An
Introductory Guide to Subwoofers
Understanding
Acoustic
Traps/Room Treatments
This page
is under construction
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NOTE, neither acoustic traps or
equalization alone are a cure for room related issues. When possible a combination of traps
and equalization will provide optional performance. The usual approach is to
start with acoustical treatments, then use EQ to fine-tune everything.
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After a room has been constructed one is limited in the methods
that can be used for treatment of acoustic related issues. After the fact
treatments fit into three general categories: Absorbers, Resonators, and
Diffusors.
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Diffusors unless they are extremely deep are impractical for
controlling low frequency sound waves.
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Absorbers
work by changing the pressure from sound waves into heat energy. The traps contain
insulation (usually fiberglass). The sound waves vibrate the fiberglass fibers
which dissipate the energy as heat.
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Absorbers can be cylindrical
or flat panels
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One of the more commonly available cylindrical traps is made by A.S.C.
Frequently this type of design has 'front' and 'back' side. The 'front' has a plastic membrane that reflects high frequencies, the 'back' has no plastic so high frequencies are absorbed. One places the trap and rotates it according to the needs of the installation.
A DIY version of this type of trap (picture below) was first designed and build by JT Cale using preformed compressed fiberglass heating/cooling duct insulation. Here's a LINK to his website.
Here's a LINK to a website by Chris Brady that offers a more comprehensive description of the build process. The link on Chris' site to 'trap placement and theory' is broken, I found THIS version on the Wayback Machine, so it's slow to load.
Doug Ploss has a nice website detailing the construction of half round traps here's a LINK. The traps in his picture below are movable and mounted on a ledger strip
And I would be remiss were I not to
mention Jon Risch's original DIY
cylindrical traps
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Standard Fiberglass, Flat Panel Traps
There are simply layers of fiberglass surrounded by a wooden frame, and covered with a layer of fabric. These are built using either standard insulating batts, or now the currently popular are panel traps using Owens Corning type 703 compressed fiberglass panels.
Chris Tsutsui's website does a good job of detailing the construction and performance of the standard trap built with raw fiberglass batts.
Pat's website also has photos and measurements of these types of traps.
Finally Dennis J has detailed photos of the initial stages of construction
Here are Steven
Helm's flat panel traps using a unique approach to the construction
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To increase the bass absorbing properties of flat panel traps they can be stand mounted and spaced away from the wall.
An example (above) of a trap using
both loose batting and OC-703 on a movable platform that can be off-set from the
wall can be found at this LINK
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NOTE: using too many of this
type of trap can lead to a room that's acoustically quite 'dead'. This is to be
avoided with 2 channel audio. With multi-channel audio it can be beneficial as
it decreases the interaction between the discrete channels.
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Resonant Membrane, Flat Panel Traps
These are similar to the standard fiberglass trap, except they have a semi-rigid membrane (usually thin plywood or something similar) spaced out away from the insulation. Some installations also install the entire trap spaced out from the wall. The thickness of the membrane, the air-gap spacing, and the depth of the insulation determine the frequency of the trap.
Ethan Winer of RealTraps.com has a DIY webpage for these types of traps. Here's another illustration of the design of this type of trap from Ethan's acoustics website.
Ethan has some rough plans for these types of traps at this LINK
And although dated Ethan has a
breakdown of the parts and costs (1995) for building the traps shown above at
this LINK. There's an
updated price list (2001) and one builder's tips at this LINK
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This is a description of another type of resonant membrane flat panel absorber. Unfortunately there are no illustrations
This is from Scott Foster's Acoustics
Forum: One type of narrow band absorber is the diaphragmatic absorber.
The diaphragm absorbs the frequencies which match its resonances. They
work by vibrating at these frequencies and turning the sound energy into heat.
The drywall in wall construction acts as a resonant panel and absorbs a
considerable amount of sound at 125 Hz. Simple panel absorbers can be built by
mounting a sheet of plywood in a frame. Just build a frame, and fill it
mineral wool, or 703, but leave a small gap between the panel and the insulation
so that the panel is free to resonate. Panel resonation can be enhanced by
reducing the point of connection between the panel and the frame by means of
narrow spacer material such as a loop of wire or welding rod run along the edge
of the frame so that the panel is perched on a thin edge. Approximate Plywood
Panel absorption peaks on a 1x4 frame 3.5 " deep are:
1/8" = 150 Hz
1/4" = 110 Hz
3/8" = 87 Hz
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Pure resonators (Helmholtz) are a device tuned to vibrate at a
given frequency. That vibration cancels specific bass frequencies in the space
around the device. The most
common resonator is the Helmholtz resonator. There are several shapes that will
function as a resonator. Like a ported subwoofer, the enclosure volume combined
with the size of the opening provides the tuning frequency. Below is a
cylindrical Helmholtz resonator made from a section of plastic pipe and tuned
with a port. This particular example is from an article on the Audioholics
website.
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Using timber slat construction, large panels, or entire
sections of walls, or ceilings can be use as Helmholtz resonators.
A website that details this kind of
design and has a spreadsheet for their design is available at this LINK.
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Here are links to some unique websites with
information about bass traps and bass trap design
http://www.bobgolds.com/AbsorptionCoefficients.htm
http://www.e-easley.com/theater/tools/fpor/
http://www.auralex.com/literature/acoustics101v3_0.pdf
http://members.shaw.ca/danhanson/Theater/tsvsaat.html
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Absorption shouldn't be the only room treatment considered. If only absorption is used the room can end up being acoustically 'dead'. To get a balanced spectrum one can incorporate diffusers and absorbers. There are several types of diffusers and all are fairly easy to construct.
Skyline diffusers are one of the easier designs to make, the primary drawback is their weight.
http://www.mhsoft.nl/DiffusorCalculator.html
http://homerecording.com/bbs/archive/index.php/f-20-p-2.html
http://www.acousticsciences.com/articles/iar89.htm
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