Do It Yourself: "The Secret Life of Speaker Isolators" By Bruce Black

Here is a great article entitled "The Secret Life of Speaker Isolators" that was published in the March 2014 volume of Recording Magazine by Bruce Black: 

Over the time that I've been doing acoustical work, I've found that there are a number of mis­ conceptions s_urrounding 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 ·a n executive toy called Balance Bolls (figure l} It's called Newton's Cradle in scientific circles and demonstrates the law of ·conservation of energy and momentum. 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 build­ ing 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 sur­ faces are attached to their framework. Any loose drywall, plywood, or similar  pan­ els 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 speak­ ers, 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 aver­ age 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 accordingly where you're standing.  This is can be particularly noticeable when planning 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. Ws easy to see how this can become a complex, messy situation. And then there's sound intrusion-what hap­ pens when this kinetic energy finds its way into the rest of your building through walls and floors to come out in ether rooms...I

Speaker Isolation Newtons Law

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, explain­ ing 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 support­ ing a given object, the greater the resistance of that interface to transferring kinet­ ic energy. This is coiled 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 could 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 sur­ face. Some spikes come with these.

Isolate It! Sorbothane Hemispheres

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 cabi­ net, 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 inter­ face. 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 theme 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  selecl­  ed 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. Anyone 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 hemi­ spheres 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 the 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 a.lso 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 silting on lop 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.

The next numbers

Now you need to figure out how much material and what hard­ ness. Sorbothane hemispheres come in three hardnesses (30, 50, and 70  durometer) and eight  sizes (0.5", 0.75 ", 1", 1.25", 1.5" 1.75" 2", 2.25", 2.5"). Each hardness in each size will optimally isolate a certain weight range. For example, a  1.25" diameter of 50 durometers 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 to smaller isolator or a lower hardness for a lighter speaker, and a larger isola­ tor 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 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 4i 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 earthquake­ prone, 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.

Isolate IT! Sorbothane Speaker Isolation

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. High­ performance speaker isolators can be installed quickly and inex­ pensively, 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 downtime as possible. This will make you happy by improving your room's sound, and make your neighbors happy by reduc­ ing sound intrusion. Try it!