I have now! Not just the video, but the audio too.

--Ethan
post #40 of 96
Thread Starter 
Quote:
Originally Posted by FOH View Post

Beautiful room, so congrats on that regard.

The back row being so, so close to the rear wall is just what it is... a tough situation to address regardless of the tools utilized. Also, you're being offered advice by all the right individuals, so all good there.

From the layman's/enthusiast's DIY perspective, I'd approach this as is being done above; with EQ but with massive and focused LF treatment too. In any space, with the mains/subs flush mounted and up front, this places increased acoustic demands on the importance of the physical characteristics of the rear boundary. Immediately, a brief examining of the rear wall surface, I can see locations under-utilized for treatment. That said, every possible square inch of that rear boundary, including the doors, could and should be utilized for some proper bass trapping.

I realize this is an issue, but how motivated are you to address it? Most draw the line, and deal with issues, however many can not simply move on ...
Is there anyway to move the rear seating forward to accommodate more space for rear wall treatment?
Is there any way to re-locate, or add additional subwoofer sources to the rear wall area?
Are you familiar with the DBA (double bass array) concept, and how it eliminates many of the negative effects associated to the rear wall?

1.) The rear wall is essentially untreated
2.) The row 3 seating is too close to the untreated rear wall
3.) The two flush mounted subs up front exacerbate the issue

Which elements are at play here,.. wrt changes?


thanks

Hi FOH

Many thanks for your comments..... you are the first to ask some of these new questions, so that's also a good start. Everyone seems to have slightly different approaches, some cheap some expensive. Many others I've spoken to have said don't worry, its a common issue with multiple rows and all 12 seats in my cinema are not always used. I have made some improvements over the last few days and the sound (back wall boom) certainly has improved I must say, please let me explain. The following is assumptions of mine only, no one here has guided me so its only experimental.

After viewing the readings of the three rows I did with the SMS EQ, the front and the back row proved to be the flattest, the middle row had the lowest dip. I assumed if I save the best reading on the SMS (front row) then then EQd with XT32 on my Integra 80.3 maybe I might have something better at the back wall. Well that proved to be correct, it was reduced significantly. The other things I experimented with which I think also helped, I didn't use the centre row at all for XT32 EQ, I did 5 mic positions at the back row, (infront of the seats away from the wall) and the last four mic positions in the front row. I thought since the readings in the middle row are bad on bass, lets just stay away from it. The other thing I've been experimenting with is just saving the crossovers at where XT32 finds them, this too has really reduced the boom on the back wall. THX says to move them all up to 80Hz, but my experiments has proven that produces more boom on the back wall, so I maybe its ok to not follow THX and just allow them to remain as XT32 finds them.......

A plan I do have however and I'm interested in your thoughts.... I'm deleting the two 10" Velodynes from up front in the next few weeks as I've ordered four SVS SB13-ULTRA that will line up in front of the stage area. Something will change, hopefully for the better all over the room, what are your thoughts on this FOB?

OK, lets get back to your questions.....

1. I am motivated in addressing it as best I can without pulling the cinema apart and rebuilding it.
2. Impossible to move the rear seating forward, the seats are on risers and built that way to accommodate the seats.
3. There is space to add subs to the rear wall but I thought that was no good, I never hear of anyone doing that, I searched for that many times.
4. I don't know anything about DBA, I will have to goggle it.
5. So my subs up front and enclosed within the stage area is making it worse, correct?



Thanks again for your thoughts, lets see what you come back with..

Regards
post #41 of 96
Thread Starter 
Quote:
Originally Posted by Ethan Winer View Post

In all honesty, you should just hire Nyal to help you. It will be money very well spent.

--Ethan

Im in NZ he isn't.
post #42 of 96
Thread Starter 
Quote:
Originally Posted by arnyk View Post

Green glue and thin wood panels may be an alternative.

http://www.greengluecompany.com/benefit/how-green-glue-works/effects-of-damping

At these low frequencies you get more involved with the architecture of the room's walls.

The whole room was constructed with two layers of sound gib board and green glue, ceiling as well.
post #43 of 96
Quote:
Originally Posted by RapalloAV View Post

Im in NZ he isn't.

