Corner Bass Traps - Always beneficial?

[edorr]

Attached is a picture of my current room. I have somewhat randomly dumped some absorbtion panels on the walls. The two panels behind my speakers are 4" bass traps. I will take possesion of a pair of Evolution Acoustics MM3 tomorrow and have sold the two subs in the picture.

http://forum.audiogon.com/cgi-bin/fr.pl?vhome&1285196540

My question is twofold. With the subs gone, I have now space for "top to bottom" triangular corner bass traps behind the speakers. Is this always a good idea, or are (REW or other) measurements required to assess if this will beneficial in my room? Should I set up and measure the new speakers first before spending some money (not soo much), on traps? Note: I also have a Trinnov room correction system to address bass issues.

If I end up buying the corner traps, what would be the best position to put the 24 x 48 panels? The ceiling would be the most obvious location. Apologies for my ignorance - I feel like a true amateur among the pro's hanging around here.

 

[Bruce B]

Bass traps are always good at room boundries. Just play some music with bass content. Now get up and walk over to the corner or up against the back wall.... you can hear the bass build-up.
I'd put bass traps in every corner of the room!

 

[amirm]

First things first. You don't have a bass trap! A 4 inch absorber is a broadband absorber that starts at a few hundred hertz and goes up. I Just simulated a 4 inch fiberglass absorber. Here is what it looks like:

As you see the response drops like a rock 300 Hz or so and by bass frequencies below 80 Hz there is no absorption. This is because these panels are put on walls where the velocity is essentially zero so their effectiveness goes away. If you put 4 inch air gap behind them and get them further out where the velocity is higher, then they work a lot better but still, even then, absorption is only 0.3 at 65 Hz.

Best thing to do is measure with REW and find the peaks and pull them down with a DSP. That will get rid of boominess. Filling in the gaps can be done with better placement of speakers or addition of subs.

Edit: I noticed you have trinnov. Let it optimize the bass. Corner bass traps are not a whole lot more efficient than above because again, they are working against the laws of physics.

 

[edorr]

First things first. You don't have a bass trap! A 4 inch absorber is a broadband absorber that starts at a few hundred hertz and goes up. I Just simulated a 4 inch fiberglass absorber. Here is what it looks like:

As you see the response drops like a rock 300 Hz or so and by bass frequencies below 80 Hz there is no absorption. This is because these panels are put on walls where the velocity is essentially zero so their effectiveness goes away. If you put 4 inch air gap behind them and get them further out where the velocity is higher, then they work a lot better but still, even then, absorption is only 0.3 at 65 Hz.

Best thing to do is measure with REW and find the peaks and pull them down with a DSP. That will get rid of boominess. Filling in the gaps can be done with better placement of speakers or addition of subs.

Edit: I noticed you have trinnov. Let it optimize the bass. Corner bass traps are not a whole lot more efficient than above because again, they are working against the laws of physics.

Sounds plausible and I cannot argue with the laws of physics, but the upshot would be the entire "bass trap industry" is selling snake oil, because no bass is actually being trapped. I find this hard to believe, but thenagain many commentators say the same about the high-end cabling market. What do I do - Tough call. May be I'll take the 48" x 24" x 4" panels from the wall and put them diagonally in the corner to simulate the triangular traps I was thinking of getting, and hear/measure what this does for in room bass response.

 

[DonH50]

Uh, Amir old chap, the absorption is really a flat line at zero if there is an air gap? Methinks thee should check the variables on that graph again...

Seriously, it depends upon the material, of course, and how much of it, etc. I do a little better with a lot of absorbers, including several 6" and 12", all spaced a few inches off the wall (which increases the absorption a bit and also their effective surface area slightly).

The real point of this post: Amir, what program did you use to simulate that? I want it.

Thanks - Don (the ever-curious)

 

[amirm]

Uh, Amir old chap, the absorption is really a flat line at zero if there is an air gap? Methinks thee should check the variables on that graph again...

No, that is just the way the simulator works if you set the air gap to 0. I didn't want to show both and have one confused with the other. Here it is with 4 inch air gap:

The real point of this post: Amir, what program did you use to simulate that? I want it.

It is a free template that runs inside excel! Here it is: http://www.whealy.com/acoustics/Porous.html

 

[amirm]

Sounds plausible and I cannot argue with the laws of physics, but the upshot would be the entire "bass trap industry" is selling snake oil, because no bass is actually being trapped.

