TRIAL AND ERROR

Dominion, Volume 26, Issue 103, 25 January 1933, Page 6

TRIAL AND ERROR

Settling the Problems of Acoustics HOW TO GO ABOUT IT Much improvement in reproduction can frequently be obtained with modern loudspeakers by such things as altering the position of the loudspeaker, either iu respect of its height above the lloor or the angle between the axis of its cone aud a line drawn from the centre of the cone to the listener; but, of course, no very definite rules can be laid down, as the acoustic properties of rooms vary so largely, not only according to their size and shape, but also according to the amount of sound-ab-sorbing material in the shape of the furniture which they contain. Thus, to ascertain the best position for a loudspeaker becomes very largely a matter of trial and error. There are, however, a few general rules which can be laid down, provided that it is borne iu mind that at any time it way bo found necessary flagrantly to break 1 one of them to suit the acoustic peculiarities of any particular room or occasion. The first thing to be borne in mind ' is that, in order to obtain the best balance between tbo two cuds of the musical register, it is desirable for listeners to sit as far as possible in Hue with the axis of the loudspeaker, or, to put it more plainly, they should sit in front of it aud not at the side, in which position a good deal of the high-note response may be lost. So marked Is the effect of high-note loss, when listeners sit outside the arc subtended by the loudspeaker cone, that the prenomenon can be put to good use where a receiver with an uneorrected or inadequately correct pentode output stage is employed; in other words, it is possible to find by experiment a position for the loudspeaker where just the right amount of highnote attenuation is experienced, thus counterbalancing the over-exuberance of the pentode iu this respect. If little or no falling-off iu high-notc reproduction is experienced when sitting at the side of the loudspeaker there are strong grounds for suspecting that its reproduction iu the upper register is not as irreproachable as might previously have been supposed. Another point of considerable importance is that it is desirable to keep the loudspeaker at the same level as the ears of the listeners when in a sitting position. Standing the loudspeaker ou an ordinary table meets the case very nicely. Many people prefer to mount the loudspeaker on a baffle placed six feet or more above the ground, but even if it Is inclined so that listeners are in line with the axis of the loudspeaker, reproduction is apt to sound rather unnatural; it. is impossible to be dogmatic on this point, however, as it may well be that iu a particular case acoustic conditions are such that they favour this particular arrangement above all others. empire service

Effect of Time Difference Iu comuieutiug on the Empire broadcasting service, a writer iu England points out that owing to time differences African listeners, being situated, ou or near the Greenwich longitude, naturally get the pick of British broadcasting at first hand. India also has a good share of. direct programmes, and even the Australians can count ou a few actual performances. But unless Canada cares to send over a special subscription for liesh-and-blood performances in the small hours it looks as if the~Doininion will have to subsist on the potted variety, though there will always'be the Empire au- . nouncer to liven things up with the news bulletin. CHARACTER OF COILS Prices Now More Moderate

The characteristics of a tuning coil depend not only upon the diameter of the "former” and the number of turns of wire, but also upon the size of the wire itself and the spacing between adjacent turns. There are, in fact, other conditions which influence the tuning characteristics of the coil, but the above are the principal ones. With modern multiple tuning circuits it is necessary to be able to produce coils which are (within at any rate a very small percentage) identical in cbaracteristics with one another. Coil manufacturers have succeeded remarkably well in turning out matched coils on a commercial basis and coils can now be had quite cheaply which are extraordinarily similar in their characteristics. Having got so far with the coils, it then becomes possible to gang the corresponding condensers so that several circuits can be turned simultaneously. Incidentally, the condensers also must be very closely alike in characteristics. By the use of pretty accurately matched colls and condensers, it is possible to tune three or even more circuits simultaneously.

OUTPUT COUPLINGS Popular Fallacy Exploded Sometimes people think that if an output transformer is used, for example,, if a high-ratio step-down transformer is employed as intermediary between tbo output of the set and a moving-coil speaker, it is unnecessary to use a choke feed for the last valve. This is not really the case, aud even when a transformer is used in this way iu the output it is still desirable to use a choke feed in (be anode, because this keeps the high tension out of the transformer and also tends to overcome back coupling and motor-boating. If a pentode valve is used iu the output stage, if: is important: to make sure that the loudspeaker used is suitable f'>r the output valve, or that the circuit is adjusted to meet the characteristics of the loudsjieaker. I have pointed out more than ouce in these notes that many people fail to get the results they should get with a pentode output stage because they do not take sufficient care to match up the valve to the loudspeaker.