It doesn't matter! I help people all over the world by phone and email, and I'm sure Nyal does the same. Or just continue asking for free advice here. biggrin.gif

--Ethan
post #44 of 96
Quote:
Originally Posted by Ethan Winer View Post

It doesn't matter! I help people all over the world by phone and email, and I'm sure Nyal does the same. Or just continue asking for free advice here. biggrin.gif

--Ethan

Yes indeedy, this kind of thing can be done very effectively remotely. It's probably not as productive as being on site but truth be told there are not many people out there in the world who understand small room acoustic analysis and design and a lot of people will not have someone close to them.

I've done same-time sessions for both diagnosis and calibration where I will access a client's computer over the internet and control an acoustic measurement rig sitting in his home theater or listening room.

I've also done staggered-time sessions where the client has taken measurements offline and sent them to me via email and in a back and forth way we have analyzed the room or calibrated the system.
post #45 of 96
Quote:
Originally Posted by RapalloAV View Post

1. I am motivated in addressing it as best I can without pulling the cinema apart and rebuilding it.
2. Impossible to move the rear seating forward, the seats are on risers and built that way to accommodate the seats.
3. There is space to add subs to the rear wall but I thought that was no good, I never hear of anyone doing that, I searched for that many times.
4. I don't know anything about DBA, I will have to goggle it.
5. So my subs up front and enclosed within the stage area is making it worse, correct?

If you want to do this yourself first you need to educate yourself on how subwoofers can be used in mode canceling arrangements. You also need to procure and learn how to use a better acoustic measurement system, one that has high resolution and can show you both the frequency and the time domains. You will need this to understand the problems properly and use it to accurately set the EQ in the SMS so it can remove the room mode ringing for modes you are not able to cancel through using multiple subwoofers. Parametric EQ is very effective at killing room modes but it must be accurately set up. Even a slightly misconfigured Q value will cause the room mode ringing not to be removed.

Rear subs are very effective IF they are deployed purposely i.e. no semi-random placement. Here's an example of how rear subs could be used in your room. From your website I got 6m as the length. That means the first axial room mode is at 28Hz. A first axial mode has a pressure peak at the front wall, a null in the center of the room and another pressure peak at the rear wall. Assuming this is a major cause of your rear wall boom then if you have one sub at the front of the room and one sub at the rear of the room and they are properly calibrated then you will cancel out the room mode. If you walk front to back in the room with a SPL meter the level should hardly change. See my intro to room mode cancellation article for more. Read the links on this page for more, particularly the links to the Blu-Ray forum articles.

Again...you will need to get a proper acoustic measurement rig. The Velodyne SMS is not high enough resolution for this kind of work.
Edited by Nyal Mellor - 4/24/13 at 4:46pm
post #46 of 96
Thread Starter 
Quote:
Originally Posted by Nyal Mellor View Post

If you want to do this yourself first you need to educate yourself on how subwoofers can be used in mode canceling arrangements. You also need to procure and learn how to use a better acoustic measurement system, one that has high resolution and can show you both the frequency and the time domains. You will need this to understand the problems properly and use it to accurately set the EQ in the SMS so it can remove the room mode ringing for modes you are not able to cancel through using multiple subwoofers. Parametric EQ is very effective at killing room modes but it must be accurately set up. Even a slightly misconfigured Q value will cause the room mode ringing not to be removed.

Rear subs are very effective IF they are deployed purposely i.e. no semi-random placement. Here's an example of how rear subs could be used in your room. From your website I got 6m as the length. That means the first axial room mode is at 28Hz. A first axial mode has a pressure peak at the front wall, a null in the center of the room and another pressure peak at the rear wall. Assuming this is a major cause of your rear wall boom then if you have one sub at the front of the room and one sub at the rear of the room and they are properly calibrated then you will cancel out the room mode. If you walk front to back in the room with a SPL meter the level should hardly change. See my intro to room mode cancellation article for more. Read the links on this page for more, particularly the links to the Blu-Ray forum articles.

Again...you will need to get a proper acoustic measurement rig. The Velodyne SMS is not high enough resolution for this kind of work.

A sub cant go on the floor at the rear of the room, there is no room. Can they be placed on the wall?
post #47 of 96
Quote:
Originally Posted by RapalloAV View Post

A sub cant go on the floor at the rear of the room, there is no room. Can they be placed on the wall?

