Description:

The velocity microphone is the result of several years of intensive research and development toward the improvement of the characteristics of microphones as used for broadcasting, and is entirely different in principle and construction from other microphones now in use. This microphone is admirably suited to studio pick-up, public address and sound reinforcement applications.

Instead of a “diaphragm” (in the commonly accepted meaning of the word), the velocity microphone contains a thin metallic ribbon suspended between the poles of a permanent magnet with its length perpendicular to, and its width in the plane of, the magnetic lines of force. The opposite ends of the ribbon are connected to a transformer which matches the impedance of the ribbon to a 250- or 50-ohm line. Sound waves reaching the ribbon vibrate it within the magnetic field set up by the magnet. The vibration of the ribbon is in exact accordance with the sound vibrations and, occurring as it does within the magnetic field, sets up corresponding alternating electric potentials across the primary of its associated transformer. These minute voltages are subsequently amplified to the power level required for broadcasting. The microphone amplifier may be located remotely from the microphone unit when necessary or desirable.

Directional Characteristics
One of the most important characteristics of the velocity microphone is its directional property. Since the ribbon is suspended in free space, sound waves approaching the microphone from a direction in the same plane as the ribbon have no effect upon it. Sound waves from either direction along an axis perpendicular to the plane of the ribbon have the maximum effect. For equal distances from the transmitter, the relative response to sound originating at various angles to the axis perpendicular to the ribbon is shown in Figure 4.

It is at once apparent that this characteristic is of considerable value in the solution of some of the difficulties usually encountered in reverberant locations by the reduction of the effect of undesired sound reflections, and in the increased possibilities of obtaining better balance, clarity, naturalness, and selectivity in sound pick-up. Extraneous direct or reflected sounds approaching the microphone from side directions will have little effect, and therefore background noises and reflected sounds in the broadcast are considerably reduced, which increases, by comparison, the quality of the direct sounds reproduced. The degree of soundproofing necessary for sound originating within the “dead zone” is, of course, dependent upon the reflecting surfaces present which may return the undesired sound to the microphone from such directions that response may be obtained.

For the same allowable reverberation pick-up the operating range of the velocity microphone is approximately 1.7 times greater than a non-directional microphone having the same sensitivity. When used for public address and sound reinforcement purposes the directional characteristic is of considerable value in reducing feed-back effects between the microphone and loudspeaker.

Sound concentrators and baffles used with condenser microphones are unnecessary with and inapplicable to the velocity microphone because of the fundamental difference in the principle of its operation. The transmitter must be used in free space where the flow of air particles is unimpeded. However, “pick-up” from the rear direction of the microphone may be eliminated by placing a baffle or shield of heavy sound absorbing material,such as heavy felt, at a distance of not less than three feet from the transmitter and so confine the “pick-up” to the area in front of the microphone.

Quality of Response
The operating range of the microphone extends from 30 cycles to 15,000 cycles. When a velocity microphone is placed close to a source of sound the low frequency response is accentuated. In view of this fact, provision is made in the Type 44-B microphone to enable the user to alter its frequency response in such a manner as to suit best the particular purpose desired; viz., the pick-up of voice (i. e., within 2 feet of the microphone), or the pick-up of music (which takes place at greater distances from the microphone). Emphasis here is placed on the fact that this feature of the Type 44-B microphone is not provided with the intention that such alterations in frequency response be made at will; i. e., between selections on a broadcast program; but is furnished for the sole purpose of supplying a microphone with the best possible characteristics for vocal pick-up or for musical pick-up. It is recommended that the frequency response be adapted to either of these types of pick-up and the use of the microphone be restricted to that type of pick-up only.

A response curve taken with and without the “V” jumper is shown in Figure 5. As will be observed from examination of this curve, there is a sharp decline at the low frequency end of the curve when the “V” jumper is used. It is to be noted that this curve was taken in a plane wave field, and that the curve is flat when the speaker is located at a distance of 1 foot from the microphone.

To alter the frequency response proceed as follows: A small circular hole will be found in the cover plate of the transformer casing. Through this hole will be visible the letter “V” (voice) or the letter “M” (music), depending on whether the microphone is at the time adapted for vocal pick-up or for musical pick-up. Vocal pick-up requires the use of a jumper (upon which appears the letter “V”), which is to be placed across the two terminals marked “M” (music), located on the terminal block within the transformer housing. Access to this terminal block is obtained by removing the cover plate of the transformer housing. When the jumper “V” is used, it connects a reactor in parallel with a part of the transformer winding (when the 250-ohm connections are used), or with all the transformer winding (when the 50-ohm output connections are used). See the schematic diagram, Figure 6.

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