Please help me stop the back wall BOOM! - AVS Forum
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post #1 of 96 Old 04-21-2013, 04:39 PM - Thread Starter
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I placed this thread in home thread in home cinema construction in Feb. but no one came back, hopefully I'm in the right place here.


My room is 6M long by 4M wide, I'm pretty impressed with my audio but the back wall has the BOOM from the subs which I don't like. I know I need bass traps but I just cant see how I can fit them into the corners of the back of the room. Please can someone offer some advise to reduce the back wall boom and make the audio similar to the other two rows in front.

My equipment is :

2x Velodyne SC-600 IF/IC 12" Subwoofer

1x Velodyne V-SC1250E AMP – 1250 watt

1x Velodyne Digital Drive SMS1

Integra DTA-70.1
Integra DHC-80.3

Clarke Synthesis 2x TST429 Platinum Transducer + Monoblock 5.3 Amp



Other specs of my room are here:
http://www.rapalloav.co.nz/blog/rapallo-home-cinema-build
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post #2 of 96 Old 04-21-2013, 08:03 PM
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This kind of thing is nearly always an issue with multi-row home theaters where one row is virtually against a wall.

Can you post some frequency response measurements, <100Hz, 1/3 oct and 1/12th oct from the different rows?

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post #3 of 96 Old 04-22-2013, 01:43 AM
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Quote:
Originally Posted by RapalloAV View Post

I placed this thread in home thread in home cinema construction in Feb. but no one came back, hopefully I'm in the right place here.


My room is 6M long by 4M wide, I'm pretty impressed with my audio but the back wall has the BOOM from the subs which I don't like. I know I need bass traps but I just cant see how I can fit them into the corners of the back of the room. Please can someone offer some advise to reduce the back wall boom and make the audio similar to the other two rows in front.

My equipment is :

2x Velodyne SC-600 IF/IC 12" Subwoofer

1x Velodyne V-SC1250E AMP – 1250 watt

1x Velodyne Digital Drive SMS1

Integra DTA-70.1
Integra DHC-80.3

Clarke Synthesis 2x TST429 Platinum Transducer + Monoblock 5.3 Amp



Other specs of my room are here:
http://www.rapalloav.co.nz/blog/rapallo-home-cinema-build

I got rid of the back wall boom by rotating the phase on the back of the sub all the way to the opposite of what it is now. Also try lowering the sub's volume a bit. That's how i got rid of the back wall boom and now the bass is punchy , tight and smooth.
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post #4 of 96 Old 04-22-2013, 03:13 AM - Thread Starter
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Quote:
Originally Posted by Nyal Mellor View Post

This kind of thing is nearly always an issue with multi-row home theaters where one row is virtually against a wall.

Can you post some frequency response measurements, <100Hz, 1/3 oct and 1/12th oct from the different rows?

I'm not sure how to give you these readings you require, can you explain how to please?
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post #5 of 96 Old 04-22-2013, 03:14 AM - Thread Starter
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Originally Posted by dazkyl View Post

I got rid of the back wall boom by rotating the phase on the back of the sub all the way to the opposite of what it is now. Also try lowering the sub's volume a bit. That's how i got rid of the back wall boom and now the bass is punchy , tight and smooth.

Can someone explain what does rotating the phase on the sub do to eliminate the back wall boom?
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post #6 of 96 Old 04-22-2013, 04:44 AM
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Quote:
Originally Posted by RapalloAV View Post

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Originally Posted by dazkyl View Post

I got rid of the back wall boom by rotating the phase on the back of the sub all the way to the opposite of what it is now. Also try lowering the sub's volume a bit. That's how i got rid of the back wall boom and now the bass is punchy , tight and smooth.

Can someone explain what does rotating the phase on the sub do to eliminate the back wall boom?

Probably not a heck of a lot!

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!

The back wall boom is often due to a standing wave that starts at the back wall. I've made them go away, but it takes a fairly thick stack of absorbing material with an equal thickness gap to the back wall. We're talking the better part of a foot here.

If you can nail the problem down to a specific frequency then you can mitigate it with a resonant bass trap.
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post #7 of 96 Old 04-22-2013, 12:09 PM - Thread Starter
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Probably not a heck of a lot!

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!

The back wall boom is often due to a standing wave that starts at the back wall. I've made them go away, but it takes a fairly thick stack of absorbing material with an equal thickness gap to the back wall. We're talking the better part of a foot here.