Yes, in / on wall, on ceiling, many options! In fact having the front subs at ground level and the rears near the ceiling will also cancel the odd (1,3, etc) order vertical axial room modes. And whilst a few years ago on wall / in wall / in ceiling subs were generally crap there have been some pretty nice subs available recently.
post #48 of 96
Quote:
Originally Posted by RapalloAV View Post

Quote:
Originally Posted by Nyal Mellor View Post

If you want to do this yourself first you need to educate yourself on how subwoofers can be used in mode canceling arrangements. You also need to procure and learn how to use a better acoustic measurement system, one that has high resolution and can show you both the frequency and the time domains. You will need this to understand the problems properly and use it to accurately set the EQ in the SMS so it can remove the room mode ringing for modes you are not able to cancel through using multiple subwoofers. Parametric EQ is very effective at killing room modes but it must be accurately set up. Even a slightly misconfigured Q value will cause the room mode ringing not to be removed.

Rear subs are very effective IF they are deployed purposely i.e. no semi-random placement. Here's an example of how rear subs could be used in your room. From your website I got 6m as the length. That means the first axial room mode is at 28Hz. A first axial mode has a pressure peak at the front wall, a null in the center of the room and another pressure peak at the rear wall. Assuming this is a major cause of your rear wall boom then if you have one sub at the front of the room and one sub at the rear of the room and they are properly calibrated then you will cancel out the room mode. If you walk front to back in the room with a SPL meter the level should hardly change. See my intro to room mode cancellation article for more. Read the links on this page for more, particularly the links to the Blu-Ray forum articles.

Again...you will need to get a proper acoustic measurement rig. The Velodyne SMS is not high enough resolution for this kind of work.

A sub cant go on the floor at the rear of the room, there is no room. Can they be placed on the wall?

I don't see why not. There are bass speaker drivers that are sold in the automotive sound market that are exceptionally thin, which may facilitate positioning subwoofers on the wall.
post #49 of 96
Quote:
Originally Posted by RapalloAV View Post

Quote:
Originally Posted by arnyk View Post

Green glue and thin wood panels may be an alternative.

http://www.greengluecompany.com/benefit/how-green-glue-works/effects-of-damping

At these low frequencies you get more involved with the architecture of the room's walls.

The whole room was constructed with two layers of sound gib board and green glue, ceiling as well.

Wow! Well, you tried.

I'm wondering whether the green glue was used to bond the panels with minimum thickness or whether a few thick beads were laid to minimally couple the panels.
post #50 of 96
Thread Starter 
Quote:
Originally Posted by arnyk View Post

Wow! Well, you tried.

I'm wondering whether the green glue was used to bond the panels with minimum thickness or whether a few thick beads were laid to minimally couple the panels.


Green Glue was applied to the four walls and ceiling exactly as per their instructions, we did not deviate from the recommendations from the company Green Glue. However it is amazing the soundproofing of the building, one hears very little at reference levels standing 1M away from the walls outside, I very much did my homework with this side of things.....

However, the most amazing thing I have discovered along the way and the largest impact of improving the "back wall boom" and without any changes in room treatments so far is:

Changing the phase of the subs to 180 deg. it seems to have removed at least 80% of that bothersome back wall boom, can you explain this phenomenon when some mentioned it wouldn't help????

If I address some form of thin bass trap on the back wall, maybe I'm almost there...
But since I have my four new SVS SB13-ULTRA on their way its best I wait before I change too much else.
post #51 of 96
Quote:
Originally Posted by RapalloAV View Post


However, the most amazing thing I have discovered along the way and the largest impact of improving the "back wall boom" and without any changes in room treatments so far is:

Changing the phase of the subs to 180 deg. it seems to have removed at least 80% of that bothersome back wall boom, can you explain this phenomenon when some mentioned it wouldn't help????

In my defense what I said was:

http://www.avsforum.com/t/1469333/please-help-me-stop-the-back-wall-boom#post_23231085

"The effect of the subwoofer phase control is largely localized to the crossover frequency. If that is the peak frequency of your boom, then go for it!

After rereading the thread I may be able to convince myself that this condition was relevant, so the observed results were allowed for in my comment and even suggested to be experimented with. ;-)
post #52 of 96
Quote:
Originally Posted by RapalloAV View Post

Changing the phase of the subs to 180 deg. it seems to have removed at least 80% of that bothersome back wall boom, can you explain this phenomenon when some mentioned it wouldn't help????

That is strange...theoretically if you are running your main speakers with a crossover at say 80Hz then which way the phase control is set on the existing sub that are all on the front wall should make very little difference at 20-40Hz. Now if you had front subs and back subs and you were flipping the polarity 180 degrees and it was making a big difference...that I would expect...

