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room treatment to reduce reverb/echo

Member for

7 years 7 months
Dear Forum, I have a home studio that I use for doing sound design for theatre, mixing acoustic theatre (theatrical installations and radio drama) and composing electro-acoustic music. I'm gradually diminishing the reverb in the room and I'd like some advice please on further steps I can take on a low budget (we renovated this house recently so I'm doing things slowly!).

The room has the dimensions: 480 x 350 x 250cm (15.74 x 11.48 x 8.2 feet). My monitors are Tannoy System 12 DMT IIs, a mid-field system. The dual concentric cones of these are placed 2m apart at ear-height with more than a metre away from the rear wall (the 350cm length) and slightly less from the side walls - I used a golden mean ratio - and they sound good where they are now, after having tries various positions in the room.

Behind the speakers is a large double doorway of glass (which can be folded away) and wooden shutters (Venetian style) which are generally exposed when I work. There's another glass/shutter window, a recessed doorway, hardwood floor, flat plaster ceiling and parallel plaster walls

Carpet is a bit of luxury item here in Brazil and I've managed to cover probably about 70% with rugs. I have 3 fairly lightweight foam mattresses placed on the walls to the sides and behind the listening position as a temporary measure.

At present, if clap my hands, I estimate the reverb time to be one second. I'd like to reduce this and I wonder what an ideal reverb time would be?

Right now I have the opportunity to buy acoustic wall panels left over from the refurbishment of a cinema. I don't their specifications yet nor the price, but wonder what sort of of wall coverage forum members would recommend? ie. what percentage - and where.

I'm not trying to isolate the sound, just improve the internal acoustics. I don't have a acoustic testing mic, but I can afford a Behringer if necessary. I have a sound meter though and software to play pink-noise, sine-sweeps etc.

Thanks!

Comments

Member for

14 years 2 months

Space Sun, 01/25/2015 - 19:27
Kurt Foster, post: 424167, member: 7836 wrote: wow! that's a crazy hard room...

And that is the biggest issue at this point. The lack of symmetry between speakers and listening position, the lack of absorption behind the speakers, the lightweight stands the speakers stand on...and all those hard surfaces. Including the window to the left which aborts symmetry and introduces leakage at low frequency.

The best thing you could do at this point, assuming you develop corner broadband and side wall absorption and an overhead cloud, would be to re-orient the equipment 180 degrees. That setting you have now is just not going to ever give you a good mix position.

Member for

7 years 7 months

Surdo Mon, 01/26/2015 - 08:33
Thanks a lot. I'll try re-orienting the equipment by 180 degrees. The stands too in the future. Would stacked bricks be OK? There is that recessed door on the rear wall which won't help the symmetry any.

I got information on the acoustic panels that are available to me at good price (i hope) - they are fibreglass insulation panels - they are much the same as the panels in the following link with slightly different dimensions (50 X 50cm):

http://www.isover.com.br/ConstrucaoCivil/Forros/Prisma

A detailed acoustic test is given here (see page 3 for a graph of the absorption coefficients): http://www.isover.com.br/uploads/produtos/9/documentos/PRISMA_Laudio_ACUSTICO_K8025.pdf

If someone could please comment on the efficacy of using these as broadband absorptive panels, I'd be very grateful. My other option will be to purchase acoustic foam.

They also have a big tub of "green glue compound" available, which I understand can be used to mount the panels: http://www.greengluecompany.com/products/noiseproofing-compound

Cheers,

Member for

8 years 7 months

pcrecord Mon, 01/26/2015 - 09:14
Space, post: 424243, member: 32398 wrote:
The best thing you could do at this point, assuming you develop corner broadband and side wall absorption and an overhead cloud, would be to re-orient the equipment 180 degrees. That setting you have now is just not going to ever give you a good mix position.

Space, I know you are the pro here.. but I don't get this one. If he turns 180degree, the wall behind his speakers won't be even because of the space going to the door. Please educate me on how this is going to be better..

Member for

7 years 4 months

Reverend Lucas Mon, 01/26/2015 - 12:48
Surdo, post: 424270, member: 47791 wrote: Thanks a lot. I'll try re-orienting the equipment by 180 degrees. The stands too in the future. Would stacked bricks be OK? There is that recessed door on the rear wall which won't help the symmetry any.

I got information on the acoustic panels that are available to me at good price (i hope) - they are fibreglass insulation panels - they are much the same as the panels in the following link with slightly different dimensions (50 X 50cm):

http://www.isover.com.br/ConstrucaoCivil/Forros/Prisma

A detailed acoustic test is given here (see page 3 for a graph of the absorption coefficients): http://www.isover.com.br/uploads/produtos/9/documentos/PRISMA_Laudio_ACUSTICO_K8025.pdf

If someone could please comment on the efficacy of using these as broadband absorptive panels, I'd be very grateful. My other option will be to purchase acoustic foam.

