The 21cm full range speaker revealed just over 3 years ago is the earliest of the Model 7 series. The cellulose pulp membrane is equipped with a peripheral carbon ring placed very close to the chassis. It is held by three carbon rods all positioned at 120° angle from each other and meeting at the top of the central bronze phase plug which is surrounded by a « flower petal » shaped bicone. The lack of suspension allows for a greater freedom and speed of movement of the membrane. Consequently, to prevent exceeding the mechanical capacity of the speaker,and possible damage to it, we recommend the use of low power electronics ( a few tube-quality watts for example).
The mechanical properties of carbon (flexibility and elasticity according to the type of effort exerted) allows the membrane to move with perfect control. The Thiele and Small characteristics measured with such a speaker clearly show that it tends towards the theoretical ideal.
Nevertheless, the response curve shows amplitude variations which need to be corrected either with a passive correction cell (speaker filter) or with a parametric equalizer.
With a resonance frequency of 24 hertz, the Fertin Acoustics Speaker equipped with just one 21 Model 7 enables us to listen to the whole of the frequency range with exceptional accuracy of tone, as much for the human voice as for musical instruments.
