Handling Noise of Microphones

Handling Noise of Microphones

Date Updated: February 2, 2012 FAQ #2845
Question:
What causes microphone handling noise and what can be done about it?
Answer:


Handling "Handling Noise"



Handling noise, a phenomenon not understood well by many, refers to any undesired noise that comes from a cable or microphone when being moved or handled. This bulletin identifies the source of the problem and presents some possible solutions.
Although handling noise comes primarily from physical handling, it is the combination of various factors that, in some cases, can correct or at least minimize the problem. In other cases, it is just physics and like gravity, we have to live with it. Let's start with noises generated at the microphone:
Pressure Gradient Noise
ImageThis type of noise is generated only in directional microphones (cardioid, supercardioid, etc.). Looking at Figure #1 we can see that a directional microphone has small ports on the backside of the capsule. These ports allow pressure equilibrium between the front and the back of the diaphragm. When sound waves reach the microphone, the diaphragm will move or sense changes in the pressure gradient, thus generating an electric current. In order to respond to very low frequencies the diaphragm has to be loose enough to move in response to those very small differences. The problem arises when the microphone is moved because the inertia of the diaphragm will make it flap briefly. This vibration caused by the movement of the microphone will generate noise in the infrasonic range, just a couple of hertz, that will overload almost all input devices. This noise manifests as a steady flat low frequency sound that stops when the microphone stops moving. It is likely that no audio is heard but there will certainly be fluctuation in the VU meter. The solution for this problem is a subsonic filter right after the microphone. Many studio microphones have this type of filter built in. For those cases where a subsonic filter is not present in the microphone, an external filter will do the trick.
Wind Noise
Similar to gradient noise, wind noise is generated by wind striking the diaphragm. This problem is most noticeable when using a microphone outdoors or in a very windy environment. It can present itself as a steady low frequency noise or even a whistle. The actual tone, type and level of the noise depends on the incidence angle of the wind with respect to the diaphragm, the worst case when the angle is completely perpendicular. Wind noise should not to be confused with the noise generated when a talker blows or breathes close to the microphone or speaks with loud plosives (P's and T's). Although the resulting noise is quite similar, breath noise and plosives can be solved by moving the microphone away from the wind blow path. The common solution for wind noise problems is a windscreen covering the grille. The only downside to a windscreen is a slight reduction in high frequency response.
Mechanical noise
All microphones consist of a capsule, a connector, and perhaps a circuit board or transformer, mounted in a handle or other enclosure. Sometimes the rubber or insulation material that holds those pieces in place comes loose or deteriorates, allowing mechanical contact between the handle and the interior pieces. When that happens any movement of the microphone generates mechanical noises that are picked up by the capsule and become part of the audio signal. The most common mechanical noise from a microphone comes from the connector. It could be a loose XLR or a poor connection between the cable and the microphone. To solve the problem, verify that the XLR is properly held in place with the screw securely tightened. In some cases changing the connector on the microphone or applying a drop of hot glue to hold the XLR in place solves the problem. If the contact between the microphone and cable is the problem, changing the cable or the female XLR at the end will generally solve this since the contacts on the female are spring-loaded and can come loose with time. All other internal noises usually require specialized tools and parts to rebuild the microphone and should be done by authorized service centers.
Internal Shock Mounts
All microphones have some sort of shock mount that isolates the capsule from the handle. These shock mounts can be made from many different materials giving them different properties, some good, some not so good. A microphone might have a shock mount with a resonant frequency in the audible spectrum that could affect the frequency response of the microphone when that particular frequency is excited. Unfortunately, the only solution to this problem is a better-designed microphone.
External Suspension
Although external suspension systems should not qualify as handling noise since the microphone is not being directly handled, there are some cases where this is applicable. These types of mounts offer some good isolation but might have different resonance frequencies in different axes. For example, it is possible that the mount is very good at absorbing side ways shocks, but a front to back shock generates a tone or vibration that is transmitted mechanically to the capsule. One way to find out if this is a problem is by tapping the suspension parts with a pencil and listening for noise. Remember that all elastic bands deteriorate with time and need replacement.
Cable noise
ImageProbably the best known and least understood of all handling noises, this noise comes directly from the cable and has nothing to do with the actual microphone (i.e. changing the microphone doesn't solve the problem). The noise can be generated in two different forms but are related to the same property of the cable. All audio cables have one or two conductors surrounded by a shield. This arrangement forms a capacitor with a value directly related to the thickness of the internal insulation separating the shield and inner conductors and the length of the cable. One of the problems occurs when the shield rubs against the inner insulation. Similar to rubbing your shoes against a carpet, static electricity is generated in the insulation and can cause some crackling noises when the cable is moved or twisted. This effect is more common in unbalanced lines and is solved with proper cable design. New technologies include an electrostatic shield in the form of a thin layer of rayon braid or conductive tape that allows discharging of these electrostatic charges (see figure #2). The other effect, sometimes referred to as being microphonic, happens when the cable is bent or slapped. When this happens the space between the outer shield and the inner conductors varies, generating a voltage difference much like a condenser microphone. ImageThis effect, known as Tribo-electric noise, is greater in twisted pair cables (also known as balanced cables - see figure #3) than in coaxial type cables but is greatly reduced with low load impedances like those found in professional microphones. As with the previous case, well-designed cables minimize this problem with a thick insulation and the use of filler strands (bits of strings) in the cable to prevent deformation.
Proximity Effect
Although proximity effect is not considered handling noise, it is important to mention it here because it is often misinterpreted as such. Proximity effect is the increase in low frequency response of a directional microphone when it gets closer to the sound source. In many cases this is desirable, but not in others. Since this is a physical property of directional microphones, the solution is to either use an omnidirectional microphone, or place the microphone further away from the source.
As we have seen, there are various sources of noise when using a microphone and cable. Some of them can be corrected; others can only be minimized. Knowing were they come from and how to identify them can help get the best possible result.


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