Straight Line Amplification?

[epigramophone]

For the 1929 season UK Columbia introduced their "Plano-Reflex" range of machines, based upon the mistaken belief that sound waves behave in exactly the same way as light beams. Nevertheless the machines play well, greatly assisted by their excellent soundboxes, the No.9 and later the No.15.

Columbia soundboxes progressed from pot metal to brass construction. HMV regressed from brass to pot metal, hence the demand among collectors for their earlier brass backed soundboxes.

[Henry]

As you rightly surmise, the "plano-reflex" concept is just that---a conceptual oversimplification of the way sound waves actually behave. The analogy to brass instruments is apt, and instructive: smoothly curved bends are better than sharp, angular ones (see the diagrams). On the road toward better sound reproduction, this one is a dead end, IMO.

[epigramophone]

Following Columbia's 1931 merger with HMV to form EMI, the larger Plano-Reflex machines were rapidly phased out. The portables lingered on for a few years, partly to fulfil Columbia's long standing contractual arrangements with Garrard for the supply of motors. The last of them were fitted with HMV motors and carrying handles, presumably to clear remaining stocks of cases.

My main reason for posting the Plano-Reflex advert was to show that techno-babble is nothing new.

[Marco Gilardetti]

As a physicist, I wouldn't be so tranchant with the plano-reflex principle.

The plano-reflex arm was introduced to improve the high end of the spectrum's efficiency. Whoever saw a horn specifically calculated for high frequencies knows that it is a very small object, thus quite distant by the linear size of a gramophone's horn, which is primarily intended to get some bass out of the record. The idea is that the horn is so un-optimised for high frequencies that a direct path (folded and mirrored) which doesnt' use the horn as an impedance adapter might be preferable altogether or would, in any case, add up and improve the sound a bit.

This is, in principle, not much different by nowadays hi-fi horn loaded loudspeakers where the front of the cone is exposed and radiates directly, while the back of the cone is conveyed through the horn and radiates indirectly:



If and how much the direct emission prevails over the horn-loaded path of sound in plano-reflex gramphones is questionable and would require, more than calculations, some precise measures or comparative listening sessions. In any case it has also to be noted that the horn path of plano-reflex gramophones is a very well calculated one, with a good approximation of the exponential envelope, except that it also has plane mirror-like surfaces at foldings. At least a fourth plane surface is not visible in the above graph and it's beneath the arm's elbow, almost hidden inside the gramophone's case. Moreover, tracking angle/overhang of the soundbox is almost perfect.

So under many aspects if not all of them, it definitely isn't an unsubstantiated arm and, as a matter of fact, whichever the cause, Columbia Viva-Tonal gramophones with Plano-reflex arms sound amazingly well. The 112a model, one unit of which I have the luck to own, is one of the most regarded portables ever, and I can personally assure everyone about its outstanding sound quality.

[zipcord]

Nice Transmission line speaker loudspeaker assembly. Transmission lines addressed the fact that the back wave (radiation from the port) is out of phase with the direct radiator. The transmission line enclosure, by nature, increases the path length of the back wave in order to get the it's radiation back in phase with the direct radiator.

The Plano Reflex, to me, is the analogy of the diffraction horn (Plano Reflex) vs. an exponential horn (Victor Talking Machines/Victrolas with the exception of the rigid arm)

[Marco Gilardetti]

That is by no means a transmission line cabinet, I'm afraid. Transmission line's ducts are larger at the speaker's back and narrower at the aperture. Ducts with a loading chamber behind the speaker and a throat that is narrower than the aperture (mouth) are horns.

Diffraction horns are shaped in such a way that sound emission is maximized in one space axis at the expense of another. The Plano-Reflex principle was not introduced in order to alter the spatial emission of sound but to improve the frequency range, and I can't understand the comparison you suggest.