CHOOSING A LOUDSPEAKER

Radio Record, Volume I, Issue 7, 2 September 1927, Page 14

CHOOSING A LOUDSPEAKER

Tt is an unfortunate fact that many amateur constructors, after having made quite a good job of a set, will go and spoil the whole thing by using an unsuitable loudspeaker. If there is bad distortion, it is a sommon orcurrence to find that the whole trouble is due to the loudspeaker This state of things is found, whether the set be home-made or from a wellknown manufacturer’s store. The main trouble is usually due to the fact that the set, quite good if properly treated, is installed in a corner "out of the way, and is fitted with a small loudspeaker--mainly so as not to attract attention, or to yive a small yol-

une. Overloading Troubles. This igs all right as long as a small output from the set is all that the loudspeaker is required to carry, but if the set is capable of giving a large output, aud the loudspeaker is being ‘pushed,’ distortion is bound to follow. ‘he same occurs if a small loudspeaker is expected to fill a ‘arge room or to provide sufficient volume for occasional dancing. The safest way to ensure satisfaction, unless small volume and a small room are to be coupled together, is to use a medium-sized loudspeaker, whether loud or small yolume is required. A medium loudspeaker will give good round, tone on both loud and weak speech and music, and if less volume is required the set can be detuned or a valve cut cnt. There are, of course, many types of loudspeaker, and it is best for the listener buying one for the first time fo have a demonstration-a comparative demonstration. --before he completes the purchase. He should hear various types and makes of _londspeakers under the same vconditionsif at all possible, on his own setbefore he finally makes his choice. He will probably be surprised at the difference between the various tones that the different loudspeakers produce. Whatever he does he should not buy his loudspeaker upon recommendation (Continued on page 15).

(CONTINUED FROM PAGE 1H.) alone. If he is advised to get a ‘'soand so,’ he should insist un hearing it before he places his order-and after hearing it, it would not be a bad plan to hear some others before finally choosing. Many and varied are the tastes ot listeners as regards the tonal quality of broadcast music, Some preter the higher frequencies to be predominant, others don’t mind = what they have, so long as they get volnme, while still more are aissatisfied if the low notes of the bassoon and organ are in any way cut off, and do not mind losing a few of the higher harmouics, A very few really study the problem, and demand real tonal purity, and these have to get it themselves, as a rule, by suiting the set to the loudspeaker, and vice-versa. Another point that may escape the listener is that concerning the impdance or resistance of the loudspeaker. This may seem a small matter, but if tonal purity is to Le obtaimed it is really essential that the resistance of the loudspeaker be suited to the impedance of the output from the set, which latter depends upon several factors, such as the anode imredance of the valve used, and so on. oughly speaking, if the resistance of the loudspeaker is about a quarter of that of the impedance of the output, the set should be capable of giving reasonably pure results. he resistance of a loudspeaker is usually somewhere about a quarter of ‘ts impedance, so that a "2000 olim’"’? loudspeaker has an impedance of abont 6000 or $000 ohnis. This is a very rough calculation, and is only very approximately correct, Lut for the average power valve having about 4000 olims impedance, the 2000 olim ‘oudspeaker should be most suitable.

The Hornless Type. Loudspeakers having only 120 ohms yresistanee will necessitate ihe use of a telephone transformer, that is, a transformer with a ratio of 10-1, or thereabouts, will have to fe inserted between the ontput to the set and the input of the loudspeaker in such a.

way that a step down in voltage {and corresponding step up in current) is obtained. ‘his is often useful where the loudspeaker is to be nsed on extension leads, though it is not necessary, as the well-known clioke system of feeding the loudspeaker, and thereby isolating it for all except the required Ll’. impulses, is quite efficient, and enables long leads to be employed without danger of TI T. leakage or howling, due to capacity leaks between the plate cirenit cf the last valve and earth.

Finally, the intending purchaser should remember that the hornless foudspeaker, as a rule, appears to give less volume than its earlier brother with the sound conduit, and this is often due, not to insensitivity, but to the fact that the sound is dispensed in all directions, and not led out in one definite direction, The golden rule for success in choosing a loudspeaker is to insist on a demonstration, and not to choose before hearing, not only of the one you fancy, but also several others. More care should be exercised in the choice of a hornless loudspeaker, especially if it is of the cone type, because blemishes in the way of high or low note suppression due to the set itself may be seriously pronounced when the londspeaker +s coupled lto the set.

Tixtreme care should therefore he taken by the purchaser, whatever loudspeaker he "has his eye on," and it cannot be too much emphasised that he should insist on a demonstration under conditions as nearly ike his own as possible, and not choose a loudspeaker solely for its heantv of line, or heeanse the firm that advertises it is "sure to turn out a good one,"