Introduction to Computer Music: Volume One

3. Microphones | page 4

Microphone placement and controls

Decisions in microphone placement should be based on numerous issues: how much of the source vs. room ambience do you want, do you want more bass response, what sort of a stereo image do you want, what is the dynamic range of the source, what are the time and phase issues in multiple mic'ing set-ups. Some of the terms below will help you in making those decisions.

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.) Therefore, you should spend a great deal of time adjusting the distance of a microphone to the source sound. The difference between 1.5 feet and 3 feet (only 15") will drop your signal strength by 3 dB, or half of its intensity!

Ambience refers to the amount of indirect signal recorded along with the source, often a mixture of room reverberation and possible other sources, such as the heating and air conditioning 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. The 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. At I.U., with its acoustically imperfect concert halls, we often joke about applying the European aesthetic of "recording the hall, not the ensemble." That is an aesthetic more suited to acoustically outstanding halls. A "2:1 rule of ambience" says that 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 study of their patterns on the previous page would make this obvious.

The proximity effect exaggerates the low-frequency content of an acoustic source when a microphone is placed near the source (within about two feet). The more directional the microphone, the greater the proximity effect will be. Also, the closer the mic, the greater the exaggeration will be. You have undoubtedly heard this effect with 'boomy' radio personality voices.

The bass rolloff control on a microphone attenuates (lessens) the intensity of bass frequencies caused by the proximity effect or floor vibrations, etc. The switch has an off position and a selection of one or two cut-off frequencies (e.g., 70 Hz, 150 Hz).

The attenuation control reduces the strength of the signal coming from the microphone transducer by 10-20 dB (0 dB means no attenuation). Often this is just a mechanical baffle. This can help prevent overloading the internal circuitry of a condenser mic when subjecting the mic to very high acoustic levels. Normally, start with no attenuation.

Feedback is a loud ringing or screaming sound created by an acoustic loop between a live microphone and loudspeakers. Solution: turn off speakers whenever recording in the same room, and use headphones to monitor recording. For sound reinforcement (i.e., amplifying sound potentially in the microphone's field), moving mics behind the speakers whenever possible is a start. 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 not without impact on the timbre of the source you are amplifying.

Pop filters are used to combat sudden explosions of air pressure caused by certain consonants like 'P' and 'D' that can cause overloads for sensitive microphones. These can cause pops or thumps. A pop filter, which is usually a circular open cell material will stop some of this when placed between the vocal source and the mic. Well-trained vocalists also have learned to move the mic away or sing plosives to the side.

Wind screens attempt to filter out wind blowing across the diaphragm of a mic, causing extremely loud unwanted pressure variations. They are usually foam coverings that go over the entire capsule, although you may see news crews with huge fuzzy things that do the same, only better, and often include a shockmount. (One type of these, used on a shotgun mic, is called a Zeppelin).

Shockmounts are used to reduced the acoustic coupling of the mic stand to the mic. Foot tappers, mechanical vibrations on a stage, and other unwanted sounds are typically transmitted to the microphone from the stand. By suspending the mic in a shockmount, with rubber bands being the only contact between the mic and the stand, a great deal of this is reduced.

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