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My bass traps don't work at all Dynamics Plugins
Old 31st August 2008
  #1
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My bass traps don't work at all

I am setting up my home theatre and the dimension of the room is 13.5ft (W) x 13.0ft (L) x 7.5ft (H). The sub-floor is covered with a carpet, all walls are finished with drywall and one of walls has a full size of curtain to cover up the window.

It is not surprise that I have a terrible bass distribution in the square room. As a result, I decided to make up bass traps to cope with the acoustic problem.

The material I used is Johns Manville 814, I stack up three sheets of 2” insulation to make up 6 inches deep panels (all four bass traps have dimensions of 4ft x 2ft x 6ins and all of them are temporarily stored in big garbage bags for a quick trial). At the same time, I made extra four 2” deep panels and again they are all in thin plastic bags.

I put up the four bass traps across each corner of the room and the rest of small panels on different walls, however, it seems there is no difference or improvement on the bass distribution. The bass is much stronger on one side of the room but weaker on the other side. It doesn’t matter what arrangement I made by putting the small panels on the same wall or on different walls, there is nothing change at all.

Can anyone give me any advice? I do believe that the plastic bags should not impact the performance of LF absorption.

Thanks
Old 31st August 2008
  #2
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andrebrito's Avatar
 

Move the sub around and see if it works. Maybe they are ineffective at low frequencies. Make measurements. For home theaters I also recommend placing diffusers.
Old 31st August 2008
  #3
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Robert Alger's Avatar
 

I think the garbage bags are working against you. I would try fabric.
Old 31st August 2008
  #4
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Quote:
Originally Posted by ck747 View Post
I made extra four 2” deep panels and again they are all in thin plastic bags.
uhhm. plastic bags = reflective, no absorption tutt just remove them
Old 31st August 2008
  #5
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The garbage bags shouldn't effect low frequencies whatsoever.

I think you just don't have enough. You're only covering slightly more than half of each corner and you're completely ignoring the entire ceiling/wall corners. Unfortunately, I think you need more trapping. The square room is working against you big time.

But I'm no acoustics expert.

I do know that if you are using the extra 2" panels flat against the walls, that they will do little for low end response. In that capacity they are more effective at knocking down reflections. But you need to put them in the right places for that to be most effective.

So I would make more traps. Also, don't underestimate what a good heavy couch up against the back or side walls will do.
Old 31st August 2008
  #6
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Thanks for the suggestion

I have already tried to put the subwoofer to two different corners and orientations. I also tried positioning all 2” panels (in a row) against the back wall or positioning each panel to different walls. However, all bass traps and panel combination did not show any improvement at all.

I am expecting some improvement even the bass traps don’t go all the way up from the floor to the ceiling. As I mentioned in the previous post, putting the fiberglass insulation in plastic bags is a temporary measure. I would like to see how it works first before I put more investment on getting fabric bags or making a frame for it.

I really don’t want to spend a few hundred dollars for the fabric bags under the current situation unless someone show me that plastic bags are bad choice for low frequency absorption (I know they are not friendly and reflective for mid and high frequencies).

I agree that I should pay more attention to the ceiling/wall corners but anyone can ensure me it will alleviate the deficiency of bass control in my square room.

Thanks
Old 31st August 2008
  #7
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avare's Avatar
 

You have triple mode build up around 96 Hz. Of course that also means you have lack of modes adjacent. There will be poor bass response no matter what you do with treatment. To give proper perspective, think of the room as 4 (~7.5') cubes together.

The best you can do is treat as many corners as possible. Remember that a room has 12 corners, not 4.

Andre
Old 1st September 2008
  #8
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Andre:

I am not an expert on the acoustic treatment. Can you please evaluate your statement.

>>> Remember that a room has 12 corners, not 4.

Are you saying that I should put an extra bass trap half way on both side walls? What is the proper distribution of bass traps other than the four corners of the room? Do you mean that no matter how many bass traps I install, the end result may make a slight difference? Is it worth putting the investment to the square room?

Thank you
Old 1st September 2008
  #9
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Ethan Winer's Avatar
 

Lightbulb

Quote:
Originally Posted by ck747 View Post
Are you saying that I should put an extra bass trap half way on both side walls?
A rectangle room has four corners where each wall meets another wall, four more where each wall meets the ceiling, and four more where each wall meets the floor.

Quote:
Is it worth putting the investment to the square room?
Do the math. heh Which do you think is better - a square room with no bass traps that totally sucks to mix in, or a square room with a bunch of bass traps that's reasonably accurate?

