Broadcasting (Oct 1931-Dec 1932)

The Xew Velocity Microphone (Continued fr phragm. Because the Velocity Microphone is a velocity-actuated device — does not utilize a diaphragm— it is subject to none of these undesirable phenomena. As a result its frequency response is a flat curve, and its reproduction is more natural than that of any microphone yet devised. Directional Characteristic THE INTENSIVE study of studio technique which many stations are making has made the directional characteristics of the microphones used of considerable importance. This results from the fact that this characteristic has two important effects. It determines, first, the placing of the artists, and second, the amount of reverberation picked up by the microphones. All previous types of microphones were very unsatisfactory in this respect. The condenser microphone, as well as more recently developed microphones in which the diaphragm supports an additional moving element, are entirely non-directional up to 2,000 cycles, while at higher frequencies they become very directional. This is due to the fact that these microphones are pressure-operated— it is a phenomena typical of all microphones using a diaphragm. Such a characteristic is, in fact, more undesirable than a pure non-directional characteristic throughout the entire range. In general, excess reverberation occurs at the lower frequencies due to the fact that the absorption characteristics of most materia] used in reducing reverberation are less efficient at the lower frequencies. Using a microphone which is non-directional at the low frequencies and directional at the higher frequencies means that the excess low-frequency reverberation will be further accentuated. This means that the amount of direct sound pick-up must be relatively great in order to keep it well above the level of the reverberated pick-up. This entails crowding of the artists about the microphone and — because of the sharp beam of such microphones at the higher frequencies — requires direction of the microphones on the point of action. Aid to Artists THE VELOCITY Microphone is particularly good in this respect. It has a marked directional characteristic which is entirely inde om page 10) pendent of frequency. Due to this characteristic the energy response of the Velocity Microphone to generally reflected sound is only onethird* that of non-directional microphones such as the condenser and other pressure-operated types. This has a very important effect on the distance at which artists may be placed from the microphone. The maximum satisfactory distance is determined by the allowable reverberation. Reverberation is the ratio of the generally reflected to the direct sound. The generally reflected sound is usually independent of the positions of source and microphone. As noted above the Velocity Microphone reduces this by a factor of three. The direct sound varies inversely as the square of the distance, hence with this microphone the artists may be placed V3 or 1.7 times further away than previous types of microphones. This advantage may be utilized in other ways. For instance, if the spacing of microphones and artists is kept the same, much less damping or absorbing materials need be used on the walls and ceiling in order to obtain with this new microphone the results previously obtained with other types. Moreover, the amount of reverberation can be adjusted as desired by turning the microphone at a slight angle to the source. This can be done with the Velocity Microphone where it could not with previous types of microphones, because it does not, like these latter, discriminate against the high frequencies when so used. The directional characteristics of this new microphone may also be used to advantage in eliminating unwanted noises. Illustrative of this is the fact that a shout at the side of it will be less audible than a whisper in front of it. Thus interfering noises filtering through a door or window — or that of an audience in the studio — may he eliminated by placing the microphone so that its dead side is toward the source of such sounds. Bi-Directionalism ANOTHER feature not before mentioned is the bi-directionalism of the Velocity Microphone. The construction, which is open both in front and back, allows pick-up equally in both directions. As a result artists may be placed equal(Continued on page 26) Frequency Measuring Service Many stations find this exact measuring service of great value for routine observation of transmitter performance and for accurately calibrating their own monitors. MEASUREMENTS WHEN YOU NEED THEM MOST R. C. A. COMMUNICATIONS, Inc.. Commercial Department A RADIO CORPORATION OF AMERICA SUBSIDIARY 66 BROAD STREET NEW YORK, N. Y. WHEN YOU MEASURE COVERAGE ♦ . don't forget percentage modulation u Field-intensity data are basic in determining coverage, but they aren't the whole story by any means. Percentage modulation is just as important a factor in pumping more power through your listeners' loud-speakers. B Consider that an increase in percentage modulation from 50 rc to 100^ will increase the loudspeaker power by four times. Yet the field intensity of the carrier remains the same. That is why the Federal Radio Commission demands that you modulate at high percentage. H But remember that unless you exercise extreme care your high-percentage modulation will be accompanied by serious distortion. Even in properly designed transmitters a wrong value of grid bias, for example, may cause trouble. B You can check on the performance of your transmitter at high-percentage modulation by means of a General Radio modulation meter. It not only measures accurately your percentage modulation, but it shows up distortion due to asymmetrical modulation of the car rier. The General Radio modulation meter is coupled to the transmitter-output circuit, its operation is in no way influenced by transmitter changes. Let us tell you more about the General Radio modulation meter. Write us for literature. OTHER INSTRUMENTS By General Radio Frequency Monitor Volume Indicator Volume Controls Audio Transformers Distortion Meter Piezo-Electric Quartz Crystals CAMBRIDGE A. MASSACHUSETTS August 15, 1932 • BROADCASTING Page 25