occurs at resonance. After the forcing frequency passes through the resonant frequency of the system, isolation begins to occur in a passive system. In an active system the isolation can begin well below the forcing frequency because the feedback is capable of removing support resonances. For this reason, many passive systems are designed with a very low resonant frequency. This point is best demonstrated by analysis of the transmissibility of an isolation system.

Transmissibility is a term that is used to describe the response of a vibration isolation system. Literally, transmissibility is the ratio of displacement of an isolated system to the input displacement. It is used to describe the effectiveness of a vibration isolation system. Transmissibility varies with frequency. A typical transmissibility curve is shown in Fig.3.

In Figure 3, the red region is the region of amplification. In this area, the isolation system is actually moving more than the ground. This occurs when the forcing frequency, say the frequency of the building is equal to the resonance of the isolation system. The green region is the isolated region where the

isolation system is moving less than the ground, i.e. the system is isolating. It is apparent that the passive system in Fig. 3 actually amplifies vibration at the resonant frequency of the isolation system. When using a passive system, one should ensure that the building resonances are not near the resonance of the passive system, otherwise the isolation systems will actually amplify as opposed to isolate. In a passive system, at frequencies less than its system resonance, no isolation takes place, and ground vibrations are transferred directly through the isolation system. At the resonance, amplification occurs, and the transmissibility is greater than unity. Once past the resonant frequency, isolation occurs. As one moves further out on the transmissibility curve, the transmissibility (T) approaches zero. In mathematical terms, the undamped transmissibility of a passive system can be represented as follows:

Damping is a term used to describe vibrational energy absorption of a system Generally, damping is accomplished by converting frictional energy into heat, resulting in an overall loss of system energy.

It can be noted from the transmissibility equation, that damping only affects the system at resonance. Damping lowers the amplitude of the transmissibility curve at resonance, but it also reduces the rate at which the transmissibility curve 'rolls off' after resonance. When designing an isolation system, it is important to maximize damping effects at resonance, while simultaneously ensuring a rapid roll off after resonance, so that isolation is achieved over a greater range of frequencies.

In an air suspension system, damping is accomplished by means of an orifice. The size and shape of this orifice directly controls the damping and resulting transmissibility of the isolation system.

HERZ uses a unique orifice configuration that minimizes transmissibility, while securing a rapid roll off of the transmissibility curve after resonance. This rapid roll off ensures that the vibration isolation system will be effective at very low frequencies.

Active isolation systems also have some resonance of the support system, like a passive system. But, unlike a passive system, the feedback loop is able to damp out the resonance of the support so it is not apparent in the transmissibility measurement. For this reason, it is possible to support the isolation system with a much stiffer system, and still isolate a broad range of frequencies. The TS and AVI systems use springs that are up to 500 times stiffer than passive isolation systems. This stiffer support makes the TS and AVI systems much easier to work on and much less susceptible to ambient sources of noise such as airborne vibrations.