Microphone Placement, Controls and Characteristics
Decisions in microphone placement should be based on numerous issues: how much of the direct source vs. reflections and room ambience is desirable, how much bass response vs. presence peak, what sort of a stereo image is best for the source and ultimate use, what is the dynamic range of the source, what are the single-mic phase issues from a reflected sound, what are the time and phase issues in multiple mic'ing set-ups. Some of the terms, concepts and options below will help in making those choices.
Inverse Square Law: The inverse square law means that at half the distance between source and mic, a microphone receives four times the sound intensity—this is in a soundfield with no reflective surfaces. Consider this the mother of all mic placement guidance. Therefore, a great deal of time should be spent adjusting the distance of a microphone to the source sound and testing it via playback, then readjusting if necessary. The difference between 1.5 feet and 3 feet (only 18") will drop your signal strength by 3 dB, or half of its intensity!
Proximity Effect: The proximity effect exaggerates the low-frequency content of an acoustic source when a pressure-gradient microphone is placed near the source (within about two feet). The more directional the microphone, the greater the proximity effect will be. The bass boost (up to 16 dB) is caused by the phase difference between the front and back of the diaphragm relative to the amplitude difference. Also, the closer the mic to the source, the greater the exaggeration will be. You have undoubtedly heard this effect with 'boomy' radio personality voices. Usually, pressure microphones, such as omnidirectional ones, exhibit little to no proximity effect.
Bass (low frequency) rolloff is a control on a microphone that attenuates (lessens) the intensity of bass frequencies caused by the proximity effect or floor vibrations transmitted through the mic stand (called rumble). The 'rolloff' means it is a slope, not a sudden drop at the rolloff frequency, as pictured here for the AKG C414 mic. The switch has 0 Hz (or 'off') position and a selection of cut-off frequencies (e.g. 40-80-160 Hz). If you desire a full-range recording, do check to make sure this is in the '0 Hz' position.
Attenuator: The attenuation control reduces the strength of the signal coming from the microphone transducer by 10-20 dB (0 dB means no attenuation) to prevent distortion. Sometimes this is just a mechanical baffle. The attenuation can help prevent overloading the internal pre-amp circuitry of a mic when subjecting the mic to very high acoustic levels. It cannot prevent the mechanical overload of the mic's diaphragm, but usually distortion is first caused by the pre-amp. Do not use this control as a quick substitute for adjusting the mic trims and fader levels on the mixing console. Normally, start with 0 dB attenuation, because recording low-volume sources with attenuation degrades the mic's signal-to-noise ratio.
Feedback is a loud ringing, screaming, often-undesirable sound created by an acoustic positive-gain loop between a live microphone and loudspeakers. Solution: turn off speakers and use headphones to monitor whenever recording in the same room. For sound reinforcement (i.e. amplifying sound potentially in the microphone's field), moving mics behind the speakers whenever possible is a start. Using mic patterns like cardioid and hypercardioid that are more directional with little or no rear lobes also is helpful, and/or making certain the rear lobes are pointed away from an potential feedback source. A graphic equalizer can be used to locate and attenuate the feedback frequencies if the microphone is not moved, as those feedback frequencies will then change. The process of locating the feedback frequencies and attenuating them is call ringing out. This is usually not without impact on the timbre of the source you are amplifying. An objective measurement of how much a mic can be amplified before it feeds back is call gain before feedback or GBF.
Colored Response: Microphones that do not linearly reproduce the frequency content of the source signal are considered colored. This is not always a bad thing. The ubiquitous Shure SM58, which is intended as a popular music vocal mic, has a presence peak that accentuates frequencies in the 2K-5K frequency range. Virtually all microphones have non-linear characteristics that often vary with the polar pattern selected. Here you can see the differences in response for the various AKG C414 patterns.
