Many audiophiles don't know it, but for the price of one not so fancy
interconnect, or about $150 dollars, you can acoustically treat your
room with excellent absorbers, including 4 wall panels AND a set of
4 bass traps.  A full blown acoustical treatment with floor to ceiling
bass traps in front, and 6-8 wall panels, along with some basic
diffusion for the rear wall will run around $470, with a significant
chunk of that in the retail bargain diffusors.

Equivalent cost of the minimal retail options exceed $1,400, and
may not work as well.  A full blown treatment using retail options may
reach $2,700 or more, and will not work any better.  Read on for
details.

Sonex vs. Fiberglass
An equivalent thickness of fiberglass (building
grade) will perform similarly to Sonex, but with slightly better low
frequency absorption.  Since fiberglass comes in thicker versions,
such as 6 1/2" and 8" thick, you can get correspondingly better LF
absorption than 4" of Sonex.  So far, so good.

But when I tried to find a suitable covering, then I ran into problems!
It seems most all fabrics are woven way too tight to allow the sound to
readily pass through to the fiberglass!  Even some velour that was being
sold as "sound absorbent covering" had a large amount of reflectance
at high and middle frequencies.  It is not just a matter of allowing the
sound waves through the cloth and into the fiberglass, but how much
of the sound reflects off the surface of the cloth, and back into the room.
At middle to high frequencies, many cloths have a fair amount of
reflectance, even some speaker grille cloth's..

I eventually found a solution and recipe for do-it-yourself sound
absorbing panels/coverings:

A rock wool panel (these run about 1" thick) for the very back, spaced
off the wall/ceiling as much as 6-8 inches (or as much spacing as can
be spared, the more the better the LF absorption), then a thickness of
fiberglass of any brand (the thicker, the better, use 8" thickness
if budget and space permits), then a layer of fairly loose polyester
batting (available at fabric stores and Wal-Marts for use in blankets)
to provide a protective layer that doesn't adversely affect the
absorption of the fiberglass (the entire absorptive layer COULD consist
of this material, but it would be cost prohibitive, on the other hand
no itch!), and then a layer of burlap as an outer covering.  Colored
burlap is available in 36" widths from fabric stores and most
Wal-Mart's.  Light colors work well with the layer of white poly
batting just underneath the burlap, although the darker colors are OK.

In actual anechoic testing, I found the burlap to have the least
reflectance and allow the most absorption by the fiberglass.

Practically speaking, it works best if a simple, minimal wooden frame
is constructed from 1X2's or 1X4's (depending on the thickness of
fiberglass), with the rock wool panels glued/stapled to the rear of
the wooden frame, with vertical frame members spaced about
14" apart (the width of the fiberglass) and the fiberglass WITH
PAPER BACKING stapled at the edges to the middle side edges of the
vertical frame members (I usually ran a 10" strip of poly batting down
the middle of the strip {roll} of fiberglass between the paper and the
rock wool panel, in order to prevent possible flapping paper noises
against the rock wool).  Then staple the poly batting over the
fiberglass, stapling to the vertical frame members (hence the need to
adjust the wood depth dimension to the fiberglass), and finally, using
brass tacks every 6" to affix the colored burlap along the vertical frame
members, and across the top and bottom.  These can be on the back
along the top and bottom, so brass is not necessary, and cheaper
tacks, nails or staples could be used there.

If you want to be sure the wooden frame does not reflect too
badly, wrap the bonded fluffy polyester batting around and along the
side of the wooden frame, and cover this all with burlap too.  Some
adjustment of spacing between frames may be needed at the two
sidemost sections to accommodate this, depending on how you have
laid out the burlap and polyester.

When this is all finished, you have a nice looking sound absorbing
panel with scallops every 14" or so that run vertically, that can be
semi-permanently located, but can leave with you as needed.  Portable
versions with sturdier frames can be built of course, and put on
rollers or wheels for adjustable studio or listening room use.

DO NOT double up on the burlap layers, as performance suffers!  Also,
remember to treat the burlap with fire retardant, wear gloves and at
least a cheap respirator mask when handling/cutting the fiberglass,
and to lightly vacuum the outer burlap surface to get up any loose
fiberglass particles.  Once covered in burlap with the poly batting
layer to prevent fiber or particle escape/contact, these panels provide
excellent performance at minimal cost and no health hazard.

I have been asked about the need for the rock wool/series 700 panels
on the rear of the fiberglass and frame.  Are they absolutely necessary?
No, but a typical absorbing panel, using 6 1/2" fiberglass AND the
rigid panel glued to the rear will be effective down to below 100 Hz.
Using only the fiberglass will result in a panel that is good down to
around 200 Hz.  Wall spacing is important too, if the panels are placed
flush against the wall, then effectiveness in the LF can be halved
compared to even just 4" of spacing.

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.
Due to the cost associated with these smaller panels, I now
recommend straight 8" fiberglass for maximum effectiveness at LF,
without the rigid panels glued to the back of the frame.

If you use at least 6" thicjness of fiberglass, then the rock wool/OC
panels are not necessary to obtain adequate low frequency
absoprtion.

+++++++++++++++++++++++++++++++++++++++++++
Rough Costs:
Fiberglass (faced) - 6" thick, 15" by 39 feet at $12.50 roll

Burlap - Dyed, with minimum levels of insect and fire protection,
available from Wal-Mart, $1.48 a yard by 36" wide.