--Ethan
Old 2nd September 2008
  #10
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Glenn Kuras's Avatar
Quote:
Originally Posted by andrebrito View Post
For home theaters I also recommend placing diffusers.
I am sure you do heh but that is not going to do anything for bass problem he has..

Quote:
The material I used is Johns Manville 814, I stack up three sheets of 2” insulation to make up 6 inches deep panels (all four bass traps have dimensions of 4ft x 2ft x 6ins and all of them are temporarily stored in big garbage bags for a quick trial). At the same time, I made extra four 2” deep panels and again they are all in thin plastic bags.
Really I think it is just a matter of putting A LOT more bass trapping in the room. Just to make sure, when making them you want to make sure you don't cover the back with any kind of wood. It should be open in the front and the back.

Glenn
Old 2nd September 2008
  #11
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Weasel9992's Avatar
 

You've already had responses from Ethan, Glenn, Andre(1) and Andre(2), so you know the right answer. That 96Hz buildup (or thereabouts) is going to take a whoooole bunch of bass trapping to even out.

We just went through something with a customer of ours that might be relevant here: what's the crossover like between your mains and your subs? If you've got a big overlap that could be part of the problem. Positioning is also critical.

Frank
Old 2nd September 2008
  #12
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andrebrito's Avatar
 

You are in a delicate situation... you need bass traps to solve those room modes BUT you cannot place much absorption in the room, particularly since this is not a control room but an home theater.

The correct way to do this, since this is related to a specific frequency would be to use membrane absorbers for the correction of room modes (that 96 Hz problem) and diffusers to give the room its appropriate sound environment for an home theater. Unfortunately this is also the most expensive way.
Old 2nd September 2008
  #13
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Glenn Kuras's Avatar
Quote:
Originally Posted by andrebrito View Post
The correct way to do this, since this is related to a specific frequency would be to use membrane absorbers for the correction of room modes (that 96 Hz problem) and diffusers to give the room its appropriate sound environment for an home theater. Unfortunately this is also the most expensive way.
Yes I will agree that using bass traps that focus on more of the low end then the high end is the way to go. If buying, ours or real traps do just that. If building use FRK to help keep the life in the room. With a room that size I guess you could use diffusion, something like OUR D1, but that really is going to be icing on the cake.

Glenn
Old 3rd September 2008
  #14
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That's an affordable suggestion to use FRK and if you don't have money you can use it, but a diffuser is always a diffuser.

The advantages are getting a larger sweet spot than a typical Control Room, since you usually have more than one person listening to music and also you want the acoustical treatment to create a sonic spatial environment, particularly if you are a fan of classical music since most concert halls use diffusers to increase the perception of sonic spaciousness and maintain reverberation in the room.
Old 3rd September 2008
  #15
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Thank you for your suggestions.

There are a few things I am going to experiment in the coming few weeks.

1) Make corner wedges (24”) on 4 vertical corners (wall/wall) from floor all the way up to the ceiling.
2) Put up two early absorption panels on both side walls.
3) Put up some foam or wedges on the wall/ceiling corner.
4) Put up some diffusers on the walls.

Questions

1) For the corners (wall/ceiling and wall/floor), do I need a full length of bass trap along the corners.
2) Can you explain to me why I need diffusers in my square room? How many diffuser panels do I need and where should they go on the walls?
3) Do I need any HF absorption panels on the back wall?
4) What are membrane absorbers?


Thank you very much
Old 3rd September 2008
  #16
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Glenn Kuras's Avatar
>1) For the corners (wall/ceiling and wall/floor), do I need a full length of bass trap along the corners.<

The more of the area you cover the better.

>2) Can you explain to me why I need diffusers in my square room? How many diffuser panels do I need and where should they go on the walls?<

Honestly in room that size I don't see the point, but the back 1/3 of the room, pretty much where ever you have wall space open, I guess would be best.

3) Do I need any HF absorption panels on the back wall?

For the back wall use THICK (4" or more) panels made of mineral wool or rigid fiberglass. You are going to want to space them the thickness of the panel, off the wall.

4) What are membrane absorbers?

There are different kinds, but the ones I was speaking of has a thin rigid substance to the front of the panel to reflect some of the highs back. This is still a broad band bass trap so it is not "TUNED" to a specific frequency. You can though build a "tuned" bass trap but honestly for a room like yours that would not be the way to start off. There are problems across the whole frequency range and the panels I was speaking of will help at those areas.

See the following page to give you some ideas on where to place diffusors, thinner panels for early reflection points with broad band bass traps in the corners. GIK Acoustics: Room Setup

Glenn
Old 3rd September 2008
  #17
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andrebrito's Avatar
 

I have a different perspective than Glenn,,,, here are my replies

1) For the corners (wall/ceiling and wall/floor), do I need a full length of bass trap along the corners.

