US Acoustical Labyrinth system introduced Aust. 1937

A description of improved sound fidelity using the US Acoustical Labyrinth system to be introduced in the 1937 Stromberg-Carlson models.

 Wireless Weekly October 16, 1936, Page 57-59.

IMPROVED FIDELITY

To Be Featured For 1937 Stromberg-Carlson Models 

 

The radio season is rather hard to define. It is generally accepted that wintertime is the best time for radio reception, the atmosphere then being free from static, and when it is cold and wet outside the family prefers the fireside and the radio. But even if you take it for granted that wintertime is the best time for radio, you still don’t define the radio season.

Is the radio season now finished, and should the new models be released now, or should they be held until March or April, the start of the next season?

 Fortunately, it doesn’t matter much, because the seasonal models are not so pronounced in radio as in other businesses. Most of the radio factories in Australia are organised on flexible lines, and they can turn out a new model whenever the time seems ripe, without any great extra cost for dies and special tools.

The winter now being over, and new models are being designed and, in some cases, released to the public. From what we have been able to discover about the new models, it appears as though the old war cry of “improved fidelity” is to be resurrected this season, with even greater push behind it and also quite an amount of actual scientific improvement.

 

WHAT IS TONE?

The whole history of the tonal quality of radio receivers is rather interesting. To run over it roughly, we would say that, when broadcasting first started, only about 14 years ago, the crystal set was widely used, and by using headphones with it, quite good tonal quality was obtained.

Then along came the valve sets, with their horn speakers, and tonal quality became very poor. It became common practice to say that a receiver had such good tonal quality that it was possible to understand nearly every word spoken by the announcer.

But about 1927 and 1928 there came a vast change in the better-class sets. Gramophone pick-ups were popular, dynamic speakers of the moving coil type sold for about £12 each, the push-pull amplifiers with 210 and 250 type valves were built by those enthusiasts who could afford the cost, and for a while, the fidelity of reproduction became greatly improved. But the improved quality was retained as a feature for only the better-class sets, until in 1930 there came the direct-coupled circuit, and with it quite good tonal quality at low cost. But the direct-coupled circuits proved bothersome in service and were soon displaced by resistance coupled audio systems with pentode output valves.
These gave a lot of volume for low cost, with greatly improved sensitivity, but very poor tonal quality. And so, things jogged along from 1931 to 1936, with a slight improvement in quality due to the use of diode detectors and improved loudspeakers.

Radio enthusiasts who wanted something better in the way of reproduction found it essential to build their own receivers or else pay a very high price. The home builders found that our circuits with push-pull output systems were capable of quite good quality. But it took radio enthusiasts to appreciate this fidelity.

The ordinary householders who wanted a receiver seemed to show a definite taste for a receiver with plenty of range and selectivity. When “tone controls” were fitted, the average buyer seemed

to prefer to have the high notes entirely eliminated to give a restful, but the soggy, type of reproduction, consisting of the frequency range from about 100 to 3000 cycles.

AVOIDING DISTORTION

Possibly the reason why the highs could not be appreciated was because of the high percentage of harmonic distortion in the reproduction. By lopping off the high notes some of the distortion was lopped off, too. And so we find that during the years 1933 to 1936 many a factory technician broke his heart trying to get improved frequency response, only to find that the buying public did not appreciate his efforts.

Now it appears as though the whole business is to be opened up again. Certain factory technicians claim that the buying public is now tired of “mellow” reproduction and that they will appreciate the clarity of wide frequency response. What is frequency response, anyway?

FREQUENCY RESPONSE

The sound which comes from a loudspeaker is considered as being made up of a number of sound waves, varying in the frequency of their waveform. Low, rumbly notes have about 50 to 100 waves per second, or cycles per second, as they are usually called. The high notes, such as the screams of the heroine in the serial, may have a. frequency of 5000 cycles per second.

Different human ears have different frequency responses, but an average is capable of hearing all sounds having a frequency between 16 and 20,000 cycles.

If our radio receiver could reproduce all of this frequency range, without any distortion, and with as much volume as the original, then the receiver would be capable of giving a reproduction of the original so life-like that you wouldn’t be able to tell the difference, perhaps. But, of course, the radio receiver can’t give forth more than it receives, and there are losses, right from the time the sound leaves the lips of the broadcasting artist before these sounds hit the microphone. By the time the microphone has handled them they have probably changed form quite a bit. Every stage of amplification in the modulators of the transmitters introduces further losses. And so you find that, no matter how good the radio set, the reproduction is never quite realistic.

If you don’t believe it, just get someone to stand alongside the receiver and switch off the set and repeat the announcer's few words. If you sit about six feet away from the set and the speaker, with your back turned and your eyes shut, you will be able to tell very definitely that the spoken voice is very different from the loud speaker’s efforts to imitate. Not that it matters much, for the ears soon become accustomed to the loudspeaker, and it becomes quite easy to understand the announcements, even if the actual speech is different from the original.

Similarly, as regards music, it is possible for a radio set with a frequency response of only a quarter of the ear’s range to reproduce a form of music that may not be very similar to the original rendition, yet it is capable of “soothing the savage breast” (or should it be, beast?). In fact, there has arisen quite a general reference to the “tone” of a radio receiver. Originally it was thought that a radio receiver should have no tone of its own; it should simply strive to reproduce the original as faithfully as possible. But the buying public has tended to judge tonal quality by its effect on them.

