The following information is published with ROUNDALAB's permission from the ROUNDALAB Reference Manual compiled by Richard & Jo Ann Lawson, 1987.
Microphone
Fred & Kay Haury
Albuquerque, New Mexico
Microphone (Mic, Mike): A microphone converts sound power (compression and rarefaction of atmospheric pressure) into audio frequency electrical signals. These electrical signals may then be amplified and used to drive loud speakers, which convert the amplified electrical signals into sound power. An output level of -58 dB is typical for the dynamic microphone used by round dance instructors. A suitable microphone will provide a relatively flat conversion response throughout the audio frequency range (60 to 15,000 Hertz). Some equalization or adjustment of frequency response of the overall Microphones-amplifier-speaker system is available by adjustment of tone/bass/treble controls of the amplifier.
A directional microphone is desirable for use with a sound system to be used in a dance hall. A directional microphone is more sensitive to sound entering directly into the face than to sounds entering at angles or passing from the sides or from the rear. The directional selectivity reduces the amplitude of undesired sounds which are directional in character, including feedback from nearby speakers. The "Cardioid" microphone is a directional microphone, so named because of the directional response pattern of the microphone which is the shape of a cardioid.
The maximum operating amplifier gain setting for a microphone at a given location may be determined by speaking into the microphone and advancing the gain control until ringing is first noticed, then backing off the gain until ringing is eliminated. The setting should be tested by aiming the microphone in various directions of probably use, to avoid ringing or oscillation from unexpected feedback when in use. The gain control should not be increased beyond the maximum operating setting.
The distance from mouth to microphone will vary depending on the user's voice level, microphone technique, angle of microphone to source, and maximum permissible gain setting. The gain control may be reduced to accommodate hall requirements when the microphone user has a high voice level and holds the microphone relative close to the mouth. A user with a weak voice will need to move the mike closer to the mouth if the gain control is at maximum operating setting and an increase in hall sound level is required.
The amount of energy activating the microphone varies inversely with the square of the distance between the microphone diaphragm and the source. When the distance from mouth to diaphragm is doubled, the energy received by the diaphragm is reduced to one-fourth. A reduction of the distance to one-half will increase the diaphragm energy by a factor of four. A reduction to space to one-fourth the initial distance will produce an energy level sixteen times greater.
Users who prefer to hold the microphone a large distance from the mouth may obtain a satisfactory sound level by speaking directly at the microphone with the gain control set for maximum permissible level. Users who hold the microphone close to the mouth may obtain best operation by speaking over the diaphragm opening and reducing the gain control setting. Close mouth users should not speak directly into the opening because initial wind blasts associated with many words will produce undesirable hisses and blips.
Never blow directly into the aperture of a microphone. A dynamic microphone is a small transducer similar in construction and appearance to a miniature speaker. The microphone cone or diaphragm is constructed of very thin metallic material with slightly more rigidity than a stiff foil. The voice coil is made with fine hair-like wire. The microphone is designed to be actuated by voice level sound power, so the range of diaphragm and voice coil travel is very small. Blowing into the aperture will cause diaphragm pressures, plus cone and coil movements to exceed design limitations. The excessive pressures can fracture or warp the diaphragm resulting in permanent disconfiguration of the cone, which will cause distortion of the voice frequency response characteristics of the microphone. Excessive voice coil travel resulting from strong wine blasts could strain and break coil connections; thereby, rendering the microphone completely inoperative.
Strong blows and jars such as dropping or striking the microphone on the palm of the hand may also cause damage to the cone or coil. Most high-quality microphones are equipped with an on-off switch. The switch is useful to silence a microphone which is plugged into a live amplifier when the microphone is not in use. The on-off switch should always be in the "off" position when the microphone is not plugged into an amplifier, especially when a microphone is being transported between locations. The "off" position place a short circuit across the voice coil terminals, which electrically "loads" the voice coil movement and inductively retards the cone's travel in response to sudden inertial changes. The loading effect provides a good measure of protection against damage should the microphone be dropped or receive other jars from careless handling.
Never hold a microphone by the cord as a pendulum. The cord was designed to provide electrical connections, not support the microphone. Such treatment will shorten the life expectancy of the cord wires and connections. Use of the microphone as a swinging pendulum may also result in striking it against nearby objects. In general, remember the microphone is designed as a delicate instrument. Treat it as such.
- Editor's comment:
- There is an old saying that states: "Ignorance is bliss!" I really appreciate the comments from the Haury's on the microphone and will not be ignorant again. Thereby, even increase the life expectancy of my microphone and help develop the feeling of "microphone technique". Being familiar with the equipment helps!
Roundalab Journal, February, 1982