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Bass Trapping Ideas for Non-Ideal Spaces

by Jeff Hedback May 26, 2009
Barnes Theater Dallas TX

Barnes Theater Dallas TX

Bass traps control low frequency issues in rooms. Simply, they are the single most effective investment toward a quality audio experience that is rarely made. Why, ‘cause they take lots space, you’ve already fought for subwoofer sq ft’s, and you just may not know how effective acoustical bass trapping devices can be! The following article will explain various types of bass traps and offer some applications for bass trapping (low frequency control) that can pay big dividends toward a better experience.

You have four major parameters to adjust/optimize perceived low frequency response:

1- The “right subwoofer(s)” is of great conversation within Audioholics, but it deserves even more attention. But, this is not the article to debate subwoofer specs.

2- Speaker Location (both subs and mains): See the numerous and excellent articles written by Gene DellaSala and his staff on this topic.

Multi Subwoofer Connection Guide

Speaker Placement Tips

3- Acoustical “Bass Trap” Panels: Type & Location are KEY. There are just a few types of traps and multiple variations from there: (1) Passive trap options that work on the velocity component of sound waves, (2) Helmholtz traps that work on the pressure component of the sound waves, (3) Diaphragmatic traps that work on the pressure component of the sound wave, (4) Membrane traps that work on the pressure component of the sound wave & (5) Active Traps.

4- Digital Room Correction. After you’ve optimized all the above, the benefits of DRC can be tremendous.

It's important to note that while this article focuses on bass traps for dealing with standing waves to improve bass problems for small room acoustics.  The best way both aesthetically and efficiency to solve bass problems is through modal manipulation using multiple subwoofers.  Once you reduce the seat-seat variance at bass frequencies, any remaining peaks can be EQ'ed out of the response to eliminate room resonances.  Low frequency bass traps can be used to supplement this solution if they are deemed to be necessary and practical.  Bass traps can be particularly effective above the subwoofers range of operation (100-400Hz) for dealing with upper bass issues.

For more information see:  History of Multi-Sub and Sound Field Management

Bass Traps vs Multi-Sub YouTube Video Discussion

Background on low frequency waves- I’ve already referred to the velocity and pressure components of the sound wave. These happen in cycles due to the frequency and the room dimensions. The maximum pressure locations occur at the sound source, when the wave makes contact with boundaries and periodically in between depending on the wavelength tennis ball bass trap.jpg(frequency). When the pressure component is at its maximum, the velocity component is at its minimum. Think about hitting a tennis ball against a wall. When the ball is at contact with the surface, its velocity is easily seen as zero and the pressure is easily seen as greatest…think how the ball flattens. If we further take the thought of the tennis ball and our goal is to stop the ball as quickly as possible. We would have our best chance to stop the ball in flight (when the velocity is greatest), but it might take some distance to come to a full stop depending on the speed of the ball. If we designed some means to stop the ball at the boundary when the pressure is greatest, the distance it takes to stop would be near zero…but you would have to do some fancy engineering to accomplish this feat. Pressure and velocity type bass traps work similar. Velocity focused traps are easier to configure and work generally better across a wider range of frequencies and applications. The downside, it can take several cubic feet of space to “trap” lowest frequencies. Pressure focused traps can give tremendous results but are more complicated systems and may only be maximally effective at specific frequencies.

Types of Traps

Kevin Barnes Theater trapping design rear vs 2.jpgPassive Bass Traps are porous absorbers. Materials such as fiberglass insulation (various densities), mineral fiber/rock wool and acoustical foam can all be effective bass traps. The specific material properties, density and thickness will determine its effectiveness in any specific location. Space=Results. You want to target the corner areas, especially where walls meet ceiling or floor. If you can sacrifice 12-17” of space out from a corner junction, you can effectively trap down to 80 Hz using several material options. If you will endear yourself to this concept and annex 2’-3’ out from the corners, you can trap down to 45 Hz depending on your material. Whenever possible, try to include such trapping into an architectural feature. Build a soffit with some down lighting and trap the upper rear wall of your theater space. Cover the vertical face with an acoustically transparent fabric. As with anything else there are details involved to maximize results. The point is that passive trapping is a reliable and proven way to control low frequencies. When used properly and creatively they are some of THE BEST investments possible in a theater.

HelmHoltz Trap 63 Hz.jpgHelmholtz traps squeeze sound through an opening(s) into a volume of space. These can be very effective at a specific frequency and can be done in surprisingly low profile assemblies. Blowing into a glass soda bottle is a Helmholtz system. The pitch you hear is the center frequency resonating back. For a couple thousand years this process has been used in arena designs under seating areas on grand scales. I have used this type of trap very effectively in rear corner areas as well as between the L & R speakers to maximize soundstage. Also, your riser itself can be a VERY effective Helmholtz bass trap. Helmholtz traps are best used in conjunction with passive traps. The downsides are if the wrong system is located in the wrong location or if is executed poorly, Barnes 71 Hz Panel Trap with millwork and acoustical graphic panel on face.jpgit won’t do anything at all and you can in fact induce resonance into your listening oasis…not good.

