Broadcasting (Oct 1931-Dec 1932)

Velocity Microphone Makes Its Bow Radically New Device Discards Diaphragm, Possesses Marked Directionalism and Increased Fidelity THE NEWEST contribution of the laboratory engineers to the rapidly progressing art of broadcasting is described authoritatively for the first time in print in this article. The Velocity Microphone, the author contends, introduces new principles of construction and operation and will shortly replace the present devices in the more progressive studios. The new instrument improves studio pick-up, increases fidelity of reproduction and is more convenient than the present type the writer asserts. By J. P. TAYLOR Transmitter Sales Engineer RCA Victor Co., Inc. FROM the early days of broadcasting the studio microphone has presented the hardest problem engineers have had to meet in their constant efforts to improve broadcast fidelity. M Taylor Early carbon J types were unreliable and of poor quality. They were improved upon, but they were never entirely satisfactory because of their high background noise and susceptibility to blasting. Meanwhile speech input and transmitting equipment capable of reproducing faithfully the range of frequencies from 30 to 10,000 cycles had been developed. A microphone of equal range was imperative. The condenser microphone was the answer. Transmitting with fair fidelity the entire range, it presented a real advance and soon became an accepted standard. Recently other types of microphones have been introduced. These have had about the same characteristics as the best condenser microphone but have had an advantage (under certain circumstances) in that they did not require a closely linked amplifier. Despite the 30 to 10,000 cycle range of the condenser microphone and other recent types of microphones, they did not satisfy the more discriminating engineers. The frequency curves by which they were judged were fairly flat — but they were made by the actuator method. In that method of calibration the pressure of the sound wave is simulated by a vibrating rod exerting a mechanical pressure on the diaphragm of the microphone. Engineers were openly doubtful of the veracity of this method; they thought they could detect in the reproduced signal whistles and lisps which could be due only to the unnatural accentuation of certain frequencies. They decided to check it by the Rayleigh disk method. A pure sound wave of known frequency and amplitude is generated by the Rayleigh disk. Since this is essentially a sound wave in free space, it makes possible very accurate measurements of microphone response. As these engineers expected, these measurements showed all available microphones to have various peaks and dips. Having proved this, they had no difficulty in determining the reason. Faults of Present Mikes ALL OF the microphones used up to this time employed a diaphragm which offered a relatively large and impeding surface to the passage of the sound waves. These waves were reflected by this surface and hence the pressure on the diaphragm was more complex than the direct sound pressure simu lated by the actuator rod. Moreover, since the diaphragm was more or less recessed, cavity resonance occurred. In addition, the dimensions were such as to cause mechanical resonance of the diaphragm at audible frequencies and pressure doubling which accentuated the higher frequency. Obviously most microphone ills could be laid directly to the use of a diaphragm. With this in mind engineers set out to develop a microphone which would be free of these shortcomings. The Velocity Microphone has been dubbed "the microphone without a diaphragm." The description is appropriate, for it emphasizes the radical difference in the construction of this new microphone. A less obvious but also important difference is that the sound waves, instead of being forced to pass around this microphone, actually pass freely through it. But more important than either of these constructional differences is the fact that it introduces an entirely new principle of microphone operation. All previous types of microphones were actuated by change of pressure on the diaphragm. They were, therefore, spoken of as being pressure-actuated. The Velocity Microphone is not. It is actuated by the velocity of the air particles. Thus it is velocity-actuated — and from this it derives its name. How It's Constructed THE MOVING element in this new microphone is a thin metallic ribbon suspended between the poles of a magnet with its length perpendicular to, and its width in the plane of the magnetic lines of force. Permanent magnets are utilized and hence no field supply is necessary. The pole pieces of these magnets are so constructed and cut away as to allow free passage of the sound waves through the microphone. The ribbon element is made of thin duralumin and is so light that its motion corresponds to the motion of the air particles. It is suspended from metal cross-pieces which in turn rest on four insulating bushings. These bushings are the only nonmetallic parts of the microphone. This construction insures that temperature and humidity changes will have no effect on the operation FIGURE 2. The Velocity Microphone mounted on a standard program stand for general studio use. Table and suspension mountings are also available. of the microphone. Moreover, it isj] sufficiently rugged that it may be J knocked over or dropped without j impairing its operation. The principle upon which the! operation of the Velocity Micro j phone depends is relatively simple. The ribbon element is caused to vi j brate by the air particles of a I sound wave. Since this vibration j occurs in a strong magnetic field there is induced in the ribbon a signal voltage corresponding to the undulations of the impressed sound waves. This signal voltage is given by the expression: E=blx where b=ftux density l=length of ribbon x=velocity of ribbon. In this expression b and 1 are, of course, constants. The velocity j of x can be shown to be independent of frequency as follows: The velocity in a mechanical system is 1 the ratio of its pressure-gradient to the accoustic impedance. Both of the latter are proportional to frequency; hence their ratio, the velocity x, is independent of requency. This being so, the signal voltage E will be independent of j frequency and the response of the microphone uniform at all frequencies in the working range. The free-wave curve of the Velocity Microphone (Fig. 1) shows this to ] be true to a close degree of approximation. Natural Reproduction THE FREQUENCY range of the I Velocity Microphone as measured by the Rayleigh disk method is shown in Fig. 1. Examination of this curve shows that it is nearly flat from the lowest audible tones to beyond 14,000 cycles. The slight falling off at higher frequencies represents a difference which would not be detected by the ear. Moreover, since it is a smooth curve it may, if desired, be compensated for in the following amplifier. For comparison curves made on other types of microphones under identical conditions are also shown in | Fig. 1. The peaks and dips which engineers found caused the whistles and lisps marring many broadcasts are plainly evident. (It should be noted that the general slope of :i 1 the condenser microphone is com ■'[ 5 pensated for in the associated : amplifier — which does not, how j ever, remove the sharp point of the ! peak.) These peaks are traceable to diaphragm resonance, cavity | £ resonance and pressure-doubling. : t All three result because of the use -! a in all previous types of micro js phones of a pressure-actuated dia t (Continued on page 25) cc NDE ;e R MICROPh ONE y \ c YNAMIC K <ICRO ?\ HC N e s \ r \ \ \ s EL OC :i r ullCROPHC ■V + 7 V )NE FIGURE 1. Frequency response of Velocity Microphone compared # with that of two types of microphones commonly used in broadcast r studios. Free wave calibration for a sound source directly in front of the microphone. (The curves for the condenser and dynamic types of microphones are from published data.) Page 10 BROADCASTING • August 15, 1932 Hi