1" X 2" white spruce 8 foot long, 92 cents each

1" X 4" white spruce 8 foot long, 98 cents each

Polyester batting, bonded fluffy loose pile, approx. 1/2 to 3/4
thickness, 45" wide, $2.17 yard

Brass plated nails, approx. 50 for 86 cents

#9 Brass Upholstery Nails, 30 for 81 cents

Tube Liquid Nails 29 OZ, $4

Tube RTV Silicone Rubber, 10 OZ, $4.50

Staples: $2 for 1,000, 9/16"

OPTIONAL: 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 for a 4 foot tall by 30" panel:  $12.50 each, minimum
investment of materials for 4 panels, for $50 total.
4' X 45" panel: $17 each, minimum investment of 4 for $68.
Some materials left over.

ASC Wall Panels are $398 for eight 2" thick by 8" wide by 48" tall
panels, good only down to about 500 Hz.
 

SONEX part 2

RE the FIre marshall:
> >cover it with fabric, nicely down, and tell the fire marshall its a wall
> >hanging (a work of art, like a painting- also not fire resistant) and not
> >an acoustical treatment. oh ya, and good luck.
>
> ....a fabric known as FR-701 panel fabric from
> Guilford of Maine.  It is fire-rated, and acoustically transparent
> (for all practical purposes).

My experience has been that many fabrics that seem acoustically
transparent may not be, and may cause significant reflections as
the sound tries to enter the absorbing material.  If ASC actually
uses the stuff, it might be OK, but remember, many of their products
are not designed for 100% absorption, but a controlled amount of
reflectance.

> Could anyone comment on when it would be appropriate to use the other
> versions of (Owens Corning) 700 series panels, such as the 705 which
> is 6 lbs. cu.ft. or the 1.58 lbs. cu.ft. for the 701.  I gather that
> the bass absorption is slightly better the denser the panel.  But how
> significant a difference between the versions?

The higher the density, the better the bass absorption, however, the
reflectance goes up proportionately in the mid and high frequencies as
the density goes up, with the lightest panels being the best in the
highs, and the heaviest best in the lows.  For use behind the wall panel
absorbers, or as simplified and minimal bass absorbers as outlined
below, I recommend at least 705 or 706. When used as the only absorber
material, I would use 703 or 704.

> I would appreciate suggestions on how to best mount the 2x4' panels to
> the wall?  Isn't absorption quality best when the panel is tight to
> the wall.  Are there any good non-permanent solutions?

Maximum bass absorption does not occur when almost any absorber is flush
against the wall, but is spaced out from the wall by some minimum
amount, with better absorption as the spacing increases to some
relatively large distance.  Many acoustic materials are specified for a
minimum spacing of 2" or 4" in order to meet specifications.

> Finally, does anyone know where to get specific plans for designing
> and building bass traps?

Both "The Handbook for Sound Engineers", 2nd Ed. Glenn M. Ballou and
'The Master Handbook of Acoustics", 3rd Ed. by F. Alton Everest
have some info on construction details of bass traps.  However, the use
of classic bass traps, also known as Helmholtz resonators, has pretty
much been superseded by broadband resistive bass control ala ASC type
tubetraps.  Homemade resistive bass traps are not that difficult to make,
and can be made almost any reasonable size.

> Thanx for the advice, I just might try to make my own based on the cost of
> new ASC stuff.  I am a little unclear on how you assemble the first panel
> you described.  Let's see if I got it, build a frame. . .no problem, then
> use the rock wool on the rear of the panel. . . no prob. . .but I'm not
> sure how you affixed the paper of the fiberglass.

It gets stapled to the inner rear edges of the frame partitions spaced
roughly 14" apart.  When completed, your acoustic sandwich is now, from
back to front:
Rock wool panel, polyester batting, paper of fiberglass roll, fiberglass,
polyester batting, burlap.  From side to side: Frame edge, wrapped with
tacked down burlap, stapled fiberglass paper, previously described
acoustic sandwich, stapled fiberglass paper, tacked down burlap over
frame divider at frame spacing (approx. 14" centers), stapled fiberglass
paper, acoustic sandwich, stapled fiberglass paper, AND either another
frame divider or the other outer edge of the whole frame.  This should
clear up the construction details for you.

Not DIY:
RPG has come out with a new inexpensive product called ProFoam.
At $50 for a basic set of 6 two foot by two foot contoured foam sheets,
they offer those who would rather not DIY an option.  Be warned that they
are not even close in LF absorption to the DIY panels, as the DIY panels
are good down to below 100 Hz when constructed according to recommended
thickness and materials, while the ProFoam is only absorbing the midrange
on up, from about 500 to 700 Hz on up.  This can skew the tonal balance of
a room severely, which is usually the problem when a sound absorbing
material that is not thick enough is used, or it is used at the wrong
locations.  For $100, you get twelve 2'X 2' squares, enough to place them
doubled up back to back to increase the apparent thickness, and provide
some automatic spacing from the walls, which increase the LF absorption
down to about 350 Hz.  Note that this is still only three patches of absorption
2'X 4' to the side of each speaker and on the back wall between the
speakers.  If you just can't or won't build them your self, then at least
double up on some of the RPG ProFoam's, Level Two for $100.
These are plain gray in color, white and colors cost over twice the price.
Sold by Audio Advisor, 1-800-942-0220

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.

Cont'd

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