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Vibration Isolation – The Final Frontier

January 29, 2013

The fourth building block of acoustics is perhaps the most overlooked, which is vibration isolation. Even people with no interest in acoustics are familiar with the problems that come from poor vibration isolation. If you’ve ever shared a wall with another whose taste in entertainment is different, you’ve experienced the effects of transmission of sound energy through the physical structure of a building.

This element breaks the physical connection between vibrating things and the structure of the room and building.


So why is this important?

Sound travels easily and efficiently through solid objects, so when speakers are sitting firmly on the floor or bolted to the wall, some of their sound energy travels through the speaker cabinet and into the physical structure of the room. This energy then easily transmits through the structure to other rooms. This is one of the reasons you hear the thump and rumble from a speaker system elsewhere in a building.

And it’s also how the rumble of roof mounted air conditioning machinery or other spinning motors can intrude into a building.

Vibration isolation is particularly important in a multi-room audio facility where each room needs to be free of intruding sound from other rooms. Or in a home theater where the sound can disturb others elsewhere in the house.

And in a situation where you have live recording, it is important to keep this sound from intruding into the studio and being recorded by the open mic.


So how does this happen?

This kind of sound transmission is best demonstrated by an executive toy called balance balls; for the more scientifically minded, this is also known as a Newton’s Cradle.

There are five balls suspended in a row within a frame. If you lift a ball at one end of the line and release it, it hits the next ball which doesn’t move. Nor do the third or fourth ball. But the fifth/last ball flies out. Amazing!

This is the law of conservation of energy and the law of momentum at work. A real-life example of your science class lessons at work. If you search “Newton’s Cradle” at wikipedia.org, you can get a full explanation, including an animation of it in action.

So in our version of this, your speaker represents the lifting and release of the ball on one side, the wall framing that connects the two side of a wall equals the intermediate balls that don’t move, and the panel on the opposite side of the wall represents the ball at the other end flying out.

Some of the energy from a vibrating speaker cone goes into the cabinet instead of the air, and from there it gets into the room structure through the solid connection between the speaker and the room structure. From there it can travel through all the framing and other physical structure to other rooms where it reemerges as disturbing thumps and rumbles.

Remember that most walls are a panel of drywall screwed to a wood or steel frame, where the portion of a drywall panel between frame members is not restrained and is therefore free to vibrate.

So when your speaker is on the floor, bolted to the wall, or otherwise attached to the room structure in one way or another (as it must), you also have a solid physical path for the energy from your speaker to get into the room’s structure.


So how do we fix this situation?

The perfect solution would be for the speaker to not touch anything. But until an antigravity device has been perfected, we’ll have to find another way.

To solve this problem, we simply break the physical connection between the speaker and the room structure by supporting the speaker with something that doesn’t provide a path for the energy.

One method that has been tried is to put your speaker on something very heavy, like a concrete block or cement slab. This is called an “inertia block”.

The idea behind this is that the higher the mass, or weight, of an object, the more energy it takes to move it. This is true, but if you think about this in terms of Newton’s Cradle, it becomes apparent that this won’t work well at all. There is still a solid connection, and there is nothing to dissipate the energy.

You still have the immutable laws of conservation of energy and conservation of momentum at work. The energy has to go somewhere (it doesn’t just “disappear”), so it transfers through the rigid inertia block just like any other solid. The block is acting like the middle balls in the Newton’s Cradle, passing the vibration energy to the room structure.

So while you will get some small frictional losses, there won’t be anywhere enough to get you the isolation you’re looking for.


On your tippie toes

Another common way to provide vibration isolation is to use something called “high loading”. That means the mass of the object is being supported on as small a footprint as possible. This is the concept behind the pointy cones you can put under your speakers, or the spikes under speakers or stands.

This creates what I like to call a “high impedance interface”. The small footprint presents an interface that has a higher resistance to the transfer of energy. But energy still gets through, especially in the vertical plane.


Pliable and squishy

But the best solution is what’s called resilient isolation. Simply put, this is placing something squishy between your speaker and whatever is supporting it. It dissipates the sound energy as heat and motion, rather than allowing it to pass through to the supporting surface. Sounds simple, but it’s important that it’s done right or you won’t get the full benefit.

The resilient isolator has to be able to squish in all three dimensions. This allows the supported speaker to vibrate without any solid connections that can provide flanking paths for the energy to bypass the isolator.

The resilient isolator also can’t be flattened by the supporting load. It needs to suspend the speaker in the same way a shock absorber works on your car. If there isn’t enough load, and the absorber is at the top of its extension, it’s a pretty solid connection and road vibration can pass through.

Similarly, if the shock absorber is completely compressed, it is again a solid connection that passes vibrations.

So in practical terms this means you can’t do what I’ve seen done so many times. If you place your speaker on a big slab of something like a sheet of rubber, it doesn’t work well because it’s not compressed enough. But if you put it on small pieces of something squishy, it’s easier to make sure the isolators are properly compressed to the middle of their range of compression. And you’ll be adding the advantage of high loading as well.

This means you have to match isolators to the weight of your speaker. The thickness, hardness, and number of isolators have to be selected so that the isolators are compressed to approximately half to two thirds of their original thickness. This means you should be able to tap the speaker and have it wiggle a little bit; this will give the range of motion in the isolator that allows it to dissipate all the vibrational energy.

My preferred technique is to place each speaker on little hemispheres or dots of material, with an isolator more or less at each corner of the speaker bottom. This placement is then adjusted to make sure the speaker is sitting level.

By far the best material to use for vibration isolation is a product called Sorbothane ™. It was designed for exactly this kind of use, and so it outperforms all other materials, like foam, neoprene, rubber, or springs.

Note that in earthquake-prone areas it is a good idea to put some sort of well anchored non-rigid restraint on each speaker, like an earthquake strap. This will help keep your speaker from becoming a projectile during an earthquake, and do it in a way so that the restraint doesn’t make a path that flanks the isolators.


The end

So now you’ve (hopefully) read about all four building blocks of acoustics, and have a better understanding of how they all work.

A good sounding listening space is not voodoo, it’s not black magic, and it’s not selling your soul to some minor deity for secret knowledge.

Good acoustics is the application of science to manipulate what happens to sound in an enclosed space, resulting in an accurate, pleasing listening experience.

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