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Old 19th August 2019
Lives for gear

As Starlight said, the material has to be compressed by the right amount in order to isolate the speaker properly.

Think of it this way: the cork (or any other isolator) is acting as a spring, and you want it to be just "bouncy" enough that it really does isolate. In terms of cars: if you look at the suspension of any car, truck, bus or whatever, you will find springs. That's what makes the ride pleasant, so that the vibrations bumps and dips in the road don't make it into the car. But you can't just use any old spring you feel like! If you take the spring out of a Smart-car suspension and try to use in on a 20 ton truck, obviously the weight of the truck will squash the spring totally flat: it won't isolate that truck from the road at all. And if you do it the other way: take the spring out of the truck and put it in the Smart-car suspension, and it won't be springy at all! It's way too solid for that, and the tiny weight of the Smart Car does not compress it at all. So once again, no isolation. If you don't use the right spring for the right job, then either the spring is overloaded with too much weight and "bottoms out", or it is under-loaded with not enough weight, and "tops out". Either way, it does not isolate.

The same with speaker isolators: You have to get the right amount of spring compression (technically, it is called "static deflection", not "compression", but it's basically the same thing) for the weight of the speaker. If your speaker is too heavy for the isolation pads, then you over-compress them, they bottom out, and they don't work. And the other way too: If your speakers are not heavy enough for the pads, then you under-compress them, they "top out", and they don't work.

So how much compression do you need? That depends on the type of spring, but as a general rule an isolation mount needs to be compressed about 10% to about 25% to be in the range for good "springiness" and good isolation. In other words, if your cork pads are 10mm thick, then when you have the speaker siting on top, they need to be compressed down to about 8mm thickness or so (2mm compression over 10mm is 20%). You can measure that with a ruler if your pads are thick enough. If not, then you'll need a micrometer.

But that's not all! There's more to it than that. Just because you got the right compression does not mean that you'll get good isolation: With the wrong compression it is guaranteed that you won't get any isolation, but even when you have the right compression, there's other factors to be considered as well, that are inherent to the nature of the "spring" that you are using: cork is not the same as rubber, is not the same as a a coiled steel spring. Factors such as internal damping, compressive stiffness, dynamic sheer, and other things can change the way it works in practice. One key factor in all of this, is frequency: your pads need to isolate properly down to at least an octave lower than that the so-called "cut-off" frequency for the speaker.

So MAYBE your cork pads are working... but likely they are not. They same would be true from chopped-up mouse pads, unknown bits of rubber, packing foam, or any other material that does not have the right characteristics for decoupling speakers. Beware of such silly advice not based on facts, science, or reality. And ignore all other advice from the same sources, too! Caveat emptor...

Personally, I pretty much always use Sorbothane pads for the speakers in rooms I design, as it's a rather amazing material. It has high damping, and very good isolation. There's nothing better that I have found for decoupling speakers. You can buy sell self-adhesive Sorbothane "hemisphere" pads that you can stick to the bottom of your speaker, but you have to get the right type! They have different types of Sorbothane with different properties ("durometer rating"), depending on the weight of your speaker, so first weigh your speakers, then buy the correct pads for that weight.

- Stuart -