Development of the Microphone
In the expansion and development of telephony it has long been recognized that a great deal of the loss of intelligibility, and distortion of the characteristics of the human voice during any telephone conversation is due to a large extent to the inferior performance of the carbon granule type of microphone, and modern standards demand that continued attention be directed to the attempt to effect improvements in this direction. It is obvious that any development carried out must involve the minimum sacrifice of sensitivity, and the question of cost of production must be continually borne in mind.
The microphone is a device actuated by power in an acoustic system, and delivering power to an electric system, and of the many forms of microphone in use today the only one suitable for ordinary line telephony is the carbon microphone which depends for its operation upon the varying resistance of carbon contacts. For the pressures encountered in speech the essential for efficient operation is that the resistance must be critical to very small changes in the position of the microphone diaphragm.
The fact that the carbon microphone is almost universal in its application to line telephony is due to its high sensitivity, and until quite recently the need for this degree of sensitivity has been of paramount importance, having far greater value than a uniform response over a wide range of frequency. Considering the advances made in telephony and the means available for upgrading speech volume on long line circuits, it may be said that the need for the very high sensitivity of the microphone, important as it is, may be subordinated to some extent in an endeavour to obtain a measure of equalization, or in other words, to improve the frequency response.
The microphone occupies a unique position in any telephone system because the success or failure of the system is judged by its performance. To be entirely successful, a conversation should be continued smoothly and without interruption and preferably in such a manner that the participants are not immediately conscious of any intervening apparatus.
Development of the microphone has been very slow, modifications of the original solid back type (White 1890) being still in use by some administrations, and from the aspect of volume efficiency the actual increase is of the order of 4 to 6 db. over a period of about 30 years. It is apparent that, unless some revolutionary new principle is evolved as the result of the work in a wide field of investigation, the maximum volume efficiency for the loose contact type of carbon microphone has been achieved.
Practically all microphones used in ordinary line telephony systems have pronounced peak frequencies which are in general somewhere near the average speech frequency, 1000-1200 c.p.s.
The position of this peak in the audio spectrum determines to a large extent the characteristic “pitch“ of the microphone, and the harmonics generated by the varying frequencies and sound pressures produced by the person speaking, influence the intelligibility. It is apparent therefore that with types of microphone having frequency responses as shown by the curves, the intelligibility varies with the character of the voice actuating the microphone, e.g. a quiet non-resonant voice will be the most pleasing and natural, whilst a powerful resonant voice gives rise to what may be termed “ blasting,” and intelligibility is adversely affected.
The articulation efficiency of a microphone is affected by many factors, some of which can be varied by methods of design, but there are a few, such as “ room noises
and” line noises “of many kinds over which the designer has no control. As microphone development is being considered, only those features which can be modified by design will be taken into account.
In the development of a microphone the factors to be taken into consideration, all of which have inter-related effects, are in general, as follows:-
Effective stiffness of diaphragm.
Effective mass of diaphragm.
Effective mass of granular charge.
Effective stiffness of granules.
Effective stiffness due to enclosures, air, etc.
Effects due to mass and stiffness of case.
The values involved may be laid out in an equivalent circuit, mass being represented by inductance and the stiffness by capacitance. Calculations may be made around any set of values to give a reasonably good approximation, but a great deal of experimental work and a series of lengthy tests have to he carried out, since any slight change in resistance or in any of the features quoted above will affect the final result.
In addition, the carbon granules have certain inherent defects or disadvantages which must be taken into account, these are briefly:-
Packing — which seriously affects output.
Breathing — a cyclic distortion resulting from “packing” and “unpacking “.
Heating — resulting from the passage of current.
Frying — Generated with the passing of current.
Ageing — Deterioration of efficiency with the passage of time.
Resistance — The average ohmic resistance in the speaking condition.
It will be appreciated that any change in the design of a microphone involves the detailed investigation of all the foregoing factors and is a long-term task.
