Radio Reproduction Improved by Moving Coil Loud Speaker

Radio Record, Volume II, Issue 16, 2 November 1928, Unnumbered Page

Radio Reproduction Improved by Moving Coil Loud Speaker

(TELEPHONE "reception, which was once popular, is rapidly falling into disuse and it is now a comparatively rare occuirence to see members of a family grouped round the fire with headphones clamped over the ears, enraptured by some item of the broadcast programme. The reason for this is threefold. First, the modern radio receiver has made such rapid progress, both in purity of reproduction and general sensitiveness, that it is now possible to work a loudspeaker from one or two valves. Secondly, improvements hve been made at the transmitting end, and the power of our broadeasting stations has, in many eases, been increased, with the result that greater yolume can be obtained with a given set. Thirdly (a most importart point), the modern loudspeaker is vastly different to that wretched contraption of a few years back, of which the man on the street was rather apt to say "it sounds like a gramophene." With the advent of loudspeaker (power) valves, and with the progress in component and set design generally, the listener began to learn that good speakers really could give quite g»od reproduction, and to realise what selfish things telephones were. One has Myici ns of the head of tte fanily sitting, pulling away at his pipe, with the only exist‘ing prir of ’phones over his ears, and contentedly listening to the broadcast programme, with the rest of the family eagerly ‘waiting their turn which, incidentally, in some ceses, never came !

Evolution of Speakers. PEAKERS have evolved rapidly, so rapidly that the listener who wants the maximum from his set must constantly change with the times. Among those who have paved the way to the modern speakers, and who merit special commendation for applied research and study are Messrs. Amplion and S. G. Brown. Through their efforts reasonably good speakers came on the market and were supported by reasonably good sets, but reproduction has not been entirely faithful. To remedy this has meant years of profitless re-search-but the fruit hag been borne and it is safe to say that with the advent of the dynamic, or moving coil loudspeaker supplied by a push-pull amplifier with signals strengthened by the screen grid valve, the acme of perfection has been reached. Invention of Moving Coil. THE idea of the moving coil was an expensive one to the General Electric Company of America. The policy of this company has been to engage a laree number of brilliant scientists. mostly young men, and. to equip them with everything they need for first-class research work, leaving them to work out their ideas without being worried every few minutes to see whether they have produced a commercial invention. — Two young Ameizans, Messrs. Rice and Kellogg, engaged by this company, were responsible for the invention of this remarkable speaker. Shortly after the invention, and certainly before it was commercialised, Mr. Harris, the

editor of the "Wireless Constructor" (England), visited the laboratories, and we will let him give his impressions of this new speaker in his own words: "T learned a great deal that afternoon about loudspeakers,"’ he writes, "and many of my illusions were shattered. I realised for the first time how hopelessly inadequate was the reproduction of the low notes by the then existing loudspeakers, and how those which seemed to reproduce the lower tones gave them wnfaithfully. I also realised how the repression or accentuation of the harmonics of a note can completely alter its character. One experiment which Mr. Kellogg showed me »brought this out very clearly. A Conclusive Experiment. "QGUSPENDED from a piece of board were a _ number of very large bottles the mouths of which projected through the board. Water at various depths had been poured in these ‘bottles and a kind of xylophone rod placed over each so that when the rods were struck one obtained a musical note which resonated in the bottle, giving rather a pleasing and rich musical effect, this being largely due to the number of harmonics of the fundamental note. Mr. Kellogg then played a few simple tunes on the scale so that I could accustom ‘my ear to the true reproduction, and then went into another room and played the same "ittle tunes before a microphone so that the output could be fed into any of the loudspeakers in the laboratory where I was listening’ | --(ContiInued on page 2).

(Continued from Front Page.) "T now had another shock. In fact I was becoming used to shocks on that memorable day, for the sounds which emerged from the various loudspeakers did not, in a single instance, even remotely resemble the tones I had heard directly from the instrument. . "In many cases the change from one loudspeaker to another produced an entirely different set of sounds, only recognisable as coming from the same

source by the contamination of the air. Mr. Rice now switched the output of the microphone over to the Rice-Kel-loge (dynamic) ) speaker and immediately the reproduction was faithful to the original. "T was then shown how important was the influence of the baffle board, for without this board reproduction was thin and poor." The writer continues to cite incidents, how, time and again, he wat amazed at the remarkable possibilities of this new speaker. It was capable

of faithfully reproducing the whole of the music scale, and harmonics, particularly when "baffied." Since that memorable occasion the dynamic speaker has won the favourable comments of a world of admirers. ¥rom one laboratory they have spread, pringing their pleasing results to an ever-widening circle of radio listeners -and now they have reached New Zealand.

How a Dynamie Cone Works. OME few weeks back an article by "Megohm" (August 31) described the technical principles of the dynamic speaker-these we will briefly re iterate. ; The name "moving coil" denotes the principle. A tiny coil attached to the apex of a relatively small cone is surrounded by a powerful magnet energised, usually, by a small current of electricity. Thus moving parts are reduced to a minimum. A great deal of play-up to half an inch-- can be provided for, so that any musical vibration can be accommodated without distortion. Any volume can be handled without the slightest detriment to qual-

ity. This type of speaker is distinctly superior to the ordinary type. Practically every speaker which has been produced in the past has been of the magnetic type, having an iron armature actuated by an electro-magnet. The fluctuating current from the receiver causes a vibratory motion of the armature, which motion is transferred to the cone, in the cone speaker, or to the column of air in the horn speaker. The outstanding fault of this design is that when the speaker is loaded above a

certain point the armature strikes the pole pieces of the magnet and causes a rattle. Another objection is that the armature responds to certain notes more readily than to others, and consequently gives uneven or distorted reproduction. Because of its principle, both these faults are impossible with the dynamic cone. i The speaker, requiring a strong current, is unsuited for weak sets, and under such conditions may even prove unsatisfactory. The solution is simple and has been described by "Pentode" in this issue-‘"Push Pull Amplification." This is undoubtedly the finest amplifier that can be used in connec-: tion with the moving coil speaker, and as, with the exception of one or two new transformers, any amplifier can be adapted, the reader who desires to obtain quality reception cannot do better than alter his amplifier and use a dynamic speaker.

The Moving Coil in New Zealand. MPOVING coil speakers are not new to New Zealand, as reference to past advertisers will prove, the Magnavox Co., now the largest manufacturers of these speakers, having already introduced their products to the New Zealand market. Up till the present the price has been beyond most pockets, but now the new models will prove to be popularly priced. A free-edged cone of this type requires a baffle to prevent the air waves, set up simultaneously by both the front and back cone interfering with one another. Great latitude is permissible in the construction of this baffle, which may range from a handsome

cabinet to a butter-box. The units now on the market (Magnavox) alge" constructed so as to allow of the easy application of the baffle, while it is stated that the American manufacturers (Magnavox) are turning’, out 1000 units a day, these are being r@adily absorbed by manufacturers of sets | who are using practically no other type. type. The popularity of the dynamic cone can be gauged from the fact that they

predominted at the Olympic Radio Exhibition which introduced the radio season in the Northern Hemisphere. Their popularity has spread _ to gramophone manufacturers, who are using this unit in the latest type of cabinet machines. Great, attention has been focused on them and great results obtained. Not the least of their attraction is the fine piece of furniture they make when enclosed in a suitable cabinet.