High and low pass filter sections have a cutoff rate that 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 4Ω speaker at 100Hz with a 6 dB filter, the net impedance will be about 5.6Ω .
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
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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