Ethan to the rescue with 60Hz dip please? - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 118 Old 08-02-2015, 03:18 PM - Thread Starter
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Ethan to the rescue with 60Hz dip please?

I hope you see this Ethan or someone else with a good grip on bass trapping. Ive asked this question in the subwoofer section but still no one is giving me an answer...


I have four SVS PB13 Ultras up front under my stage, these cant be moved.
I have three rows of seating over risers, these cant be moved.
I have all my back wall treated up to 12" in areas thick.
I have all under my stage area treated walls/corners 16" thick rockwool.
There is no other places in the room to place bass traps except say super chucks in the corners behind the screen and speakers, as in the image supplied. These would be approx. 8' high. I could also do all the floor/wall junctions behind the screen. Would these reduce the 60Hz dip I have which is approx. 9db? If so how much could I get from this, 2/3dbs?


One of the measurements show with and without EQ.
My speakers with screen frame.
Someone else's corner super chunks


Many thanks in advance.
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post #2 of 118 Old 08-02-2015, 06:27 PM
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Send him a PM or contact him through his website, www.realtraps.com

A floorplan and/or room dimensions would be really helpful...

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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post #3 of 118 Old 08-02-2015, 07:24 PM - Thread Starter
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Originally Posted by DonH50 View Post
Send him a PM or contact him through his website, www.realtraps.com

A floorplan and/or room dimensions would be really helpful...

Thanks for that but he might think its a bit pushy contacting him, I hope he finds this.


BTW. you can see the build and construction in my signature.


Here are the plans.
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post #4 of 118 Old 08-02-2015, 08:32 PM
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Trust me, he won't think you are being pushy, he has a business and helping people like you (and me) design the optimum room treatment is what he does. He does not monitor AVS all the time; drop him an email. Or PM through this site.

60 Hz is fairly low so you will need pretty thick treatments, ideally with an air gap, to provide absorption at that low a frequency. You have pretty thick absorption already, maybe not in the right dimension? The wavelength of a 60 Hz signal is about 18.7833' (18' 9.4") so you are looking for a room dimension around 9.5' or so. Is the ceiling treated?

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post #5 of 118 Old 08-02-2015, 09:42 PM - Thread Starter
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Quote:
Originally Posted by DonH50 View Post
Trust me, he won't think you are being pushy, he has a business and helping people like you (and me) design the optimum room treatment is what he does. He does not monitor AVS all the time; drop him an email. Or PM through this site.

60 Hz is fairly low so you will need pretty thick treatments, ideally with an air gap, to provide absorption at that low a frequency. You have pretty thick absorption already, maybe not in the right dimension? The wavelength of a 60 Hz signal is about 1.7833' (18' 9.4") so you are looking for a room dimension around 9.5' or so. Is the ceiling treated?

Ok thank you, I will try and contact Ethan...


Most of the ceiling is covered with just 2" thick to catch reflections, it would do nothing for the bass this low.. All the walls and ceiling was constructed with double sound GIB with green glue between, plus normal inwall insulation.

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post #6 of 118 Old 08-03-2015, 06:28 AM
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Very nice room. I can see why you would not want to move any of the subs but too bad you can not move your subs as that would definitely have an effect. Normally with 4 subs it is suggested to have a sub at each wall or in each room corner for flatter bass frequencies at all seats.

Can you not put two subs in the bottom part of the projection area? You would have to create an opening into the room. However before doing that I would test it with putting two subs in the doorway areas.
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post #7 of 118 Old 08-03-2015, 08:45 AM
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Murray - that is a very nice theater you have there! You must be proud! I LOVE that lighting and great seats, too!

I may be mistaken, but my understanding is that peaks in bass response can be relatively easily addressed with EQ and/or traps, but there is relatively little that can be done with nulls. Nulls tend to be caused by standing waves as a result of physical room dimensions.

I used pink fluffy in my corner traps (see here: https://www.avsforum.com/forum/19-ded...l#post22132736). The conversation before this points highlights some of the benefits of using less-dense materials for bass traps.

Good luck with your project!
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post #8 of 118 Old 08-03-2015, 08:49 AM
 
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Nulls tend to be caused by standing waves as a result of physical room dimensions.
Nulls, and for that matter most response issues, tend to be caused more by boundary reflections than room modes, as already pointed out in the other thread the OP has running on the same topic. Read this, and note the authorship:
http://realtraps.com/art_modes.htm
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post #9 of 118 Old 08-03-2015, 09:22 AM
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Poor wording on my part, Bill. It's been a while since I've read Ethan's work...
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post #10 of 118 Old 08-03-2015, 12:19 PM
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There is no other places in the room to place bass traps except say super chucks in the corners behind the screen and speakers, as in the image supplied. These would be approx. 8' high. I could also do all the floor/wall junctions behind the screen. Would these reduce the 60Hz dip I have which is approx. 9db? If so how much could I get from this, 2/3dbs?
I'm glad to help.

