THE PRACTICAL RADIO JOURNAL 26 th Year of Publication No. 905. FRIDAY, JANUARY 1sT, 1937. VOL. XL. No. 1. Proprietors : ILIFFE & SONS LTD. Editor : HUGH S. POCOCK. The New Year Editorial, Advertising and Publishing Offices : Promise of Many Developments DORSET HOUSE, STAMFORD STREET, LONDON, S.E.s. 7 might have been supposed that, should pave the way for conditions of Telephone : Waterloo 3333 (So lines). wireless being no longer in its reception, especially in large towns, Telegrams: "Ethaworld, Sedist, London.' infancy, and broadcasting having better than anything which has hitherto COVENTRY : Hertford Street. now been with us for some fifteen been enjoyed. The probability exists, Telegrams: Telephone: "Autocar, Coventry." 52 ro Coventry. years, the year which we enter upon also, that changes in wavelength alloca- with this issue would show promise of tion in Europe may come about, BIRMINGHAM: no remarkable changes or develop- resulting in an improvement, at least Guildhall Buildings, Navigation Street, L ments of importance. Yet from all with the larger of : transmitters, Telegrams Telephone : "Autopress, Birmingham." . 2971 Midland (q lines). appearances the very reverse will apply, reception . conditions as far as ad- because no time in of MANCHESTER : 260, Deansgate, 3. at the history jacent- channel interference is con- Telegrams : Telephone : wireless have there been more positive cerned. "Muffe, Manchester." Blackfriars (4 4412 lines). indications of really important achieve- Promises have been made that, in GLASGOW : 26B, Renfield Street, C.2. ments in view. the broadcast programme field, 1937 Telegrams: " Inge, Glasgow." Telephone: Central 4857. Short -wave developments, both in will see an increase in the exchange of PUBLISHED WEEKLY. ENTERED AS SECOND the broadcasting and general corn- national, programmes, thus helping to CLASS MATTER AT NEW YORK, N.Y. munication fields, are being planned improve cultural relations as between Subscription Rates : on a big scale in almost every country one nation and another. Home, ¡s is. 8d. ; Canada, LI Is. 8d. ; other in the world and new services are countries, £I 3s. sod. per annum. promised for early in the New Year. Receiver Progress As many of the circuits and apparatus described in These The B.B.C. has several additional pages are covered by patents, readers are advised, before In the sphere of reception it is making use of them, to satisfy themselves that they would transmitters in preparation for an not be infringing patents. im provement in the Empire service, difficult to anticipate how important and these transmitters may be expected may be the changes which may develop. to be in operation during the year. Short -wave reception circuits will un- doubtedly improve and many refine- Television Hopes ments which have already been CONTENTS disclosed in theory may be expected Page Television has passed through its to find their way into practical applica- earliest stage of development as a tion. The electron Editorial Comment. .. 1 multiplier, the service with fair success, and if tele- development of which in connection Microphones : Pressure and Vel- vision is to provide a lasting service in with television has been greatly stimu- ocity .. 2 the future, then it is certain that 1937 lated, still holds promise of staging a Broadcast Research 5 Receiver must see important programme changes revolution in amplifying circuits ; A New Detector Circuit . 6 in addition to improvements on both whether or not progress in this direction Broadcasting in the Balkans 9 the transmitting and receiving side. will be sufficiently rapid to promise any If no means are found to improve the radical changes in the nature of re- Current Topics . 11 programmes of the television service ceivers during the year it would be Ferranti Arcadia Set 12 Review out of present funds, then an alternative unwise to anticipate, but that in the Unbiased . 1-1 event of 1937 might be a new financing electron multiplier we have a potential The Beam Power Tube . 15 scheme. revolutionary influence in receiver de- Listeners' Guide for the Week 16 With the commencement of the sign will not be disputed. Principal Broadcasting Stations 18 year the B.B.C. enters upon a new The Wireless World enters this year lease of Brilliant Television 20 ten years' renewal of its upon its twenty- seventh year of pub- Charter. lication, and we may hope that the Broadcast Brevities.. 22 1937 should show the introduction record of events which its pages will Letters to the Editor 23 of the much -discussed legislation to chronicle will make this one of the Recent Inventions . .. .. 