More on Microphones by Michael Williams, (www.williamsmmad.com)   △   <   ∧   >

8.6 - MICROPHONE MOUNTING AND SUSPENSION SYSTEMS

One would expect that architects and engineers would take great care in the design and location of the professional recording, television or broadcast studio in an attempt to reduce external noise to a minimum - that is not just airborne noise, but also mechanical vibration of the building structure itself. However I think we all have anecdotes of recording situations where traffic rumble transmitted through the building structure has been disturbing, or even the ‘well known’ studio where the Underground (metro or subway) actually passes underneath the studio! In order to isolate the microphone from this external mechanical vibration, some form of suspension must be used to filter or decouple the unwanted vibration of the microphone stand and clip, from the microphone itself. This is also necessary when using a microphone on a boom or fish-pole in order to prevent transmission of handling noise to the microphone.

The fundamental resonant frequency of any suspension system must be lower than the external vibration frequency. If the resonant frequency of the suspension system is higher than the external vibration frequency, then it will even tend to amplify vibration. Most present day microphones are made to be as light in weight as possible, this means that the elasticity of the suspension system must be very flexible in order to bring down the fundamental resonance frequency. It is also necessary to introduce a degree of damping into the suspension system, so as to prevent large amplitude vibrations of the microphone assembly when the external vibration frequency is near the resonant frequency of the suspension system.

That is the theory, in practice it is very difficult to achieve a perfectly designed suspension system. A typical tubular microphone housing and capsule weigh about 80 grams. With the standard suspension system (a microphone clip with 4 elastic loops to an annular ring) will produce a resonant frequency of about 10Hz. Damping will be introduced by the microphone cable, which obviously will vary considerably with respect to the size and nature of the cable, but the material and design of the ‘elastic’ suspension can also improve the damping efficiency. The desirable elasticity and therefore resonant frequency should also reflect the conditions of usage. If rumble is being transmitted through a stationary microphone stand on the floor of a studio, the suspension system can support a much lower fundamental resonant frequency compared to a microphone suspension system on the end of a boom arm which can be ‘thrown around’ quite brutally on occasions. Fortunately with building rumble, the frequencies transmitted are indeed almost subsonic and therefore need a very low fundamental resonant frequency for the suspension system to be efficient, whereas on a boom arm the handling noises are at relatively higher frequencies and therefore the suspension can be firmer.