Maybe what you are hearing as back wall boom is not the 20-40Hz band but somewhere else around 60-100Hz. Then what you say would make sense. The thing I wonder though is if you havemade things better for just the back row and worse elsewhere?
post #53 of 96
Thread Starter 
Quote:
Originally Posted by Nyal Mellor View Post

That is strange...theoretically if you are running your main speakers with a crossover at say 80Hz then which way the phase control is set on the existing sub that are all on the front wall should make very little difference at 20-40Hz. Now if you had front subs and back subs and you were flipping the polarity 180 degrees and it was making a big difference...that I would expect...

Maybe what you are hearing as back wall boom is not the 20-40Hz band but somewhere else around 60-100Hz. Then what you say would make sense. The thing I wonder though is if you havemade things better for just the back row and worse elsewhere?

Not only the back wall is better, the other two front rows are too, I seem to have much more punchy bass in the room, the over all effect to the ear is much more pleasing and most of that muddy sound has improved.

I would also like to investigate a thinner membrane bass trap in the three areas on the back wall that have Auralex foam in them at present, is this a good idea or not?
post #54 of 96
Thread Starter 
Quote:
Originally Posted by arnyk View Post

In my defense what I said was:

http://www.avsforum.com/t/1469333/please-help-me-stop-the-back-wall-boom#post_23231085

"The effect of the subwoofer phase control is largely localized to the crossover frequency. If that is the peak frequency of your boom, then go for it!

After rereading the thread I may be able to convince myself that this condition was relevant, so the observed results were allowed for in my comment and even suggested to be experimented with. ;-)

Thank you Arnold for that...

I wasn't sure who mentioned that in the beginning, so it was you.
Well I can prove your were right, Ive tested it over and over again, with the phase set at 0 and back to 180 deg. Its 100% better set to 180, cleaner, removes the muddy sound, punchier, and the boom on the back wall has dropped considerably!

These are good signs and at least a start in the right direction. I just have to wait for my four new subs to arrive, then I will decommission the Velodynes and see if the room inproves even more.
post #55 of 96
I guess you are taking the experiment and see approach. That's cool and it seems like you are making progress.

Personally I like to design so that seat-to-seat variability in bass response is low before the theater is even built. If the room is designed properly with the right number of subs in the right places then you will have low seat-to-seat variability and global EQ can be used to flatten out any remaining peaks and even bring up the level of dips a little.

Here's an example of a predicted frequency response across a single row in our demo theater. This was for four subs, two front, two rear:


Here's what is measured like post install with some separate delays applied to a front and rear sub array but without any EQ (unsmoothed measurement btw):


The modeling technique we use is I have found not particularly useful for predicting absolute frequency response (as you can see there are some differences between the predicted and post although also note the modeling assumes the subs are all in phase whereas I purposely delayed the rear subs to cancel some of the room modes which would have greatly affected the 35Hz ish response values). However I have found it is very useful at predicting the seat-to-seat differences...and in a multi-seat home theater with EQ capability and the skill to dial it in that's all you really need.

If your bass issues are in the 60-100Hz range as it seems from the results of your experiments then yes sealed box membrane traps will work. I think from memory (not having done the calcs) you will need more like 6" depth though to be effective at 60Hz. These things need to be tuned and so really you need to figure out what your problem frequency is. 4" of a fiberglass panel on the wall isn't going to do much at 60Hz mounted flat on a wall even if you put a membrane on the front of it.
Edited by Nyal Mellor - 4/25/13 at 5:09pm
post #56 of 96
Thread Starter 
Quote:
Originally Posted by Nyal Mellor View Post

I guess you are taking the experiment and see approach. That's cool and it seems like you are making progress.

Personally I like to design so that seat-to-seat variability in bass response is low before the theater is even built. If the room is designed properly with the right number of subs in the right places then you will have low seat-to-seat variability and global EQ can be used to flatten out any remaining peaks and even bring up the level of dips a little.

Here's an example of a predicted frequency response across a single row in our demo theater:


Here's what is measured like post install with some separate delays applied to a front and rear sub array but without any EQ (unsmoothed measurement btw):


The modeling technique we use is I have found not particularly useful for predicting absolute frequency response (as you can see there are some differences between the predicted and post). However I have found it is very useful at predicting the seat-to-seat differences...and in a multi-seat home theater with EQ capability and the skill to dial it in that's all you really need.

