- · 大为兄,看上去你可读英语吧?(作者:Jim Wang 时间: 点击数: 90)
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TUBE BASS TRAPS
Bass traps are usually either of the resistive absorption type
(ASC tube traps), or of the Helmholtz resonator type. The bass
traps described in earlier posts by others do not correspond to either
type, and based on the construction described, might be considered
more correctly high frequency absorbers and low frequency diffusers
(the round shape).
While they would provide some benefit, even at low frequencies, they
would be much more effective if they were constructed to fully absorb
low frequencies as much as possible.
Since Helmholtz resonators are very much room and frequency specific,
the more general purpose and portable approach would be to roll your
own homemade tube traps. These can be built in several different ways.
Method #1: Duplicate the ASC tube traps (1st generation). This can be
done by doing the following: Obtain some rolls of "hog fence" wire,
preferably not rusted. This is the rolls of animal fencing that run
about 3 feet wide (it utrns out that 4 feet and five feet tall are also
pretty common), and have rectangular openings between galvanized
steel wires, with the openings about 3/4" wide by 3 or 4" tall.
Chicken wire, the hexagonal looking stuff, is not stiff enough.
Decide what size trap you wish to make: you can duplicate the commercial
sizes for the approx. same cutoff frequencies, or make an extra-large
version to really soak up those low, lows. Realize that sizes over
approx. 16 to 18" get unwieldy and more difficult to assemble, and a
16" diameter by 3 foot trap will soak up a lot of bass if properly
constructed, to below 80 Hz.
Once you have a size, cut out 3 circles of wood from 3/4 plywood or
particle board approx. 3 1/2" to 4" smaller than that diameter, and
2 circles of wood that exact diameter. Glue a smaller circle one each to
the center of the two larger circles, and keep the 3rd smaller circle for later.
Take the relatively stiff "hog wire", cut 1 1/4" from one end, and bend
it around the two smaller circles of wood, with the large wooden circles
uncovered by wire on the outside, and free at either end, while including
the third smaller circle of wood near the middle of the cylinder of wire.
The third circle will provide support for the middle of the roll, and
allow the fiberglass to be compressed properly without buckling the wire.
If making a 4 or 5 foot tall cylinder, then use two internal spacers.
These internal support discs need to have holes in them, so as to allow
pressure to equalize from one cavity to the other. For a 16" diameter
trap, and a nominal support disc diameter of 12.25", two 2.5" holes
or one 3.5" hole are the minimum. If you cut out the center so as to leave
3" of wood around the edge of the disc, this would be approx. a 6" hole.
Use staples and/or liquid nails to affix the wire grid to the edges of
the smaller wooden circles. Now take 6 1/2" thick unfaced fiberglass,
and loosely wrap it around the wire grid, with none hanging past the
ends of the caps. The fiberglass should be laid in so that none is
overlapped, and no gaps are showing. Use adhesive or glue to help seal
any gaps, keeping the adhesive away from the surface, and more toward
the inside. IN ORDER FOR THE TRAP TO WORK PROPERLY, THE
FIBERGLASS _MUST_ ALLOW NO DIRECT AIR LEAKS! In other
words, no holes, gaps or seams should be present. Butt the fiberglass
up against itself firmly, and make sure it covers the inner circle of wood.
Now for the hard part: take another layer of 'hog wire" and wrap/staple
glue it around the edge of the larger end circles compressing the
fiberglass uniformly between the two wire grid layers. It is this
compression of the fiberglass that makes the closed end cylindrical
trap so effective. Too much compression, and it begins to reflect the
low frequencies partially, too little compression, and it does not fully
absorb the bass frequencies. If 8" thick fiberglass is compressed to 2",
it starts to be too much compression, and is very difficult to assemble.
The outer layer of hog wire can now be covered with polyester batting,
stretched around the outside, without any gaps or seams. This blunts the
raw edges of the hog wire, and seals the fiberglass fibers inside.
Now, if you wish to duplicate the ASC devices, get some very limp
plastic film (like the plastic bags dry cleaners use for covering the
clothes). Plastic drop cloths for painting are usually thin and limp
enough. Place the film on one half side of the now cylindrical
assembly, then cover with burlap. If you orient the burlap seam with
respect to the partially reflective side (the side with the plastic film
on it), you will always know which side is the partially reflective one.
