a fixed filter, whereas tone controls (trebble and bass) are variable filters

The rate, expressed in dB per octave, at which audio signals are attenuated as frequencies move into the crossover range. A high attenuation rate, e.g. 24 dB/octave means that there is little interaction between adjacent loudspeaker drivers. Low attenuation rates, e.g. 6 dB/octave allow adjacent drivers to operate simultaneously over a wide frequency range. See: Crossover, Octave.

The rate at which a driver attenuates as it starts to see unwanted frequencies. Specified as dB/oct.The higher the number, the steeper the slope, which results in a narrower zone where the sound transitions from one driver to another.

Describes the steepness of a crossover filter. Expressed as " dB/octave." For example, a subwoofer with a crossover frequency of 80 Hz and a slope of 6 dB/octave would allow audio frequencies at 160 Hz (an octave above 80 Hz) into the subwoofer, but signals at 160 Hz would be reduced in amplitude by 6 dB. A slope of 12 dB/octave would also allow 160 Hz into the subwoofer, but the amplitude would be reduced by 12 dB. The most common crossover slopes are 12 dB/octave, 18 dB/octave, and 24 dB/octave. Crossover slopes are also referred to as "first-order" (6 dB/octave), "second-order" (12 dB/octave), "third-order" (18 dB/octave), and "fourth-order" (24 dB/octave). The "steeper" slopes (such as 24 dB/octave) split the frequency spectrum more sharply and produce less overlap between the two frequency bands, but they also cause phase anomalies.

The rate at which the crossover attenuates the blocked frequencies. Slope is expressed as decibels per octave. A 6 dB/octave crossover reduces power by 6 dB in every octave starting at the crossover point. An octave is double the crossover point when you're going higher in frequency, and 1/2 the crossover point when you're going lower in frequency. (For example, an octave above A440 is 880 Hz, an octave below is 220 Hz.) With slopes of 12 dB and higher, you'll hear little output beyond the crossover point.

The rate of attenuation expressed in decibels of change for every octave away from the crossover frequency.

High and low pass filters used for speakers do not cut-off frequencies like brick walls. The rolloff occurs over a number of octaves. Common filter slopes for speakers are 1st through 4th order corresponding to 6db/oct to 24db/oct. For example, a 1st. order, 6db/oct high pass filter at 100hz will pass 6db less energy at 50Hz and 12db less energy at 25Hz. Within the common 1st through 4th filters there is an endless variety of types including Butterworth, Linkwitz-Riley, Bessel, Chebychev, etc. Salesmen and product literature will sometimes make claims of clear superiority for the filter used in the product they are trying to sell. Since the subject fills books, suffice it to say that there is no one best filter, it depends on application and intended outcome. Good designers use the filters required to get the optimum performance from the system.