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The 1501AL-1 employs a pure pulp straight sided cone body
attached to an outer suspension made of special foamed rubber.
This suspension material has the longevity and frequency-response
characteristics of traditional rubber surrounds, but with a low density
very close to that of foam surrounds. This material was chosen to
preserve the transient detail of musical signals while still providing
the movement required for high output at low frequencies. Unlike
traditional foam surrounds, the foamed rubber has a very long
service life and will not deteriorate. The cone consists of a special
layered paper-pulp matrix with proprietary Aquaplas damping,
which offers more pistonic behavior throughout the woofer’s
operating bandwidth, and controlled cone breakup beyond it.

The 1501AL-2 uses an “accordion pleated” high strength Poly-
Cotton cloth outer suspension. This material is then coated with a
sealing/damping compound to render it air-tight and provide the
appropriate amount of damping control. This surround is attached

to a composite cone body consisting of an inner and outer layer of
pure pulp stock with an inner foam injection core. This sandwich
construction creates an incredibly stiff, but still relatively light cone
body with extremely good distortion characteristics.

Both woofers employ dual inverted Nomex

spiders for the

cancellation of even-order distortion components. The design
and material of these spiders have been further optimized for the
DD65000 and DD67000 systems to offer even greater freedom
of motion and dynamic range, specifically at low playback levels.
All suspension elements are tailored for maximum mechanical
displacement linearity.

All together, these design factors provide reduced harmonic
distortions at very low and very high acoustic output, improved
power handling, reduced power compression, and more
consistent spectral balance with varying input drive level.

476mg, 476Be high-Frequency
compression driver

The 476 family of high-frequency compression drivers make
use of a 4-inch (100mm)-diameter formed diaphragm with a
4-inch (100mm) aluminum edge-wound voice coil, operating into
JBL’s existing rapid-flare-type, coherent-wave phasing plug. The
476Mg (DD65000) has a 95% pure magnesium alloy diaphragm.
The 476Be (DD67000) has a 98% pure beryllium diaphragm.
The use of an efficient neodymium rare-earth motor structure
with a new copper-sleeved polepiece maintains maximum gap
flux and reduced coil inductance at a minimal size and weight.
The combination of these features has resulted in drivers that can
deliver superior sound quality with very little distortion and power
compression, regardless of acoustic power output.

A high-purity copper sleeve is used for the polepiece. This greatly
improves the electrical conductivity of the polepiece for lower coil
inductance and thus greater high-frequency output at 15kHz and
above. The copper-sleeved pole piece quickly wicks away heat
generated by the voice coil, thereby contributing to a reduction
in dynamic power compression. To compensate for the higher
resistance caused by the use of the copper-sleeved polepiece, a
new magnet with special high-grade and high-temperature-grade
neodymium has been used.

The phasing plug is of JBL’s traditional rapid-flare, coherent-wave
four-slot design. This coherent-wave design shapes the wave
output, producing a truly coincident wave front as the sound
enters the Bi-Radial horn.

The diaphragm is formed of very high purity magnesium alloy
foil (476Mg, DD65000) or pure beryllium foil (476Be, DD67000)
that is manufactured with a proprietary high-temperature and
pressure-forming process. This process enables the integrated
JBL diamond-pattern surround to be formed as one piece with the
dome. Compared to other methods, forming the diaphragms out

of sheets of magnesium or beryllium foil yields greater reliability and
resistance to failure due to fatigue. If breakage ever does occur, the
diaphragm does not shatter into pieces or harmful dust. Beryllium
has a very high stiffness-to-density ratio of about five times that
of aluminum, magnesium, titanium or iron. This maintains pistonic
behavior up to 20,000Hz, eliminating diaphragm modal breakup
and keeping the upper frequency response very smooth, with
minimal distortion spikes. Compared to the 475Nd compression
driver used in JBL’s original K2 S9500 system, this is about a 45
percent reduction in moving mass. With such a low mass, the
moving assembly is able to respond even more quickly to musical
transients, further enhancing the reproduction of musical detail
and microdynamic nuances.

Like the beryllium diaphragm in the 476Be, a proprietary thermal
forming process is used to form thin magnesium alloy sheets into
a 100mm magnesium dome shape of exacting dimensions for
the 476Mg. This includes the integrated JBL diamond surround.
The 95% pure magnesium alloy diaphragm of the 476Mg also
uses small percentages of aluminum, zinc, and other alloys to
improve characteristics such as diaphragm forming, fatigue
strength, corrosive and acoustic properties. Magnesium has very
low density, slightly lower than even beryllium, but with much lower
stiffness. However, with such a low density the thickness of the
magnesium alloy diaphragm can be increased to 130 microns,
greatly improving its stiffness. In fact, when compared to the
titanium diaphragms used in JBL’s original K2 S9500 and M9500
systems, this new magnesium alloy diaphragm is 2.5 times thicker
but maintains about the same moving mass of 3.4 grams. This is
beneficial because with high stiffness, pistonic behavior extends
up to higher frequencies than is possible with current aluminum
or titanium diaphragms. Another benefit of the magnesium alloy is
greater internal loss or damping when compared to aluminum or
titanium diaphragms. This greater internal loss quickly minimizes
the amplitude of resonances when the driver is asked to produce
frequencies beyond its acoustic limits.