Bass Trapping Ideas for Non-Ideal Spaces
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
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 
(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
Passive
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
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, 
it
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.
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 
serious 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!
UltraTouch (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
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