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ACTIVE OR
PASSIVE CROSSOVER
A crossover network for a subwoofer has the
primary job of filtering out high frequency
program that is above the intended operating
range of the sub. This application is known as a
low pass filter, also sometimes referred to as a
high cut filter. As a matter of either
convenience or system integration the low pass
filter is sometimes combined with a second
filter that removes low frequencies from the
high range speakers. The name for that filter
is- wait for it- a high pass filter (or low
cut). Whatever bandpass we are shaping, there
are two approaches to the application of these
concepts. The most common approach is the
passive network, which in various configurations
is by far the most common for loudspeaker
systems in general. A passive network refers to
the use of high power handling capacity
components AFTER the amplifier and before the
speaker load itself. While there are some
advantages as far as flexibility and cost, there
are some significant downsides as well. The
insertion loss that results from the inductive
components lowers system efficiency, and passive
filter circuits can also affect phase response
and damping.
An active network derives its name from the
placement of the filter BEFORE the amplifier;
the network is designed to operate at line
level. The formulas that are used to calculate
the crossover are similar in theory to the
passive designs, but the voltage and current
involved are obviously much lower. There is
consequently much less of the associated
distortion and loss that we have described for
the passive network, plus a few other
significant advantages. Active crossovers send
the filtered output to the amplifier responsible
for each particular frequency range. Although
there are cost, space, and complexity issues
with using multiple amps, the advantages are
improved headroom, lower distortion, and much
greater control.
SUBWOOFER DRIVER
SELECTION
The selection of the actual subwoofer driver
must by necessity be considered throughout the
process of choosing the actual type of
enclosure. Each enclosure type can demand that a
driver of a particular specification is used in
order to provide the performance that is
expected. In general, sealed box designs tend to
expect a driver to have a relatively high Qts,
low Fs, very good xmax, and relatively soft
suspension. Vented boxes will usually expect the
driver to have a somewhat lower Qts, and
remember that Fs will need to also be fairly low
since you cannot usually tune the box below that
frequency. Horn loaded designs do not need a
driver to have a low resonance because the
length of the path of expansion, and the mouth
area of the horn, determine low frequency
cutoff.
A useful formula to determine the suitability of
a particular driver for a particular box is the
Equivalent Bandwidth Product (EBP), also known
as the Equivalent Bandwidth Ratio (EBR). It is
determined by taking the driver free air
resonance (Fs) and dividing it by the electrical
Q (Qes). If the resulting number is around 50 or
less, you are looking at a driver that is best
suited to a sealed type box. If the EBP is
between 50 and around 100 or more, the speaker
will work best in a vented enclosure. Horn
loaded designs will perform best with an EBP of
150 or even higher.
Look closely at xmax values and any indicators
of excursion limits for the driver that you are
evaluating. The lowest musical octave that the
subwoofer has to reproduce will cause far more
stress than any higher frequencies. Linear long
excursion capabilities combined with good cone
control will provide high output low distortion
sub bass. Most modern subwoofers will run out of
mechanical power handling before they exceed
their thermal power handling capacity.
Some users prefer to mount their subwoofer in a
"down-firing" orientation, although this
particular configuration is not that common for
automobile applications. The cone mass of the
driver must be taken into consideration, as the
tendency for the cone to settle downward can
move the voice coil out of the gap and affect
xmax and excursion capabilities. The formula
that should be used to determine if a subwoofer
is acceptable for this kind of arrangement is:
As long as the amount of "sag" does not exceed
5% of xmax, the driver will work just fine in a
horizontal configuration.
SINGLE OR DUAL
VOICE COILS
A conventional woofer has a single voice coil
that moves in the magnetic field and provides
the motive force for the cone. A dual voice coil
has a second coil wound concentrically with the
first coil, and has its own separate
terminations. The primary advantage of this
arrangement is the ability of the dual voice
coil subwoofer to accept both left and right
amplifier outputs in a single driver. This can
be of obvious benefit in compact, low cost
applications since bass is non-directional
anyway. An alternative feature of the dual coil
speaker is that choices can now be made to most
effectively connect the subwoofer to the
amplifier. If for example the dual coils are
rated at 4 ohms each, you can decide whether to
connect them in series (8 ohms) or parallel (2
ohms). This leads to even greater flexibility
because the connected voice coil pairs can be
connected to additional voice coil pairs to
achieve a specific target impedance. |
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