LOUD-SPEAKERS

Sun (Auckland), Volume 1, Issue 41, 11 May 1927, Page 10

LOUD-SPEAKERS

(By

“RAYTHEON.")

Many and varied are the reports one hears of the merits, or otherwise, of a given loud-speaker. One listener will “boost" the loud-speaker he himself uses as the one and only out of the several he has tried. A friend asks his opinion as he purposes to instal a set himself and taking the previous listener’s advice buys the speaker he has heard so much about. However, possibly in a week or so, after mastering the little peculiarities of his set, he finds results not quite as he anticipated. The music may be quite good; the voice perfect. He may discuss the matter with some other friends until finally he comes to the decision that it is the loud-speaker after all which is not giving him exactly the satisfaction he desired. Why? The answer to this is comparatively simple. No two persons receive quite the same effect from a given loudspeaker. All loud-speakers have certain inherent characteristics of their own, just the same as is found with the human ear. Some speakers favour low notes, others reproduce uniformally over a fairly wide scale of frequencies, and yet others are a dismal failure on low bass notes or with highpitched sopranos. Thus we have one of the greatest reasons why one person will commend, and another condemn, a given speaker. Obviously to secure the greatest satisfaction the buyer should find one to match his own ears. Just at this point let us digress a little. To persons with normal hearing that, is, average hearing, there .s very little apparent change in intensity of sounds between the frequencies of 500 to 2,500 cycles per second. In other words, the response of the ear to sounds of equal volume is fairly uniform throughout this range of pitch. However, below 500 cycles the sensitiveness of the normal ear is very much lessened; at 200 cycles it is only fair, comparatively insensitive at 60 cycles, and frequencies lower still feeling often supersedes hearing. At the other end of the scale, that is, where the high notes exceed 4.000 cycles frequency, the ear sensitiveness again lessens, but it does not become serichis until another octave i passed, and about 8,000 cycles is reached. Thus it is seen that most of the musical tones appreciated by the average ear fall within the band extending from 40 to 7,000 cycles. In listening to the composition of these which lakes a chord the human ear loses very little of the high frequencies; but it is considerably handicapped on frequencies below 200 cycles. Returning to the subject ai-i ta--as a basis the tonal or “distortion" frequency curve of a free-edge cone type speaker. The response to signals between 1,000 and 7,000 cycles is found to be comparatively uniform, -r similar to the normal human ear. Between 400 and 1,000 cycles, however, there is a considerable increse in the amplitude and correspondingly in the volume of sound. From the viewpoint of the average listener this is undesirable as the ear shows no great lack of sensitiveness until the frequency falls below 400 cycles. This is just where the amplitude of the suond produced by the speaker also takes a sudden drop. Consequently the low notes are heard too feebly because both the ear and the loud-speaker are inefficient at this end of the scale. It would sound just as if one were sitting too far away from the orchestra. However, take a person whose hearing is of an unusual nature, and hears notes of low frequency better than the average. To such a speaker with the class of curve just stated he would be admirably suited. However, to take another instance, anyone whose ears respond consistently to all frequencies is also abnormal. To such a speaker which registers the high and the lov frequencies less strongly and the central range with comparatively more volume than the average listener requires, is to be commended. A speaker with the response curve as mentioned would possibly be presumed to be very poor. Such a test curve, however, as loud-speaker curves go, is a very good one indeed. If you think for a moment under most conditions an orchestra in a hall appears somewhat weak on the low tones, just for the reasons given and consequently the drop below 400 cycles is not so serious a matter after all for average requirements of the normal ear. In any case the average ear cannot distinguish a change in the amplitude of sound less than ten per cent. Bass instruments in an orchestra, when broadcasting, should be given dominance so that the desired luwfrequency background will be present at the receiving end. This all leads to an interesting point. Any orchestra may sound vastly different to one person to what it does to another. The same applies to loudspeakers. In other words it may be said that it is “earpoint," and not viewpoint, which are at variance, and although on electrical test the speaker under judgment may be found perfect in reproduction, yet due to individual differences of hearing the speaker satisfying you lamentably fails to please another. Loud-Speaker Efficiency Although there are certainly loudspeakers which are distinctly superior, there are others just as poor. Usually we are inclined to prefer those which favour the bass notes. All the same in respect to electro-acoustic efficiency, there is practically no difference. Very few loud-speakers show’ an efficiency greater than two per cent. The average speaker operated under normal conditions wastes 99 per cent, of the energy put into it. This loss is due to dissipation taking place in the horn or diaphram, as the case may be, base and baffle if there is one, excess damping in the mechanical parts, electrical “eddy currents,” “hysteresis” (magnetism), loss, and resistance loss. However the little that does g?et through is fairly effective, and so until some engineer finds a way whereby this loss can be considerably reduced, this inefficiency needs to be forgotten. Regarding the principles of the horn or the cone speaker, there is not a large difference. In tonal characteristics there is a striking similarity. This after all is one of the most important points. The electrical and mechanical properties of both are much alike, w'hile both are limited practically to the same conditions. With the horn speaker the electrical unit is magnetically, sometimes mechanically, connected to a small diaphram which vibrates the air column in the horn itself. In the cone speaker the driving unit is coupled direct to the diaphram itself, w’hich is of a large diameter compared to that used in the horn type speaker. Choice of Loud-Speakers In regard to horn speakers it may be said that the shape and length of the horn particularly effect the tone of the speaker. Usually the ordinary type of horn loud-speaker is inclined to be a bit weak on low tones, and a little too strong on the higher tones. The bass notes become more prominent as the length of the horn is increased. Theoretically the low notes can only be faithfully reproduced when the horn is made 20ft. long. Such a size of course is out of the question for , ordinary work.- However, one from

