Can I fix a room decay of 1500ms?

Scion

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Jul 7, 2016
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Hopefully this is the right forum to ask the question: My room rings like a bell. It is mostly concrete and the REW waterfall shows a decay time of nearly 1500ms.http://www.whatsbestforum.com/images/icons/icon9.png I really don't want to rebuild it so I'm hoping to fix it with a number of bass traps.

The room is 20' x 15', with 16" soffits on the sides and back walls. I'm thinking of starting with triangular, mineral wool absorbers under the soffits (13" on wall x 16" underside x 20" diagonal), with a run of about 14' on each side. I understand that bass absorption needs lots of depth but I'm thinking that some of the sound travelling along the length axis would see a 14' deep absorber.

My question is whether this is worth the effort and a good place to start? There is a wall behind my AT screen for other bass traps but I thought that doing the soffits first would help me choose later the best type of absorber on the front wall.

Many thanks.
Ion
Atta
 

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Cincy2

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The best advice I can give you is to call Acoustic Science Corp (ASC) and get their engineers to recommend a set of treatments for your room. I've been using Tube Traps and related products for 20 years and have yet to have a problem they couldn't fix. Be prepared to give them a detailed diagram of the room dimensions, speaker locations and models, listening chair location etc. I have no affiliation with the company at all except that I am a very satisfied customer.

Cincy
 

Scion

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Jul 7, 2016
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Thanks Cincy. I've sent ASC an email to see what they can offer.

Does anyone else have experience in reducing highly resonant rooms? For example, any absorbers that can either go behind the screen wall or under soffits, since those are spaces for traps which won't intrude into the room.

Ion
 
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amirm

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Your reverberation time at 400 Hz is 0.4 seconds. So be careful about adding too much absorption as may get an overly dead room.

On the bass, triangular absorbers are not going to do much at the lower frequencies (below 100 Hz). Length does help but then you face the above situation.

If you have depth behind your AT screen, that would be the place I would target. It would be invisible to your room decor and can do a better job.

Also, seating and speaker location optimization can help.
 

microstrip

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IMHO the single waterfall picture as shown can be very difficult to interpret and can even be misleading. I would suggest using the decay functions of REW and looking at decay times at the defaults bands to get a general view. To check consistency we should repeat the measurements at several positions of microphone and speakers. Decay time should be measured with an omnidirectional source - I usually use the stereo pair of speakers in mono back to back. Just my humble .02!
 

Scion

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Jul 7, 2016
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Absolutely marvelous feedback, thank you. It gives me hope I'm not stuck with this room.....roomm.....roommm....

On the question of the measurement, you're right in that I had the microphone in only one position i.e., centered between the two MLPs on the front row. In the Room Sim function, the two positions on either side had very similar graphs so I assumed that would also be the case in the testing. I'm intrigued to re-test with the 'real' sitting positions. But I'm not clear how/why I should move the L/C/R speakers to a 'back to back' position.

With regard to the ineffectiveness of soffit triangles, should I consider substituting a series of pressure based membrane units instead of mineral wool velocity absorbers? My initial concern was concern was they might be too narrow in their frequency bands, unlike a more broadband fibreglass unit.

Finally, a more general question about sequencing. I was thinking of adding absorbers in the soffits and ceiling first since the possibilities there are more limited; and then re-measuring the room. I could target the remaining problem frequencies more accurately e.g., optimising the 40cm gap behind the screen, adding a Modex plate on the rear wall, building a riser as a bass trap, etc. Am I over-thinking this or just build the damn thing...?

Thanks again for your inputs.
Ion
 

microstrip

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As I wrote I use only the LR speaker back to back. I put them in the center line of the room, about one third length and the microphone at two thirds.

I have built membrane bass traps tuned to my room some years ago. They were very efficient, but very hard to tune - I had to built three box sizes till I got the proper tuning. Please see this old thread in WBF http://www.whatsbestforum.com/showthread.php?7710-Diaphragmatic-versus-membrane-bass-traps and links in the posts. BTW, the boxes were enormous - 2´x 1.5´x 6 ' - but the results were really good.
 

JackD201

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Hi Scion

Could you show a picture of your room and describe the construction of your walls and ceiling? This could help a lot in recommending treatments that are the least obtrusive visually.
 

Scion

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Hi Scion

Could you show a picture of your room and describe the construction of your walls and ceiling? This could help a lot in recommending treatments that are the least obtrusive visually.

Thanks Jack. The room is 20.2 x 15.4 x 8.6 (2673 ft³). The walls are concrete blocks, surfaced with 80mm plastered insulation board. The floor is poured concrete. The ceiling is plasterboard under wooden joists filled with foam.

