Educational screen & audio-visual guide (c1956-1971])

on the floor will help establish the limits employed. Upon playback of the tape the optimum limits for best results can be determined. In regular use it has been found helpful to position the mic and then to affix two cardboard silhouettes in the shape of footprints to the floor to enable student speakers to automatically establish the proper relationship. Height of microphone in relation to the performer is important. Ordinarily, the voice is projected from the mouth at a slight downward slant. This means that the mic should be lowered as the distance from the performer is increased. 3. When setting up for a recording session in a particular location it is well to test acoustics. This may be done by clapping the hands sharply. If persistent echoes result, undesirable reverberation is present. Measures taken to reduce this effect involve lowering of shades to cover glass areas, closing of drapes or hanging absorbing cloth panels near the microphone position. In some instances it is helpful to suppress the bass frequencies through the use of the tone control (also for playback) and to have the performer work somewhat closer to the mic to reduce acoustical interference. 4. Many microphones are adversely affected by humidity and should be stored in a relatively dry place. Also, it is unwise to allow the performer to work so close to the mic that his moist breath directly impinges upon the unit. Crystal microphones are particularly vulnerable to shocks and can be destroyed easily if measures are not taken to prevent accidental dropping, etc. For AV Coordinators Acquisition of microphones involves consideration of I number of factors other than price alone. Even an jxpensive microphone can be ill-adapted to a particuar application. The following items point up criteria nd characteristics important in making a selection: L. Three popular types of microphones are available to meet most requirements. a. Crystal microphone— is most economically priced, has low to moderate frequency response characteristics, but is vulnerable to extremes of heat, humidity and shock. Some types are sealed against humidity. Crystal mics can be employed in instances where high quality is not a requisite. b. Ceramic microphone— has essentially the same frequency response of the crystal and is slightly higher in price, but is resistant to heat, humidity and shock factors. c. Dynamic microphone— has a uniform and extended frequency response and is rugged and long-lasting but bears a higher price tag. This type is essential for applications where quality reproduction is involved. Impedance matching transfonuers are available to permit use of a low-impedance circuit where a long microphone cable is involved, or a high-impedance circuit where a relatively short cable is employed. Some microphones have built-in components to achieve this effect without additional accessories. (Continued on next page) omnidirectional Informal interviews conducted in a relatively quiet location can be picked up most satisfactorily by an omnidirectional microphone. This arrangement provides freedom of movement for several speakers. Shown below is an example of this type — a crystal, in this cage. For outdoor use, where the microphone cord is of considerable length, it is necessary to employ a low-impedance microphone to re<luce signal loss and eliminate buzz. The type of microphone will depend on the application intended and the particular area of use. MHiCATIONAL SCREEN AND AUDIOVISUAL GuiDE — ApRIL, 1961 177