You do need bass traps but the type depends of your situation ! If your problems are only a specific frequency you don't need broadband bass traps that act in the ENTIRE low frequency range but tuned absorbers to decrease in a specific frequency or frequencies. If the problem is several frequencies then I would recommend using broadband bass traps. If you cannot afford or build tuned absorbers use broadband, it is better than nothing.

2) Can you explain to me why I need diffusers in my square room? How many diffuser panels do I need and where should they go on the walls?

One thing people in these and other forums need to understand, and I'm sure others agree with me, is that the first thing you do when you evaluate or design a space is to define its acoustical goal. A control room is used for critical listening where you are having an analytical approach to sound. You want the Control Room to be as neutral as possible. A home theater is an approach for an existent space (concert hall, cinema, theater) in a smaller one. You don't analyze sound in an home theater, you use it as an entertainment space. Home theaters need diffusers to "simulate" the acoustics of concert hall, to increase the sweet point and give the listener a sense of spaciousness in the room. Home theaters usually have higher RT60 than Control Rooms. This means also using less absorption but using diffusers to correct acoustical problems such as flutter echos.

Is it more expensive to use diffusers in a room ? Yes it is.

3) Do I need any HF absorption panels on the back wall?

On back wall I usually suggest diffusers or thick absorption panels. You can use HF absorption if the budget is short.

4) What are membrane absorbers?

Membrane absorbers act at certain frequencies.. check this page

RPG Diffusor Systems

And check this page for some home theater projects



Old 3rd September 2008
  #18
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Glenn Kuras's Avatar
Quote:
2) Can you explain to me why I need diffusers in my square room? How many diffuser panels do I need and where should they go on the walls?

One thing people in these and other forums need to understand, and I'm sure others agree with me, is that the first thing you do when you evaluate or design a space is to define its acoustical goal. A control room is used for critical listening where you are having an analytical approach to sound. You want to the room to be as neutral as possible. A home theater is an approach for an existent space (concert hall, cinema, theater) in a smaller one. You don't analyze sound in an home theater, you use it as an entertainment space. Home theaters need diffusers to "simulate" the acoustics of concert hall, to increase the sweet point and give the listener a sense of spaciousness in the room. Home theaters usually have higher RT60 than Control Rooms. This means also using less absorption but using diffusers to correct acoustical problems such as flutter echos.
I would agree with you, but you are going to play HELL ""simulate" the acoustics of concert hall, to increase the sweet point and give the listener a sense of spaciousness in the room." in a 13.5ft (W) x 13.0ft (L) x 7.5ft (H). You do understand this in feet not in METERS. heh Trust me, I am HUGE promoter of diffusion but in the right room. Not to say that it would not work in this room but would not me my first focus.

Quote:
If you cannot afford or build tuned absorbers use broadband, it is better than nothing.
heh

Glenn
Old 3rd September 2008
  #19
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That is way I use the "" on the word simulate. I'm perfectly aware of the differences between a concert hall and a home theater. The idea is to give some acoustical spaciousness to the room, not to imitate literally the acoustics of a concert hall. I'm also aware that is a small room. Yet I don't see a problem to use diffusers in the room since it is not intended to be used as a control room. As simple as that.

Otherwise you can ask the folks at RPG why the hell some of the most known acoustical designers are doing rooms like this, and why RPG for home theaters suggests to use diffusers in the side and back walls... and that is the main reason I told before. A home theater is NOT a control room.



Old 3rd September 2008
  #20
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Glenn Kuras's Avatar
Quote:
Originally Posted by andrebrito View Post
That is way I use the "" on the word simulate. I'm perfectly aware of the differences between a concert hall and a home theater. The idea is to give some acoustical spaciousness to the room, not to imitate literally the acoustics of a concert hall. I'm also aware that is a small room. Yet I don't see a problem to use diffusers in the room since it is not intended to be used as a control room. As simple as that.

Otherwise you can ask the folks at RPG why the hell some of the most known acoustical designers are doing rooms like this, and why RPG for home theaters suggests to use diffusers in the side and back walls... and that is the main reason I told before. A home theater is NOT a control room.



Why do I have a feeling these rooms are larger then 13.5ft (W) x 13.0ft (L) x 7.5ft (H).heh and just because you show me a cool picture has nothing to do about the sound.
I have nothing but the highest respect for RPG but we have added to more then one of there rooms. Like I said and will say it again, I myself like diffusion for all rooms (yes not all rooms are treated the same) but in smaller rooms it is more of a "icing on the cake". I guess it just depends on how much icing you want. heh

Glenn
Old 3rd September 2008
  #21
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Bass absorbtion is required in EVERY small room - sorry. It's not only to help deal with frequency response aberrations, but also to bring decay time curves into line.