Diaphragmatic Traps rely on a sealed volume of air that is faced with a rigid material that vibrates. Plywood and drywall are common face materials for diaphragmatic. The center frequency is determined by the material, its thickness and the airspace. The “Q” (or bandwidth of affected frequencies) can and should be widened to address more frequencies by filling the space with a porous absorber (see above), but you trade maximum efficiency at center frequency. The diaphragmatic (sometimes called panel trap) has similar hazards as the Helmholtz. These are essentially simple yet extremely precise systems…execution is everything.

Membranes Traps are similar to diaphragmatic traps but the face is not rigid, it has a membrane quality. Mass-loaded Vinyl is typically used in this application, however even lower density & thinner plastics are used in some membrane traps. Again, the right system has to be located where the modal pressure of that frequency exists.bag end e trap.gif

Active/Electronic traps: I only know of one, the Bag End E-Trap System. This uses a dedicated subwoofer & electronics to pinpoint two desired modal resonances. This product actually comes with software to aid in the process. The system then emits a counter wave to diminish these resonances…really does work and takes a lot less space than many of the above. Oh, and this is not an additional subwoofer for your system, it is just addressing modal resonance…actually pretty cool!

Location, Location, Location

Possibly the only area of life that is more dependent on location than real estate is bass trapping. Let’s boil this down and make it easy as 1, 2 & 3. (1) All modes are active in Trihedral corners where two walls meet the floor or ceiling, (2) many modes are active in Dihedral corners where two surfaces meet and (3) the center of a wall that has 90 degree corners will have a pressure peak. As you add open passages, “L” extensions, vaulted roof profiles and other common floor plan situations the locations to attack with trapping get less predictable.

Modes…why do these acoustics people always talk about modes?

In the simplest terms, the dimensions of your space will react with specific frequencies (wavelengths) to create peaks & dips. Your structure will affect how dramatic these peaks & dips are. Place yourself in a peak and your head may ring like a bell, sit in a null and you have an anemic “blah” of an experience. The goal is to get these peaks & dips as even as possible BEFORE Digital Room Correction using appropriate bass trapping.

OK then “I’m starting to get this”. “What and from whom?” I will highlight several manufacturers and better performing options

Auralex Acoustics offers several options in the passive trap category. The VENUS bass traps and the MegaLENRD bass traps are effective performers (below 60 Hz).

GIK Acoustics offers several passive traps. The Monster Trap, Tri-Trap and Pillar Trap all are VERY useful products for RPG Modex Plate 35 Hz traps.jpgserious modal issues (below 60 Hz).

Primacoustics offers both passive and membrane type traps, some down to 60 Hz.

RPG Inc offers some of the most technologically engineered traps including membrane Modex traps, and the very sophisticated Modex Plate traps. Wonderful tools that can perform as low as 35 Hz.

RealTraps specializes in various membrane traps. Their testing data is derived in a different manner than the others, but are worth close consideration.

Did you say DIY?:

Fiberglass comes in several densities. Typical pink can be used but more typically the 3-4# and 6-7# versions are used for acoustical control. In general, the denser the better when tackling the lowest frequencies. But, location and mass can override density at times.

Rockwool & Mineral Wool are another common material usually used in higher density 4# or 8# versions. Both acoustical fiberglass and mineral wool can be tough to source…they exist, but all said and done you may be better off letting someone else fabricate.

The internet famous Superchunk method of spanning 45 degree across a corner floor to ceiling with 3# density rigid fiberglass (703) is proven!

Passive Trap Soffit with UltraTouch.jpgUltraTouch (see www.BondedLogic.com) is a personal favorite material of mine to use…very effective, versatile and green. It’s common application is an alternate house insulation, but the acoustical benefits are tremendous.

There are books and spreadsheets available to design Helmholtz & Diaphragmatic/Membrane traps…I don’t suggest the weekend offender tackle these…stick to Passive types.

What can I expect from proper bass trapping?

1) You can expect to reduce the amplitude of modal peaks up to 10 dB. 6dB would be wonderful for tough “critters”.

2) You can expect a noticeably more even bass response throughout the room by even non “Audioholic” listeners.

3) You will hear the harmonic series of your content be more coherent, become more musical and you will hear improved mid and hi frequency voicing from your system. Yes…improved Lo Frequency Response yields better sound all around.

4) If the bass is right then everything else is tight!

The above information may not allow you to unleash your own plan for optimal bass trapping, but it may point you toward that result. Proper bass trapping is a 100% guaranteed investment.

