Minimizing mic noise
Sep 1, 2009 12:00 PM, By Chris Woolf
Broadcast microphone suspension innovations prove that better techniques are available.
Figure 1
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Microphones are designed to be heard, but they should ideally be translators of sound rather than originators. Some self-noise is inherent in every microphone, such as the jostling of air molecules against the diaphragm or the thermal noise in the electronics of an impedance converter. However, these contributions usually just a continuous bland hiss will be very low compared with the wanted sound. What irritates listeners far more is the much higher levels of erratic, impulsive noise that are often mechanically transmitted to the microphone.
Microphones detect sound by sensing the movement of a light diaphragm relative to the fixed, massive reference of a back plate or a magnet. However, such movement can also be generated if the reference moves for example, if the microphone or its cable is struck while the diaphragm is held still by inertial forces (such as the mass of air resting against it). Unfortunately, no microphone can distinguish between the first type of movement, usually caused by a sound we want to record or broadcast, and the second, which is the result of unwanted vibrations.
Obviously, the aim should be to keep the microphone body completely free from external, mechanically transmitted noise, but that is not so simple. Microphones have to be physically supported in space and are most often connected to the outside world via a cable and both routes provide a pathway for mechanical noise. The cable is a highly significant conduit, and using thin, flexible cable and a properly anchored decoupling loop can reduce the noise it transmits. Dealing with the support is trickier.
Springs and masses
Figure 2
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The efficacy of mic suspension at reducing unwanted vibrations is dependent on the frequency of the vibrations. Handling noise, which includes that transmitted through poles and stands, tends to have a spectrum that is tilted heavily toward low frequencies. As the frequency increases, it becomes progressively harder to move the physical mass of a microphone body; thus, handling noise rarely has much content above a few hundred hertz unless the microphone is very small and light, such as a subminiature personal type. However, the low frequencies are also the hardest ones to eradicate.
A typical mass/spring system can isolate suspension. It consists of an oscillating mass exhibiting compliance (the ease with which the mass is set in motion) and damping (which is the dissipation of energy from the system). Mic suspension complies with the rules of physics associated with such a system. One of these is that given the fixed mass of a microphone, the lower the frequency of the handling noise, the further suspension has to be able to wobble to give a particular degree of isolation.
To isolate effectively from low-frequency handling noise, ideal suspension should have high compliance and damping. This ensures that the microphone can move many centimeters, but will be brought back gently to its original position with minimal overshoot (perfectly damped). This sounds great except that practical high-compliance suspensions can also set up the microphone to wobble up, down and sideways rather wildly and make it difficult to control on a boom pole, for example. Fortunately, microphones are not as sensitive to handling noise on every axis. The one axial to the diaphragm, Z, is far more important than the other two, X (sideways) and Y (up and down), so sophisticated suspension can exploit this to give both good isolation and good control.
Importance of resonance
Figure 3
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Every spring system has a natural resonant frequency, and at the point of resonance, a stimulus will be accentuated rather than damped. This fundamental frequency also includes harmonics that will repeat the same behavior at higher frequencies, though in an increasingly damped fashion. Because resonance is rarely identical on each axis, microphones can frequently buck and yaw rather vigorously near these particular frequencies. However, at above roughly three times the resonant frequency, the fairground-ride behavior calms down, and it is possible to make a suspension isolate with reasonable efficiency. (See Figure 1 on page 17.)
The resonance point is important in defining how suspension functions and needs to be as low as possible. Ideally, it should be less than one-third of 20Hz (the lower limit of human hearing), so the mount is isolating at the lowest frequency we can hear. With large, heavy microphones, this is feasible (though not always easy). With lighter microphones, it gets progressively more difficult; the spring element has to be much more compliant to match. With the small, light designs beloved of location recorders, getting the fundamental below 20Hz can take some ingenuity.
Practical considerations
Figure 4
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The biggest problem is combining this high compliance with sufficient control to prevent the microphone from wobbling around so vigorously that it starts to generate extra handling noise, becomes uncomfortable or distracting for artists underneath, or even crashes into the basket of a windshield. Being able to tailor different degrees of compliance into each axis and to dampen any oscillations heavily so the mount doesn't ring are key factors. (Ringing here is used in the sense of long-term oscillation after an energy impulse, not necessarily to mean producing a bell-like tone).
Another problem is nonlinearity. Rubber bands, cushions or diaphragms are extremely common in suspensions, but their behavior under tension is severely nonlinear. As with stretched guitar strings, they become harder to move the tighter they get the compliance reduces drastically so the resonant frequency rises sharply with displacement. This means that the suspension works dramatically differently for small or large movements.
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