4ft. to 6ft. w’ill prove very satisfactory, and there are on the market to-day speakers so built to secure such a length in the horn.

The diaphragm or cone loud-speaker exerts pressure on the air, just as the column of air which is caused to vibrate in a horn loud-speaker. The frequency-characteristics of the cone type, however, are a little better than the average horn type. This is due in the main to inherent resonant qualities in the shape of the cone itself. As regards the reproduction of the low notes, the diameter is a ruling factor, as the greater the diameter the better will the notes respond.

As regards relative merits, it will be seen that the main difference exists in design only. A horn speaker properly designed as mentioned here can equal a cone speaker for the reproduction of a broad band of musical frequencies, but a small cone speaker will generally prove a better reproducer of bass notes than a small horn speaker. In closing it may be said that it will be found some speakers will work better with one set rather than another. In this regard it may be mentioned that as far as possible the impedance of the last valve in the set should be the same as the impedance of the speaker. Otherwise it is policy to use a special output transformer or impedance unit for beat results. An output transformer also will protect your speaker from burn-outs. Checking Your Wiring

This little tip used by “Raytheon” ever since he wired his first valve set some years since, and which lias always been rigidly adhered to whenever construction is being done may prove useful to others who like to construct their own. If a kit set of components is purchased there is, or should be, a complete blue print showing all wires. As each wire is put into place and soldered, fill in that particular wire on the print with a blue pencil crayon. It will save much trouble later on, and more than anything usually means the set will “go” first try when completed. If no diagram has been supplied, sketch a rough one out in pencil beforehand and fill over with the blue pencil as the work proceeds. Eight Common Radio Fallacies A contemporary publishes the following list, which may be of interest under an article entitled “Popular Delusions about Radio.” 1. That long-distance reception is possible only at night. 2. That the greater the static is the less the signal strength, and viceversa. 3. That the colder the night is the better is the reception. 4. That a regenerative receiver can “re-broadcast” a programme. 5. That a receiver with six or eight valves must necessarily get better results than a set with four or five valves. 6. That a long or high antenna is more efficient than a short or low one. 7. That better reception is attained by the burning of filaments of valves to the limit. 8. That distant programmes are better than local programmes.