Projection engineering (Jan 1932-Mar 1933)

APRIL, 1932 Page II microphone in public-address systems and in radio broadcast installations. Operation of the Double-Button Microphone The diaphragm is always connected to a tap brought out in the exact electrical center of the primary winding of the microphone transformer in the manner shown, and the carbon electrodes are connected to the two ends of the winding. This places the resistance of each button across one-half of the transformer winding, and thereby creates a balanced circuit that helps greatly to reduce the distortion produced by even harmonics. A sound wave will cause the microphone diaphragm to move backward and forward, thus alternately decreasing the pressure on the carbon granules in one chamber while increasing the pressure on the granules in the other chamber. Due to the variations in the resistance of the chambers, this causes the current through one side of the transmitter and one side of the split winding of the transformer to decrease and the current through the other side of the transmitter and the other side of the transformer winding to increase, and vice versa. These current changes do not oppose, but aid each other and add together in the transformer, inducing a voltage in the secondary winding that is twice as great as it would be if only one cell of the transmitter were used. So long as the resistance of each side of the microphone diaphragm remains constant, the steady battery current, and any fluctuations in that steady current, will not induce a voltage in the secondary of the transformer because it will not produce a magnetic flux in the transformer core, due to the fact that the currents flowing through the two halves of the primary winding are in opposition and balance each other. This prevents saturation of the core by the steady d-c. current, which could readily happen in the microphone circuit shown in Fig. 2 if the transformer is not properly designed. The value of this steady current flowing through the transmitter at all times is regulated by means of the series rheostat or potentiometer in the center lead of the transformer circuit. Usually this current is adjusted to a value of about twenty-five milliamperes per button, but often satisfactory results can be obtained with currents as low as ten or fifteen milliamperes, depending on the microphone used. A lower current is preferable because it is less likely to cause burning or arcing of the carbon granules and it is better for the delicate contact surfaces. As the microphone ages from steady use, its resistance increases with a resultant loss of COMPENSATING DIAPHDA-v DIAPHRAGM Fig. 5. Crosssectional view of the 394 type condenser micro phone. DAMPING PLATE GROOVE Courtesy Bell Telephone Labs. sensitiveness. As we have seen, the double-button transmitter is much less sensitive than the single-button transmitter, due mainly to the stretching of the diaphragm, and so a certain amount of audio-frequency amplification is necessary to bring the speech current produced by a double-button transmitter up to a useful value. The Condenser Microphone The condenser microphone, which is composed of a sound pickup device called a condenser transmitter and an amplifier, is a later evolution. It was first described by E. C. Wente, who developed it. It produces speech of high quality and is the most perfect microphone in use at the present time. The condenser microphone has several disadvantages, though, for it is more complicated in construction than the carbon transmitter and more difficult to maintain. Its sensitiveness is so much lower than even the double-button carbon transmitter that it is necessary to use at least one stage of audio amplification to bring it up to an approximation of the same level of energy output, and even then its output is about six decibels lower than that of the twobutton carbon microphone, being about minus thirty to forty db. It is true that there is a slight tube hiss from a condenser microphone, but that is not so annoying as the carbon hiss from a carbon microphone, and so, on the whole, there is far less background noise present in the condenser microphone than in the carbon microphone. The two-button microphone is still used widely, nevertheless, because it is more rugged and compact and that less batteries and associated equipment are needed with it. However, the condenser microphone is very durable and is not so easily affected by sudden temperature changes or mechanical shocks as the carbon microphone. It can be moved about while in operation without creating noise. Also, there is no danger of "blasting," and it can be op erated in any position, while the carbon microphone should be used only in an upright position. The condenser transmitter operates on a principle that is entirely different from that of the carbon microphone. Instead of varying its resistance and thereby altering the flow of current in the circuit in which it is connected, the condenser transmitter changes its capacity under the influence of the sound waves and in that way varies the flow of current into it from a source of steady e.m.f. in the circuit in which it is operating. This change in current in the condenser circuit is so very minute that the only satisfactory way it can be used is by causing it to produce a change in potential in the grid circuit of a vacuum tube. Without its associated amplifier, the condenser transmitter could not be used, because the impedance of the condenser transmitter would not match the impedance of the speech input equipment, and so they are always to be found together. The amplifier serves to couple the condenser transmitter, which operates most efficiently into a circuit having an impedance in the neighborhood of fifty million ohms, to a 200-ohni transmission line. Actually, if only a single-stage amplifier is employed, the electrical level at the condenser transmitter is approximately six db. higher than the level at the output of the condenser transmitter amplifier, despite the gain at the amplifier, due to the extreme difference in impedance of the two circuits. The condenser-like pickup device — the condenser transmitter — is sometimes known as the "head" ; and the associated amplifier is called the condenser transmitter amplifier. The titles for these two parts of the microphone are abbreviated CT and CTA in general practice. In the second article of this series, the internal construction of the condenser microphone and the theory of its operation will be discussed, but before closing this article, it might be well to