以下是一篇解绍1500AL喇叭开发过程的文章,以及这款喇叭需要达到的一重要参数是失真。它是从1999年开始立项开发的,大约在2003年完成。而且这款喇叭并不是凭空开发的,它是在1400ND基础上搞的。而1400ND是1989开发的喇叭用在K2-9500上。
请大家重点关注,他们解释的失真的原因以及他们应对的方法。 当然,希望声学楼里的高手,对这些做一个评价。最好能提出你的解决问题的方法,供大家学习。
Development of the 1500AL traces back to the first K2 low frequency
driver, the 1400Nd. That driver was originally developed in 1989 for the
K2-S9500 by Doug Button, currently Vice President of Research and Development at
JBL Professional. The intent was to go “all out” in the design of a linear, low
distortion, high output driver. From Doug’s perspective, there was only one way
to achieve the lowest possible distortion – an underhung design. Underhung
designs use a voice coil that is considerably shorter than the magnetic gap in
which it is suspended. Most dynamic loudspeakers use a voice coil that extends
well above and below the magnetic gap to make the most use of the magnetic
energy. However, there is a price in increased distortion. It has to do with the
fact that the coil is not acted upon symmetrically by the permanent field once
it is displaced from the rest position. Conversely, in an underhung design, the
full coil is always immersed in a constant strength magnetic field since it
never leaves the gap.
Underhung designs are not common
in dynamic loudspeakers due to their cost. The fact that the coil height is less
than the gap height means that there are always portions of the permanent field
that are not in contact with the coil. This is wasted magnetic energy and
requires an inordinately strong magnet to ensure reasonable sensitivity. To gain
the necessary magnetic flux levels, Doug specified the very first use of
neodymium magnets for low frequency transducers. Neodymium magnets have
approximately 10 times the magnetic energy by weight as ferrite magnets. Their
use resulted in a relatively compact driver that was exceptionally free from
distortion, yet had high sensitivity and high output.
The 1400Nd driver, and subsequent
K2 systems that employed it, were a stunning success. However, Doug was not
completely satisfied. Classic underhung designs have a unique frequency response
anomaly that is due to the coil topology. It regards eddy currents set up by the
moving coil in the gap. These currents act as the secondary in a transformer to
effectively short out the coil circuit leading to a loss in voltage drive. The
net result is a “swayback” response curve through the midband with about a 2db
drop in output. While this can be dealt with in network compensation, the purist
in Doug wanted that energy back. Addressing this would be the starting point for
the 1500AL design.
Design work for the K2-S9800 and
its transducers began in 1999. The focal point of this system was to be an
Alnico magnet woofer with an underhung coil. Alnico had been the magnetic
material of choice for JBL from its very inception until the late 1970’s. In
1978, a civil war in Zaire resulted in the temporary embargo of the commercial
supply of cobalt that was a primary ingredient in Alnico. Over a one year
period, virtually every loudspeaker manufacturer was forced to switch to more
readily available ferrite magnets.
Initially, this magnetic material
presented problems compared to Alnico. It was less powerful by weight, requiring
much more magnetic material than an equivalent Alnico. It was more susceptible
to flux modulation caused by voice coil currents and more susceptible to flux
loss due to heating. None of these constraints were absolute. Through innovative
engineering (eg. SFG, VGC and SVG motors), JBL was able to develop ferrite
magnet drivers that exceeded the performance of the previous Alnico versions.
Nonetheless, the ferrite
developments led to an interesting question. What would happen if the same level
of innovation applied to ferrite designs were applied to Alnico motors? As
described above, Alnico starts off with a number of intrinsic advantages
compared to other magnetic materials. By leveraging these advantages, it should
be possible to achieve new levels of accuracy.
This became the design goal for
the 1500AL. The intent was to end up with a bass driver with the lowest
distortion possible while retaining the efficiency and dynamic response that
were the hallmarks of JBL. However, there was one major disadvantage in Alnico
loudspeakers that would have to be addressed – the susceptibility to permanent
demagnetization due to overpowering.
This phenomenon is a result of
variability in strength of the permanent magnetic field caused by interference
from the voice coil’s electromagnetic field. This is referred to as flux
modulation and is a leading cause of distortion in any dynamic loudspeaker. The
electromagnetic field generated in the coil pushes against the global magnetic
field set up by the permanent magnet and return circuit, causing it to “bend”.
Under normal operating conditions, Alnico magnets actually resist this bending
better than most other magnet materials. However, should this shift become large
enough, it will exceed the coercivity of Alnico and cause it to partially
demagnetize.