Yes, adding 8-foot high chunk style bass traps in the front wall-wall and floor-wall corners will absolutely help! How wide can you make them across their front face? A lot of people make traps two feet wide, but if you can make them even wider they'll work better at 60 Hz. Three feet or even four feet would be ideal. I can't promise how many dB improvement you'll get, but I bet it will be more than only 2 or 3 dB. Can you cover the entire front wall behind the screen with yet more thick insulation? That will help further.

--Ethan
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post #11 of 118 Old 08-03-2015, 12:35 PM - Thread Starter
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Originally Posted by Ethan Winer View Post
I'm glad to help.

Yes, adding 8-foot high chunk style bass traps in the front wall-wall and floor-wall corners will absolutely help! How wide can you make them across their front face? A lot of people make traps two feet wide, but if you can make them even wider they'll work better at 60 Hz. Three feet or even four feet would be ideal. I can't promise how many dB improvement you'll get, but I bet it will be more than only 2 or 3 dB. Can you cover the entire front wall behind the screen with yet more thick insulation? That will help further.

--Ethan

Ethan thank you so much, absolutely wonderful to hear these words upon waking up here in NZ!!!!!
You have been the person I've been looking for as no on else seems to have been able to answer this question Ive been asking.


I have a couple more questions to ask you on the subject before I take on this final project in behind the screen, but first I need to sort out a couple of images to go along with those question.


Thank you again Ethan, I hope this also helps others who also suffer similar problems.

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post #12 of 118 Old 08-03-2015, 01:47 PM - Thread Starter
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Quote:
Originally Posted by Ethan Winer View Post
I'm glad to help.

Yes, adding 8-foot high chunk style bass traps in the front wall-wall and floor-wall corners will absolutely help! How wide can you make them across their front face? A lot of people make traps two feet wide, but if you can make them even wider they'll work better at 60 Hz. Three feet or even four feet would be ideal. I can't promise how many dB improvement you'll get, but I bet it will be more than only 2 or 3 dB. Can you cover the entire front wall behind the screen with yet more thick insulation? That will help further.

--Ethan

Here you go Ethan....


Behind the speakers we have 100mm ridged rookwool withpool liner glued to it, done exactly to Nyals plans. The hardest part is all the speaker frames are glued and screwed to the stage floor and ceiling. Its hard to get in behind, so the only way really is to remove the speakers for the L&R and climb through the hole. One would pass through the rookwool to the other who stacks them in place. Getting any full sheets of rockwool in behind the L&R speakers looks pretty much impossible. We might however be able to remove the top part of the baffle front to pass rockwool through to the back. Looks like the super chunks could only have a face of approx. 600 after looking at the plans.


In the last two weeks I stuffed four sheet thick of 100mm rockwool in behind the subs, this has brought it to 16" thick, this really helped the situation. You will see in the images the subs before the rockwool was installed. That area is now full!
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post #13 of 118 Old 08-03-2015, 03:25 PM
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Ethan to the rescue with 60Hz dip please?

I wonder if a properly placed helmholz resonator tuned to 60 hz will do the trick but you will need to find the place in the room where you have a 60 hz peak that is causing the cancellation at your listening position.


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post #14 of 118 Old 08-03-2015, 03:36 PM - Thread Starter
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I wonder if a properly placed helmholz resonator tuned to 60 hz will do the trick but you will need to find the place in the room where you have a 60 hz peak that is causing the cancellation at your listening position.


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It would be fantastic if I the two areas between the left and right speakers would work for trapping as this area is so easy to get to now that all the baffles are locked in. I could go really deep here and the access is easy.
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post #15 of 118 Old 08-03-2015, 05:20 PM
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Actually, a number of people could have answered, but you specifically requested Ethan. Not a bad thing by any means...

If you have a single frequency a resonator (Helmholtz, perforated, active, whatever) would be fine but it still may need to be large.

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post #16 of 118 Old 08-03-2015, 06:35 PM - Thread Starter
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Actually, a number of people could have answered, but you specifically requested Ethan. Not a bad thing by any means...

If you have a single frequency a resonator (Helmholtz, perforated, active, whatever) would be fine but it still may need to be large.
Anyone is more than welcome to contribute ideas regarding bass traps for my situation, as long as I don't hear the usual place two subs in the rear which I cant do


I only asked for Ethan as I had asked the same question in the sub bass thread a week ago and no one answered. I even PM'd someone from another large acoustic company (no names mentioned) a week ago, he too never responded.
Finally I asked for help from Ethan, hoping an expert like him would offer help......


