Rockford Fosgate 100X2 Owner's Manual page 18

igh and low pass filter sections have a cutoff rate that
H
increases as the number of components in the filter
increases. This means that a complex filter with a number
of parts will stop the unwanted frequencies faster than a
simple filter. The amount of reduction is measured in dB
per octave. The most common filters used in speaker
crossovers, as stated above, are 6 dB per octave which
uses one component per filter. Placing this filter in series
with the circuit will reduce power to the speaker by 6 dB
every octave. When passive crossover components are
used in multiple speaker systems, the crossover system's
impedance must be considered along with the speaker's
impedance when determining amplifier loading.
For 6 dB systems (where a single filter [highpass, lowpass
or bandpass] is used in series) the net impedance of the
system will be increased and problems are unlikely. For
example, if you are using a 4 speaker at 100Hz with a 6
dB filter, the net impedance will be about 5.6 .
12 dB per octave systems (where two filters are used in a
series for each speaker) can cause trouble. A 12 dB
choke/capacitor pair forms a series-resonant circuit to
ground. This impedance at resonance is determined by
the speaker's dynamic independence. For example: if the
speaker is open or disconnected, the crossover input
impedance can approach ZERO at resonance resulting in
A DIRECT SHORT!
Since speaker impedance varies considerably from rated
values at various frequencies, it is common to find that
"standardized" 12 dB per octave crossovers (which are
designed for pure resistor loads) have serious impedance
dips near their crossover frequencies, possibly causing
overheating.
We recommend you use caution when using 12 dB per
octave crossover systems unless they are specifically
designed to have minimal impedance variations for the
speakers in use.
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