JBL had done extensive work in
stabilizing flux modulation during the conversion to ferrite magnet motors.
Ferrite is much more susceptible to flux modulation than Alnico. The coercivity
is much higher so that extreme modulation will not result in demagnetization.
However, under normal operating conditions, this flux modulation will manifest
itself as second order harmonic distortion. By installing a copper shorting ring
around the base of the pole piece, JBL engineers were able to reduce flux
modulation, and therefore distortion levels in ferrite motors, to very low
levels.
Doug Button realized that this
same technology could be extended to control flux modulation to such a degree
that the coercivity in an Alnico magnet would never be exceeded. He specified a
huge copper shorting ring that would buck any changes in the global field that
was induced by the voice coil signal. The result was that the major drawback of
using Alnico for a loudspeaker magnet had been completely eliminated.
Nonetheless, Doug was not done in
setting the design parameters for the 1500AL. There remained the previously
identified issue of midband loss in output and a more subtle form of flux
modulation that was not addressed with the large shorting ring. Doug would
devise an ingenious solution that would resolve both of these issues – a
lamination of copper and steel rings applied to the outside diameter of the
voice coil gap.
Before detailing this solution, it
is worthwhile understanding the phenomenon of “local” flux modulation. The
variability in the overall strength of the permanent magnetic field is referred
to as “global” flux modulation and this was controlled through a massive
shorting ring. However local variance in the flux field was determined to exist
in the voice coil gap, independent of variance in the global field. It was
particularly an issue for an underhung design like the 1500AL which had an
inordinately deep gap. The AC field generated by the voice coil would cause
variance in the gap strength from top to bottom that dynamically resulted in a
teeter tottering effect. Doug realized that another copper shorting ring placed
in the gap would address this phenomenon. However, it came to him as an
inspiration, that by interspersing steel coils in between a series of copper
coils, he could break up the eddy currents that were causing the midband loss.
Thus, came the specification for a series of laminated coils of alternating
copper and steel construction that would be applied to the outside diameter of
the coil gap.
With this final attribute, the
conceptual design for the 1500AL was set. However, there was now the formidable
task of developing the concept into a production driver. Responsibility for this
would fall upon Jerry Moro, Senior Transducer Engineer for JBL Consumer
Products. Jerry recognized that for the dynamic range requirement of modern
digital sources, the driver would have to accommodate large excursions. A large
coil would be required, but to maintain the underhung topology, there would need
to be a gap depth of unprecedented proportions. Ultimately, a 0.8” long coil
would be suspended in a 1.5” deep gap. This would accommodate 1” of peak to peak
excursion. The magnetic energy requirements to provide a reasonable flux density
in such a deep gap was enormous. It resulted in an Alnico magnet of massive
proportions. An Alnico slug weighing over 5lbs became the core of the motor
structure. The overall structure would weigh over 30lbs.
The large excursion requirements
presented unique suspension demands. Normally, the surround and spider are
designed to act together to provide restoring force and damping for the cone
movement. However, with these large excursions, the different construction of
the surround and spider would result in non symmetric response to deflections
and therefore distortion. Jerry addressed this by designing a mirror imaged pair
of spiders that acted symmetrically and thus allowed the surround to be
optimized for linearity. Mirror imaging the spiders resulted in any distortion
components being cancelled out.
A major focus of Jerry’s design
for the 1500AL was to minimize power compression. Power compression regards a
reduction in output as power levels increase. It is the biggest factor in
restricting dynamic response. Therefore, mitigating it to the maximum degree
became a design objective. Heat is by far the major culprit behind dynamic
compression. It causes energy losses in the permanent magnetic field and
impedance changes in the coil that conspire to reduce output as heat builds up.
Jerry designed a series of measures to dissipate heat.
First, the pole piece and magnet
have three channels embedded along their outer circumference. Air trapped behind
the dustcap is forced past the coil and out these channels to convectively cool
the coil. Next, an aluminum motor cap completely surrounds the magnetic return
structure. A series of ribs draws heat away from the iron pot to the cap which
acts as a large heat sink. Finally, there is an air gap between the cap and the
pot structure that is vented in the rear. The motion of the spiders pumps air
into this gap to provide further convective cooling.
The net result is a woofer that
arguably has lower levels of power compression than any other loudspeaker of its
size. It can sustain maximum output levels of 118db with minimal distortion.
Testing at a 110db output level revealed midband distortion to be down 50db, or
around 0.3%. In summary, the 1500AL sets an unprecedented standard for accuracy,
dynamic response and freedom from distortion
[此贴子已经被作者于2009-04-01 10:30:38编辑过]