Sectional View of Microphone Inset N.7752
The production of the carbon granules is a carefully controlled laboratory process primarily the dependence upon a natural form of raw
material - the highest grade anthracite - is at present an indisputable necessity. The selection of the material for the hard, shiny, short grained, non-porous portions is tedious but very important. This precedes the processing which involves preliminary crushing, sifting to size, extraction of dust, roasting to drive off unwanted volatile matter and to obtain the correct resistance characteristics, final sieving to size. removal of flats or slivers and treatment to remove all magnetic particles.
F or the maintenance of microphone efficiency, every batch of granules produced is subject to close control and careful test before final issue to the assembly shop for actual microphone production.
In considering the design of the granule chamber, attention must be given to the size and shape of the chamber and the disposition of the electrodes ; the ideal chamber is probably that where the fall of the carbon granules in any position of the microphone results in the same pressure being exercised on the electrodes and between the granules, but it appears at present that there is little possibility of this condition being satisfied. The semi-spherical chamber appears to be a satisfactory compromise and this has been embodied in the N.7750 and N.7753A types of microphone, in which pressures exerted in all directions are nearly equal, except when the microphone is held face downwards. The complete immersion of both electrodes in the mass of carbon granules is a necessary feature which effectively prevents any possibility of a complete disconnection with the microphone held in any position.
The closure to the carbon chamber may be carried out in several ways ; the two best known methods are to arrange the closure so that an electrode system affixed to the main diaphragm may move freely through a hole in the centre of the closure (Fig. 2, section N.7752).
This method has two main disadvantages:-
If the hole through the closure disc or discs is very slightly too large some of the granules may escape into the outer case, on the other hand, if the hole is too small it will exercise a restraining effect on the diaphragm system with the consequent loss of efficiency.
The chamber may be completely closed by a diaphragm of mica or other suitable material which is rigidly fixed to the main diaphragm, thus forming a definite closure effectively sealing the granule chamber.
By arranging this form of closure to have a definite mass and stiffness, a counter resonance is produced, acting against the natural resonance of the main diaphragm and resulting in improved intelligibility; a step in the direction of equalization which is a very desirable feature. Fig. 3, N.7753A, shows a section of microphone embodying this feature.
Concerning method (2), doubts from a technical point of view have been expressed regarding the completely closed, hermetically sealed chamber. Long experience satisfies the opinion that there are no serious disadvantages ; changes of atmospheric pressure at the diaphragm are of a relatively low order with normal variations in barometric pressure, and the slight damping which may be experienced can, with suitable design of carbon granule chamber and diaphragm type closure, be arranged to give advantages in the direction of an improvement in articulation.
Attention has also been directed to the question of insulation coating within the granule chamber. This was a coating of hard stoving enamel on the inside of the chamber and metal electrode mounts. Occasional troubles were experienced due to the hard carbon granules piercing the enamel, causing a large increase in microphone noise and eventual short circuit of the microphone. A composite construction of the case was evolved and has proved to be a definite improvement (Fig. 3).
The trend in the development of the N.7753A microphone has been to provide a component that will give satisfactory service in tropical climates and in situations where moist conditions prevail. Even in normal temperature surroundings any microphone is subject to the impact of moisture ; it is desirable therefore that the so-called tropical type be adopted as standard.
Developments in the matter of finishes have been made in collaboration with the Services and conditions have been satisfied where the microphones have passed the most severe tropical tests, including the subjection to intensive salt spray for extended periods. This experience has been used to advantage in the production of microphones.
Sectional View of Microphone Inset N. 7753A
The N.7753A type of microphone is interchangeable with the N.7752
(P.O. No. 13) in practically every case and no circuit modification is necessary. It may be used in local battery type instrument circuits, and on heavy C.B. systems, also by modifying the granule charge to have a higher resistance, it can be used on the low current exchange feeding systems of the condenser relay type having high resistance feeding coils.
The N.7742A type may also be utilized in the same conditions but owing to its smaller dimensions it is not interchangeable with N.7752 or N.7753A.
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