They also have a big tub of "green glue compound" available, which I understand can be used to mount the panels: http://www.greengluecompany.com/products/noiseproofing-compound

Cheers,

I don't speak Portuguese, but from what I can tell this looks to be typical of 25 mm (1") thick absorptive panels. They're useful as 'broadband absorbers' in the midrange to high frequencies, but quickly fall off below 500 Hz. 25 mm material isn't effective for low frequencies, but using multiple layers per panel and spacing the panels from the wall will increase absorption in the low end.

Green Glue is a damping compound used in construction for 'soundproofing.' It isn't useful in acoustically treating rooms.

Member for

14 years 2 months

Space Mon, 01/26/2015 - 17:24
pcrecord, post: 424271, member: 46460 wrote: Space, I know you are the pro here.. but I don't get this one. If he turns 180degree, the wall behind his speakers won't be even because of the space going to the door. Please educate me on how this is going to be better..

The problem with this room is that it has many areas that leak. In the current configuration, the window to the left and the open door bi-fold area are at a point that should be a hard boundary with some absorption to reduce early reflections PLUS low frequency absorption in the front corners.

Even if he did that, the window destroys the required symmetry and adds an acoustical hole that will not be matched on the opposite side and the bi-fold does not, as Kurt stated, add any diffusion especially at this close a range so it is the worse of two evils.

To rearrange the equipment 180 degrees gains the hard boundaries back on both the side and behind the speaker walls while reducing the issues down to the door. The door can be massed up to match the existing wall areas and you now have an area that is better to start in then was the other end.

What you need the most in a mixing environment are:

Symmetry;
A balanced equilateral triangle for your speakers and head to fit in;
A mass laden stand for each speaker to over come vibrations introduced from either direction;
Absorption directly to the left and right of the listening position centered on the listeners ears;
An overhead cloud to reduce the early reflections;
Typically back wall low frequency absorption to once again stop early and multiple reflections and catch low frequency waves.

These are the basics. Differences in the room sheathing, dimensions and gear will put you into round two of the fight in the attempt to tame or eradicate intrusive rays and waves from the listening position.

Member for

21 years

audiokid Mon, 01/26/2015 - 22:16
pcrecord, post: 424175, member: 46460 wrote: Some ideas from Sweetwater rep :
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Nice one Marco, I feel like I'm watching myself here. Mitch and I have an near identical approach right down the treatment, TV, piano upstairs, tracking acoustic G and vocals while using larger facilities for those bigger room requirements.
With the exception to my 2 DAW approach, a lot of the gear we use is the same. We share the same hybrid approach, "scaling down on some area's while up scaling quality".
I have a Large VocalBooth Amp enclosure in a corner room that I'll leave open when I can. Its packed with treatment.
My Studio is fitted with Real Traps etc..
It works excellent for me.

I'm even testing how the Apollo 16 integrates with my Neos and Dangerous Master.

Thanks for posting this one. (y)

Member for

7 years 7 months

Surdo Tue, 01/27/2015 - 01:04
Perhaps it didn't come out well in the photo, but there is a balanced triangle formed by the speakers and the listener position, however no, it's not exactly an equilateral triangle. It's slightly elongated: 210cm distance between the speaker axes and slightly more (225) between each speaker-face and the listener. This is done following the recommendations of Tannoy for the DMT Series speakers, where the speakers are also meant to be angled between 10 and 15 degrees off-axis (to "give the optimum spread of the HF information").

Re. absorption behind the speakers, when I can, those double doors are wide open, which forms a 100% absorptive panel at the rear (there's a marauding 5 month-old german shepherd puppy outside which is why it was closed when I took the photo).

Re. the side window, perhaps I can make a demountable rig with absorptive panels to cover the window, with a matching panel on the right. Would this help?

Not sure how the door recess can be "massed up", although I could potentially put a second solid-wood door to cover the door opening. The current door opens inwards towards the door cavity.

Re. speaker stands, I'll look into something done with bricks.

Member for

20 years 6 months

Ethan Winer Tue, 01/27/2015 - 10:47
audiokid, post: 424182, member: 1 wrote: I'm going to ask Ethan. Stand by...
:D
Thanks for the heads-up Chris. RealTraps bass traps are not tuned - they are "broadband bass traps" meaning they absorb mostly bass, but over a wide range of bass frequencies. They do use a membrane to increase LF absorption and decrease HF absorption, but it's not the same type of resonant membrane as the old-school tuned membrane traps in this article:

[[url=http://[/URL]="http://www.ethanwin…"]Build a Better Bass Trap[/]="http://www.ethanwin…"]Build a Better Bass Trap[/]

In a room that's only 15.74 x 11.48 x 8.2 feet the main problem is bass peaks and nulls and ringing, and early reflections. RT60 isn't as relevant, and isn't quite the correct metric anyway.

As for buying used panels from a cinema, that might be brilliant or a total waste of money. It depends on the thickness of the panels and what they're made of.

--Ethan

Member for

7 years 7 months

Surdo Tue, 01/27/2015 - 11:45
Thanks. The panels from the cinema are made of glass fibres - unfortunately the site describing them seems to be down at the moment (see links in message #27 above) but I think they are 25mm thick and from memory they have reasonable absorption between 500Hz and 4kHz. The Reverend Lucas above suggested doubling them up or mounting them away from the wall to improve low frequency absorption.

