Recording - March 2014 - 54

If you can place your speaker on a compressible, elastic
material, you can isolate your cabinet nearly as well as if it
floated. This is called resilient isolation and uses "visco-elastic"
materials. Unlike cones and spikes, these resilient materials will
give you isolation in all three dimensions. In a properly selected and installed resilient isolator, nearly all of the energy would
be dissipated as heat in the isolator. This is the basis of speaker isolation platforms made of acoustical foam, either alone or
combined with an inertia block for even greater efficiency at
damping motion.
But it is important to select the right material. Some acoustical
foam can disintegrate over time, and non-fire-rated urethane foam,
such as packing foam, can be very flammable, giving off toxic
fumes when it burns. Rubber tends to harden over time, making it
less like an isolator and more like an inertia block. Any one of these
will lead to a deterioration of a room's sound as they age.
But there actually is a better material to use. It is called
Sorbothane™. It provides better vibration isolation than any other
material; it doesn't break down over time; it doesn't deform like other
materials. It's just as if it was made for this purpose... which it was.
But before you run off and buy a sheet of Sorbothane (or
anything, for that matter), remember that the efficiency
improvement from high loading is very real, and will further
improve the performance of any vibration isolating material.
Common wisdom may say to put a sheet of rubber or foam
under the cabinet, but you'll get the greatest benefit from using
small pieces of material under each of the speaker's corners.
Bumpers ("hemispheres") work best, as shown in Figures 3
through 6. Sorbothane is available in several sizes of hemispheres that are perfect for this application.

Juggling the numbers
This raises the question of just what size those hemispheres
should be. And the answer is, "it depends".
Getting the optimum isolation for your speaker cabinets
requires some simple math. Any material used for vibration isolation needs to be compressed a certain amount-not too little,
not too much-to get full benefit from it. Like a car's shock
absorber, it needs to be compressed into its optimum range to
be effective. In our application, a speaker isn't heavy enough to
compress a whole sheet properly.
The first number to get is your speaker's weight. This should
be accurate to within a pound; you can weigh the speaker or
look in the owner's manual. You also need to include the weight
of anything else that will be placed on the speaker.
Large speaker systems like those used in theaters and dubbing
stages usually have the mid- and high-frequency drivers and
horns sitting on top of the low-frequency cabinet. There's no
need to put isolators under each of these sections, just the entire
stack. But the total weight of all the pieces is the key number in
figuring out the quantity and type of isolators to use.
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RECORDING March 2014

The next numbers
Now you need to figure out how much material and what hardness. Sorbothane hemispheres come in three hardnesses (30, 50,
and 70 durometer) and six sizes (3/4", 1", 11/4", 11/2", 2", and
21/2"). Each hardness in each size will optimally isolate a certain
weight range. For example, a 11/4" diameter of 50 durometer is
rated for 5 to 10 lbs. per isolator. You need four per speaker for
stability, so a set of four of these will support a speaker that is 20
to 40 lbs. All these numbers interact linearly, so use a smaller isolator or a lower hardness for a lighter speaker, and a larger isolator or higher hardness for a heavier speaker.
For heavy speakers, you can add more isolators to increase
the weight range of the group of isolators to match the weight
of the speaker. Subwoofers usually have the greatest power and
the lowest frequencies, making them the greatest offenders, so
remember to isolate them too.
The load specifications for Sorbothane can be found at
www.isolateit.com in their product descriptions. You can also
purchase Sorbothane hemispheres there.
Putting them in place
Once you have calculated the proper size, durometer, and
quantity of isolators, all that remains is to install them. Evenly
spacing them beneath your speaker cabinet is a good starting
point. Once installed, visually check to see that the cabinet is
level; cockeyed speakers look terrible. Some cabinets are heavier in front, for example, so you may need to adjust the isolators' positioning to level it out. See Figures 4, 5, and 6.
Check to make sure no other point on the speaker cabinet
touches anything, as this will provide a flanking path around the
isolators for the kinetic energy. When the cabinet is touched, it
should bob just a little bit. And check the speaker wires to make
sure they're loose and not also providing a flanking path.
And as a Californian acoustician, let me remind you of an
easily overlooked point! If you live in an area that's earthquakeprone, heavy speakers should be restrained by non-rigid straps

or chains. The restraints shouldn't be rigid, so they don't provide
another flanking path for energy to get into the structure. But
they will prevent the speakers from falling or flying, and potentially injuring or killing someone, in a strong quake.
Final thoughts
There are things that speaker isolators won't fix, most importantly
rattles, buzzes or sound intrusion caused by sound transmitted through
the air. In that case, the energy that you listen to is the same energy
that causes the problem. You'll have to work on the room structure or
fixtures, or, like your parents may have told you, turn it down.
Otherwise, the benefits of using speaker isolators designed
specifically for this purpose can bring very real benefits. Highperformance speaker isolators can be installed quickly and inexpensively, greatly improving the sound quality in your room, and
solving a number of pesky problems. The investment of time and
money is very small-not a bad thing since we all want our
rooms to sound as good as possible, with as little expense and
down time as possible. This will make you happy by improving
your room's sound, and make your neighbors happy by reducing sound intrusion. Try it!

http://www.isolateit.com

Recording - March 2014

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