Hi guys - Malcolm's page no longer works
but I did save his page.
This is his stuff on absorbers.
"
ACOUSTIC FLATS; WHAT, HOW, WHY, AND WHERE.
There is a saying among recording engineers that the setup is 80% of a
mixer's work, and the rest is politics. That's overdrawn, and we do
move a knob from time to time, but there's a lot of truth to the
perception that a mixer with a bad setup is dead meat, and one with
a good setup has got at least a fighting chance of doing good work.
A mixer's setup is generally tailored to his (or her) style, the shape
and size of the studio, and the kind of music being recorded, but all
setups have a few basic purposes in common. Specifically, the setup has
got to allow the musicians to play decently, the sound of one instrument
or section must not trash the sound of another, and it would be real
nice if the mixer could see the players. Visual cues are very helpful
when you don't know who's got the next solo.
All three of these criteria involve the use of acoustic flats. The
direct reason for using them is generally (not always) to improve
isolation between mikes, but their physical presence affects sight
lines between the mixer and the musicians as well as between the
players themselves, and to a lesser extent, the ability of one
player to hear another. Throw in the fact that they're the biggest
and most awkward tools in the studio and flats start to look like a
pretty big deal.
Flats, that is, or screens; not baffles. The only baffling thing
about acoustic flats is that despite their having been utilized in
studio setups since the 1930's, there is no apparent standard as to
how they should be made or employed. In fact, there is very little
published material concerning them one way or another.
This commucations gap has given rise to a widely held view that flats
are absorbent, and that their purpose is to isolate instruments by
blocking sound travel between one and another. Neither notion is
actually true, although the latter commonly appears to be the case.
Wrapping musicians in Fiberglas (TM) often works, but not the way
generally thought. It's a matter of introducing more absorption into
the studio than can be hung on the walls, and from that standpoint the
flats might as well be suspended from the ceiling.
Obviously, there's some tricky stuff involved here, and the writer has
spent an embarrassingly long time straining out a few facts from a barrel
of fiction with respect to these simple looking bits of studio furniture.
To begin at the beginning, flats come in two fundamental types,
reflective and absorbent, and are most useful for controlling the
local sound environment of instruments. Sort of an acoustical micro
climate for each instrument or section.
The optimum acoustical environment varies from one instrument to another,
and a number of them work best in live enclosures. Strings, for example,
French horns, real harps, and strange as it might seem, full-fuzz
overloaded guitar amps that drive the whole room into resonance and get
into every mike on the floor.
Reflective flats allow the mixer to work these and other instruments
or sections in a live area rather than a separate live room, which helps
no end with communications between players.
For strings and horns, the individual instruments blend over a much
shorter distance than otherwise, so you can get a section sound miked
at a few feet instead of a few yards, and the section will produce
far more sound than it would in a relatively dead studio. A harp is
typical of instruments that sound too raspy for close miking, and the
live enclosure smooths out the sound at a reasonable mike distance.
As to the guitar amp, a resonant sound can be achieved at much lower
volume in a small live enclosure than in the whole studio, and mostly
kept out of the main room. Eliminates the isolation problem altogether.
In passing, when confronted with working three or four people in an
enormous studio, a few live flats behind them will reduce the
perceived acoustical size of the room, and make them feel less lonesome.
Also works for overdubbing, which has justifiably been described as a
solitary vice.
While we're on the subject of live flats, they're made of 1/4 inch
tempered Masonite, (TM) usually set in 2x4 frames with four foot braced
cross piece legs to keep them upright. Castors are a must, because
these things are used to build small rooms, and they have to be
surprisingly large in order to work. Generally speaking they'll run
six to eight feet wide by a minimum of eight feet high. Twelve is better.
On the plus side, they're cheap, take up almost no storage room at the
back of the studio, and will multiply and smooth the sound of instruments
in a way that nothing else can. Recommended.
Absorbent flats are the opposite of reflectors in every way. They are low,
thick, and made as compact as possible. Their primary purpose is to present
a dead surface to cardioid mikes.
