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[Editors Note: Revised this really old page in 2003, but the basic info is the same]
I designed and built an effective isolation system for my equipment,
and it worked out so well that some people wanted plans, so here they are.
The Sandbox is a simple, yet highly effective isolation system based on
proven mechanical principals. In one frame it provides an air-spring
support to free the platform from external vibrations and a sand
resonance control bed to absorb airborne and equipment-generated noise.
The result is a clean sonic environment where your audio equipment can
operate at it's highest possible level.
Like most good audio tweaks, the Sandbox requires some work to install
and maintain, but the results are definitely worth the effort.
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The Sandbox
Audio Isolation System |
How It Works
The air isolation of the lower section of the box isolates the Sandbox
from rack or support vibrations, while the sand tray damps vibrations
absorbed by the equipment (via acoustic energy) or produced by equipment
motors. The acrylic load panel transfers the vibrations to the sand, which
damps the energy by rubbing grains against one another and generating tiny
amounts of friction heat. In addition, the weight of the sand (and components)
lowers the resonant frequency of the air suspension well below 20 Hz, the
bottom component of human hearing range.
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Construction
The Sandbox is sized to fit nearly all audio components and shelves, and
has a very simple construction. It's basically and open box with a recessed bottom.
You can build it from the plan shown at left.
Steve Friedman at Chewing Dog Furniture built the prototypes for the Sandbox,
and will be happy to build a standard model for $180.00. It has an MDF shelf
and black painted oak sides with an acrylic equipment platform. Corners are
mitered and biscuit-jointed together and the shelf and sides are dadoed
and glued together.
Sandbox customizations can be easily made by Steve during construction for additional fees:
Alternate sizes with larger or smaller footprints, or taller sides for more sand.
Stained, Clear, or Unfinished wood sides.
Maple, Mahogany, or exotic wood side panels, even veneer trim.
Custom corner joints (box joints, dovetails, etc.)
Call or eMail for a quote.
Sandbox Dimensions
20.5 by 19 by 3.5 inches overall
19 by 17.5 bed size
Approximately 45 lbs loaded
Components
The Sandbox frame
An air or spring support system
Approximately 30 pounds of clean, dry sand
The Sandbox acrylic load platform
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Putting the Sandbox Together
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Making Raquetball Supports
The easiest support method for the box is to buy a supply of raquetballs
and cut them in half. You will need 5 to 12 halves (3 to 6 balls) depending
on the weight of the components. Use a very sharp razorknife to cut the balls
along the faint seam on each ball. BE VERY CAREFUL doing this. I prefer to
insert the knife into the seam and then rotate the ball while pulling the
knife out in a ripping motion along the seam direction. If the ball does
not cut cleanly, your blade is too dull and needs to be changed (it's OK,
you don't need really clean edges, but you may want to place the cleanest
edges along the front since they may be visible along the bottom edge of the box).
Place the balls in an even grid with the spherical side up on the support
shelf arranged evenly where the bottom sides of the box will contain the array
and set the shelf in place. The box should sit high on the balls without a
load of sand or equipment.
Grades of Sand
There are many sources of quality sand to fill the upper box.
The most important requirement is that the sand be dry, which may take
some searching. Many building supply stores sell grades of masonry sand
(used to make cement or mortar) or "play sand" (for children's sandboxes).
If possible, coose the densest sand. You will need between 20 and 40
pounds of sand, depending on density and load panel thickness.
Filling and Smoothing the Sand
This step can be messy, so it's better to add sand slowly than to
try and scree off an overfilled box. Basically, you want to end with a
tightly packed, very even load of sand which is 1/4 inch below the upper
edges of the box. You have to make a simple tool to do this correctly,
a scree. This is a piece of cardboard or corrugated at least 20 inches long.
Measure off 17 inches in the middle of this panel and then notch the corners
up 1/4 of an inch for the width outside this measure. When you are done,
check the fit across the short width; the notched corners should rest on
the top of the box sides with the 17 inch section hanging down evenly 1/4
inch below the top. Move the scree across the box to check for binding, there
should be slight pressure along the sides.
Now begin adding sand at one end and work your way across, getting the
sand roughly smooth as you go. Once the box is filled evenly take a final
pass to get everthing as flat as possible with no voids. Again, the load
panel transfers the vibrations to the sand, which damps the vibrations by
rubbing against other grains and generating tiny amounts of friction heat.
Voids or high spots will reduce effectiveness.
Finally, drop the load panel on top of the sand and press it down evenly
to seat it on the surface. Make sure the load panel edges are centered
in the box, with even gaps all around.
Settling
Sand is a chaotic medium and you can expect the load to settle
over time. The greatest changes will occur over the first week or so,
after which you will want to add more sand and re-scree the surface for
better perfomance. Removing a heavy stack of equipment may be a lot of
trouble, so here is a way to speed up the process vibration under load.
