Firestone’s AirMount isolators are a very unique form of gas spring, essentially strong rubberized outer walls capped on either end by metal caps. By filling the inner area with a mass of compressed air and shaping the outer wall correctly, the result is a compact device capable of up to nine inches of static deflection with spring rates lower than a coil spring and an installed height that would be impossibly small with any solid isolator.
The AirMount is also incredibly versatile compared to a coiled spring; by adjusting the pressure of the air within the isolator — which can be done on the fly — the device can easily adapt to loads from dozens to thousands of pounds without needing to be switched out or manually adjusted. No solid spring could hope to achieve the same kind of flexibility.
Furthermore, the ability to adjust the volume of air inside the AirMount on the fly gives you the ability to maintain a level surface: as the load shifts, the air springs can be adjusted to maintain a constant height and thus a level surface for the load. This variability also means the AirMount can be used across a wide variety of loads with constant efficiency — very much the opposite of a coil spring, which loses isolation efficiency if the supported load decreases even by a small amount.
Also, the AirMount is a remarkably compliant with vibratory motion. That means that you don’t need to add inertia mass to your loads. That means no expensive structural aluminum framing to cope with the added mass. The AirMount has a low natural frequency and a large travel range to boot, further reducing the need for inertia mass.
In fact, the already low natural frequency of the AirMount can be reduced even further with the addition of an auxiliary reservoir — below 1 Hz. To accomplish the same frequency with a coil spring, you’d have to have a real static deflection of nine inches; a spring offering that much deflection would have to be so long as to be nearly impossible to stabilize.