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The Secret Life of Speaker Isolators
Recording Magazine - March 2014 - by Bruce Black
Over the time that I’ve been doing acoustical work, I’ve found that there are a number of misconceptions
surrounding just what speaker isolators are, what they’re supposed to give you, how they work, and why one
kind works better than another.
Common wisdom says you just throw a slab of rubber or foam or something under your speaker, and like Merlin
waving his wand, it “makes your speakers sound better.” Wrong—there’s a lot more to it than that. By
understanding what is going on, you can install speaker isolation that is truly effective, and costs
the least. So let’s deconstruct this.
The problem
Low frequencies require lots of Watts to move the cone that moves the air that makes the SPL that Jack wants.
No speaker system is 100% efficient in transferring electrical power to acoustical power, so some of the juice
from the power amp ends up transferring to the speaker cabinet as kinetic energy (i.e., vibration). This kinetic
energy migrates from the cabinet to whatever it is sitting on, and through that into the entire room structure.
Energy then transfers along all the points of solid contact (“interfaces”) in the room’s structure, by virtue of
one object being in tight contact with another. This transfer is very efficient because the weight and contact
surface area of the speaker keeps it tight to the surface it sits on, and nails and screws tightly clamp all the
pieces of the building structure together. These interfaces have very little attenuation, providing a path for the
kinetic energy to infiltrate the entire building. A very simple device demonstrates this: Remember an executive
toy called Balance Balls (Figure 1)? It’s called Newton’s Cradle in scientific circles, and demonstrates the law
of conservation of energy and momentum (en.wikipedia.org/wiki/Newton’s_cradle). Five balls are suspended in a cradle;
if you lift and release the ball at one end, the ones in the middle remain still, while the one at the opposite end
flies out. Like magic! This is how sound energy can permeate through a building structure with almost no attenuation.
This energy finds release in room surfaces. No drywall or paneling surface is completely rigid, so when this energy
reaches them, they vibrate. This, in turn, makes the air vibrate—which, as we all know, is sound.
The effect
And so the trouble begins. The first thing you find out is how well the room surfaces are attached to their
framework. Any loose drywall, plywood, or similar panels will buzz, and loose lighting fixtures and decorations will
rattle. Each panel vibrates at a resonant frequency, so specific frequencies are enhanced. This acts like a
mechanical equalizer that changes the response of your room, in different ways in different locations in the room.
And don’t bother trying to fix it with EQ in your audio path—you’re working on the wrong thing. But there’s more.
When this sound combines with the direct sound from the speakers, you get a comb filter effect from the two waveforms
combining. This, too, alters your room’s frequency response. Since each point in the room will have a different
response due to amplitude and phase/time differences between the two signals, you can only equalize from a vague
point called the “listener’s position”, or average multiple positions, and hope the rest of the room kind of follows
along. Unfortunately most rooms aren’t this cooperative. But wait! There’s even more. From the listener’s
perspective, the sound from this secondary source comes from a different direction, so it can alter the “real”
sounds’ position in the sonic panorama, smearing its localization in a way that varies according to where you’re
standing. This is can be particularly noticeable when panning a sound or with a phantom center, as these require
good localization to be effective. All this from one vibrating panel! Now add in more vibrating surfaces, multiple
speakers all blaring away at once and the ever-present room resonances. It’s easy to see how this can become a complex, messy situation. And then there’s sound intrusion—what happens
when this kinetic energy finds its way into the rest of your building through walls and floors to come out in other
rooms...!
Breaking the chain
As you now know, the path for kinetic energy from your speaker to your room surfaces is a chain of multiple
interfaces. If you could completely break just one of the interfaces in this chain, the problem would be solved. In
situations like this, the simplest and most effective solution is to isolate the source from its supporting surface.
In our case, this is the speaker cabinet. And what’s nice about this is it can be done quickly and inexpensively.
It ain’t heavy—it’s my isolator.
A number of different ways have been tried to isolate a speaker cabinet. One is to
put something really heavy between your cabinet and whatever it sits on. This is called an inertia block. The idea
behind the inertia block is that the more something weighs, the more energy it takes to move it. This is Newton’s
First Law in practice—a body at rest stays at rest until an unbalanced force acts upon it, i.e. more weight requires
more force to overcome its inertia. While I’ve read anecdotal reports of sonic improvement with this method, it
doesn’t address the whole situation. With inertia blocks there are other laws at work, specifically the conservation
of momentum and the conservation of energy I spoke of. And no block movement is required to transmit sound. Once
again we’re back with the Newton’s Cradle. So like the balls in Newton’s Cradle, the kinetic energy from a speaker
cabinet can still be transmitted to the supporting surface through the inertia block, explaining its less-than-
optimal performance. So in our application, increasing inertia alone only has a limited effect.
On needles and pins
Another concept in speaker isolation is that the smaller the contact area supporting a given object, the greater
the resistance of that interface to transferring kinetic energy. This is called high loading in acoustical-science
circles. So theoretically, if you could support your speaker cabinet on an infinitely small point, no kinetic energy
could cross the interface. That ideal situation is not possible in our world, but if we restate the concept as “a
smaller area of contact will have a higher resistance to energy transfer”, we now have something that can be put
into practice. Since only the point of contact needs to be as small as possible, you can isolate your speaker using
pointed cones or spikes on your speakers or speaker stands without being concerned about the shape of the body of
the spike or cone. It’s the point that matters. See Figure 2.
This works better than the inertia block, but there are still two issues that limit the performance of cones or
spikes. The first is that the point can dig into the surface of the speaker or the floor, which is usually made
from wood or some other material that a sharp point can penetrate. It may not be much initially, but over time the
point will dig deeper, increasing the surface area of contact, and reducing its attenuation. This is fixed by
putting something hard, like a small, hard metal disc, between the point and the surface. Some spikes come with
these. But there’s another way the efficiencies of cones and spikes are compromised. Kinetic energy traveling from
side to side or front to back in the cabinet sees a high impedance interface where the point contacts the cabinet, as
this energy is traveling at right angles to the axis of the spike or cone. But there’s also energy traveling in the
cabinet’s vertical axis—for reasons I won’t go into here, it has an easier time crossing the point of contact.
Remember Newton’s Cradle again? The energy passes through the tiny point of contact between each two spheres, moving
in the direction of the interface. So spikes and cones do reduce the energy migration. A great improvement, but we can
do much better.
Float like a butterfly...
Let’s say you could make your speakers float in midair. That would certainly isolate them! There would be no point
of contact, and no path for energy moving in any direction. We can’t make this happen, but believe it or not, we
can come very close. 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..... 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......
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.....
Final thoughts
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!
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