Microphonic Machinations

Dec 1, 2001 12:00 PM, By Scott Wilkinson

A microphone's job is simple: it converts an acoustic sound into an electrical signal that corresponds to the original waveform as closely as possible. That signal then can be processed, mixed with other audio signals, and recorded.

Acoustic sounds occur when something vibrates at a frequency between 20 Hz and 20 kHz in air or another medium. That creates small regions of high and low pressure around the vibrating source. As the molecules in those regions move in response to the changing pressure, they jostle nearby molecules, and that causes the regions of high and low pressure to expand outward.

Once the regions of changing pressure reach a microphone, they impinge on a flexible diaphragm within the mic, causing it to vibrate in response. That physical vibration is then converted into an electrical signal, which is sent to a mixer, a signal processor, or another device. The difference between types of mics is the specific manner in which the conversion is performed.

PHYSICAL ATTRIBUTES

Before I discuss the various mic types, here are some concepts that are common to all. As mentioned previously, all microphones include a diaphragm, which is usually mounted in something called a capsule. The capsule is mounted in an outer case along with any support electronics. Some cases can be handheld or standmounted for stage use, whereas others must be mounted on a stand. Standmounted mics sometimes include a shockmount, which isolates the mic from unwanted vibrations in the stand.

In most cases, the capsule is located behind a screen of some sort that lets the acoustic sound enter while protecting the diaphragm from physical damage. That screen often includes a layer of foam to reduce wind noise and vocal pops, though an external pop screen is usually more effective in the latter application.

Once the acoustic sound has been converted to an electrical signal, it is conveyed to another device along a cable. Some inexpensive mics include a permanent cable that terminates in a 2-conductor, ¼-inch or ⅛-inch phone plug. The cable includes one central conductor that carries the audio signal surrounded by another conductor (called the shield) that connects to ground. However, that type of cable is susceptible to induced hum and other environmental noise. As a result, most professional and semipro mics use a 3-conductor XLR connector at the end of a balanced cable.

FREQUENCY RESPONSE

The frequency range within which a microphone accurately translates the sound-pressure level (SPL) of acoustic sounds into electrical signal levels is called its frequency response, which is measured in decibels (dB) over a range of frequencies. But what does “accurately” mean? For a given SPL, the output-signal level typically varies by no more than ±3 dB from its nominal level. That is normally depicted in a graph of frequency versus output level (see Fig. 1). A mic with a flat frequency response generates the same audio signal level for a sound of any frequency within the specified range at a given SPL.

However, most mics don't exhibit a flat frequency response, partly because making such a mic is expensive and partly because a frequency response that's uneven can be of some benefit. For example, many vocal mics boost the upper frequencies; that is the presence peak shown in Fig. 1, and it helps improve the intelligibility of words. However, a presence peak can exaggerate a shrill upper vocal range.

At the low end, the frequency response of a vocal mic often falls off below 100 Hz. Because the human voice can't produce frequencies that low, there is no reason to make a mic that reproduces them accurately. Instrument mics generally fall off below 50 Hz. However, the low-end response of many mics can be greatly enhanced by moving the sound source close to the mic. That bass boost is the proximity effect in Fig. 1, and it helps radio announcers achieve their characteristically deep sound. However, moving too close to the mic increases breath noises and vocal pops.

PICKUP PATTERNS

All mics exhibit a pickup pattern, which determines how the mic responds to sounds at different frequencies coming from different directions. An omnidirectional mic responds more or less equally to sounds coming from any direction. That pickup pattern is particularly well suited for ambient mics, which are used to pick up the sound of the room in which an acoustic source is radiating.

In many cases, omnidirectional mics are not used in live performance because they pick up sounds from all directions, which can lead to feedback. However, omni mics are generally less susceptible to wind and breath noise, and they tend to have a relatively flat frequency response with no pronounced peaks, which can actually help to avoid feedback. Omni mics also tend to have excellent low-frequency response, and they do not exhibit the proximity effect.

If a mic does not respond equally to sounds from any direction, it is called a directional mic. There are several types of directional mics, most of which respond best to sounds coming from directly in front of the mic's capsule. (The main exception is the middle-side, or M-S mic, which contains two capsules. M-S mics pick up sounds from both sides as well as the front.) Sounds that strike the mic at its most sensitive spot are on-axis; sounds from any other direction are off-axis. Directional mics are prone to the proximity effect, and their frequency response is normally less flat than in omni designs.

The pickup pattern of any microphone can be depicted in a polar graph (see Fig. 2). In this type of graph, the mic's axis is defined as 0 degrees (usually located at the top of the graph), and the outer circle defines a flat frequency response. The smaller, inner circles represent a drop in frequency response. The curve within the graph indicates how the mic responds to sounds from different directions. An omnidirectional mic's polar pattern forms a circle. Keep in mind that although polar patterns are conventionally graphed in two dimensions from a bird's-eye view, the mic's actual pickup pattern is three-dimensional.

The most popular type of directional mic is the cardioid (see Fig. 2a). Its polar pattern resembles an inverted heart — hence the name. The mic is most responsive to on-axis sounds, whereas off-axis sounds are attenuated; sounds from 180 degrees off-axis are almost completely rejected. In addition, notice that the polar pattern changes slightly at different frequencies. The extra curves give a rough idea of a mic's frequency response and pickup pattern in one graph.

A supercardioid mic is often used in live performance because it rejects more sound from the sides than a cardioid design (see Fig. 2b). However, it does have some response to sounds coming from 180 degrees, as indicated by the small rear pickup lobe. Another variation of that design, called the hypercardioid, is even more directional.

Some mics exhibit a bidirectional or figure-8 pickup pattern (see Fig. 2c), so-called for obvious reasons. Those mics are most sensitive to sounds from the front and rear, rejecting sounds from the sides. That works well for miking two sources (such as two toms in a drum kit or two singers facing each other) with one mic.