Sadly it is true. From Dr. Toole's CEDIA presentation:

""Sometimes these (modular bass absorbers) are called "bass traps." The problem with the name is that some of them don't "trap" much of anything excpet cash from unwiiting purchasers."

I find this hard to believe, but thenagain many commentators say the same about the high-end cabling market. What do I do - Tough call. May be I'll take the 48" x 24" x 4" panels from the wall and put them diagonally in the corner to simulate the triangular traps I was thinking of getting, and hear/measure what this does for in room bass response.

Yes, that is a good way to test them. Recently someone tested corner traps he built before and after on AVS.

Here is before:

After adding "superchunks"

Needless to say, he was very unhappy with the results. Here is how he built them: "The superchunks are about 5.5' tall, a few inches off the floor (to match my wall panels), to about a foot from the ceiling. They are 36" faced and filled with pink fluffy insulation"

 

[MylesBAstor]

What about these panels Amir that I use in my room?

http://www.ultrasystem.com/usfeaturedPanelsTechInfo.html

I've had people over and hear an inch or two in placement makes a world of difference.

 

[AudioExplorations]

Best thing to do is measure with REW and find the peaks and pull them down with a DSP. That will get rid of boominess. Filling in the gaps can be done with better placement of speakers or addition of subs.

This is how I currently address my room issues and it is VERY effective. In the future when I move to a dedicated listening room I would like to get rid of the DSP and address bass issues with acoustic treatment. The material properties are of utmost importance (specifically the resistance to airflow). The 'pink foam' used in the example is likely the reason why there is so little improvement. Also, I believe it may be best to address specific room issues with tuned narrow range bass trapping along the relevant XYZ dimensions of the problematic modes.

 

[bblue]

What about these panels Amir that I use in my room?

http://www.ultrasystem.com/usfeaturedPanelsTechInfo.html

I've had people over and hear an inch or two in placement makes a world of difference.

I've wondered about those. The claims seem to be too effective! Have you done any measurements with and without, or moved from point A to point B?

--Bill

 

[microstrip]

Unhappily the internet if filled with misinformation about bass traps. As Amir as shown, when using a simple porous bass absorber the use of a significant gap length is mandatory, making it very room unfriendly and also taking large areas, as the absorption is limited.

The Master Handbook of Acoustics by F. Alton Everest is a very good source of information about bass traps. And do not expect to get all the critical information about building the best bass traps for free.

My bass trap plans do not show how to construct the added features of my company's for-sale panel traps. But obviously the plans show the basics. I'm more interested in helping folks than making a killing in business. More to the point, building panel traps is a lot of work, and even doing all the labor yourself the materials are still expensive. So I'm hoping all but the poorest folks will realize they can buy MiniTraps for a similar investment and a whole lot less work!

http://recording.org/studio-construction-and-acoustics-forum/11408-dissecting-rpg-modex-corner-bass-trap-2.html

 

[bblue]

Best thing to do is measure with REW and find the peaks and pull them down with a DSP. That will get rid of boominess. Filling in the gaps can be done with better placement of speakers or addition of subs.

Amrim,

Are you advocating NOT correcting room problems acoustically (as much as possible) before you apply EQ? That's completely counter to commonly practiced room tuning. And you can't undo the audio character of resonance at a certain frequency, by EQing it down at that frequency. It is still resonant. Same thing with dips caused by nulls. They still have the sonic character of nulls.

--Bill

 

[MylesBAstor]

I've wondered about those. The claims seem to be too effective! Have you done any measurements with and without, or moved from point A to point B?

--Bill

No I haven't.

 

[microstrip]

What about these panels Amir that I use in my room?

http://www.ultrasystem.com/usfeaturedPanelsTechInfo.html

I've had people over and hear an inch or two in placement makes a world of difference.

Myles,

Considering their size they can not affect the bass frequencies significantly, unless they have reasonable mass and your walls are panel type - then they will change the resonance frequency of your room intrinsic bass trap. Or most probably you are just listening to their effect in the medium or high frequencies.

 

[edorr]

Sadly it is true. From Dr. Toole's CEDIA presentation:

""Sometimes these (modular bass absorbers) are called "bass traps." The problem with the name is that some of them don't "trap" much of anything excpet cash from unwiiting purchasers."


Yes, that is a good way to test them. Recently someone tested corner traps he built before and after on AVS.