24 control electrical interference, and this most interesting years of its existence. Wireless World, January ist, 1937 PRESSURE Microphones AND VELOCITY Main Characteristics and Applications By F. N. G. LEEVERS, B.Sc., A.C.G.I. of the Two Types IT has been frequently stated that of below a mean value which is the atmo- all the links in the broadcasting trans- AN explanation of the essential spheric pressure. It is this alternating mission and reception chain the micro- difference between the two variation of pressure to which pressure phone microphones and the loud speaker are by far principal types of microphone and respond. the weakest as regards both efficiency and The air pressure at any point is indi- a discussion of the properties, fidelity. Although they occupy such cated in the diagram by the density of the particularly with regard to direc- extreme positions in the chain, the reasons particles ; hence variation of pressure due for their shortcom- tional characteristics of examples to sound waves involves the movement of ings arise from the in common use. the air particles towards or away from the NO SOUND very similarity of A B source of sound. Particles cannot move in functions. any other directiop unless their Fig. 1. - Dis- influenced by they are both SOUND ......... ......... placement of air another source of sound, and they will electro- acoustic de- P R P R particles under only move about a mean position which vices, and while the influence of sound waves. remains constant. Except when very close electrical measure- to the sound source, the velocity of the ments of a high order of accuracy can be the air particles. This re- grouping is shown displacement of the air particles is pro- made with comparatively simple and inex- in the accompanying table. portional to the variation of pressure. pensive apparatus, the same cannot be Reference to a suitable illustration of the From the last paragraph it can be seen said of acoustic measurements. Further, propagation of sound in air will indicate that a microphone which responds to acoustic conditions have a very important the fundamental differences between the " particle velocity " will only indicate the effect on the results obtained both in two types of microphones. In Fig. i the component of this velocity' which lies along laboratory measurement and in practical line of dots indicates the positions of air its " major," or sensitive, axis. Since use. particles along a straight path radiating pressure microphones respond equally to it is perhaps not surprising, therefore, from a source of that in the case of microphones the choice sound situated to the of types suitable for commercial use was, left. When no sound until a few years ago, very limited, being is emitted from the confined almost entirely to various types source the particles of carbon transmitter. In recent years, will be equally however, the rapid increase in the variety spaced, as indicated of uses to which the microphone is put, A B. When a sound many of them involving very adverse con- of given frequency is sslDaNti ditions, has led to intense research, and emitted by the 1111111111m. this has resulted in the production of a source, waves of number of new designs, in which good high and low pres- frequency and amplitude characteristics, sure will radiate from EQUIVALENT MICROPHONES OF THE PRESSURE AND VELOCITY TYPES. Electrical Pressure Type. Velocity Type. Characteristic. (a (b) Electro -Dynamic r. Moving Coil Ribbon 2. Inductor Piezo- Electric Sound Cell Differential Capacity Non- directional Differential Condenser Condenser together with low noise level, have been Fig. 2.- Diagrams a and b show a velocity achieved. They include the " ribbon," ribbon microphone in the " inductor," the moving coil, the front elevation and crystal, the differential crystal, the con- section, while c is a pressure ribbon micro- denser, and the differential condenser Dl ,//////ae microphones. These various types are phone with enclosed back. A moving -coil (e) usually regarded as belonging to categories type is shown in (d) determined by their method of converting diagram d. acoustic to electrical energy-i.e., they belong to ' one or other of the electro- it-i.e., they will traverse our particular sound waves arriving from any direction dynamic, piezo -electric, or capacity types. row of air particles from left to right, as (neglecting special effects due to size and In practice, however, it is far more indicated at P and R respectively. At a shape of the microphone itself), the direc- useful to group them under the two main fixed point anywhere along our line of tional characteristic, or " polar field," headings of pressure and velocity, accord- particles the pressure will appear merely constitutes the major , difference between ing to whethe they function by virtue of to rise and fall as each wave passes. The the performances of the two types. the pressure variation or the velocity of pressure will actuálly alternate above and Having outlined the different acoustic ist, 1937 MmIlsoo 3 JANUARY WopIldt Microphones- construction of a typical moving-coil very low, so that several cells are usually operations involved in the two types of microphone, and a close resemblance to a assembled in series-parallel to increase the microphone, let us return to the three main small moving -coil loud speaker is evident.