They can be low because most instruments project out and up at about 30
degrees, so most mikes are positioned above the instruments and angled
down at about 30 degrees. O.K., brass is an exception, but given a
chance, they'll play downward and bounce off the floor at the usual angle.
No help in recording, but the general case for "up at 30" holds pretty
well and a mike basically pointed at the floor behind a musician doesn't
need a 12 foot absorber behind him.
Getting back to first principles, musicians play better when they can see
and hear each other, the mixer is better off being able to see everybody
clearly, and most soaker flats don't have to be tall.
To put some numbers on this, the average player sitting in the usual
undertaker's chair is about four feet high. Eyes and ears will fall at
about 3' 8", and mouth about 3' 4". That puts even a flute a bit over 3
feet off the floor, and a screen higher than that is pure waste.
Drummers sit a little higher, and singers are their own animals, but the
majority of screens need not and should not be much over 3 feet. That's
too high to sit on and about right for an ash tray, but most importantly,
just below sight and sound lines. Since the drummer is the player most
likely to be jacketed in Fiberglas, if not actually locked away in a
tiny room somewhere, the drummer's acoustical micro climate is a rational
starting point for a discussion of how flats can be used to make life
easier for a mixer. The drum kit is also an extreme case, as it
constitutes a sizable section, and on small sessions you're likely to
have more mikes on drums than are used for everything else.
There is a public perception that the problem with drums lies in their
getting into everything else, but as the working mixer knows, it's
usually the other way around. All those drum mikes are open, and drums
aren't half as loud as most people think.
A drum booth takes care of both problems, but in addition to cooking
the drummer, it destroys communications between the drummer and everybody
else, and a booth big enough to accommodate the wavelengths of a floor
tom, let alone a bass drum, will run at least 12 feet square. Anything
less makes the drums sound funny, so a booth starts a session with two
strikes against both the drummer and the mixer.
It's more practical to put the drums in the middle of the floor, in a
four sided anechoic enclosure.
Three of those sides are screens, and the fourth is a thick drum rug.
Shag works well in this application, as it's about as thick as you can
get. Thickness is critical because the performance of an open absorber
(as opposed to a tuned box or panel absorber) at low frequencies is
chiefly dependent on it's thickness. With one or two exceptions noted
below, the mass and rigidity of the material have very little effect on
absorption.
Interestingly enough, there are no absorption figures published for
frequencies below 125 cycles.
The reason for that, given by one of the labs dedicated to generating
those figures, is that the anechoic chambers aren't big enough to
accommodate the wavelengths involved. Bass, defined as 60 cycles,
has a wavelength of over 18 feet, and it takes more than one wave to
get good measurements. That same lab has expressed the opinion, however,
that doubling the thickness of an open absorber should halve it's low
frequency cutoff point. The cheapest, lightest, and handiest open absorber
material commonly available is unfaced Fiberglas wool. It is also a first
class acoustical material both in terms of absolute absorption, at 1
Sabin per square foot and flat response, within .7 Db from 250 Hz
through 4000 Hz.
Absorption figures are published for several thicknesses of Fiberglas
batts. The deepest of them is 3.5" (R11) which shows a 25% drop in
absorption at 125 cycles when mounted 16 inches off the nearest wall.
When placed directly on a wall the drop at 125 Hz increases to 230% so
for movable screens any solid backing should avoided. As stated above,
the figures for 6-1/4" (R-19) batts should produce similar figures at
60 Hz. Presumably a foot (R-38) would yield even absorption down to 40
cycles or so, but since these things have to be handled by real people
in real studios, 6 inches seems a reasonable compromise. If for some
reason an absorber must be mounted on a wall, 6 inches of semirigid
Fiberglas board (Corning 703) will result in flat absorption to 60 Hz,
but it's both heavier and harder to work with than the soft stuff. For
those reasons, boards are best left to wall treatment, where they work
very well indeed.