Hot melt can be used to seal the burlap seam, or some sort of
sewing or stitching used.
Burlap can be made fire retardant either by using a commercial
fire-retardant chemical treatment, or for DIY, a 1/2 saturated solution
of borax or calcium chloride will provide good fire retardancy. Note
that dark colored burlap will probably show white streaks from these
treatments.
For those who are using burlap and wish to fireproof it there is a
company called Flameseal (http://www.flameseal.com) in Texas
which makes a spray-on product to fireproof fabrics, its approx.
$30.00 per gallon.
The end result is a 3 foot tall XX" diameter cylinder. The end caps
can be finished in Formica, etc., and for height, stack two up.
Additional comments and instructions are available at:
http://www.AudioAsylum.com/audio/tweaks/messages/17498.html
on building the full-blown wire fencing versions.
2. Quick and dirty homemade tube traps
Obtain some sheets of 1" thick rigid high-density fiberglass panels, or
better yet, rock wool panels. If the panels are only medium density,
double up on them and size accordingly for the inner/outer panels.
Determine a size based on an equilateral triangle (side panels of 24"
or smaller recommended, make them sub-multiples of the sheet width,
e.g.,24",16",12")and once size is determined, cut the panels to size
(these can be 8 feet tall if the panels come 8' by 4', but a full 8 feet
will probably be too unwieldy). The shape can be a right triangle so
as to fit in a corner better, size the panels accordingly. For a full
use of a 48" wide panel, 14' on the sides, and 20" across the hypotenuse.
Glue (silicone rubber or liquid nails) these together at the edges
forming an equilateral triangle or a right triangle, and cut 3/4"
wooden end caps in the appropriate size/shape to cap the ends and glue
the triangle of panels to the end caps. To take some of the "edge" off
of the acute angles, wrap a single layer of 3/4" minimum loosely spun
bonded polyester batting around the fiberglass panels (this also aids HF
absorption), and then wrap with fire-retardant treated burlap. If you
want some partial reflectance/diffusion, wrap a limp plastic sheet
around one corner of the triangle and half way across the two sides.
Either one of these will easily outperform the previously posted cardboard
tubes filled with fiberglass and covered with carpet, and at a lower cost,
providing a broadband absorption over a wide and extended LF range.
Recent checks on 700 series pricing and availability has lead to the
discovery that, to the consumer, the only size that is readily available
is a 2' by 4' panel, in 1", 2" or 3" thickness's. In some area's of the
country, rock wool panels are still available, but not everywhere.
For quick and dirty DIY bass traps, the 1" thick 705 panels will work, and
a carton of 12 panels will make 4 triangular bass traps four feet high,
roughly equivalent to 12 to 14" diameter round bass traps made using
hog wire.
++++++++++++++++++++++++++++++++++++++++++++++++
Rough Costs, based on a 14" outer diameter trap:
Fiberglass, unfaced - 6 1/2" thick, 15" wide by 39 feet long approx. $13.50
(Enough for approx. 4 traps, 2 and a half once around strips stacked up)
Hog Wire, 14 gauge - 36" by 50 feet, $17 Enough for approx. 6-7 traps
Particle Board - 4'X 4' sheet 3/4" $7 (enough for approx. three 14" traps),
4'X 8' $13 enough for 6 traps
Polyester batting, bonded fluffy loose density approx. 3/4 to 1" thick,
45" wide, $2.17 a yard.
Burlap - Dyed, with minimum levels of insect and fire protection,
available from Wal-Mart, $1.48 a yard by 36" wide.
Tube Liquid Nails 29 OZ, $4
Tube RTV Silicone Rubber, 10 OZ, $4.50
Staples, 9/16", $2 for 1,000
One small can brown or black paint, $5
Alternative Approach: Owens Corning 705 semi-rigid fiberglass panels,
2' by 4', by 1", 12 to a carton for approx. $92, not including S&H.
Rough Total per trap, minimum of 4 14" diameter by 36" high traps,
some materials left over: $19 each.
Minimum investment approx. $77 for four, with materials left over.
Costs to do 6 or 8 is slightly less per, due to better materials usage.