If you can nail the problem down to a specific frequency then you can mitigate it with a resonant bass trap.

It looks like there are two peaks that show up on the SMS graph, round 30 and 40Hz, Im crossing over at 100Hz
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post #8 of 96 Old 04-22-2013, 12:50 PM
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I know I need bass traps but I just cant see how I can fit them into the corners of the back of the room.

If you simply replace that 2-inch thick foam with proper bass traps, that will make a huge improvement. Not just the upper part of the wall near the ceiling corner, but the entire center area as well. Additional bass traps in the front of the room (and elsewhere) will surely help further.

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post #9 of 96 Old 04-22-2013, 12:58 PM
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Originally Posted by RapalloAV View Post

I'm not sure how to give you these readings you require, can you explain how to please?

So you have the Velodyne SMS? Post pics for us of what you are seeing! Do you have Room EQ Wizard or any other acoustic measurement product?

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post #10 of 96 Old 04-22-2013, 01:02 PM
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Originally Posted by Ethan Winer View Post

If you simply replace that 2-inch thick foam with proper bass traps, that will make a huge improvement. Not just the upper part of the wall near the ceiling corner, but the entire center area as well. Additional bass traps in the front of the room (and elsewhere) will surely help further.

--Ethan

In all fairness Ethan 99% of bass traps are going to do nothing for a 20 or 30Hz issue which it seems like the OP has from his recent post. Most commercial bass traps are really effective only in the 100Hz + region. There are a few that go down to 70Hz ish, and one or two that go down to 30Hz with consistent absorption.

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post #11 of 96 Old 04-22-2013, 01:08 PM
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Originally Posted by Nyal Mellor View Post

In all fairness Ethan 99% of bass traps are going to do nothing for a 20 or 30Hz issue which it seems like the OP has from his recent post.

He said 30 and 40 Hz, but yeah, that's pretty low. However, I imagine he has other "boom" problems at higher bass frequencies. Further, good bass traps can still make a big improvement that low if you have enough of them and put them in the right place. The graphs below from my Hearing is Believing video show a real improvement as low as 40 Hz.

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post #12 of 96 Old 04-22-2013, 01:15 PM - Thread Starter
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Originally Posted by Nyal Mellor View Post

So you have the Velodyne SMS? Post pics for us of what you are seeing! Do you have Room EQ Wizard or any other acoustic measurement product?

We first measured the room with the 5 mics but still had a similar result. Last night I just used one mic in the best seat with the auto EQ on the SMS and the result is maxed out in some areas. There are three rows of four seats, I set the mic up in the middle row. I also tried it in the back row but it maxes out even earlier and the result is worse.

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post #13 of 96 Old 04-22-2013, 02:17 PM
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Quote:
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If you simply replace that 2-inch thick foam with proper bass traps, that will make a huge improvement. Not just the upper part of the wall near the ceiling corner, but the entire center area as well. Additional bass traps in the front of the room (and elsewhere) will surely help further.

--Ethan

In all fairness Ethan 99% of bass traps are going to do nothing for a 20 or 30Hz issue which it seems like the OP has from his recent post. Most commercial bass traps are really effective only in the 100Hz + region. There are a few that go down to 70Hz ish, and one or two that go down to 30Hz with consistent absorption.


IME there is a tendency to mistakenly equate absorption curves with frequency response curves. IME the fact that the degree of absorption has dropped to say 10% of maximum is not solid evidence that the absorber has totally lost its effectiveness. It may be an error to forget that absorbers have two dimensions, area and thickness and not just thickness.

That said, investigating resonant and diaphragm absorbers makes more sense at very low frequencies.
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post #14 of 96 Old 04-22-2013, 03:57 PM
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Originally Posted by RapalloAV View Post

We first measured the room with the 5 mics but still had a similar result. Last night I just used one mic in the best seat with the auto EQ on the SMS and the result is maxed out in some areas. There are three rows of four seats, I set the mic up in the middle row. I also tried it in the back row but it maxes out even earlier and the result is worse.


You need to post a photo of the onscreen RTA measurement, not just the EQ settings!

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post #15 of 96 Old 04-22-2013, 04:10 PM
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Originally Posted by arnyk View Post

IME there is a tendency to mistakenly equate absorption curves with frequency response curves. IME the fact that the degree of absorption has dropped to say 10% of maximum is not solid evidence that the absorber has totally lost its effectiveness. It may be an error to forget that absorbers have two dimensions, area and thickness and not just thickness.

