Voigt "Light Coil Twin" Loudspeaker Diaphragm

Voigt "Light Coil Twin" Loudspeaker Diaphragm.

This light coil twin diaphragm is inter-changeable with earlier Voigt diaphragms and a liner can be supplied for the air gap of existing magnets.

Reviewed in Wireless World, March 9, 1939, Page 223.

To take full advantage of recent improvements in the quality of some of the BBC transmissions, particularly from the Alexandra Palace, the frequency response of the Voigt twin diaphragm loudspeaker has been extended in the upper register and lifted to maintain the general level of the output up to at least 12,000 cycles even when the unit is used with the domestic corner horn.

It will be appreciated that this is a more difficult task than producing a level response on the axis, for the corner horn is designed to give even distribution of high frequencies through an angle of 90 degrees in the horizontal plane and also for a considerable range in the vertical.

There can be no doubt that the work involved has been worthwhile, and as the result of a week's listening to all types of programs, we are satisfied that the lily has, in fact, been gilded. The transient response is as good as ever and the bass chamber in its latest form extends the response below the natural cut-off of the horn smoothly and with no hint of artificial resonance.

Every type of transmission has the authentic mark of the original and, without the sense of sight, a listener coming into a darkened room during a good pianoforte or quartet recital might easily expect to find the actual performers in the room when the lights were switched on.

For some BBC transmissions, however, there is an excess of top and a filter can be supplied with the loudspeaker to compensate for such inequalities in the characteristics of microphones, etc.

The technical details of the changes which have brought about this improvement in performance may be of interest to those who possess or who are familiar with the original twin diaphragm. All the modifications have been in the direction of lightening the mass of those parts of the diaphragm which are in motion at high frequencies.

The centring threads are no longer attached to the speech coil former and being relieved of this radial strain it has been reduced to a thickness of only 0.003 inch. As in previous diaphragms, the layers of the speech coil are wound inside and outside the paper former.

The speech coil has been lightened by the use of a finer gauge of aluminium wire, but the DC resistance remains approximately as before. The magnet gap has been reduced by a liner ½ mm thick and the flux density has consequently gone up from about 16,500 to 18,000 lines /cm². Since the poles are supersaturated the coil inductance is negligible and this in turn has resulted in a further increase in the high-frequency output. Incidentally, the clearances between speech coil and pole pieces have not been reduced by these alterations.

The price of the light coil twin unit, including components for the transmitter correction network, is £16, and corner horns in white wood are available at £12 15s. for the simple H.C. corner horn, or £17 5s. for the domestic corner horn with the bass chamber.

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