The secrets of bass trap placement
Wednesday, February 9, 2011 at 1:34PM One of the things I get asked quite often is 'where is the best place to put bass traps and why?'. This short article will explain a little about what bass traps do and how they work before going on to discuss the three different places where bass traps can be placed.
Bass Traps and room modes
The most effective place to add bass traps is in the corners of the room since all room modes have a pressure high or node in the corner. The three key points about room modes are summarized below, but if you want to know more be sure to read the detailed article Room Modes and Room Mode Calculators on our main website.
- ALL rooms have naturally occuring resonance frequencies - variously called room modes, standing waves, eigentone, eigenmodes or modal resonances.
- Room modes are the main cause of acoustic distortion below 300Hz or so, causing peaks and dips in the frequency response of 20dB or more.
- Modal resonances store energy and decay slowly compared to nearby frequencies causing audible problems such as 'one note bass' and 'boominess'.
How Bass Traps work
Bass traps are acoustic absorbers designed specifically to be effective down into the frequency range where room modes cause problems. Typical 3" thick fiberglass absorber panels start to lose effectiveness at around 250Hz whereas most of our modal resonance problems occur below 150Hz in the so called 'sparsely populated modal region'. Bass traps typically incorporate elements such as membranes, airspaces and additional thicknesses of fiberglass (up to 6" or more) to extend absorption down into the double digit range.
The three places where Bass Traps can be installed
There are three main places where bass traps can be installed:
1. Tri-corner, where the two walls meet the ceiling or the floor. 
Tri-corner bass trapThere are 8 tri-corners in a square or rectangular room, although placing bass traps at the four on the floor may not be practical. Placing a bass trap in this location affects all room modes. There are specific triangular bass traps designed for installation in the tri-corners, such as the Primacoustic Cumulus.
Corner mount bass trap2. A wall / wall corner, such as the interface of the side and back wall. There are 4 corners in a square or rectangular room. Placing a bass trap in a wall / wall corner will affect the main length and width modes and some of the height modes. Stacking two 4' tall corner mount bass traps such as the Primacoustic MaxTrap, one above the other, in the corner can affect all room modes, since the tri-corner location will also be covered in a standard 8ft ceilinged house. RPG Modex Corner are also very effective in the wall / wall corner location.
3. A wall or ceiling. There is plenty of wall and ceiling area available in most rooms for installing bass traps such as the Primacoustic FullTrap, the RPG Modex Module or the amazing RPG Modex Plate. Generally a wall mount bass trap will most strongly impact the room modes caused by the two parallel surfaces. Placing a bass trap on the side wall, for example, will mainly affect the width mode.
Wall mount bass trap
Are there any other things you want to know about bass trap placement or bass traps in general? Let us know by adding a comment or contacting us.
Acoustic Frontiers sells bass traps and corner bass traps in our online store. We offer free shipping on all orders over $399 to the lower 48 states of the USA.
Reader Comments (17)
Hi, I have a room of 3m x 3m x 3. A perfect cube. I have a closet covering a side wall almost completly. So the available space is 3x2x3. I have a lot of cancelation and boming in bass. Im wondering where would be the best places to put the bass traps. 2 of the corners are not available for placing as there is a door and also the closet. Any sugestions would be apreciated
Thanks
Hi Fernando, thanks for asking this great question! That is going to be a very hard room to fix because all the room modes are going to coincide creating a significant peak in the frequency response and extended ringing in the time domain. It is the extended ringing that is the main cause of the boominess you are observing.
One place I always like to put acoustic treatment is on the ceiling. You can use the wall / ceiling corner and the wall / wall / ceiling corner for starters. After that I would recommend adding bass trapping to the ceiling. As much as you can really!
The other option that I have written about in one of my blog posts is to use multiple subwoofers in a mode cancelling arrangement.
Thanks, and please do let me know if there are any more questions!
Hi, I have a room that is 17.5 feet wide, 13 feet deep, 8 feet tall, with draped glass windows on the right side, and open on the left side, and carpet. The walls are concrete, and the ceiling is concrete with acoustic coating. The speakers are located on the front wall (17.5 feet wide) and the listening position is near the back wall. I have serious bass problems, particularly at around 60hz and 100hz.
I installed 4 foam bass traps on each of the wall/ceiling corners of the front and back walls, and 16 foam studio tiles on the each of the front and back walls as well. Is there anything else I do or better locations for bass traps that would help get this under control?
Hi John
First of all thanks for leaving a comment! It's always reassuring to know that people are really interested in getting good sound and are aware of the impact room acoustics has on this.