That said, investigating resonant and diaphragm absorbers makes more sense at very low frequencies.

Absorption curves generated from random absorption data collected in acoustical testing facilities becomes increasingly suspect the lower down in frequency you go so I'm not relying on that data.

Some good bass trap testing has been done by people like Dirk Noy from WSDG which has been published in the AES. The methods are similar I suspect to what Ethan is doing which is real world testing.

Speaking from this kind of real world experience with all kinds of commercial products from RealTraps, Primacoustic, RPG, etc I can tell you that unless you basically cover your whole room with bass traps like the room in the video Ethan posted then you are not going to have much effect on that 20-40Hz region, even if you custom design panel resonators or use the best off the shelf products like the RPG Modex Plate with the 2.5mm steel or the MSR SpringTrap.

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post #16 of 96 Old 04-22-2013, 04:45 PM - Thread Starter
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Originally Posted by Nyal Mellor View Post

Absorption curves generated from random absorption data collected in acoustical testing facilities becomes increasingly suspect the lower down in frequency you go so I'm not relying on that data.

Some good bass trap testing has been done by people like Dirk Noy from WSDG which has been published in the AES. The methods are similar I suspect to what Ethan is doing which is real world testing.

Speaking from this kind of real world experience with all kinds of commercial products from RealTraps, Primacoustic, RPG, etc I can tell you that unless you basically cover your whole room with bass traps like the room in the video Ethan posted then you are not going to have much effect on that 20-40Hz region, even if you custom design panel resonators or use the best off the shelf products like the RPG Modex Plate with the 2.5mm steel or the MSR SpringTrap.

Guys I can see that you all really know a lot about this, way more than I do, but:

I think this is something that maybe I just cant fix and maybe I'm just going to have to live with the back wall boom. I'm not about to wreck the look of this room with bass traps all over the wall surfaces as the room was built for "looks" and I have no desire to turn it into a mixing room with padded walls. This is a real shame as I would like to rid the back wall boom, but not at the expensive of destroying the looks of the cinema.

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post #17 of 96 Old 04-22-2013, 05:59 PM
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You can have both form and function, depends on how serious you are.

Measurements can determine course of action.


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post #18 of 96 Old 04-22-2013, 07:05 PM
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Originally Posted by RapalloAV View Post

Guys I can see that you all really know a lot about this, way more than I do, but:

I think this is something that maybe I just cant fix and maybe I'm just going to have to live with the back wall boom. I'm not about to wreck the look of this room with bass traps all over the wall surfaces as the room was built for "looks" and I have no desire to turn it into a mixing room with padded walls. This is a real shame as I would like to rid the back wall boom, but not at the expensive of destroying the looks of the cinema.


It is a nice looking room!

One solution would be to measure each row (assuming in each row the low bass is consistent across the row, which depends on how you positioned your subwoofers...) and either a) find a compromise by EQing out the bass boom in the rear row, which may have negative effects in the other two rows or b) setting up multiple EQ profiles in the SMS and selecting the right profile depending on whether the rear row was occupied or not. That way most of the time when the back row is not in use the front two rows would have better bass, and when the theater is full the bass in the front rows might suffer a bit but at least the rear row isn't overwhelmed.

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post #19 of 96 Old 04-22-2013, 10:27 PM - Thread Starter
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I did three separate auto EQs on my three rows of seats with the SMS tonight to see what's going on in the room. I came up with some interesting results, it looks like the most important row (centre) is the worst. Audyssey XT32 on the Integra 80.3 must help clean things up to an extent after the EQ on the SMS, but how much I don't know.


The images are placed in this order, first row, middle row, back row.

FIRST ROW


MIDDLE ROW


BACK ROW


Very interested in your thoughts, can I improve this someway without turning the room into a padded cell?
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post #20 of 96 Old 04-23-2013, 12:07 AM
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Sorry to link to a different forum, but its only a link to a post and not a thread; http://www.gearslutz.com/board/7597560-post146.html

Ethan posted his measurements in the same thread a few pages before that post as well. This is a test we did with our Scopus Tuned Trap (along with broadband treatments). Significant improvement at 35 Hz was had with traps. Now, 30 Hz or lower, I don't expect these same types of treatment - really any type of treatment to work other than better sub positioning, better phase timing, and room correction EQs (the good ones, at least). But 30 - 40 Hz is still controllable in a room.