Setup the Sandbox and center it on top of a speaker, then play loud music
for a day or so (you may want to make sure your neighbors and family are
not around); the vibration will provide mechanical energy for the grains
to reorient and compact. Another method is to use a vibrating massage unit,
or an electric hair trimmer to add energy to the box.
Adding Components
With the sandbox in place you can place your audio components on top.
Set them in place carefully, if you slide the load panel around then you
will need to re-center it. Some audiophiles recommend that an isolation
platform be used for each individual component, but the proliferation of
loading devices such as clamp-racks show that loading of the cases of
components can reduce their sensitivity to vibration. The Sandbox will
work effectively with no special cones or loading devices, and the effect
is transferred into a stack up to four components high. You are welcome
to try alternate arrangements and see what works best for you. Hey,
let us know what you find.
Float height
Wait to hook cables up until you read this. The ideal is to have a
1/8 inch gap all the way around the base of the box and a jiggly, waterbed
feel to the loaded system. The Raquetballs will settle over the first couple
of weeks, so make sure the gap is larger (3/16 to 1/4 inch) when you
finish setup. If needed, place a spacer under the balls to lift them
higher; cork drink coasters work well. You may need to readjust after you
load the box, or later after things have settled. You can test the resonant
frequency and damping of your installed system by gently pushing one corner
down about 1/8 inch and suddenly releasing the force. The platform should
rebound slowly and bounce once or twice at a 1 to 3 Hz rate
(1 to 3 oscillations per second), slower is better.
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Sandbox Tweaks
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The Sandbox is designed to allow a lot of flexibility in installation so
you can 'tweak' it to fit your requirements. Here are some ideas to get you started
Inner Tubes Air Support and Support Zones
An alternative suspension method is a single 17 inch motorcycle
innertube, easily purchased from any motorcycle dealership,
or wheelbarrow innertubes from a hardware store. Turn the tube
inside out so that the stem exits the outside and attach a valve
stem extension (available at any hardware or auto parts supply store).
Push the extension through one of the side holes for the lower section so
you can attach a hand pump and inflate the tube in place. Inflate the tube
so that the platform lifts approximately 1/8 inch all the way around the
base. If the platform is uneven, you can supplement the lift by adding
cut raquetballs under the low end of the box. The downside of innertubes
is that they all leak slowly via osmosis and must be checked and filled frequently.
You'll notice that we've shown two holes so you can try two lift zones,
by using two wheelbarrow inner tubes from a hardware store, one for each side. Most audio
equipment (especially source components) are heavier on the side
where the power supply is located. Using two zones will allow
more lift pressure to balance the load evenly.
Sorbothane
The elastomeric material Sorbothane is very useful for absorbing
vibration. This material has a very high hysteresis, which means it
has very low rebound. Panels of adhesive gel can be attached to the top
center of the load platform to add damping to the load panel. Don't try
putting this sticky gel substance on the bottom in contact with the sand, or you'll
get a big mess. Another use would be to substitute sorbathane feet for
the raquetballs beneath the box.
Alternate Platform Panels
The standard Sandbox load platform is a .250 inch thick polycarbonate panel.
Polycarbonate was chosen for it's excellent vibration damping characteristics.
It can be modified with a sorbathane top treatment as above, or the
corners can be partially drilled for cone location. You could easily
cut a replacement panel from another material and try your own experiments
with effects of thickness, etc. Another idea suggested by Steve would be to
cut an oversized panel and route in a notch to completely cover the
sand while clearing the top edges of the box. If you try this be careful
that the load panel is fully floating, i.e., that is not touching the outside
frame of the box at any point, or the sand isolation will be compromised.
Covering Sand Edges
You could try making the oversized load panel described above,
or you could attach some kind of flashing to the load panel, such as
strips of open-cell foam, black tape, garage door weatherstripping, etc.
to cover the sand gap. Again, take care not to create a bridge where
vibration from the outside box is conducted onto the load panel, or
the isolation effect will be reduced. Also, be sure your sand is completely
dry if you try to seal it in, as wet sand can mildew or worse.
Colored sand, available from some craft stores, can be added around
the edges as part of the final screeing process if you want the sand gap
to be dark grey, etc.
Iron or Lead Pellets
Finally, you can try alternate fillings to the sand recommended above,
such as iron pellets, BBs, lead, etc. to increase mass. Using a ferrous (iron/steel)
material will also provide some RFI/EMI shielding effect for your components.
DO NOT use iron filings; since all audio equipment is electromagnetic,
the filings will inevitably be drawn into your electronics with potentially
disastrous results. You may want to try a combination of iron pellets for
the first .75 inch and a sand layer above them for better damping.
Support Frames
Atlantis makes an excellent and affordable amplifier floor stand,
the Reference, which makes a great support for a Sandbox. The Atlantis
stand uses a welded flat strap iron subframe with spikes supporting
a 19 inch by 19 inch shelf.
If you remove the shelf and spikes, you can place a set of cut raquetballs
on top of the welded steel frame and set the Sandbox on top with the sides
covering the frame. Works great! Other shelf designs may also be adapted.
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