If the radio set has a wide frequency response they refer to it as “not soothing,” and father will probably say after a hard day at the office, that the radio sounds harsh, and he wants it turned down to its softest position while he reads the paper.

All of this thoroughly shocks radio and music enthusiasts who work out the distortion percentages of their reproduction, who spend £12 on a speaker instead of 30/- in order to get the highs between 6000 and 7500, and who strive for “high fidelity”.

A very solid drive after high fidelity started in the United States last year, and specifications were laid down for a standard of high fidelity, the principal being a frequency response flat within 2 decimals from 50 to 7500 cycles, a power output of 15 watts with not more than 5 percent, total harmonic distortion, and so on.

 Candidly, the whole scheme was a failure so far as mass production and bulk sales were concerned. Not 5 percent, of the receivers sold during the past twelve months in the United States, could claim to come up to these specifications, if for no other reason than cost. In order to get such high fidelity the cost is greatly increased, and, as has been proved time and again in USA, the people want cheap radios.

DRAWBACKS OF FIDELITY

There are several serious drawbacks to high fidelity, the first and foremost being the way in which a high-fidelity receiver reproduces static and electrical noise with such fidelity, that static crashes through extra hard when a high-frequency response is allowed.

Then there is the question of transmission technique. Many Sydney stations sound fine on an ordinary receiver, but played on a set with a satisfactory low note response there is a heavy background of hum in the carrier, and with a high note response the studio noises, line noises, sizzles, and whines of converters and so on spoil the effect. The calls of the newspaper boys in Market street and the toots of the cars become prominent in the background of the news sessions from 2BL (not that this is unpleasant, in fact, it adds realism).

ADVANTAGES

On the other hand, the marketing of a receiver with fidelity is a good scheme for any factory, for it gives further scope. There are some people who do appreciate fidelity, and this type of buyer will be a wonderful advertisement, proudly displaying the receiver to everyone who visits the house, talking about it incessantly, and so on.

Tone control can be fitted so that noise and static can be smoothed over, and “mellow” reproduction obtained when desired.

The more you hear of a good quality receiver the better you get to like it.

If we were sentenced to suffer the penalty of being locked in a prison cell with a radio set going for twelve hours a day, for example, we would infinitely prefer fidelity. The thump, thump of soggy “mellow” tone gets on your nerves after a time.

EDUCATION NECESSARY

Higher pitched tonal quality seems as though it is going to be a noticeable feature in the new season’s receiver, and the public should be educated to appreciate it. The first thing to remember is that a tone control is fitted, and if highs are not wanted they can be lopped out according to taste. The extra high note response is being offered as something that is quite free, and if you don’t want these free highs you can cut them out very readily. When a thunderstorm looms up on the horizon, the tone control will prove valuable to cut down the fidelity with which the crashes of static are reproduced.

If you own a high-fidelity set you will also want a constant reminder that the stations with obsolete equipment and with worn-out records don’t really sound any worse on a good set. On a set of comparatively poor frequency response, all stations sound the same, and you can’t even tell the difference in wide range and third dimension recordings.

But with a set having decent high note response, the difference in the stations and in the recordings becomes more pronounced, and you find that the owner of the set spends a lot of time turning from one station to another in search of something that sounds alright. It is a difficult job to convince them that the poor stations don’t really sound any worse on a good set.

DISTORTION

Probably to the average ear a far more distressing feature of reproduction is harmonic distortion, and whereas a certain amount of frequency discrimination may not be unpleasant, in fact, may be quite satisfactory, a high percentage of harmonic distortion is certain to meet with disapproval, even from listeners without any musical or technical knowledge.

This point brings to light the care which is needed when a set is designed for higher fidelity. If the high notes are to be reproduced, it becomes most important to see that harmonic distortion is as low as practicable, as otherwise there will be a higher percentage of distortion in the so-called high-fidelity set, which will make the quality sound harsh to all types of listeners.

THE ACOUSTICAL LABYRINTH

One of the first of the new models to be released is the Stromberg-Carlson model, which is being widely advertised as having an acoustical labyrinth. Already a number of readers have asked us what this acoustical labyrinth really amounts to and what it can do. Fortunately, the Stromberg people have prepared a booklet on the subject, which they will be pleased to send to anyone who asks for it.

In a nutshell, the acoustical labyrinth consists of cabinet treatment designed to do two main things, firstly to provide a more effective baffle for the speaker; and, secondly, to stop the sound waves from the back of the cone of the speaker from being reflected from the wall behind the cabinet to upset the fundamental sound waves issuing from the front of the speaker.

Undoubtedly, the acoustical labyrinth idea is sound, both technically and practically, and quite a few firms are making plans for special cabinet treatment of this type, some utilising the sound waves from the back of the speaker to reinforce the high note response.

We feel sure that Stromberg’s is going to meet with the success that their effort deserves, but at the same time, they are bound to have a certain amount of the usual trials of any pioneer,  particularly at the hands of the buyers who have no musical or technical, knowledge, and whose ears have become accustomed to the ordinary ‘canned music” type of reproduction.

The reason for this is not so much the acoustical labyrinth itself, but the fact that the Stromberg designers have made a definite effort to offer a much wider frequency response than normal.

Another thing that the ordinary listener seldom seems to appreciate is the need for fairly high-volume output. It is quite impossible to obtain brilliance of reproduction with a low volume level unless the response is faked.

The excellence of reproduction at high volume levels tends to give an impression of a falling off in quality at low volumes, just another difficulty that is sure to be encountered when the new type of reproduction is pushed into unlearned hands.

 

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