I don't want to interject from Nyal's help with your Velodyne system nor do I wish to say he doesn't know when/where to place absorption and when to use other methods, its clear he understands acoustics to a good degree...Just want to point the above out so people aren't thinking that if an acoustics company says you can treat 40 Hz, which you can very effectively, they aren't a "shill".

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post #21 of 96 Old 04-23-2013, 08:09 AM
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and that's a really pretty cinema!
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post #22 of 96 Old 04-23-2013, 10:19 AM
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Quote:
Originally Posted by RapalloAV View Post

I did three separate auto EQs on my three rows of seats with the SMS tonight to see what's going on in the room. I came up with some interesting results, it looks like the most important row (centre) is the worst. Audyssey XT32 on the Integra 80.3 must help clean things up to an extent after the EQ on the SMS, but how much I don't know.


The images are placed in this order, first row, middle row, back row.

FIRST ROW


MIDDLE ROW


BACK ROW


Very interested in your thoughts, can I improve this someway without turning the room into a padded cell?

If you look at the graphs it seems like there are a couple of issues. Like you say there is some difference in the <40Hz SPL across the three rows. It looks to be on the order of 6dB or so (it's difficult to interpret those graphs accurately!). Unfortunately it also looks like you have exhausted the amount of cut you can apply in the Velodyne SMS to those bands. It's been a while since I used a SMS but from memory they have a shelf filter you can use (AFAIK it's called 'contour') and also a subsonic filter that you could move up to 40Hz and put a gentle slope on, like 6dB/oct - both these would help you reduce the bass level further. What you'd be looking to do would be to take the bass out of the back row just enough until the excess SPL was gone and then hopefully you'd still be left with something listenable at the front row...

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post #23 of 96 Old 04-23-2013, 12:03 PM - Thread Starter
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Interesting thing I did after this last night was:

I left the back row EQ set on the SMS and just saved that one, then I did the XT32 on the 80.3. I reduced the cross overs from the 100Hz (that I usually increase them to) after XT32 and this time moved them all up just 70Hz (THX recommends 80 for my THX speakers) I then moved the phase to 180 (which someone here recommended) the results were MUCH improved in the back row.

m not sure what part of this might have helped me, can someone lend a hand please????
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post #24 of 96 Old 04-23-2013, 12:56 PM
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Originally Posted by RapalloAV View Post

I'm not about to wreck the look of this room with bass traps all over the wall surfaces as the room was built for "looks"

I submit that solid black color commercial bass traps will look nicer than the red foam on your rear wall. At least they'll blend in better. As was also posted:
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You can have both form and function, depends on how serious you are.

The finest high-end theaters do indeed have "bass traps all over the wall surfaces," but the traps are hidden behind fabric false walls. So it's not only how serious you are, but also how much you're willing to spend. Me, I'm as serious as it gets, but I'm also a cheap skate. Thankfully, my wife and I both actually like the high-tech look of acoustic treatment! See the photo of my living room system below. (You should see the back of the room!) biggrin.gif

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post #25 of 96 Old 04-23-2013, 01:17 PM - Thread Starter
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I know you know plenty about acoustics and I appreciate you trying to help by offering your knowledge. I'm sure all that padding in your room does wonders to your sound but your room doesn't look like a cinema to me.The look of my cinema would be wrecked if I did that I'm sorry to say. I spent over 250K building my cinema, I didn't have the guidance back then about "back wall boom", my mistake, but I'm learning and as I said right at the beginning of this thread I don't want to wreck the room. For me I don't like the look of all those bass traps all over the walls and ceiling.

I have the back wall that I could do something on and that's the only wall I will touch, I would do it if I thought it could help but some here don't seem to think so. What are your thoughts, would it help or not, I don't want to go down that path if it does nothing more than what I have.

Nayal seems to have a different approach and that's what I'm trying to explore at this stage.
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post #26 of 96 Old 04-23-2013, 01:22 PM
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Originally Posted by RapalloAV View Post

II'm sure all that padding in your room does wonders to your sound but your room doesn't look like a cinema to me.

Understood fully, though once the lights go down it doesn't matter. Again, I like the look, so to me it's not even an issue.
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I spent over 250K building my cinema

OMG, please tell me you'll look into doing a proper treatment, then covering that treatment to hide the panels. I can't imagine spending that much money and not having audio perfection as well as visual beauty.
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I have the back wall that I could do something on and that's the only wall I will touch, I would do it if I thought it could help but some here don't seem to think so. What are your thoughts, would it help or not

I already told you that treating that wall with high quality bass traps will surely help. It's not a complete solution, but it's a step in the right direction. If you don't want to see the traps, even a simple wood frame with stretched fabric will hide them completely.

