A microphone is a transducer which converts sound pressure waves to closely corresponding electrical impulses.
Pickup PatternsA microphone will be designed and constructed to possess a certain pickup pattern. This is also called a polar pattern, the term being derived from the way in which pickup patterns are graphically plotted.
The names given to the basic pattern types are more or less self-explanatory. An omnidirectional pattern indicates that the microphone picks up sound equally from all directions, within the frequency range of its sensitivity. Other patterns exhibit some degree of off-axis rejection, as plotted on a polar graph. An analogy is used to describe some of these patterns, owing to the resemblance to a heart-shape which their plots produce. These include the cardiod, hypercardiod and supercardiod patterns.
The ideal of extreme off-axis rejection is unidirectionality. While no actual microphone is ideally unidirectional (which would be undesirable anyway, as such a pickup pattern would in practice render the microphone insensitive except at a mathematical point exactly on axis), microphone designs aspire to approach this ideal to some extent. The purpose of manipulating the pickup pattern of a microphone in this way is to emphasise the sound coming directly to its front, while de-emphasising extraneous sounds in the immediate vicinity. This serves to help isolate sound sources in the recording environment, and to reduce acoustical feedback in live amplified situations.
Types and Characteristics of MicrophonesSeveral types of microphones are used in recording situations. While each design type will tend to perform in a certain way, individual models of any particular type will vary greatly in terms of performance. This section briefly describes the physical principles, general performance characteristics and typical applications of the various microphone types.
Dynamic MicrophoneDynamic microphones operate on a passive electromagnetic principle employing a thin diaphragm to pick up sound pressure waves. The output of the element is relatively high, but since no internal amplification is used, dynamic microphones require a high gain at the input stage of a preamp. Furthermore, the mass of the dynamic element, including the diaphragm and coil, which are required to move in response to sound pressure, tends to be comparitively high. The net result is a relatively noisy signal, slow transient response and narrow frequency response. For these reasons, in the recording environment they will typically be reserved for loud sound sources for which accurate reproduction of the attack and detailing of the high end are not critical. The effect of these qualities can in fact be desirable when applied to certain sound sources. Dynamic microphones tend to be rugged, and so find their true niche in live performance situations.
Condenser MicrophoneCondenser microphones operate on an active electrostatic principle, in which a capacitor is used to produce electrical output. The element of a condenser microphone, called a capsule, produces a relatively low output signal, and so a preamplification stage is built in. Condenser microphones exhibit comparitively fast transient responses and wide frequency responses, and are in practice quieter than dynamic microphones. They are the most widely used type of microphone in the recording environment. Special cases of the condenser type include the electret microphone and the pressure zone microphone (PZM).
Ribbon MicrophonesLike dynamic microphones, ribbon microphones operate on a passive electromagnetic principle, but employ a metallic ribbon sensitive to sound pressure. Ribbon microphones tend to be delicate and expensive, and are not widely used. Nevertheless, the peculiar movement of the ribbon in response to sound produces a signal such that the reproduced sound is warm and full of character.