Hedback Designed Acoustics is a niche-based acoustical design firm focused on Pro Studios, Performance Driven Residential A/V & Small-Format Commercial Projects. Every space has a unique story to tell where “Imagination intersects Physics”, “Innovation yields Performance” & “Aesthetics create Accuracy”.

Phone: 317-863-0753

Website: www.HedbackDesignedAcoustics.com


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Recent Forum Posts:

Jeff Hedback posts on June 01, 2009 13:15
That's adventurous (placing mineral fiber under sofa).

interesting sub placement research by Floyd Toole confirms that a single corner loaded sub verses two front corner subs results in variances + 35 dB, like you're experiencing.

The best 2-sub location is 50% back both sidewalls (180 degrees out of phase). This would, in theory minimize your primary axial mode issue the best.
Doubt that's even possible with sofa, etc…

You do have a tough situation Dave. Sorry no easy answers.

Hopefully the SMS-1 can come back into the equation successfully.
Davemcc posts on June 01, 2009 12:43
Thanks Jeff. The front right location actually measured worse than the front left corner. The peak was several dB higher. You can see in one pic that I tried to run two subs, a Hsu STF-3 with my PB12-Plus. Even this did little to tame that peak.

I wish I could put some traps in the back left corner but that is the front door of the house. When the front door opens, there is literally only the width of the door frame between the front door and the side wall. That doesn't leave any room for the thickness of traps required to tame the frequencies involved. I have been considering opening up the bottom of my sofa and stuffing 6“ of mineral wool into the sofa frame, essentially giving me a horizontal trap about 6' x 2' x 6” thick.

Virtually all of my active eq attempts so far have been focussed on creating the maximim cut at 40Hz and leaving the rest pretty much alone, with minimal or no boost. It looks like my best bet is to spend the time and figure out why my SMS-1 isn't giving me the results I've come to expect. Tuning bass is time consuming and expensive when you start with a room that sucks this bad.
Jeff Hedback posts on June 01, 2009 09:36
There is a similar answer to both Davemcc and captiankirk28: symmetry.

captainkirk28: yes a single bass trap will help, but if at all possible try to implement bass traps symmmetrically.

Dave…I've reviewed the links and I can identify the nearly unbudgeable 40Hz (actually 38 Hz) issue. Now, in reality your open spaces on the right side of the room could be helpful. If at all possible can you (or have you) located sub in front right corner? Your current location of front left sub is making the worst of the given situation.

As far as trapping, the rear left corner should have as aggressive as posisble trapping across that corner (like the mineral fiber traps you previously described).

The main pressure points of your 38Hz issue are the halfway points of the sidewalls and as one is glass you're not going to be too successful trapping there (actually some of that 38Hz energy is going through the glass thus weakening the mode).

Options: I've been able to identify that the only seating location that improves your concern substantially is the halfway point front to back..this does worsen the 75-80 Hz zone (but your trapping is more effective in that region)…OK, nice analysis but not reasonable.

Options to consider:
- rear wall/ceiling soffit trap…would have to be 3-4' but could certainly be designed/built to be acoustically effective and architecturally interesting (aesthetically as well).
- commercial traps: the RPG Modex Plate does work. The 35 Hz version exists…pricey though, likely $2K-ish retail/ea.
- the Bag End E trap is a very good option…again, has a cost/benefit equation involved.

My guess is that your room correction had to be so aggressive to address this peak that you lost headroom and overall dynamics of system. If you can manually go in a reduce peak but not allow typical auto-resolve process which raised dips…you might get a more natural response.

In summary…most curious if moving sub to front right corner helps.
captiankirk28 posts on May 31, 2009 21:17
Great read, i have to get some of those bass traps but the wife is the problem as they dont look good enough.

Would it help to have a corner type in the same corner as my sub? I could put one back their and never see it.
Davemcc posts on May 31, 2009 18:00
Jeff Hedback, post: 575034
As you likely know already, a high quality digital parametric might be able to slice that specific issue down…maybe better than trapping.

I also have an SMS-1. I don't think it's working properly and I've taken in out of the system. I had some very good initial results with the SMS-1 but it took some very significant eq cuts along with the passive traps to achieve nice flat bass response. But that's a whole other time consuming process to diagnose what the SMS-1 is or isn't doing and I really haven't had the time. I've been thinking lately that it may be easier just to fix the room.

The room is 11' W x 14' 8“ L x 7' 11-1/2” H. The system arranged along one of the 11' walls. The two traps are at the front corners of the 11' wall. The sub is corner loaded at the front left because there is simply nowhere else to put it in this small room that is also our main living room. Some pics here.

Many more pics here. It shows the system in a different configuration, but the also shows the room better, doorways, closet, windows, etc.


Graphs of the room showing it's native response.

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