Best of the year 2012-2013 - page 5

Manger now.
"
The 9-pointed star-shaped damper
on both the front and back of our flat
driver reduces the amplitude of edge reflections. The diaphragm itself
uses a special inlaid ring which matches the wave impedance of the
material like a coax cable does in electronics. Mechanically the voice coil
employs two windings on a single layer spaced adjacent. These windings
receive the same signal but run in opposite directions. Their placement
means that when one winding is inside the air gap, the other one isn't.
Their counter winding compensates for the electromotive force and
cancels out back EMF. This generates our very fast 13μs rise time without
overhang but also allows for the ±3.5mm Xmax which is quite high for a
full-range
driver.
"
Laser Doppler Interferometry
images show how our resistively controlled
diaphragm begins to concentrically activate from within the small area
encircled by the 70mm voice coil. The coincident frequency of such a
driver occurs where the propagation velocity of the bending wave
traveling on the diaphragm attains to the speed of sound in air. This
propagation velocity increases with frequency and makes the coincident
frequency ultrasonic or 80kHz in our case to eliminate the phase problems
which trouble membranes with higher bending stiffness. By changing the
texturized pattern and thickness of the membrane in a controlled fashion,
we exploit shearing forces and control the travel of frequencies depending
on their sound velocity. With rising frequencies, smaller and smaller areas
of the diaphragm are activated. Low fr equencies involve the entire
surface. Due to the driver's geometry there is no room for the voice coil
leads on the inside. Hence four straight wire lengths are visible on the
outside. This voice coil is a mere 0 .4g in mass and its induction is a
minuscule 18μH. That's far lower than any cable or signal -path inductance.
We build the driver in 8/4Ω versions but in the MSMs1 use the 8Ω unit
whose voice coil is a bit lighter.
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