ASC Prices: One 11" diameter by 4 foot tall tube trap: $248 retail, four
tube traps: $992
Latest Super Quick & Dirty Recipe
The triangular versions I outline were just a way to do a quick and dirty version for the less labor
inclined and/or impatient amoung us. They are not quite as effective as the round animal fence
versions, which duplicate the effects of the retail bass traps quite well.
The middle support disc also helps reduce any tendency for an internal resonance developing, and
while not strictly necessary for the triangular versions (the panel flexing of the OC panels was a part
of their design), it wouldn't really help that much.
If one wanted to use the triangular version, then if an internal divider was tacked into place leaving
air gaps along the sides, then they would break-up any internal waves, and not keep the panels
from flexing. A section of the fiberglass board would do nicely.
New Quick & Dirty Recipe
Thanks to my less motivated friends, I have a new recipe:
buy three rolls of 15" wide fiberglass (virtually any thickness, as the rolls are longer for the thinner
stuff, and the total diameter ends up about the same, it can be faced or unfaced) insulation, stack
them on top of one another, leaving them in the plastic bags, and wrap burlap or grill cloth around
them. For better HF absorption, wrap polyester batting around one side or the entire sides. You can
use something like duct tape to hold the three bags of fiberglass together, and to tape the polyester
batting into place, etc. How involved you wish to get is up to you. They should be placed in the
corners, as for all bass traps.
This will work almost as well as the wire fence version in my recipe, at perhaps 70-80% of the bass
absortion, with no resonances, or problems. The cost is greater, at about $50 each, if you wrap them
in polyester batting, and cover in burlap. However, they can be in the corners in a matter of minutes
instead of hours/days, and for a lot less labor. If you only get enough for two in the front corners to start,
then you can later use these same rolls to make up wallpanels and the full blown bass traps using the
wire animal fencing cylinder versions. If you plan to do this, make sure you get the appropriate number
of faced and unfaced rolls, and mix them symmetrically on each side of the room.
Additional information on the Super Quick & Dirty versions are available at:
http://www.AudioAsylum.com/audio/general/messages/70817.html
-Fire-Proofing-
Burlap, or virtually any other cloth can be made fire-retardant by soaking
it in a commercial fire retardant sold for the purpose. For those who
are using burlap and wish to fireproof it there is a company called
Flameseal (http://www.flameseal.com) in Texas which makes a
spray-on product to fireproof fabrics, its approx. $30.00 per gallon.
If you are interested in a DIY treatment, there are two possible
alternatives: borax and calcium carbonate. Borax (get the pure stuff,
not with soap) is readily available, and calcium carbonate is the super
ice melting salt that comes as little white round balls of salt.
The only problem with these is that on dark colored fabrics, they will
leave white streaks/residue that is visible in the concentrations needed
to be effective. If light colors of burlap are involved, then it is
not nearly as much of a problem.
Or, you can just use the Guileford fabric, but be aware that it is VERY
expensive!
A NOTE ON ROCK WOOL (Mineral Fiber)
Apparently, rock wool is no longer readily available throughout the
country, as the primary manufacturer has quit making it. A near
equivalent would be an Owen Corning 706 or 705 panel, which
should work almost as well. Owen Corning 700 series panels are
now readily available to the consumer only in 2' by 4' size.
Cartons of 12 panels in the 1" size.
More Details on Taming Room Acoustics
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Q: I visualize them as similar to rather thick Roomtunes panels
that looks like Vandersteen speakers! Am I right or wrong?
A: Essentially. The looks can vary a little, depending on the type
of burlap, the colors, the tacks/nails/studs used to tack down
the burlap, etc. Other cloth than burlap can be used, but at a
possible (very likely) performance penalty. Speaker grille cloth
probably wouldn't be too bad, and would allow some choice in weave
and colors over the burlap.
Q: How tall should the panels ideally be? I understand that the
width is about 15" w/c corresponds to the width of the fiberglass.
A: For use as spot absorbers, the panels can run the height of the
wall, or as a minimum, only the middle 4 feet. I would recommend
a minimum of two widths, or about 30" wide to be as effective as
possible.
Q: Is cotton batting a good replacement for polyester?
A: No. Cotton batting is usually too dense, and will cause excessive
reflectance. It also tends to settle, lump up and in general,
become a pain to deal with.
Q: How many panels are needed to effectively treat the whole
room? Is it necessary to fill the entire backwall with them?