Given all the above, a proper absorbent flat would be nothing more than
6 inches of Fiberglas behind a musician, and no higher than necessary for
the mike pattern. Which leaves the problem of how to get it there.
No problem. You'll need 3/4' plywood, 2x2's, hardware cloth, burlap, 4"
hinges, and 4- 5" casters.
Hardware cloth amounts to very big window screen. It's made of galvanized
iron wire welded at each intersection, with spacings of 1/4, 3/8, or 1/2
inch. Standard width is 3 feet, cost is 50 to 65 cents per square foot.
It is strong enough to use as a hammock, is slightly springy, and makes
an excellent enclosing material. Burlap comes in a couple of textures, and
several colors. It doesn't sag or shift once it's mounted, looks good,
and overcomes some small problems with Fiberglas, which gets a little
weird at very high frequencies. Cost is about $1.20 a yard for 3 feet wide
and $1.60 for 52 inch wide. Very durable material.
Do not cheap out on the casters. Those rotten little one inchers can't be
pushed over a cigarette butt, let alone a bundle of mike cables. Three
to five inch light/medium duty casters cost about a buck an inch, and save
incredible amounts of frustration every day.
So: Rip some 3/4" plywood into 6" strips and use them to build an open 6"
frame, screwing the corners together with 2x2x6" blocks. Drywall screws
are terrific for this. Get some 1/4 to 1/2 inch hardware cloth and staple
it on one side of the frame. Drop in the Fiberglas, and cover the second
side with another length of hardware cloth. With the hardware cloth in
place, the box is self supporting, as the cloth is quite rigid. If you
feel the need , the box can be braced with 2x2's, but they add weight,
and probably won't be needed. Now wrap the whole thing in burlap,
staple it down, and you've got a near perfect absorber.
Perfect, but terribly clumsy. Light though these flats may be, carrying
them around is a pretty dumb idea. Also, they tend to fall over on
people's instruments, and you don't need that.
Since flats are normally used three at a time in "U" shaped formations,
the obvious solution is to build them that way using hinges between the
three sections, and add casters for mobility.
The hinges can be installed at the edges of each section if the studio
has enough room to store the composite flat in the open position. For
smaller rooms, they can be built with 6" double-swing hinges or with a
6" plywood strip with hinges on both sides. Either system will allow the
composite flat to be stored in a very compact zig zag position. Four
casters are customary, with two on the center section and one at the end
of each wing. Small touches include screen door handles to give the
engineer something solid to grab, and a set of screen door hooks to hold
the composite screen closed for storage and moving. One inch aluminum
right angle stock can be used as outside trim on the edges, and looks
pretty glitzy, but it's a lot of work. Standard wood outside trim is
easier, and untrimmed is acceptable. Depends on the image projected by
the studio.
A studio will normally need several sets of dead screen sets. The
normal compliment is listed below.
2 for vocals, 6W x 4H' sections. Six feet is plenty if you place the
vocal mikes high and point them down as in film practice. That avoids pop
and proximity problems, reduces sibilance and intake breath sounds, and
gives the vocalist a clear sight path to the lyrics and the band. Helpful.
1 for drums, 8 to 10W x 3H' center, wings at half width. Allow passage room
for the drummer at one side of the kit. If nothing more, the screen avoids
multiple wall reflections back to the drummer.
2 or 3 for acoustical instruments, and sometimes open backed amplifiers.
4W x 3H.
1 for piano, 5H x 8W overall. This one can be two sections. Actual
heights will be up a few inches because of the casters. The gap has no
effect.
Since each flat absorbs on both sides, the set listed above will add about
500 Sabins of absorption to the studio (and improve isolation) whether
they are in use or not. They work even when they aren't working.
In summary, the control of isolation is a major part of an engineer's
professional effort, and the use of correctly designed, well constructed
screens can reduce a mixer's setup time as well as greatly improving that
control.
-30-
Words:2411"
Shame his site has gone. Hope he doesn't mind me posting that.
Cheers
