voigt-pa: Voight Twin Diaphragm Unit

Extracted from the Lowther Voigt Radio, Season 1936-37, Page 18.

Voight Twin Diaphragm Unit

Specifications

 

Very little can be said in the space available to describe fully all the various points and advantages of the Voigt moving coil driving unit. First and foremost, it is noteworthy that high electro-acoustic efficiency is believed to be between 25 and 35%, that is to say, that for a given input the sound output would be about 5 dB above the average unit.

Under working conditions across a 2 mm gap used N.P.L. tests reveal 17,000 lines per sq. cm.

In practice, with such an intense magnetic field, the inductance of the moving coil is substantially less when the field is switched on than when it is off.
As a result, the dynamic impedance at 8,000 cycles is only about 50 Ω which compares very favorably with an impedance of 30 to 35 Ω at normal musical frequencies.

This not only permits accurate matching with the output valve and also the maximum transference of electrical energy, but there will also be no tendency to choke back the higher harmonics.

To reproduce transients "aperiodically" that is, without delay introduced when the resonance of any kind is used to build up the vibration it is necessary that the currents should not be wasted in overcoming the inertia of the moving parts. Since the available force on the coil is partly used to overcome the inertia of moving parts, it will be obvious that by making the inertia as low as possible the maximum force will be left to do useful work.

In the Voigt Loudspeaker, this inertia is made as low as possible by using aluminium wire for the speech coil. There are, therefore. Three reasons why the efficiency of high notes is greater than with ordinary loudspeakers.

Two types of diaphragms are available the standard and the twin. The standard diaphragm responds efficiently up to above 4000 cycles.

The twin diaphragm is slightly different from the standard and in particular, is provided with a second truncated cone that deals with frequencies above 4000 cycles.

The Unit when fitted with normal diaphragms (standard or twin) and used on a suitable horn is intended to handle an input from a few milliwatts up to that required to produce sound output estimated at 2 by 10⁷ ergs/sec. This, power is about half that of a nine to fifteen-piece jazz orchestra and owing to the high efficiency of the Voigt speaker is obtainable with an electric input of 6 to 8 watts.

Alternatively, when greater handling capacity is required, a unit fitted with a double power diaphragm may be used. Such a double power diaphragm is incorporated in the Protected Public address unit.

At very low frequencies, i.e. well below the horn "cut-off, the intense magnetic field helps to prevent excessive vibration of the coil. This is because the back E.M.F. produced by movement in the strong field opposes the current through the coil, thus reducing it.

Boominess in reproduction and danger of rattle or damage due to excessive displacement are thus avoided.

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