4 Introduction to Microphone Mounting and Placement
Having now established the various microphone construction types and the variety of characteristics and polar patterns, this section deals with placement techniques for specific applications.
4.1 Hand Held
The majority of microphones produced today are intended to be used hand-held and are of the cardioid pattern, particularly those intended for sound reinforcement (PA systems). Being hand held they are at risk of being dropped so they are generally of sturdy construction and shock resistant. The sound element is well isolated to greatly reduce handling noise and they have protective shields with an inner head lining that can, on some microphones, be removed and washed. However, omnidirectional microphones are inherently less susceptible to handling noise than cardioids, and make a good alternative as a hand-heldreporter’s mic for vox-pop type interviews where proximity can be more important than the directional characteristics. Indeed, there is less chance of an in-experienced presenter getting either the interviewee or themselves “off-mic” when using an omni.
4.2 Stand-mounted
Where you don’t have a spare pair of hands to hold a microphone, or where the mic does not need to track the action, a fixed mic stand is a useful accessory. Whether on a floor-stand, or when a hand-held ‘boom’ (long pole) is used to enable the operator to get the microphone as close as possible to the sound source, it is best to attach the microphone using a rubber-based shock mount designed to keep vibration interference to a minimum. While stand-mounted microphones are highly recommended, one type of stand mounted microphone that should be avoided – as much as is possible – is the one attached to the video camera! It is prone to picking up the vibrations of the camera mechanisms, the zoom controls and our less than gentle handling!
4.3 Radio Microphones
Simply put, a radio microphone is a matched pair of a radio transmitter and receiver units, which carry the signal from any of the aforementioned microphone types to the camera or sound recording device. Radio mics give a freedom of movement between camera and performer that no other mic can give as there is no restrictive ‘umbilical’ cable attaching them together. That said, licensing regulations limit the transmitter power to quite a low level compared to many radio communications devices, and other external factors like walls and structural metalwork can affect the RF signal strength reaching the receiver, resulting in momentary signal drop-outs when moving around. The better systems use a “diversity” system with two aerials on the receiver, which automatically switches to whichever one is picking up the stronger signal, helping to eliminate drop-outs within the normal operating range.
Radio mics can have the transmitter built into, or plugged onto the bottom of, a hand-held type of mic (for vocalists or interviewers), but for video work are commonly configured as belt-pack transmitters, connected to lavalier microphones. These microphones are usually clipped on the subject’s chest, therefore keeping the distance between the sound source and the microphone fairly constant. Again, most lavaliers can be mounted upside down, to cut down problems caused by the speaker’s breath. A good practical tip when microphones must be hidden from view is to use toupee-tape to firmly attach the lavalier microphone to the subject as this cuts down on the background noise caused by the person’s clothing moving against the microphone.
Radio receivers and transmitters can also be used in conjunction with microphone mixers, where the output from the mixer is then fed to the input on the radio transmitter and then on to the receiver at the camera position or main visual mixing desk. This negates the need to run long cables on location from the point of sound to the filming position. This will usually require a special “line level” input cable for the radio mic transmitter, or the use of attenuating in-line “pads” to reduce the line level down to mic level.
As a final word of caution when using radio microphones, the better systems offer the ability to adjust the input gain on the transmitter to cope with different microphones and the dialogue levels expected. If set incorrectly, it is therefore possible to overload the transmitter before the signal reaches the receiver, never mind your camera or recording device. Adjusting the level on the recording device will not cure this problem, so care must be taken when setting the input gain on the transmitter. The cheaper systems without gain controls will either use fixed gain or an AGC/limiter system, both of which can be overloaded and degrade the audio. At the other extreme, setting the input gain too low will impair the signal-to-noise ratio
4.4 Practical Applications
Whatever the microphone-type is used there are a number of best practices that should be adopted. >break> i) Keep to a minimum the distance between the microphone and the subject being recorded. (but don’t get so close as to cause breath “popping”. 1-2ft is generally a good distance).
ii) Develop an understanding of the working angles of the polar patterns and attributes of your microphones, so you can keep the sound source directly on-axis and avoid any unwanted off-axis sound.
iii) Endeavour to keep the distance between the sound source and the microphone constant. Take every opportunity to test and play with different types of microphone – they are not all the same and nothing beats the ‘hands on – ears out’ approach. If you do experiment you may be pleasantly surprised at the difference the right microphone makes.
Susceptibility to wind noise
The air pressure changes generated by sound waves are generally so low that although they are heard by the ears, they are not felt as moving air by the body. Even at a really loud event like a rock concert you are unlikely to feel the air moving unless you place your hand right in front of a bass speaker cabinet where you can actually see the speaker cones moving. The diaphragms in microphones are designed to respond to these tiny changes in air pressure, which cause equally tiny amounts of physical movement, so it’s not difficult to imagine that air movement (e.g. wind) that we can feel with our bodies will have quite a disruptive effect on such a delicate instrument, pushing the diaphragm out of its normal operating range and giving rise to all manner of unwanted signal output.
At the onset of disruption, wind noise starts to manifest itself as low frequency rumble which can be filtered out electronically with a high-pass or bass-cut EQ filter, but as higher pressure levels buffet the diaphragm, harmonic noise is generated right acrossthe audio frequency spectrum and is generally impossible to remove with an electronic filter on the camera or in post- production.
This is why microphones need protecting from any conditions where moving air may hit the diaphragm. This can sometimes be achieved by careful positioning, or the use of windshields to isolate the diaphragm, deflect most of the moving air, and slow down any moving air which penetrates the outer layer of the windshield. In this respect bigger is usually better, and a full size “basket” windshield will give better protection to a shotgun mic used outdoors than a smaller foam device, although indoors foam windshields are normally adequate to cope with more minor drafts. Many hand-held mics have a windshield built into the head, but these are only intended for indoor use and even these can still suffer from the effects of breath (popping) when used very close to the mouth (ie less than a foot).