There are a couple of problems with your setup:
1) a room with concrete walls and ceiling is going to have problems with room modes since the room is not providing any supporting low frequency absorption. A sheetrock (called gypsum board or plasterboard in other areas of the world) / wooden stud construction actually flexes and is a reasonably good absorber of low frequency energy. So bass trapping, and likely tuned absorbers, are going to be required in your space to bring the low frequency decay under control
2) a listening position against or very close to a wall puts that seat in an area where all room modes have a pressure node (these correspond to peaks in the frequency response since they are where energy builds up)
3) foam bass traps absorbers are what is called a broadband bass absorber and typically only absorb down to maybe 150Hz or even only 200Hz with any real effect. The foam tiles will not do anything for bass absorption. A useful measurement that shows absorption over the frequency spectrum is the absorption coefficient. A coefficient of 1 indicates that all incident energy is absorbed in that frequency range. In our opinion all manufacturers of acoustical products should produce a table showing absorption coefficient since it very clearly shows where a product is effective and where it is not.
A good place to start would be to use a room mode calculator - input your room dimensions and see if the peaks you are observing correspond to room modes. I am very sure that they will be. Once you've confirmed this then you can decide what bass trapping is required to combat the issues at those frequencies. For your space in particular you will need to seriously upgrade your bass trapping. Products like the Primacoustic MaxTrap are effective down to about 70Hz. Below that you need to use custom built bass traps or the RPG Modex Plate. Acoustic Frontiers can help with both products - just get in touch!
The other important thing to know is that positioning of both loudspeaker and listener (but particularly listener is an effective tactic to deal with resonances). If you can find a null for the mode in question then you can put your listening seat in that location. Just beware that you might have put yourself in the peak for another mode by moving your seat though! Nulls can be found either by playing a sine wave tone at that frequency and walking around the room to find the peaks (where the sound is louder) and nulls (where the sound disappears) or by using an acoustic measurement product such as XTZ Room Analyzer.
The best combination is normally to add bass trapping and to seek a position for the listening seat where the frequency response is most even. A third option is to use subwoofers in a mode cancelling arrangement.
Hope that helps! Nyal
Hi,
I'm not sure I understand how the room mode calculator is displaying the frequencies, since it displays 28 values and I'm not sure if these all apply to all axes, however several of these are in the ranges that I have noticeable trouble with as evidenced by the SPL meter with test tones. In particular though, I notice that the ~60hz peak is the one that is most physically uncomfortable to listen to.
I am discovering too late that I bought the wrong type of bass traps for the frequencies that are most problematic, but given your recommendation of other bass traps, I am left wondering where I would install them. The problem is that my room does not really have vertical corners where traps can be installed as seems to be the standard recommendation. Can I expect the same benefits with these bass traps installed on the back wall or on the horizontal corners between the front/back walls and ceiling where the foam is currently placed?
I have experimented with moving the speakers and listening position as much as practicality allows, although I have noticed what you describe which is that the problem frequency merely shifts, and to alleviate the uncomfortable deep bass pressure, the speakers have to be 5-6 feet into the 13 foot deep room. I am interested to read more about the mode cancelling subwoofer arrangement that you describe.
Thanks again
Hi John,
Good to hear from you again!
The RealTraps calculator shows the length, width and height modes separately. Even better is to find one of the room mode spreadsheets floating around on the internet which show all of the modes and their order.
The best placement of bass traps does depend on which surfaces are causing the problematic standing waves to form. You want to place the bass traps in a location where they in the pressure peaks for those standing waves. For example if you can associate your problem around 60Hz with, say, the 2nd length mode of the room, then you know the best place to put bass traps to deal with that standing wave will be anywhere on the front and back walls.
Its worth knowing that Acoustic Frontiers does offer free consultations to help out with just this kind of problem. And of course our main reason for being in business is to professionally solve exactly these type of issues.
The mode cancelling subwoofer arrangement is very effective but also requires that you know which surfaces are causing the problematic standing waves. You place one subwoofer in a peak for the mode and the other in a null, and voila, the room mode ceases to exist! See this short article for more information.
Nyal
I will definitely try out a free consultation before spending any more money on the wrong products, or installing in the wrong positions. Expert advice is of tremendous value. Since I currently have three non-identical subwoofers to experiment with, I'll also be trying the cancellation approach. Does this require that one of the subs be used with inverted polarity, or is this achieved by placement alone? The other thing that comes to mind, is whether it may be possible to use a single subwoofer on an opposing wall from the speakers that are creating the bass boominess. That is to say, rather than multiple subs, can a single sub cancel out the modes created by large full range speakers? It is great to know there are some options available and this site has proven to be a great resource.
Cheers
Hey John
The subs are run with normal polarity - it is putting them in opposite 'poles' of the standing wave that creates the mode cancelling effects.