Recent photos with screen temporarily mounted:
Without screen.jpg With screen.jpg

Initially had three areas in mind:
1. The soffits pictured are 13" wide and 14' long. I thought I could put a row of pressure based traps underneath them without taking up too much room space.
2. Similarly, the gap between the screen and front wall is nearly 16" and could hide some deep traps.
3. The ceiling has less space since the projector is flush mounted, until I get closer to the screen and then have about 4 inches.

Afterwards, I thought to re-measure the room decay and look at resonating plate for the back wall and tube traps in corners.

All advice is gratefully received.
Ion
 

JackD201

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Ahhh I see now! Concrete blocks with insulation board directly plastered on them will basically still behave like concrete. That's the bad news. If you had put the insulation board on studs and filled the cavity (about 2" deep if you used 2"x2"s) with rock wool then that would have decreased your very low bass decay time considerably. To keep the insulation board from rattling you could have used something like green glue to mass damp the rear and used silicone on the stud points. In a nutshell, every wall becomes a pseudo Pressure trap, pseudo because they aren't air tight but close enough. The more expensive but more predictable way would have been to mount them on metal rails and used isolation clips. If you don't want to go rip everything up again, I see for delay humps you might want to address with tuned helmholtz resonators for those specific frequencies. Hope this helps.
 

Scion

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Jul 7, 2016
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Hi Jack

I understand, but I'm afraid it was not my decision to build the room this way. However, given my starting point, I'm trying to ascertain if I can dampen the room to a manageable level with bass traps (with respect, that was my initial question). If not, then I may very well have to bite the bullet and re-build the walls. But I would prefer not to.

Has anyone successfully tamed this amount of room resonance? How did they do it?

Cheers
Ion
 

Scion

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Jul 7, 2016
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As I wrote I use only the LR speaker back to back. I put them in the center line of the room, about one third length and the microphone at two thirds.

I have built membrane bass traps tuned to my room some years ago. They were very efficient, but very hard to tune - I had to built three box sizes till I got the proper tuning. Please see this old thread in WBF http://www.whatsbestforum.com/showthread.php?7710-Diaphragmatic-versus-membrane-bass-traps and links in the posts. BTW, the boxes were enormous - 2´x 1.5´x 6 ' - but the results were really good.

Very useful thread, thanks. Did you consider vinyl for the membrane in lieu of a plywood face? Are there pros/cons in choosing between MLV and plywood e.g., Q, or minimum size for effective trap?

Ion
 

FrantzM

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^^^^ wot he sed!!!!!

+1 including the orthograph.

It would be wise to combine two approaches. The low bass region: 100 Hz and under is very difficult to treat .. Not untreatable but difficult and often costly... Multi subs plus DSP will help nad could be the cure immensely in this area. You will likely enjoy the best bass you have ever heard... Then >100 Hz use the services of ASC or of RealTrap. to smooth out the rest.
 

Bjorn

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Why not contact RPG in Kent, which are close to you. They have bass traps that are both effective at low frequencies and quite broadband. Sure, they aren't cheap as porous material but it's an investment for life.
http://www.rpgeurope.com/

The room should be measured first to find the right locations for the bass traps.
 

Rodney Gold

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the DSP alone will give you great results at sitting position ... not so great elsewhere in the room
The multisubs actually smooth the whole room , great bass throughout the room.
whatever yoiu do , you DO NOT want flat bass.. the curve you aiming at is a rise from 150hz downward , to about +6db at 20-30hz..ie a house curve.
 

Bjorn

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the DSP alone will give you great results at sitting position ... not so great elsewhere in the room
The multisubs actually smooth the whole room , great bass throughout the room.
whatever yoiu do , you DO NOT want flat bass.. the curve you aiming at is a rise from 150hz downward , to about +6db at 20-30hz..ie a house curve.
If one only cares about frequency response, DSP or multiple subs will work well.

However, the time domain is equally if not more important. And in order to achieve a great time domain in the bass, physical treatment is necessary. DSP/room correction will actually introduce phase wraps where the response isn't minimum phase. This may not be very audible at the very low frequencies but they higher in frequency you get the more audible is becomes. The result is often a very unnatural sound, compared to what physical treatment gives you.
 

amirm

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However, the time domain is equally if not more important. And in order to achieve a great time domain in the bass, physical treatment is necessary. DSP/room correction will actually introduce phase wraps where the response isn't minimum phase. This may not be very audible at the very low frequencies but they higher in frequency you get the more audible is becomes. The result is often a very unnatural sound, compared to what physical treatment gives you.
We were talking about below 100 Hz solutions. And there, peaks in general are minimum phase. And regardless, if you get a DSP solution (and some automated ones) you can easily toggle them on and off to hear the difference. If it sounds worse, undo the change. Indeed it is pretty educational to learn such effects rather than go by graphs and technical terms.

In general, "time domain" correctness is more there to please the eye than the ear. :)
 

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