I would love a tuned Helmoltz trap or similar if they would work in the yellow areas specified between the L&R speakers, but I have no idea about this sort of thing. I just have to wait for Ethan to comment again as I know its evening in the USA.
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post #17 of 118 Old 08-03-2015, 07:03 PM - Thread Starter
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Very interesting observation how bass trapping can affect the low end....
After I stuffed the area below the stage with four layers of 100mm (4") thick rockwool, it proved to me bass trapping this thick does impact well on the low end. This gave me a total thickness of 16".
See the before and after measures and what happened to the peeks and dips.
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post #18 of 118 Old 08-04-2015, 07:33 AM
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Of course it works, it just takes a lot of it at lower frequencies (LF's). I am not sure there's a lot of data showing absorption with very thick layers. A wavelength at 60 Hz is about 18.8' (5.7 m) so thin absorbers won't do much -- you are doing the right stuff.

Be sure you don't seal the openings. Seems obvious, but I have seen it done. One example was a stepped (tiered) seating area stuffed with insulation, then the workers sealed it off with a kickplate before they put the carpet down. The mistake was not noticed in time and the carpet and seats had to be pulled to get the kickplates out and restretch the carpet (with extra tack strips to hold it down over the openings). Then he decided to use acoustic fabric instead, more rework...

If you add triangles to the corners as Ethan suggested pay attention to his comment about their width. You only get their full thickness in the middle, where they go all the way into the corner. At the edges they dwindle to zero. If you can make them 1 m or more wide they will do much better at 60 Hz. That means they will take up 1 m along each wall coming out of the corner, natch. Since you are trying to absorb low frequencies, it helps to have area as well as depth. Unfortunately it does not work both ways; you cannot make up for a lack of depth by increasing the area. A 1' x 1' square 10' deep won't do much as a broadband absorber despite it's depth, just not enough cross section with the sound waves, and a 10' x 10' region only 2" deep will not absorb LF very well (though HF's would be significantly suppressed).

A Helmholtz can work great but is a little more difficult to design and is not a broadband absorber. It could work great in notching 60 Hz (and multiples), but you'll need some way to tune it, because it is likely that the frequency will shift a bit when you finish out the room with all the furniture and such. Furniture and people also act as absorbers, though less so at LF.

One thing you need to decide is whether to add membranes to the absorbers so HF's are not deadened so much. That comes down to your preference. A dead room will more accurately reproduce the recording; any ambiance is from the source and not the room. I like a dead room, but many do not. Membranes on the outside of the absorbers will reflect higher frequencies but still absorb bass. Getting the right balance can be tricky. A mix of diffusers and absorbers helps since the diffusers break up standing waves without absorbing all the energy, but diffusers are generally much more costly (unless you DIY) and because they also need depth and area for low frequencies are generally impractical for the bass. But, they can make the room much more "live" in the midrange and up, while still providing even frequency response. My preference would be a mixture of both.

Finally, sound treatment within the room is not the same thing as isolation. My media room has thick walls (6" inside studs), filled with mineral wool, then the inner walls (and ceiling -- not floor, it is concrete basement) float on Kinetics clips and are double-layers of 3/4" wall board. Everything caulked and sealed. HVAC is provided by a mini-split unit so there is no ducting from the media room to the rest of the house.

I'll drop off this; Ethan is an expert in this field and I am not. I do have significant experience and a couple of grad classes but that was fairly long ago. It will be interesting to see your space evolve and the final construction and measurements.

FWIWFM/IME/IMO/etc. - Don

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post #19 of 118 Old 08-04-2015, 10:26 AM
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Quote:
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Very interesting observation how bass trapping can affect the low end....
After I stuffed the area below the stage with four layers of 100mm (4") thick rockwool, it proved to me bass trapping this thick does impact well on the low end. This gave me a total thickness of 16".
See the before and after measures and what happened to the peeks and dips.
Thanks for posting these measurements. I plan to build a 16" deep front wall next year (partly so I can build my front subs into it) and I have been trying to find out about baffle walls, bass traps and how effective just filling it with rockwool might be. Your measurements really show how much smoother the response is without resorting to subs at the back of the room (which we already discussed in the other thread and I know isn't possible in your room).

Another thanks for the tip about the pond liner material; I understand that the rockwool can take too much of the HF out of the room, especially at 16" deep, so I can cover the front as you have.

Hope you don't mind me tagging along on your thread.

