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Like the 1500AL, the 435Be stands at the forefront of a long legacy of
continual advancement in transducer engineering. However, understanding
its significance requires an understanding of the evolution of compression
drivers at JBL.
From the company's inception, and through to the late 1970's, JBL relied
on compression driver technology that was virtually unchanged from that
established by the engineers and scientists at ATT Labs in the 1920's and
1930's. There were basically two types of compression drivers in JBL's
catalog – the large format driver and the small format driver. The large
format driver (best represented by the 375) used a massive Alnico magnet
to energize a circuit that held a 4” diameter aluminum diaphragm. The
small format drivers (175, LE85) used a 1 3/4” diameter aluminum diaphragm
in significantly smaller motor structures. Both types used an integral
aluminum surround drawn from the same single piece of aluminum that formed
the diaphragm.
Large Format Driver (375)
© Harman International
,
Courtesy Mark Gander and John Eargle
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Not surprisingly, the large format drivers were intended for high output.
However, there was a tradeoff in high frequency extension. Due to the
higher mass of its large diaphragm, its response was limited to 10khz. The
small format drivers, with their lighter diaphragms could extend response
up to 15khz, but could not match the output levels of their larger
counterparts.
The first significant innovation to these designs occurred in 1979 with
JBL's introduction of the diamond surround. These surrounds take advantage
of a phenomenon called ”“parasitic resonance”. The aluminum surrounds have
a natural resonant frequency. Forming the surround into a diamond pattern
pushes the second resonance out in frequency to result in a broad band
response. This allowed the large format drivers to match the frequency
extension of the small drivers.
However, it was soon discovered that the diamond surrounds limited power
handling. The diamond peaks are more susceptible to stress concentration
and failure. This led JBL to pioneer the use of titanium as a diaphragm
material in 1982. Titanium is an order of magnitude more resistant to
fatigue failures than aluminum. With this new diaphragm, JBL now had
compression drivers with exceptional output and extension.
However, it was recognized that there were compromises with the new
diaphragms. Titanium does not have the internal damping of aluminum and
thus has marginally higher distortion levels. The diamond surrounds, while
extending frequency response, do so at the expense of transient response.
Further, due to its lower stiffness, titanium goes into breakup at a lower
frequency.
This issue of breakup is worth elaboration. Ideally, a dynamic loudspeaker
diaphragm should act as a piston, with all points in uniform motion.
However, since diaphragms are not infinitely rigid, there will be a
condition at which the forces acting upon it cause oscillating deflections
resulting in different points on the surface moving in different
directions. Under this condition, the diaphragm is said to be in breakup,
and there is an attendant increase in distortion. Both aluminum and
titanium compression driver diaphragms are in breakup for much of their
response. On a large format driver, the breakup modes for aluminum
diaphragms occur as low as 7000hz, and for titanium diaphragms, as
low as 4000hz.
In 1999, Doug Button began development
on a new series of compression drivers that would result in the 435Be.
With the 435Be, Doug wanted to address both the bandwidth and output
requirements without compromise in distortion. The goal was to have a
driver that was pistonic throughout its bandwidth and have extension that
did not rely on parasitic resonance. The solution was in a different
diaphragm material – beryllium. The use of beryllium in compression
drivers was not new. The TAD division of Pioneer had been producing such
drivers for many years. However, the approach and design objectives set
for the 435Be were unique.
435Be Beryllium Diaphragms
© Don McRitchie
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To ensure pistonic response, Doug
specified a smaller 3" diameter. A beryllium diaphragm of this size
ensured that breakup modes would be above 15.5khz. A thin layer of
Aquaplas was applied to the back of the diaphragm to damp spurious
resonances. The diaphragm was also light enough for extension to that
frequency without parasitic resonance. Total moving mass was only 1 gram,
with the diaphragm less than 0.5 grams. This was less than a third the
weight of previous materials.
While the switch to beryllium addressed
distortion and extension issues, it would not address the requirement for
high output since a surround formed out of this same material has much
less fatigue resistance than the titanium it replaced. Doug addressed this
in a very innovative way, with a unique composite design. He specified a
kapton surround that would be immune to fatigue failure. However, previous
composite designs always proved problematic due to the need to have the
diaphragm, voice coil former and surround all attach at the same point.
Doug solved this issue by having the kapton surround formed into a deep
well just before the point where it attaches to the diaphragm. The voice
coil was dropped into this well, which eliminated the need for a former,
and thus, only the surround had to be fastened to the diaphragm. This
resulted in a very robust design with reduced moving mass.
Doug next worked to minimize the weight
of the overall driver. The use neodymium allowed for a very small magnet.
The unique external magnet topology resulted in an extremely efficient
magnetic circuit with an absolute minimum of material. It resulted in a
gap flux density of 2 Tesla in a driver that weighed less than 3lbs. In
comparison, JBL's large format compression drivers weigh as much as 30lbs
to achieve the same gap flux density.
435Be Cross-Section
© Harman International
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Another unique feature of this driver is
that it does not have a traditional throat. The phase plug terminates at
the driver exit. It results in an effective flare rate of 550hz.
Previously, virtually every compression driver made had a 180hz flare rate
whose origin dates back to the original AT&T Labs designs from the 1930's.
This low rate was necessary to accommodate the low cross-over points used
in early two-way loudspeakers. However, this low rate compromised high
frequency performance. Given that there was no need for such low frequency
output for the 435Be, the flare rate could be optimized to result in a 6db
drop in second harmonic distortion.
The final result was the highest
performing high frequency driver ever produced by JBL. The stiff, light
beryllium diaphragm has such excellent transient response that its sonic
character is more near an electrostatic design than previous compression
drivers. Distortion was reduced to levels never previously attained at
JBL. All of this was accomplished with a driver whose output, and thus
dynamic range, were fully in keeping with the hallmarks of the JBL brand.
© 2004 Don
McRitchie
based on information provided
by Doug Button
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