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Antique Phonograph News
Canadian Antique Phonograph Society


Sep-Oct 2008

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Some Notes on Orthophonic Era Soundboxes
by Thomas Rhodes

After the advent of "Victor Day" on Monday, November 2, 1925, the accepted thinking about the nature and format of the garden variety talking machine was forever changed. No longer was the public, which was already embracing the home radio receiver in ever increasing numbers, content with the essentially Edwardian upright Victrola. They were not necessarily deserting the phonograph field but were decidedly restive over the long reign of a mechanical record player that had been bestowed with only incremental and largely cosmetic betterments over the course of nearly twenty years! It was not that the sound of early radios was luring them away; in fact, many early radio horns had more limited output than even a minor league phonograph. The appeal of apparently endless and costfree entertainment, with no spring-winding chores, needles to misplace or fragile disks to break, proved of immense appeal; strong enough to challenge the talking machines’ long hold on domestic entertainment.

No. If the talking machine were to survive, two paths had to be taken. One, to "civilize the beast" by making radio simply a part of a "combination" instrument. Two, to better the sound of the talking machine so that its appeal would be strengthened and its traditional patronage regained. The Victor Orthophonic line admirably accomplished both these goals and literally gave Victor a new lease on life. One of the features of the new Orthophonic instruments was its soundbox, like nothing previously offered by Victor or any other firm.

In this short piece we will therefore examine this soundbox and its effect on design thinking among competing manufacturers, in the general quest to update and improve talking machine technology in the "Phonic Era".

Beginning Remarks

A talking machine is nothing more, from an engineering standpoint, than a mechanical loud-speaking telephone transmitting a recorded message. Given the underlying and fundamental similarities of these two devices, Western Electric engineers were able, using circuit analogy design precepts, to reconfigure the standard upright phonograph into a form that made the fullest and most efficient use of its given size and features. When the outside-horn machines of the earliest gramophone era began to be replaced by insidehorn cabinet instruments, we had, in the words of Compton Mackenzie of Gramophone fame, "acoustics subordinated to cabinet work". In essence, for nearly two decades Victor and its competitors sold what has been dismissively called "phono-furniture".

Then, with the design work on an "improved mechanical phonograph" begun at Western Electric in 1922 by a small team led by Henry C. Harrison and including Paul B. Flanders, Edward L. Norton and Theodore Osmer, with substantial input from loudspeaker pioneer Albert L Thuras, the cabinet-enclosed talking machine was not rejected but instead raised to a lofty level of applied acoustic engineering. Aside from the greatly enlarged horn, calculated to precise logarithmic formulas, the outstanding feature of the new Orthophonic line was its vastly bettered soundbox.

Brief Engineering Basics

A soundbox is a mechanical transducer, which converts the potential energy engraved into the wavy spiral groove of the disk into kinetic energy, by the agency of the record spinning under its needle-arm assembly. This movement of the needle-arm or stylus bar, in essence a pivoted armature, is transferred to the diaphragm which, fastened securely to the armature, must necessarily itself move in concert with the imparted groove modulations. Now, the area behind the diaphragm, enclosed by the outer wall of the backplate, causes the soundbox to act like a compression-type loud speaker, meaning that the armature/diaphragm assembly works into this enclosed and deliberately constricted space, in the fashion of an air pump, forcing wave motion from the backplate through the throat and into the tone arm, where it is channelled into the horn assembly. The horn (and the tone arm in talking machines with a tapered arm, like the Victor and Victrola) acts as a coupling between the soundbox and the air within the room. It does this by being a gradually expanding chamber that transforms the incoming sound waves of small size and high velocity into outgoing larger waves of slower velocity. At this point the sound waves
leave the horn mouth and radiate into the surrounding air. A horn therefore, no matter what its size or shape, depends upon being driven by the soundbox, itself a mechanical analog to a loud speaker motor. If the soundbox is poorly designed or otherwise inefficient as a transducing or converting agent, the output of its associated horn will likewise be poor.