Member for

14 years 2 months

Space Tue, 01/27/2015 - 18:14
Surdo, post: 424293, member: 47791 wrote:

Re....those double doors are wide open, which forms a 100% absorptive panel at the rear (there's a marauding 5 month-old german shepherd puppy outside which is why it was closed when I took the photo).

It is a big hole where your music leaks out. Here is what we all try to attain. An area that captures ALL the sound. This requires hard boundaries. A louvered bifold is not a hard boundary it is a leakage area that will allow the music a path out of the room especially low frequency.

This means that you make poor mixing judgement errors in under or over compensating for this audible gain or loss.

At the listening position you want to capture all the sound. All the direct sound. In order to do this you have to create a contiguous area with hard boundaries that do not let any of the music out of this environment.

Surdo, post: 424293, member: 47791 wrote:
Re. the side window, perhaps I can make a demountable rig with absorptive panels to cover the window, with a matching panel on the right. Would this help?

You can develop a window plug that matches the same mass as the exterior assembly on the outside and matches the interior sheathing on the inside and plug that window. That will essentially remove that path. But you still have to deal with the actual 4 foot wide by 80 inch hole that the light weight bifold door introduces.

Now, you are free to do as your situation might seem to suggest, but the leakage at the existing end you are using is going to cost more in trying to match it up to the opposite end.

And...after re-orientating 180 degrees you get the added benefit of having a path for lower frequency waves to escape by having that bifold door way behind you.

Member for

7 years 7 months

Surdo Wed, 01/28/2015 - 01:54
OK, but getting back to the door cavity. When the speakers etc are rotated by 180 degrees, the left speaker will have a big hole behind it. The door, there at the back of the cavity, is not even solid board, but that can be fixed. And I can potentially install a second solid wood door to enclose the hole. That's the best I can think of.

I did once try putting the speakers along the longer side of the room, on the side with no window, but it sounded terrible - very boomy.

Member for

8 years 9 months

DonnyThompson Wed, 01/28/2015 - 03:54
correct me if I'm wrong here, but doesn't an open doorway or window provide 1 Sabin of low frequency absorption/dissipation? In short, is it not in fact acting like a gigantic bass trap, and could it not skew the bajeezus out of the acoustic fingerprint of your room?

Sabin as Unit of Sound Absorption

The unit of sound absorption is square meter, referring to the area of open window. This unit stems from the fact that sound energy traveling toward an open window in a room will not be reflected at all, but completely disappear in the open air outside. The effect would be the same if the open window would be replaced with 100 % absorbing material of the same dimensions.


source: http://acoustics-engineering.com/html/sabin.html

Member for

20 years 6 months

Ethan Winer Wed, 01/28/2015 - 07:42
Surdo, panels that are one inch thick (25 mm) are useful only for reflection control. Even doubling them up won't do any good for corner bass traps. Now, stacking four of them adjacent would be fine.

Yes Donny, an opening to the outdoors is a perfect bass trap. An opening indoors could be excellent or only so-so, depending on what's beyond. But generally an opening is a Good Thing if it's in a useful place.

--Ethan

Member for

14 years 2 months

Space Wed, 01/28/2015 - 18:08
DonnyThompson, post: 424348, member: 46114 wrote: correct me if I'm wrong here, but doesn't an open doorway or window provide 1 Sabin of low frequency absorption/dissipation? In short, is it not in fact acting like a gigantic bass trap, and could it not skew the bajeezus out of the acoustic fingerprint of your room?


No idea about Sabins, I am an Auburn fan. But an open door or window is an escape for the longer frequencies so is in effect a bass trap. Conditions matter as Ethan has stated, it might improve or it could couple and present more issues.

What happens is the low frequencies leave and the high frequency bounces off the glass and introduces a mix dicision to the brain that says, "no lf...all high frequency."

So the mixer boosts the bass and cuts the highs on the channel in question.

The position of the open area is crucial, it is better suited at the backside of the listening position. At the front side it will defiantly screw up what you are hearing. The leakage of a thin sheet of glass is comparable to a hole, low frequencies will go right thru it, and the placement of a window in want of a balanced symmetrical area has to be considered strongly.

Member for

19 years 2 months

Kurt Foster Wed, 01/28/2015 - 18:13
Space, post: 424399, member: 32398 wrote: The leakage of a thin sheet of glass is comparable to a hole, low frequencies will go right thru it, and the placement of a window in want of a balanced symmetrical area has to be considered strongly.

what about a window that is the center of the wall? could one take advantage of the bass loss through the window as a bass trap or is the loss at different frequencies too erratic?

Member for

14 years 2 months

Space Wed, 01/28/2015 - 18:34
If the window is thin enough, bass moves into the tracking room assuming one is present. My personal opinion is that you want all the frequency spectrum to come to the listener position first, so a window, if it were in front of me as we have all seen in high end studios, should be a decoupled laminated glass window with the speakers a good distance from the glass.

This will still introduce high end reflections but is also one of those places where you have to know your room and you have to know your gear.
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