Yes - a home theater and a control room are 2 very different things. That said, a home theater still needs to have the room taken out of the equation more than a 2 channel listening room. Why? Because in a home theater environment, we have additional channels available to recreate a specifice surround space whereas with 2 channel, we need to help simulate it.

In a 13x13 room, the only places I'd potentially see a viable use for diffusion are:

- Side wall reflection points (not my choice but it's an option as long as the diffusion is sufficiently large to reach down into the vocal range - most isn't)
- Ceiling relfection points - this can help to bring a feeling of a taller space to short rooms
- Rear half of the side walls - this is to create a larger feeling of space and to assist in creating a more diffuse surround stage.

That said, on the rear wall, no way, sorry. Proper seating position for the first row (forget the 2nd row if any) is going to place the listener approx 5' from the rear wall, WAY too close for diffusion to be effective. Also, there will be bass nulls to deal with off the rear wall.

Front wall - sorry, again, no way. Front wall is to be dead in multi-channel to avoid information from the surround field coming off the front wall and contaminating the front soundstage. Also, most of the time, some absorption capable of dealing with close speaker proximity and the resultant SBIR is required due to cramped positioning in HT's and in small rooms - this is both.

Don't get me wrong - I have nothing against diffusion. When used properly and in the right locations, it can be a marvelous tool that can really help the sound of a room. That said, it's not a one size fits all solution. VERY few PRACTICAL sized diffusers are available that will do down well into the male vocal range. That's a killer right there for many placement options.

Bryan
Old 3rd September 2008
  #22
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Quote:
Bass absorbtion is required in EVERY small room - sorry. It's not only to help deal with frequency response aberrations, but also to bring decay time curves into line.
I did NOT say the room did not required bass traps. It does having in account the problems he refer to. The question is what kind of bass traps he needs. And that is another question.

And affirmations like that EVERY small room needs bass traps are a bit too risky. First you need to define what a small room is. Then what is the goal of the room and possible acoustical problems. Then we can discuss things from there. If the room is 6 per 8 meters (18 ft per 24 ft) would you consider a small room or not ?


By the way, perfect decay curves are not linear in terms of pressure levels where reverberation time is extrapolated. And in order to bring those curves into a perfect decay at low frequencies you would need lots of absorption. Of course bass traps smooths the oscillations due to beats between room modes. Meaning that you would end up a dead room, inappropriate for an home theater. I'm not against using bass traps, nor I'm in favour of them. If the room needs it, so be it. But I'm against this philosophy of placing bass traps everywhere without even considering the acoustical goal of the room and other possible parameters.


Glenn, pictures are pictures and sound is sound. And I'm aware of that. But RPG does have several papers on the subject, even some available at their site. They have researched a lot on this field as well. Yes they want to sell diffusers and they do have a marketing strategy like any other acoustical company does in the market. But they do invest a lot and have great partnerships in the research field.

If you think the acoustical designers (who are not RPG workers by the way) choose that quantity of diffusers to be placed in that room, with high costs to the client, just for the "looks" and not for some valid scientific reason, then you are accusing them of something serious.

Now if you guys asks me what is the affordable way to go for an home theater, I would say use only absorption, if that is your budget. And I understand GS is all about making things work at low cost. No harm on that. I'm all in favor of that.
Old 3rd September 2008
  #23
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Glenn Kuras's Avatar
Quote:
If you think the acoustical designers (who are not RPG workers by the way) choose that quantity of diffusers to be placed in that room, with high costs to the client, just for the "looks" and not for some valid scientific reason, then you are accusing them of something serious.

Are you running for president? How could you even draw that from what I said? Talking about that, I have had the opportunity to visit many rooms that the client THOUGHT they where getting correct advise but turned out to be crap. I must admit the rooms looked pretty darn cool, but sounded like dog poop. I don't believe it was because the designer was dishonest but just had more of this "one solution for every room" way of thinking.
disclaimer: I am not talking about RPG or anyone part of RPG OR YOU.

I am Glenn and I support this statement. lol lol heh

Quote:
Now if you guys asks me what is the affordable way to go for an home theater, I would say use only absorption, if that is your budget. And I understand GS is all about making things work at low cost. No harm on that. I'm all in favor of that.
I really think you are missing the point and I say that with the up most respect. What we are recommending is NOT based on budget, but what is best for the room. I do though respect our thoughts though, which honestly we are not that far apart.

Glenn
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