In the mechanical period (sometimes called the "acoustic era"), the customary soundbox employed a diaphragm made of mica (micaschist), a natural geologic substance easily split or cloven into thin plates suitable for the purpose. This occurred before the development of reliable plastics, so the early gramophone engineers can hardly be faulted for the choice. However, from the standpoint of sound transmission, the mica-equipped soundbox has several drawbacks. One, a given mica disk itself has only so much mechanical compliance or yield factor. Too thin a disk will not hold up under repeated use and will necessarily be
prone to damage and distortion. Two, the need to securely clamp the mica disk both for positional stability and to prevent air leaks around the gaskets, further limits its compliance. Three, the clamping pressure used tended to either raise or lower the natural vibrational period or periods of the mica disk, adding unpleasant resonances to the frequency band of interest. Four, the mica diaphragm is driven at one point only, where it was attached to the needle-arm. This tends to produce a diaphragm action that bends the mica disk mostly at its center, with movement falling off as it reaches the periphery. Such an action might be sufficient for reproducing the customary acoustically recorded disk, with its rather limited frequency and dynamic range, but is unsuitable for transmitting input from an electrically cut record.

An Early High Compliance SoundBox

Joseph P. Maxfield, an MIT graduate hired, along with colleague H. C. Harrison, by Dr. Frank Baldwin Jewett into Western Electric in 1914, had witnessed for patent on August 23, 1917, an improved soundbox designed with much higher compliance than the garden variety mica- equipped models. Its diaphragm, of differential thickness made (like those patented by an older Western Electric inventor Henry C. Egerton for Western Electric loudspeakers) from molded fabric, was not only of better yield characteristics, but had less annoying resonances as well. The diaphragm of this early Maxfield soundbox, like those designed by H. C. Egerton, had a thicker center portion for "plunger" action, and a thinner outer periphery for long excursion properties. The salient features of this soundbox, although embodied in forms different from the later Harrison-authored diaphragm, no doubt influenced the latter’s thinking in terms of performance goals.

The prevailing model customarily employed by acoustic scientists of this period was that of a straight pipe or tube driven by a diaphragm acting as an infinitely smooth piston. This theoretical tube produced no wave-form distortion either at the driving end or at the open end. Sound conducted through this tube was to be an orderly succession of perfect planar waves, which obviously existed only in laboratory studies, not in the rough and tumble of the commercial talking machine world! Henry Harrison found, however, that if a body intermediate between needle-arm attachment point and diaphragm were introduced, the diaphragm tended to act with much more plunger action, like the ideal piston. This interposed body or structure tended to spread out the motion from the armature, affording the desired broader action in the diaphragm, which in turn caused more air to be displaced compared to the center-biased mica disk. This was especially critical in the transmission of lower frequencies or bass, just like the wide excursion cone speakers of the upcoming Electrola and Panatrope models, then also in development. Thus was born the spider of the eventual Orthophonic soundbox that in turn led to the cone intermediate element of the Brunswick soundbox and the dome structure of the Columbia VivaTonal models!

Better Diaphragm Materials

It was not enough, however, just to place an additional structural member before the main diaphragm. Even with such a structure, whether spider, cone or dome, the elastic limits of mica could not have been overcome. What was needed was a new material, thin enough for compliance yet reasonably free from unwanted bending and "billowing". Egerton and Maxfield chose fabric strengthened with phenolic, a good choice for low resonance during their gestation period of 1916-1917. However, there was another telephone tradition which could lend itself to the cause. Aluminium diaphragms had been used in telephone handsets years before, and in the early 1920s, during the Harrison investigations, a relatively new aluminium alloy called duralumin (known technically as alloy 17 S-T) made with 4% copper, 0.5% manganese and an equal measure of magnesium, came into industrial usage. This alloy proved to have the ideal properties sought by Harrison, namely strength combined with flexibility. After the advent of the Orthophonic line in the fall of 1925, Victor’s chief rivals, Brunswick and Columbia, followed suit, discarding without regret their micaequipped soundboxes in favor of formed light alloy diaphragms. In our next instalment we will examine the particular features of each of these soundboxes, to determine how they bettered the performance of the new lines of large-horn mechanical disk instruments.

Concluding Note:

This researcher would like to thank Mr. Keith Wright for encouraging me to write this brief soundbox piece and for featuring it in this fine publication. Most of the data in this article was adapted from my book-in-preparation, The Orthophonic Victrola in Word and Picture, specifically the chapters entitled "Henry Harrison and His Talking Machine World" and "Competing Machines" Readers wishing to learn more about talking machine acoustics are encouraged, if they have not already done so, to visit the outstanding Victor-Victrola Page maintained by my correspondent Paul C. Edie. Mr. Edie, an acoustic engineer of wide learning and deep understanding, features an invaluable chapter entitled "An Introduction To Vibro- Acoustics" that should be required reading for any serious talking machine collector!