If your bass issues are in the 60-100Hz range as it seems from the results of your experiments then yes sealed box membrane traps will work. I think from memory (not having done the calcs) you will need more like 6" depth though to be effective at 60Hz. These things need to be tuned and so really you need to figure out what your problem frequency is. 4" of a fiberglass panel on the wall isn't going to do much at 60Hz mounted flat on a wall even if you put a membrane on the front of it.

Ok If 4" doesn't work its no use me wasting the time and money on even investigating that further.......
I really think I need to see what happens when the new subs arrive.
post #57 of 96
Quote:
Originally Posted by RapalloAV View Post

Ok If 4" doesn't work its no use me wasting the time and money on even investigating that further.......
I really think I need to see what happens when the new subs arrive.

Yes and whilst you are at it get an acoustic measurement rig setup like an Omnimic, XTZ Room Analyzer or a Room EQ Wizard setup!
post #58 of 96
Quote:
Originally Posted by Nyal Mellor View Post

4" of a fiberglass panel on the wall isn't going to do much at 60Hz mounted flat on a wall even if you put a membrane on the front of it.

The problem frequencies from reflections off a rear wall are from 0 Hz to 20 KHz, so anything is better than nothing. In this case a real bass trap four inches thick is vastly better than sculpted foam only two inches thick (that behaves more like rigid fiberglass only one inch thick). Further, a good bass trap that's only four inches thick can still absorb about half at 60 Hz. See the first Absorption Coefficients table on this page for MondoTraps flat on a wall:

RealTraps Product Data

Yes, it's not 100 percent absorption, but even 50 percent will reduce a peak from 6 dB down to 4.5 dB, and (possibly more important) raise a null from -30 dB up to only -10 dB. If four inches is all he has available, I say go for it. It is absolutely not a waste of time! biggrin.gif

--Ethan
post #59 of 96
Thread Starter 
Quote:
Originally Posted by Ethan Winer View Post

The problem frequencies from reflections off a rear wall are from 0 Hz to 20 KHz, so anything is better than nothing. In this case a real bass trap four inches thick is vastly better than sculpted foam only two inches thick (that behaves more like rigid fiberglass only one inch thick). Further, a good bass trap that's only four inches thick can still absorb about half at 60 Hz. See the first Absorption Coefficients table on this page for MondoTraps flat on a wall:

RealTraps Product Data

Yes, it's not 100 percent absorption, but even 50 percent will reduce a peak from 6 dB down to 4.5 dB, and (possibly more important) raise a null from -30 dB up to only -10 dB. If four inches is all he has available, I say go for it. It is absolutely not a waste of time! biggrin.gif

--Ethan

Ok Ethan looks like Im sold!

As you know I only have those three spaces on the back wall that are currently picture framed with 2" timber, and the Auralex fits into. I need to use these as a starting point. I can build them out to 4" with traps but I will have to make them so they fit nicely into the spaces. Im thinking the larger area could be made up of say four traps snuggly fitting into that space and then one each in the two smaller areas. I would cover the traps with black fabric so the wall would have a nice upholstered look.



This I can at least start with this while I wait for my four new SB13- Ultras to arrive.
Am I on the right track?

Is there a tutorial that you know of that would be good to follow?
I know there are many on Youtube....

Thank you!
post #60 of 96
Quote:
Originally Posted by Ethan Winer View Post

The problem frequencies from reflections off a rear wall are from 0 Hz to 20 KHz, so anything is better than nothing. In this case a real bass trap four inches thick is vastly better than sculpted foam only two inches thick (that behaves more like rigid fiberglass only one inch thick). Further, a good bass trap that's only four inches thick can still absorb about half at 60 Hz. See the first Absorption Coefficients table on this page for MondoTraps flat on a wall:

RealTraps Product Data

Yes, it's not 100 percent absorption, but even 50 percent will reduce a peak from 6 dB down to 4.5 dB, and (possibly more important) raise a null from -30 dB up to only -10 dB. If four inches is all he has available, I say go for it. It is absolutely not a waste of time! biggrin.gif

--Ethan

Yes you are quite right, pretty much anything is going to be better than foam! I do like foam for somethings, like facing screen walls or using as spacers for other acoustical panels but those uses are mostly for it's structural properties, color options and texture, not it's acoustical performance.

Given the choice though I would use another product that was still working down at 60Hz in that location (e.g. modex plate). If the OP wants to use DIY with standard fiberglass then I'd try and face it with a membrane or a fiberglass-membrane-fiberglass sandwich. I like mass loaded vinyl for this kind of application, thin masonite also works, and I'm sure you have other suggestions also.
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