Minimizing mic noise
Sep 1, 2009 12:00 PM, By Chris Woolf
Broadcast microphone suspension innovations prove that better techniques are available.
New approaches
Figure 5
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Two entirely new designs for suspensions have appeared in recent years that have rethought the problem of keeping microphones cushioned and quiet, as well as have introduced some alternative solutions to simple bits of rubber. Both can tailor the compliance on each prime axis, and both have sufficiently linear action to give effective isolation for the large displacements that occur at low frequencies.
The French OSIX design, derived from research originally funded by Stefan van den Burg and now handled by Cinela, uses a wire spring curved in a hoop shape that crosses over itself with the microphone inside the hoop. (See Figure 2.) The single coil can bend forward and backward, enabling a high Z compliance. For up, down and sideways movements, the loop distorts, which requires more force and makes the suspension stiffer in these directions.
Figure 6
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Metal springs have little self-damping, so the OSIX has to provide separate damping elements in the form of rubber bands twisted around the fixing points. This arrangement dampens all movements that require the spring to twist at its anchors, but is less effective at dealing with twisting in the wire itself. This means that there is some degree of translation of movements from one axis to another, though the OSIX is far from weak in this respect.
As a classic design of spring, mass and damper, the suspension is best tuned to some degree, so OSIX products are marketed for individual microphones. They are essentially inflexible in this respect, and this, coupled with the complex assembly, makes them moderately expensive. They also suffer from a degree of fragility — the delicate spring is easily bent — but they are highly regarded by users.
Figure 7
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The other design, researched and developed at roughly the same time in the UK, is the lyre suspension, so-called because the elastic element has the shape of the classical instrument. (See Figure 3.) Although the overall outline of the lyre is roughly circular like the OSIX, it has a different operating principle. Z compliance is governed largely by the long path length of the recurved arms, which allow a high degree of Z-axis movement, while X-axis compliance is much reduced; to move in this plane, the arms of the mount would have to tighten their sideways curl. Y-axis movement is also restricted, because both pairs of arms have to move simultaneously to raise or lower the central clip.
The lyre's shape determines the compliance rather than the material used, but the latter is important for the damping. Unlike the OSIX, the lyre can have integral damping, which simplifies the design and tends to calm any translations of movements from one axis to another. The lyre's use of a moldable shape memory material brings two other benefits: It is virtually unbreakable and enables the addition of an integrated clip for the microphone, minimizing assembly costs. This makes the design suitable for a wide range of microphones. It also allows the design to be scalable across a wide range of sizes, because these plastics retain their strength and toughness even when very thin, whereas rubber bands and wire springs soon become excessively delicate.
Future of suspensions
Development in suspensions has been an extremely slow process. There have been a tiny handful of new ideas since the Shure Donut in 1970, but nothing compared to the flood of improvements in microphones in the same period. It is likely that suspensions based on rubber bands, diaphragms and webs will be with us for a while, because they are simple to conceive and inexpensive to produce, and less inventive manufacturers and less discriminating users are unlikely to give up such designs in the short term. But both the OSIX and lyre suspensions have shown that much better techniques are now available and have set a higher level of performance that new contenders will have to reach.
Chris Woolf is an independent design engineer who has worked with companies including Rycote and Schoeps.
Performance measurements
Figure 8
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The plots reproduced here show the performance of various types of suspension with rising frequency, as measured along the Z axis of the microphone. Note that the upturn in the response at just below 2kHz is a typical artifact of the fast Fourier transform measuring system used; it was left in only to prove that these are real plots.
In each case, the black trace is the performance with no isolation; a shaker is used to rattle the microphone directly. The red trace shows what happens when the shaker does the same thing but via the suspension. The blue shading gives a quick visual indication of the benefits the suspension gives, and the 50Hz notes at how low a frequency the mount manages to isolate. Notice that the red trace always rises above the black at very low frequencies, showing that the mounts do indeed amplify movement under these conditions.
Figure 9
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Figures a, b, c and d show the same microphone mounted in diaphragm, web, band and lyre mounts, respectively. The mounts used are typical ones, but without any attempt to optimize performance.
Figures e and f show similar plots using a compact microphone in OSIX and lyre mounts. The microphone is much smaller and lighter, which makes isolation more difficult. The performance of both mounts is broadly the same, but note that the OSIX has a significant extra mass (65g as opposed to 40g), which helps to lower the resonant frequency — something the lyre suspension does not need.
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