"Don't believe everything you read on the internet". William Shakespeare 1615

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post #20 of 118 Old 08-04-2015, 11:15 AM
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I just have to wait for Ethan to comment again as I know its evening in the USA.
You're getting plenty of good advice, beyond my earlier suggestions. Rigid insulation 10 cm thick flat against a wall is okay, but thicker is much better if you want to target 60 Hz. Even using 10 cm material with a 10-20 cm gap will be better than flat on the wall.

--Ethan
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post #21 of 118 Old 08-04-2015, 01:08 PM - Thread Starter
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Originally Posted by Ethan Winer View Post
You're getting plenty of good advice, beyond my earlier suggestions. Rigid insulation 10 cm thick flat against a wall is okay, but thicker is much better if you want to target 60 Hz. Even using 10 cm material with a 10-20 cm gap will be better than flat on the wall.

--Ethan
Thank you Ethan and everyone else. Im going to buy four bales of rockwool today which have four sheets 100mm thick by 600x1200 per bale. Im going to just stack these wrapped on the stage between the left and right speakers with a gap off the wall and measure. This is easy before I install them properly, at least the mesurements will give me an idea what they might do. Im hoping this might give me enough for the 60Hz without trying to get in behind the speaker baffles. They will be 16" thick!


Stay tuned!

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post #22 of 118 Old 08-04-2015, 09:21 PM - Thread Starter
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Ok here we go.
Today I bought four bales of ridged rockwool 3 per pack 4" thick 1200 x 600. I wanted to see if there was another change in the room with them just wrapped leaning up against the two side walls.
There is a slight improvement in the 60Hz dip as you will see in the measurements. But this I hope will get better up on the stage area as presently they are only 12" thick, three panels 100mm each in the pack. My plan is to make them up to 16" in parts behind the screen, more where I can. Its pretty cramped in behind the screen with all the framing etc, but Im going to pack in as much as I can. I believe this is a good idea, please tell me otherwise before I start tomorrow. Take a look at the measurements and see what you think?

I can also double up some of the existing rockwool that has the pond liner over on the back wall to make it even thicker. Is it better to remove that pond liner or just place more rockwool directly in front of it?


Since Im planning on 16" thick in parts, is it still better to space this off the wall or direct to the wall?
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post #23 of 118 Old 08-04-2015, 09:30 PM - Thread Starter
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Originally Posted by Kelvin1965S View Post
Hope you don't mind me tagging along on your thread.

Your more than welcome to come for the ride, I hope my experiments will also help you when your ready to start. The more you can learn here the better.


I just wish I knew all the things Ive learnt in the forums before I even planned my cinema, we live and learn....
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post #24 of 118 Old 08-05-2015, 06:29 AM
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The pond liner is reflecting higher-frequency sounds to make the room more "live". This is a mixed blessing; it makes the room sound better to most folk, but adds ambiance not in the original recording. A preference thing. If you like it, then pull the pond liner out in front of all the mineral wool. If you want to dampen everything, put the mineral wool in front of the pond liner (or get rid of it altogether).

An air gap always improves the performance of these type of absorbers but there are diminishing returns... Here's a hand-waving explanation, not 100% accurate but should convey the idea. The absorber works by converting sound energy into heat. The energy is a function of velocity. Velocity at a boundary (wall, floor, ceiling) is zero (the waves are stopped by the wall, assuming it is not flexing, natch -- an idealized assumption) so effectively no absorption happens right at the wall (no kinetic energy, all potential, remembering basic physics). Move a few inches off, the sound waves are moving again, and the absorber now has energy it can absorb.

All that said, with 16" of material you might not see much improvement with a small air gap. You have the stuff to try it out: prop the mineral wool right against the wall and measure, then use something to space it a few inches off the wall (maybe scrap pieces of the mineral wool) and measure again. Any difference?

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post #25 of 118 Old 08-05-2015, 12:08 PM - Thread Starter
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Originally Posted by DonH50 View Post
The pond liner is reflecting higher-frequency sounds to make the room more "live". This is a mixed blessing; it makes the room sound better to most folk, but adds ambiance not in the original recording. A preference thing. If you like it, then pull the pond liner out in front of all the mineral wool. If you want to dampen everything, put the mineral wool in front of the pond liner (or get rid of it altogether).

An air gap always improves the performance of these type of absorbers but there are diminishing returns... Here's a hand-waving explanation, not 100% accurate but should convey the idea. The absorber works by converting sound energy into heat. The energy is a function of velocity. Velocity at a boundary (wall, floor, ceiling) is zero (the waves are stopped by the wall, assuming it is not flexing, natch -- an idealized assumption) so effectively no absorption happens right at the wall (no kinetic energy, all potential, remembering basic physics). Move a few inches off, the sound waves are moving again, and the absorber now has energy it can absorb.