Hole-in-the-Middle Effect, generally undesirable, can occur when coincident microphones are pointed at too great an angle from the source and/or set too far apart. While this may offer greater stereo separation, it also leads to weaker pickup of the source sound directly in front of the mic array compared to those off-axis.
Spot mics, also called accent mics, are close mics used in multitrack recordings or sound reinforcement situations to allow engineers to feature a particular instrument or instrument group. For recordings, several of the problems that crop up with spot mics are time/phase differences from the house mics (this can be easily corrected with digital editing), differences in ambience (artificial reverb can help) and bleed from other sources that one didn't want to highlight (acoustic baffles can help). There is also the problem of mechanical transmission of sound from foot tapping or stage vibration, so shock mounts are helpful. Even correcting for these difficulties, this author has experienced 'harshness' and other disagreeable issues when using spot mics for all but limited use.
Outriggers, usually hung off to the sides parallel to the center array of mics, are often used in multitrack recording to pick up signals from a wide ensemble such as an orchestra or choir when using a narrow center array such as a Decca Tree.
Ambience refers to the amount of indirect signal recorded along with the source, often a mixture of room reverberation and possibly other unwanted sources, such as the HVAC vents. While there is no hard and fast rule for the 'perfect' spot to place your mics, a knowledge of the inverse square law, the microphone patterns, and a lot of experimentation before recording a final take is recommended. An overly ambient, weak-sourced recording is very difficult to undo. Reverb, for example, can be added artificially but not taken away when recorded from the actual environment. On the other hand, mic'ing a source too closely may pick up unwanted artifacts from the source, such as bow friction, heavy breathing, keys clicking, etc. In the past, the European aesthetic was in part "record the hall, not the ensemble." That is an aesthetic more suited to acoustically outstanding halls. A 2:1 rule of ambience states a cardioid microphone must be placed two times the distance from a source as an omni to capture the same amount of room ambience. A view of their patterns would make the reason clear.
Ambient mic: An additional mic or mics that are hung farther away from the source so the amount of desired room ambience can be adjusted in the mix.
Ambisonic mic'ing: Ambisonics are a spherical 360°surround format, developed in the 1970's, that specifies all dimensional axes, including those above and below the listener (left, right, up, down, front, back) and treats them all equally (isotropic). Because the playback speaker arrangement is precisely specified, so is the format for recording. Either separate mics, or dedicated ambisonic mics (such as a first-order tetrahedral mic ) are used with precise angles. Higher-order ambisonics gives greater spatial resolution to the source material. The ambisonic signal must be encoded when recorded and decoded when played back, since the signal is not in channel format, but in what is called B-format.
Acoustic Baffle: A technique for achieving greater stereo separation with less cross-talk between channels by placing a sound-absorbent physical barrier between close mics. Harvey Fletcher experimented with baffles for his AB mic'ing technique in the 1930's. A very cool modern example of baffling is the Jecklin disc technique, whereby a soft circular disc is placed between two omni microphones, each 3" away from the disc and angled out by 20°.
Mircrophone Impedance: If you are using or purchasing a professional-level mic, preamp and board, this is not something a composer would normally be too concerned about. Impedance is a measurement of resistance in the output or input of a device, measured in ohms (Ω). The higher the ohms, the higher the resistance and impedance rating. For microphones data sheets, the specs will usually give at least the output impedance, and for pre-amps or board inputs, the input impedance. Professional microphones will normally have output impedances in the 50-250Ω range, considered low, and above 20,000Ω (perhaps a $5 karaoke mic) is considered high (high impedance mics can lead to hums and electrical interference in longer mic cable runs). On the other end, preamps, which form a circuit with an attached microphone, whether separate or part of a board input channel, will best have input load impedences of at least 5 to 10 times the mic output impedence, so normally greater than 1,000-2,000Ω's. An example: the AKG C414 XLS mic has an output impedance of 200Ω, and the mic preamp input load impedence of a Yamaha 01v96 board is rated at up to 3000Ω, so that would be a good match.