I certainly hope not!
A: It is not necessary to fill the room with them, or even the back
wall. Look at ads for Roomtunes, ASC tubetraps, etc. or contact
these companies and request their literature, being sure to ask
about actual use of the product in a room and placement details.
Q: How should these panels be placed? I know they should be
at least 6-8" away from the wall but where behind the speakers?
On the space between the speakers? On or near the corners?
A: The usual minimum game plan is to use a 30" wide by 48" tall by
4" deep or more panel directly behind each loudspeaker (all panels
spaced from 2" to 6" off the walls), then one each on the mirror
points on the side walls. The mirror point on the side walls is
the point where a mirror flat against the wall reflects the
speakers image to you at your normal seating location. Two
panels for the rear wall IN THE CORNERS if not using tube trap
type bass absorbers, angled at 45 degrees. These rear corner
panels would be most effective if they were floor to ceiling,
and they could be only 15" or so wide, instead of 30".
Next step up in treatment would involve a pair of panels on the
ceiling at the mirror points, possibly a center panel (normal
size) in the center of the rear wall. If the floor is hard
with no carpeting, then removable/portable panels on the floor
at the mirror points (spaced appropriately) would help a lot.
These can be smaller sizes too, about 15" wide by 2 feet for
better portability and ease of storage when done listening.
DIY resistive bass traps would be a third step, with at least
one 3 foot tall cylinder of at least 10-12" diameter in each
corner. Better bass smoothing and absorption can be had if they
are made 14-16" in diameter, and two are stacked in each corner,
or at least two stacked in the front.
Anything beyond this requires a true acoustic analysis of the
room as a whole, including all the surfaces, etc.
Q: Since I have small monitor type speakers that have no
deep bass to speak off (and no serious bass problems) that
means that I can get away with closer spacing of the panel
against the wall, and relatively thin panels say 3-4" thick
only for midrange/treble absorption especially at the front
wall (wall closest to listening position). Is this correct?
A: Yes. Use of less than full band wall absorbers will shift
the tonal balance though, and make the room/speakers sound warmer.
This may not be a bad thing in your case.
Q: How can a panel of this size be placed half way up the wall,
4 or so inches away from it. Special hardware? Long,
beefy bolts with spacers?
A: You can easily add legs to the rear of the frame, of
sufficient height to get them off the floor, some 1X1's or
1X2's, and then glue/nail some 1X2's with the desired length
for wall spacing to the back/side of the frame.
See ASCII diagram below.
Wall
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-About Diffusors-
RPG started it, SysDevGrp carried the concept to different
price points and Auralex made it even more affordable. Some
acoustics experts insist that a soundly designed listening
room must contain some scientifically designed sound
diffusors. QRD (Quadratic Residue Diffusors) diffusors were
introduced by RPG, then the Art Diffusors by SysDevGrp, and
then T'Fusor by Auralex.
Placing a diffusor array at either the speaker end, middle of
the wall, or the listening end, middle of the wall can
sometimes make a room bloom into a really great listening
environment.
For a retail solution, Auralex has the T-Fusor for about $220
(includes S&H) for four 2 foot by 2 foot panels that are 5 1/2"
deep. This will create a 4 foot by 4 foot diffusor area on a
wall, and would be considered a minimum amount of diffusion to
provide any real improvements. They are plastic molded sheets,
meant to have the rear filled in with fiberglass or polyester
batting to combat the hollow plastic sound that would
otherwise result from a thin sheet of plastic. Careful use
of some of the expanding foam sold for filling/insulating gaps
and cracks would probably be superior to these loose fillings
for stiffening and damping the plastic walls of the diffusor.
For comparison, a 4X4 foot square of QRD diffusor would run
over $700, and a 45X45 inch square of Art Diffusor, Series E
runs $500.
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(作者:Jim Wang 时间:2003-08-25 10:16:13)- Here we see the basic framework, consisting of the ends caps, the middle support, and the wire fence wrapped around the inner edge of the end caps and the middle support. Wire fence with closer spacings would be preffered to the wide spacings shown, but this kind can be made to work OK.
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(作者:大为 时间:2003-08-25 20:34:44)- Jim,
Thank you very much for your hard work. This is very helpful for me.Thanks again.- Jim,