All that said, with 16" of material you might not see much improvement with a small air gap. You have the stuff to try it out: prop the mineral wool right against the wall and measure, then use something to space it a few inches off the wall (maybe scrap pieces of the mineral wool) and measure again. Any difference?

HTH - Don

Thank you Don for that added help!
Also many thanks for the explanation regarding the pond liner. The room to me always has seemed to "live". Especially with my Klipsch speakers which are very bright to start with. Ive decided now after reading your post to just go over the pond liner and cover it.


Many thanks...


I will report back.

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post #26 of 118 Old 08-19-2015, 02:04 PM - Thread Starter
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Well guys Im finally back to report!


Its taken me sometime to report back as my findings were not as favourable as when I first started this journey.
I have discovered that all the rockwool in the world will just not solve "all" the problems in some rooms sadly to say.
I have stuffed on this last venture $2000.00 of rigid rockwool into every nook and cranny in the cinema to try and alleviate the 58Hz dip I had/have. Ive exhausted the experiment now, the dip is till there.
Its a room thing and its obvious that bass trapping can only do so much, its not a miracle cure for all aliments
I could keep adding more but I know from my measurements anymore rockwool added is doing absolutely NOTHING!
I have some more in the garage, but have stopped!
The whole of my front wall is now covered, some areas up to 20" thick, its insane!
Most of the rear wall is also treated up to 12" thick, there is not an inch left in the room to treat!


I have done everything else, crossovers, phase etc etc etc etc etc etc.....
Tried everything in the book, the dip will not go away without subs in the back row, I know!


As you will see the width of the dip is slightly narrower than before, but its still there in all its glory!


I ran a 63Hz tone, you can defiantly here the lack of sound in the MLP. Move ahead just 12" and its back loud and clear, so a dip as small as mine is still heard, or better to say not heard......


See the results after $2000 of rockwool. If they were store bought bass traps the cost could be double!


These rooms are a massive problem.......
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post #27 of 118 Old 08-19-2015, 03:44 PM
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It is almost impossible to solve a room mode by absorption. You can reduce it a bit, but at 60 Hz it would be tough to eliminate. If it is the height mode, even more so, as it's hard to stuff that much material into a normal ceiling.

Remember that, while you can clearly hear it with a test tone, it will not be as noticeable in actual music and movies because there is much else going on and you'd only notch out that narrow band. On the plus side, 60 Hz power line noise will also be attenuated; it's a feature!
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post #28 of 118 Old 08-19-2015, 03:56 PM - Thread Starter
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Originally Posted by DonH50 View Post
It is almost impossible to solve a room mode by absorption. You can reduce it a bit, but at 60 Hz it would be tough to eliminate. If it is the height mode, even more so, as it's hard to stuff that much material into a normal ceiling.

Remember that, while you can clearly hear it with a test tone, it will not be as noticeable in actual music and movies because there is much else going on and you'd only notch out that narrow band. On the plus side, 60 Hz power line noise will also be attenuated; it's a feature!

Thanks Don for your support, its encouraging.
Yes you are dead right, I have now proven that all the absorption in the world will never totally eliminate a room mode like this.


The strange thing is when I keep raising the crossovers on the two mains, the null starts to disappear, this proves to me that's its not the main speakers reacting to the room it must be just the subs, am I correct in thinking so?


I could run the mains with a 40Hz crossover but it just doesn't sound as great this way!
Take a look at this...
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post #29 of 118 Old 08-19-2015, 03:59 PM - Thread Starter
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On the plus side, 60 Hz power line noise will also be attenuated; it's a feature!

Sorry Don, forgot to ask what does this mean?

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post #30 of 118 Old 08-19-2015, 04:47 PM
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The strange thing is when I keep raising the crossovers on the two mains, the null starts to disappear, this proves to me that's its not the main speakers reacting to the room it must be just the subs, am I correct in thinking so?
Not so strange. A source of bass at a room boundary will excite all the modes perpendicular to the boundary. As Don mentioned, your 60Hz cancellation hints at a roughly 3m dimension. If that's the height of your room, then placing the source of bass (subs) at a room boundary (on the floor) will excite all of the room's height modes.

The first height mode is around 60Hz, with its null at the midpoint of room height. If your measuring mic is near that height, like ear height for someone seated in the second row, then that could explain the 60Hz dip.

Since your main speakers aren't at the floor or ceiling boundary, they aren't exciting this mode. Which is why you don't see that dip when you pass the sound to them at a lower crossover point.
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