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Crossover
networks use filters
in high pass low pass
pairs. These may be
symmetric or assymetric
and may (usually significantly)
involve driver characteristics
which affect slope,
shape, and rolloff
at both the high and
low end. All of these
and many variations
for which no particular
name has been applied
also exist. The elliptic
differs from the others
mentioned in having
"zeros"
in the stop band.
This means the roll
off shifts between
two rates and can
even result, if the
parameters are so
adjusted, to show
increases in drive
in the stop band.
Since most drivers
would sound excellent
with no crossover
at all but would burn
out and overload almost
immediately, crossovers
are necessary to prevent
low frequencies which
are beyond a midrange
or Tweeters capability
from entering them.
It is also then necessary
to use many drivers
to cover the frequency
range to enable frequency
and load sharing and
power handling. Some
cone type drivers
can achieve nearly
full range and play
adequately loud in
car and home environments.
Generally some bass
and highs must be
sacrificed. This type
is found in many OEM
cars due to low cost
and compact installation.
The
Butterworth and Linkwitz-Riley
(L-R) differ not in
configuration but
in the setting of
the values and order
of the sections. The
Butterworth gives
a flat response on
axis but has a power
response increase
in the crossover region.
The L-R damps the
response somewhat
more and decreases
overlap to provide
a compromise in power
and frequency response.
The L-R is actually
a combination of two
Butterworth sections.
The
drivers usually have
to be inverted in
phase relative to
each other in the
Butterworth and L-R.
The overlap in the
Butterworth can often
also be reduced to
provide good results
especially in a reverberant
environment such as
found in the automotive
sphere.
Because
the ear-brain can
separate the sound
of reverberation from
the sound of ringing
it makes an audible
difference to provide
crossover designs
that give better transient
response. Transient
response degrades
with greater order
in practice. This
audibly gives a system
that has a character
which remains embedded
in the music over
all musical sources.
This character can
overlay real information
in the source thereby
obscuring it. Usually
the losses are barely
noticeable up to an
order of three. Beyond
this the system can
develop a character
that is kind to various
sources in that harshness
is minimized but then
information is lost
when the very best
recordings are used.
The
louder the system
will be run the more
appropriate higher
order designs become.
The elliptic filter
by allowing typically
.1% to 1% of the input
power to "bleed
through" out
of band gives faster
roll off rates in
the crossover region
without degrading
transient response.
The gain is typically
one to two orders
of roll off added
at no penalty.
This gives a very
clean sound and good
power handling without
overlaying the sound
with excess character.
The roll off of woofer
power response is
typically 24 db per
octave and sometimes
can reach 36 db or
more if the driver
is taken to the limit.
This typically happens
in most cases. The
sound this makes is
mitigated by the fact
that part of the drop
(12db/octave) is due
to dispersion reduction.
The roll-off rate
is actually staggered
in the majority of
cases. Metal cone
drivers display somewhat
different character,
must be expertly employed
and are an exception
to this rule of thumb.
Use
of the elliptic in
a fourth order configuration
gives a transient
response slightly
better than a third
order Butterworth
and a roll-off rate
slightly better than
a fourth order of
the same type. The
same advantages apply
to the L-R but in
somewhat different
proportions depending
on the order.
Low
passing woofers with
high order filters
to the Tweeter aggravates
an already steep power
response roll off
in that driver by
itself - an exception
is in a pure woofer
application where
the crossover frequency
is much lower than
the natural cutoff
of the driver. The
net crossover in the
CDT system is designed
to interface the mid
woofer to the soft
dome Tweeter to give
optimum overall performance
and consistent results.
Only a loss in power
handling and a custom
in-car install and
unusually delicate
main drivers (not
rugged miniature woofers)
can provide a smoother
more detailed easy
listening experience.
This can be accomplished
with cone drivers
that are both full
range, crossed over
to and interfacing
with; a Tweeter that
uses its own mechanical
properties to augment
filter response. Each
car requires different
positions and settings
and the settings are
hyper critical. Although
the networks are simple
there are more of
them. Operating such
a system with a high
power subwoofer, especially
the band pass type,
is also a bad idea.
The mid-high sound
cannot keep up. Even
marginally powerful
systems will stress
drivers like this.
This is however an
optimum approach for
easy listening OEM
cars. Pure, custom
quality only, aftermarket
product is also feasible
but the market for
such systems hardly
exists. Most people
stick with the systems
found in their new
cars. The value of
such a system has
to be demonstrated
to see if the prospective
buyer can hear the
difference, and understand
the high custom labor
expense including
the potential need
to alter the car's
interior. - usually
not an option.
Maximum sound quality
can be so configured
by providing special
large expensive exotic
drivers not easily
installed - to achieve
a demonstration of
very detailed and
moderately loud (by
autosound
standards) sound.
Some show cars like
this have been attempted
with varying success.
The CDT system gives
a wide margin of power
and precision with
the easiest installation
possible. Overlay
character is held
to a minimum giving
unprecedented results
short of a full all
out custom show car
- with a genious for
a designer - on an
unlimited budget.
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