[Editors Note: Very very old page... original version was circa Y2K. Rebuilt it in 2003, and again in 2022]
In 1999 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.
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.
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.
20.5 by 19 by 3.5 inches overall
19 by 17.5 bed size
Approximately 45 lbs loaded
The Sandbox frame
An air or spring support system
Approximately 30 pounds of clean, dry sand
The Sandbox acrylic load platform
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.
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.
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.
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.
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.
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.
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.