The New Line Array for 2003 - Page 2
Damping treatment on the truss.
After the basic array structure was completed I used a trusty chime knocker to
attempt test and determine the resonance profile of the assembly by tapping and
listening to the resonance profile. The
closet-rod spine seemed to be a potential problem area, but how to attempt to
damp such an open triangular-truss assembly? Having a lot more time than money
now, which was why I didn't build the structures from resonance-free
"unobtainium" in the first place, I thought of that old product out of Washington
state, "Acoustical Magic."
"Acoustical Magic" was sold as a paint-on damping compound, described as
an 'elastic borosilicate' compound? Hmmm? An internet search turns up vestiges
of "Acoustical Magic", but apparently the company which manufactured it is long
gone. A trip to the local "Lowe's" turned up "Kool Seal- Elastomeric Roof
Coating." Looks like heavy white paint to me.
I had on-hand a large amount of 80 grit white silica sand from a local
masonary supply yard. 80 grit sand is very fine, the kind of sand you might
imagine in an hourglass. I think I used about a 4:1 mixture, by volume, of the
paint-to-sand. It produced a non-settling mixture which could be painted on.
The paint is water-based, so drying time was relatively quick.
What you see is in the photo is three coats of the mixture on everything but the
spine, which
had four coats - the painting process was a horror! Tapping again with the chime knocker,
after the mixture had dried, did produce a dramatic change in the resonance
profile. Whether that change will provide any virtue at all, given the forces
involved on the structure when the drivers are exhibiting a large Xmax is
unclear?
Rear gluing of the drivers.
I'd always planned on gluing the rear of the drivers' magnet assemblies to the
truss-spine. The glue is silicone seal, hence there is some chance of cutting
through it to remove a driver in the future.
The texture of the dried DIY acoustical magic is shown in the photograph.
It does dry to a sort-of hard concrete-like density. The surface is definitely
'shark skin' texture, i.e. like 80 grit sandpaper.
With the added thickness to the spine from the coating of damping mixture,
the drivers' magnets were a flush fit when screwed in to the baffle front.
Square-drive wood screws with large 'collar-heads' were used to mount the
drivers. All holes were pre-drilled. A battery-powered screw driver with
clutch seemed to be mandatory what with all the screws that were used in the
structures.
Note the number in felt-pen ink on the magnet assembly: a foreshadowing of more
obsessive-compulsive issues related to the drivers described below.
The drivers were alternated as to orientation of the electrical connectors
in the structure. Each driver had a nominal 3 ohm DC resistance, and six
drivers each were wired in series, alternating up the array, and then each set
of six drivers-in-series wired with each other in parallel, for a nominal 9 ohm
DC resistance for the entire array.
A spare piece of Gortz flat-conductor was run down the center-rear of the
truss spine for the electrical return of the series-parallel arrangement.
Teflon-insulated sliver-plated 16 gauge wire was used to interconnect the
drivers.
The completed line array.
The baffle front of a completed line array is shown. The mounting flanges
of the RD-75 driver were emulated with 3/4" x 3/4" x 1/8" aluminum angle stock.
The 'integrated' feature of this emulation is that the mounting screws for the
angle stock are also used to further anchor the 1" x 1" side rails to the 1" x
1" cross-pieces on the rear side of the baffle. Lots of screws were used in addtion
to the epoxy as a nicety. The mounting flanges allow for 3/4" thick material to be
attached to the line array flush with the front-face of the baffle on either side.
The black finish is "Krylon-Contractor" brand "Tread and Grip" paint system.
This relativity new product can be used to achieve a modern sort of 'crackle
finish' or at least a textured surface, which has a bit of damping functionality
also. A thick coating can be built up with multiple applications.
But what of the drivers?
The 'infinite grief' drivers were purchased from the local "Apex Jr."
surplus outlet in 1998. Apex Jr. was a well-known supplier to Bass List members
at the time I was a member of that forum in 1998.
They are 6 5/8" in diameter, an as can be seen from the photo have
a large inverted, very thick 'dust cap' - clearly meant as mass loading for
the driver. When I first received 28 of the drivers, oh so long ago, their foam
surrounds were in good shape! Then I got involved in 'other matters' and they
sat in the garage for years.
Rudi Blondia had measured the parameters of the drivers for Apex, Jr. as
having a Fs of ~37Hz and a Qt of ~0.70. Clearly of Japan-origin, the drivers
were reputedly manufactured by ToneGen? They really looked to be of
reasonable quality, given their ostensible intended purpose - as the 'subwoofers'
for autosound 'bazooka' systems ;o) Most of all, the price was right then for me to
purchase 28 (which included 4 spares). I had thought they might make good
dipole drivers in a line array that could closely match the height of the RD-75s
and the true ribbons in an array of 12.
Then to my dismay in late 2002 I discovered that the foam surrounds were
starting to deteriorate. Four of the drivers had tears in their surrounds. I
could neither see myself abandoning them nor installing new surrounds. Previously
I'd purchased from Nick McKinney at "Lambda Acoustics" a quart each of both his
white(clear) and black "Moyen" damping compounds for cone drivers. Could these
compounds be used to repair and/or restore the foam surrounds to functionality?
The simple answer was yes, the long answer was an extended period of
restoration madness which ulimately led to my mass-loading the drivers to
achieve a close match on the Fs value. That story follows.
The New Line Array for 2003 - Page 1.
The New Line Array for 2003 - Page 3 - Next Page.
The New Line Array for 2003 - Page 4.
Acoustic Line Source Research - Table of Contents.