It may be possible to use a single sub opposite say a full range speaker as long as there is significant acoustic overlap in the region of the mode in question AND the placement puts the speakers and sub in opposite poles for the standing wave. For example, if the mode you want to cancel is at 70Hz then you need both speakers and sub to be playing at pretty much the same level at that frequency. Normally if you use a crossover between sub and mains then this will not be the case since the output of one is rolled off in the pass band of the other. Furthermore the placement of the mains is typically in a location where soundstaging and imaging is the best, and this placement is rarely going to coincide with a pole for the mode you are attempting to cancel...
Nyal
Thanks Nyal, great article.
You mention that some bass traps have a membrane.
We have found that a foil membrane on the front of a glasswool slab really improves the low frequency absorption.
Those of you thinking of making or purchasing bass traps might want to bear this in mind.
And avoid acoustic foam traps if you really want them to make a difference in your room - glasswool / mineral fibre performs much better!
Will
Thanks Will for your comment... are you referring to the aluminium foil on FRK type slabs?
Any type of membrane will have the effect of transforming sound pressure into movement. That movement is then damped by the fiberglass or glasswool core, which turns it into heat. This is particularly useful since most of the time we put our bass traps in areas of high pressure but low sound velocity (i.e. near the walls) so membrane based traps help to increase absorption by working with the physics of the situation.
Where in the UK are you based? I spent 8 years in London before moving over to California. Can't say I miss the weather one little bit!
Hi Nyall
Well I know the FRK slabs are popular in USA but we don't have Owens Corning in the UK so we put the foil on ourselves.
We are not too far from London - we are near Winchester in Hampshire. EQAcoustics.com for any of your readers in Europe that would like to get in touch.
I know what you mean about the weather - winter is just setting in here. Time to wrap ourselves in fibreglass and stay warm!!
Best regards,
Will
We are building our own bass traps and acoustical panels at our church. We seem to have problems with low frequencies. Our back wall is 50' wide with a cathedral acoustical tile ceiling. I'm wondering about placement for our bass traps? I know that the corners are a better place for them. But, that back wall is very big and flat. The distance from the stage to the back wall is roughly 45'. Do we also add bass traps in the field of that wall? Do I make all bass traps 6"?
Jesse
Hi Jesse, thanks for your comment!
Have you done any acoustical measurements of your church? It might be worth doing some before building a lot of bass traps to understand what the issues are in your space. There are low cost options for acoustical measurement including XTZ Room Analyzer II Pro and my remote Acoustical Diagnosis Service. Generally big rooms such as the one you describe don't have problematic issues in the low frequencies - the issues are more related to sounds taking a long time to decay in the lower midrange and midrange frequencies causing issues with speech intelligibility in particular.
In answer to your question the best place for bass traps depends on exactly what room modes you are trying to address. But your proposed location on the back wall and thickness would seem like a good start. Generally the thicker the better and if you can put some kind of hard membrane like hardboard or thick plastic over the top of fiberglass/rockwool then that also increases low frequency absorption at the cost of reduced midrange and high frequency absorption.
Good luck! Nyal
Hi great stuff here!
I was just wondering;
i have a 16' wide 32' long with 12' ceilings that gradually slope up to about 13' in the back. but it is only the full length on one side. about 20' on the left side wall 32.5 on the right. and this happens about the middle or the room width wise
i have 2 6inch thick 2x4 panels on the floor tri corners (front wall behind mixer and speakers) (O/C 703 rigid fibreglass)) and a 1st reflection panel on either side wall 2x4x 3inch thick (703)
my question is if i were to make 2 more 6 inch bass panels would putting them in the 4 tricorners be better than standing them on top of aech other leaving the top corners untreated? also do i need a cloud with 12' ceiling? sorry also have panels behind the speakers .
Thx a bunch
(speakers are 3' out from wall)
Darin
Hi Darin - I wouldn't worry too much about the decision of whether to stack the bass traps or put them in the tri-corners. It's hard to add too much bass absorption!
I was planning to buy the Cumulus bass traps for a 7 X 7 Whisper Room. Asking some of my peers what their thoughts on this were, I got the following response:
I would think a traditional 24 x48 corner bass trap would be a lot more effective than those Cumulus ones simply because they're so much bigger... mass is what absorbs bass. According to the specs page, the Cumulus panels don't deal with anything below 100Hz... I wouldn't think they'd be much help in a Whisper Room.
Would you mind commenting on this?
Hi Ben, thanks for your question.
I am not familiar with Whisper Rooms but from a quick google it appears they are for voiceover recording? In which case it's unlikely that you would need bass absorption below 80Hz, since that's as low a frequency as vocals go. And most of the energy is actually in the 150Hz to 400Hz range. So products like the Primacoustic Cumulus will be fine for your needs.
Mass makes the difference for sound transmission but is not important for sound absorption e.g. you could have a very heavy piece of wood that would do nothing for sound absorption.
Hope that helps!
Nyal