Here is before:

After adding "superchunks"

Needless to say, he was very unhappy with the results. Here is how he built them: "The superchunks are about 5.5' tall, a few inches off the floor (to match my wall panels), to about a foot from the ceiling. They are 36" faced and filled with pink fluffy insulation"

This article reports some reasonable, measurable improvement from passive treatment, including the exact type of corner traps I had my eyes on - admittedly not much is happening below 180Hz, but 5db improvement at 180Hz is quite impressive .... The improvement are not just in frequency response, but also decay and impulse response. In fact, this article inspired me to do some fiddling with more passive treatment, before applying DRC with the Trinnov.

http://www.computeraudiophile.com/blogs/mitchco/importance-timbre-sound-reproduction-systems-222/

Multiple subs to address dips are out of the question for me, so (near) perfection is not achievable for me. In my current setup, I measure a 12db dip somewhere in the low frequency range, and I am boosting it by 6db with the Trinnov. Curious to see what the full range Evolution Acoustics will do in my room.

 

[amirm]

What about these panels Amir that I use in my room?

http://www.ultrasystem.com/usfeaturedPanelsTechInfo.html

I've had people over and hear an inch or two in placement makes a world of difference.

It says they are pressure based devices which is the right type for near wall placement. There, velocity is small or equal to 0 where pressure is near or at maximum. So these devices work better there.

 

[amirm]

This article reports some reasonable, measurable improvement from passive treatment, including the exact type of corner traps I had my eyes on - admittedly not much is happening below 180Hz, but 5db improvement at 180Hz is quite impressive .... The improvement are not just in frequency response, but also decay and impulse response. In fact, this article inspired me to do some fiddling with more passive treatment, before applying DRC with the Trinnov.

http://www.computeraudiophile.com/blogs/mitchco/importance-timbre-sound-reproduction-systems-222/

I have read other articles Mitch has written and they are usually very sound technically. Unfortunately this one is not one of them despite the amazing effort he has put in it. It starts off with assumptions that I call "forum myths" and then goes on from there. I have to go and mow the field before it gets too hot . So more comments later. For now, I would not copy what pros do in the studio as he is patterning his work after. They have different needs and hearing sensitivity distinct from consumers and audiophiles alike.

Multiple subs to address dips are out of the question for me, so (near) perfection is not achievable for me. In my current setup, I measure a 12db dip somewhere in the low frequency range, and I am boosting it by 6db with the Trinnov. Curious to see what the full range Evolution Acoustics will do in my room.

The right location for the EAs will help a lot. See you can place them 25% in from walls. Doing so reduces two of the modes, in essence making them like two subs at those locations. Mike Lavigne has/had them that way.

 

[AudioExplorations]

The right location for the EAs will help a lot. See you can place them 25% in from walls. Doing so reduces two of the modes, in essence making them like two subs at those locations. Mike Lavigne has/had them that way.

I'm wondering if for a rectangular room you couldn't have a positioning rule of thumb (for subs or LF woofers of speakers) in terms of % along the length and width of the room that would work in all cases seeing as the standing wave pattern always takes more or less the same shape in this geometry.

Am I right in assuming you want the woofers situated as close to 0 instantaneous pressure as possible? This shows the the first through 4th modes along a single dimension.

 

[slowGEEZR]

Edit: I noticed you have trinnov. Let it optimize the bass. Corner bass traps are not a whole lot more efficient than above because again, they are working against the laws of physics.

Amir, are you sure about this statement? I believe corner bass traps work by using the laws of physics and are much more efficient than what you show. I only base this on what happened in my own listening room. I had, at the time, some Martin Logan Aerius speakers and I was disappointed with the total lack of perceived bass at the listening position. It was like my speaker woofers were disconnected; a very thin sound. I ordered two GIK Tri Traps (bass traps), placed them in the front corners and voila, the best bass I've ever heard from the MLs appeared. It actually sounded like I added a subwoofer in the room. The difference was amazing, not subtle at all.

 

[microstrip]

(...) Unfortunately this one is not one of them despite the amazing effort he has put in it. It starts off with assumptions that I call "forum myths" and then goes on from there. I have to go and mow the field before it gets too hot . So more comments later.

Amir,
Can I assume you have also read the F.Toole book? I forgot to refer that "Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms" has an excellent section on the acoustics of small and medium size listening rooms, referring to the limitations of the methods used in acoustics for large spaces when (mis)applied to small volumes.