A microphone is a device that transforms sound pressure into electrical energy. In most types of microphone, the sound pressure acts upon a thin plate or diaphragm, setting it into vibration, and this mechanical motion is then utilised to produce electrical effects. The chief types are described below. The various designs are also built to incorporate desired directional pickup properties.
One of the earliest microphones, and the type still most generally used in telephone practice, depends for its action on the fact that the electrical resistance between carbon granules in contact with each other varies with the contact pressure. The above figure shows a simplified sectional view of a single-cell or single-button carbon microphone, as is used in some telephone sets. A small brass cup contains two polished carbon discs, one fastened solidly in the cup and the other attached to the diaphragm. The space between the discs is partly filled with carbon granules, and as the diaphragm vibrates in response to the sound waves striking it, the varying pressure on the granules causes changes in the electrical resistance between the buttons. The microphone circuit is shown in the same figure, and from this it is seen that variation of microphone resistance will alter the current through the transformer primary, and so will set up induced voltages in the secondary.By proper choice of diaphragm stiffness and mass, the moving system can be made to resonate near the m1ddle of the speech range of frequencies. When this is done, the electrical output is large enough to operate a receiver over a considerable length of line. Without requiring amplification the frequency response is then not very uniform, although it is entirely adequate for speech reproduction.
By using a very light diaphragm, tightly stretched, the frequency response is greatly improved, but at the expense of sensitivity. Carbon microphones with this type of construction, and having two buttons, of carbon cells, were used extensively in early broadcast.
Another type of microphone, widely used in public-address systems, depends for its action on the piezoelectric effect possessed by certain crystals, for example Rochelle salt. The term piezoelectric effect refers to the fact that when pressure is applied on the crystal in the proper direction, electrical potentials are produced between opposite faces of the crystal. The sound-cell type of microphone contains an assemblage of small crystals of this type, so connected that their piezoelectric potentials are in series. The sound falls on the crystals and vibrates them directly. The electrical output is quite small, but the frequency range and uniformity of response are excellent.'In another type of crystal microphone, a metal diaphragm is coupled mechanically to a crystal of Rochelle salt in such 'a way that vibration of the diaphragm causes a twisting of the crystal, and thereby the generation of a voltage at the terminals. This type has much greater, output than the sound cell, but the frequency response is limited by the inertia and stiffness of the diaphragm and the associated driving members.
The crystal microphone has high output impedance, making it well suited to direct connection to vacuum tube amplifiers.
Several types of microphones depend for their action upon the voltage induced in a conductor moving in a magnetic field. The dynamic or moving-coil micro- phone contains a small coil attached to a diaphragm, so arranged that when the diaphragm vibrates, the coil moves back and forth in a radial magnetic field. It thus generates the output voltage. By careful design of the moving element, and by making use of air-chamber resonance, it is possible to obtain a nearly uniform response from 40 cycle to 10,000 cycle. An incidental advantage is that the output impedance of the microphone is low, and the microphone cable is less sensitive to hum pickup than in the case of the crystal microphones. A transformer is required to couple such a low-impedance source to the high-impedance input of a vacuum tube grid.
In this type, the moving element is a very thin and flexible aluminum ribbon, upon which the sound waves act directly. It vibrates in a transverse magnetic field and generates an electromotive force on the ribbon. The ribbon impedance is so low that a small step-up transformer is included in the microphone mounting to raise the impedance to a level suitable for transmission over a line. Most ribbon microphones respond to air-particle velocity.In the sound wave, rather than to sound pressure. These are referred to as velocity microphones. They can be made to have excellent frequency characteristics.
A reproducer is a device for converting electrical energy into sound. As in the case of microphones, this transformation usually involves an intermediate mechanical motion.
The ordinary telephone receiver is the most commonly known acoustic device. A modified form, the watchcase type used in radio operators headsets, is shown in the above figure. Two small coils are wound on soft-iron pole pieces, which are attached to the poles of a permanent magnet. The pole pieces attract the steel diaphragm with a steady pull caused by the permanent magnet, and with an alternating force set up by the voice currents flowing in the coils. The diaphragm is set into vibration, and sets up sound waves in the air in contact with it. The permanent magnet is necessary to avoid distortion in the output, since the diaphragm would be attracted twice in each cycle if only the a.c. attraction were present.The ordinary receiver used with telephone instruments. Telephone receiver is wound for about 70 ohm resistance, and it has a definite resonance peak near 1,000 cycle, for the sake of sensitivity. By winding with many turns of fine wire, the sensitivity to weak currents can be greatly increased, and such receivers are very useful as indicators in a.c. bridges and for radio communication systems.
The most common type of loudspeaker is shown schematically in the above figure. The moving coil, situated in a powerful radial magnetic field, carries the operating current. The reaction of the signal current with the magnetic field causes the coil to move back and forth along its axis. In this motion, it carries with it the paper cone radiator. The cone is supported at its outer edge by a flexible suspension, and at least at the lower frequencies, it moves as a rigid piston, without appreciable bending or deformation.The result is a very effective transformation of the electrical input into sound energy radiated from the surface of the cone.
The radiation from the rear surface of the cone is opposite in phase to that from the front surface, and it is the function of the baffle shown in the figure to prevent these two effects from canceling each other. The baffle will be effective at any frequency for which the distance from the front of the cone, around the edge of the baffle, to the rear edge of the cone, is greater than a half wavelength of the sound. For example, at 100 cycle the wavelength is l = 1,100/100 ft = 11 ft and the distance from front to back of the cone around the baffle edge should not be less than about 5.5 ft. Various means for meeting this requirement, employing curved baffles or horns, are used to return the rear radiation in phase with that from the speaker front. This is important because speaker efficiencies falloff at low frequencies, and by adding the radiation from the rear to that from the front, an increased efficiency can be obtained to improve low-frequency response. One way in which this is done is to provide a planned path, usually involving a port or hole in the baffle, which will return radiation in the proper phase at some low frequency.Cone speakers are also inefficient at high frequencies because of their large mass. This difficulty is often overcome in high-fidelity systems by employing two speakers, a cone and baffle to provide the low frequencies up to possibly 1,000 cycle to 1,600 cycle, and a small high frequency horn to carry the frequencies above. The amplifier output is divided into two frequency ranges by a dividing network or "crossover network" employing filter principles. Thus, the low-frequency speaker or "woofer" receives only the low frequency band and the high-frequency speaker or "tweeter" receives only the higher frequencies. Some of the imperfections of the system are due to imperfect division or phase shift in the crossover network. But the net effect is still beneficial. Systems of unusual perfection may employ three or four speakers with a corresponding number of frequency bands.