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post #27 of 96 Old 04-23-2013, 01:46 PM - Thread Starter
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Originally Posted by Ethan Winer View Post

I already told you that treating that wall with high quality bass traps will surely help. It's not a complete solution, but it's a step in the right direction. If you don't want to see the traps, even a simple wood frame with stretched fabric will hide them completely.

--Ethan

Ok lets explore the back wall treatment. I don't mind if 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?

If so what design should they be?
They need an air space in the back then the padding then the fabric to cover, have I got this right?

BTW.
I forgot to mention.... 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?
My image is amazing and the blacks are something else!
It just wouldn't look this good with white/cream colours on the ceiling and walls.....
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post #28 of 96 Old 04-23-2013, 06:05 PM
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Originally Posted by RapalloAV View Post

Ok lets explore the back wall treatment. I don't mind if 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?

If so what design should they be?
They need an air space in the back then the padding then the fabric to cover, have I got this right?

BTW.
I forgot to mention.... 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?
My image is amazing and the blacks are something else!
It just wouldn't look this good with white/cream colours on the ceiling and walls.....

Honestly speaking...20-40Hz issues are not easy to fix at all. The only commercial acoustic treatment product I know of that is working down in the nether regions of the bass and would fit with your depth needs is the RPG Modex Plate Type 2. Generally speaking I and others have had success with some panel resonator type traps (like the GIK Scopus I guess though I have not used them yet or DIYed) but they need to be more like 12" to be effective that low. Maybe someone else knows a magic solution. I'd be very keen to know!

It would be easy for you to waste a lot of time and or money DIYing or buying commercial products that might not solve the issue. The only real way to know for sure what will work is to do some in depth acoustical analysis and through that work out what combination of solutions might give you what you want. Of course this level of analysis is not free as it takes time to do it properly. My gut would say it will probably involve some combination of acoustic treatment, additional subs and EQ.

Most people only use 1 row in their theater in anger. Is it that important to have superb bass across all three rows?

Master of Minions, Acoustic Frontiers. We specialize in the design and creation of high performance listening rooms, home theaters and project studios for discerning audio/video enthusiasts.
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post #29 of 96 Old 04-23-2013, 08:08 PM - Thread Starter
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Originally Posted by Nyal Mellor View Post

Honestly speaking...20-40Hz issues are not easy to fix at all. The only commercial acoustic treatment product I know of that is working down in the nether regions of the bass and would fit with your depth needs is the RPG Modex Plate Type 2. Generally speaking I and others have had success with some panel resonator type traps (like the GIK Scopus I guess though I have not used them yet or DIYed) but they need to be more like 12" to be effective that low. Maybe someone else knows a magic solution. I'd be very keen to know!

It would be easy for you to waste a lot of time and or money DIYing or buying commercial products that might not solve the issue. The only real way to know for sure what will work is to do some in depth acoustical analysis and through that work out what combination of solutions might give you what you want. Of course this level of analysis is not free as it takes time to do it properly. My gut would say it will probably involve some combination of acoustic treatment, additional subs and EQ.

Most people only use 1 row in their theater in anger. Is it that important to have superb bass across all three rows?

This Modex plate looks great, do you think it might improve my back wall?
Have you used this product before?
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post #30 of 96 Old 04-23-2013, 08:26 PM
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Originally Posted by Nyal Mellor View Post

Generally speaking I and others have had success with some panel resonator type traps (like the GIK Scopus I guess though I have not used them yet or DIYed) but they need to be more like 12" to be effective that low. Maybe someone else knows a magic solution. I'd be very keen to know!

As would I! And yes, you've got the thickness correct - our T40 is 10.25" thick and 35 Hz would probably be around 12" thick.
Quote:
Originally Posted by Nyal Mellor View Post

It would be easy for you to waste a lot of time and or money DIYing or buying commercial products that might not solve the issue. The only real way to know for sure what will work is to do some in depth acoustical analysis and through that work out what combination of solutions might give you what you want. Of course this level of analysis is not free as it takes time to do it properly. My gut would say it will probably involve some combination of acoustic treatment, additional subs and EQ.

Most people only use 1 row in their theater in anger. Is it that important to have superb bass across all three rows?

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.

Alexander Reynolds
GIK Acoustics
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