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Microphones

The overall performance of a sound system starts with proper microphone selection. Microphones are selected based on the type of sound source (voice, musical instrument, etc.) and to the sound system.

Dynamic and Condenser Microphones

A transducer is a device that changes energy from one form into another, in this case, acoustic energy into electrical energy. It is the part of the microphone that actually picks up sound and converts it into an electrical signal. There are some of the basic capabilities of the microphone. The two most common types are dynamic and condenser. Although there are other operating principles used in microphones (such as ribbon, crystal, carbon, etc.) these are used primarily in communications systems or are of historical interest only. They are rarely encountered in worship facility sound applications.

Dynamic microphones employ a diaphragm/voice magnet assembly which forms a miniature sound- driven electrical generator. Sound waves strike a thin plastic membrane (diaphragm) which vibrates in response. It is the motion of the voice coil in this magnetic field which generates the electrical signal corresponding to the sound picked up by a dynamic microphone. Dynamic microphones have relatively simple construction and are therefore economical and rugged. They can provide excellent sound quality and good specifications in all areas of microphone performance. In particular, they can handle extremely high sound levels.

Condenser microphones are based on an electrically- charged diaphragm back-plate assembly which forms a sound-sensitive capacitor. Here, sound waves vibrate a very thin metal or metal-coated-plastic diaphragm. The diaphragm is mounted just in front of a rigid "back-plate" (metal or metal-coated ceramic). In electrical terms, this assembly or element is known as a capacitor (historically called a "condenser"), which has the ability to store a charge or voltage. When the element is charged, an electric field is created between the diaphragm and the back-plate, proportional to the spacing between them. It is the variation of this spacing, due to the motion of the diaphragm relative to the back-plate that produces the electrical signal corresponding to the sound picked up by a condenser microphone.

All condenser microphones contain additional circuitry to match the electrical output of the element to typical microphone inputs. This requires that all condenser microphones be powered: either by batteries or by "phantom" power (a method of supplying power to a microphone through the microphone cable itself).

Condenser microphones are more complex than dynamics and tend to be somewhat more costly. However, condensers can readily be made with higher sensitivity and can provide a smoother, more natural sound, particularly at high frequencies. Flat frequency response and extended frequency range are much easier to obtain in a condenser. In addition, condenser microphones can be made very small without significant loss of performance.

Frequency Response

The frequency response of a microphone is defined by the range of sound (from lowest to highest frequency) that it can reproduce, and by its variation in output within that range. It is the frequency response that determines the basic "sound" of the microphone.

Directionality: How does the microphone respond to sound from different directions?

The directional characteristic of a microphone is defined as the variation of its output when it is oriented at different angles to the direction of the sound. It determines how best to place the microphone relative to the sound source(s) in order to enhance pickup of desired sound and to minimize pickup of undesired sound. The polar pattern of a microphone is the graphical representation of its directionality. The two most common directional types are omni directional and unidirectional.

A microphone that exhibits the same output regardless of its orientation to the sound source will show on a polar graph as a smooth circle and is said to have an omni directional pattern. This indicates that the microphone is equally sensitive to sound coming from all directions. An omni directional microphone can therefore pick up sound from a wide area, but cannot be "aimed" to favor one sound over another. A unidirectional microphone, on the other hand, is most sensitive to sound coming from only one direction. On a polar graph, this will appear as a rounded but non- circular figure. The most common type of unidirectional microphone is called a cardioid, because of its heart- shaped polar pattern.

A cardioid type is most sensitive to sound coming from in front of the microphone (the bottom of the "heart"). On the polar graph this is at 0 degrees, or "on-axis". It is less sensitive to sound reaching the microphone from the sides ("off-axis"), and the direction of least sensitivity is toward the rear (the notch at the top of the "heart"). For any microphone, the direction of least sensitivity (minimum output) is called the null area. For a cardioid pattern, this is at 180 degrees or directly behind the microphone.

Thus, a unidirectional microphone may be aimed at a desired, direct sound by orienting its axis toward the sound. It may also be aimed away from an undesired, direct sound by orienting its null angle toward the sound. In addition, a unidirectional microphone picks up less ambient sound than an omni directional, due to its overall lower sensitivity at the sides and rear. For example, a cardioid picks up only one-third as much ambient sound as an omni directional type.

Microphone placement and output

Sounds can be categorized as desired or undesired and that the sound field, or total sound in a space, is made up of both direct sound and ambient sound. The level of direct sound decreases with distance (the inverse-square law) while ambient sound stays at a constant level. The critical distance is the distance (from the sound source) at which the level of direct sound has fallen to the level of the ambient sound. Critical distance is determined by the loudness of the direct sound relative to the loudness of the ambient sound. A quiet talker in a noisy room has a short critical distance while a loud talker in a quiet room has a longer critical distance. In practice, microphones must be placed much closer than the critical distance to get an acceptable ratio of direct-to-ambient sound.