A band-pass filter is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range. An example of an analogue electronic band-pass filter is an RLC circuit (a resistor-inductor-capacitor circuit). These filters can also be created by combining a low-pass filter with a high-pass filter.
The 4th order or sealed rear chamber bandpass system is basically a sealed enclosure system with the addition of an acoustic filter in front of the driver. The resulting system usually provides a lower cutoff frequency, the tradeoff being a larger enclosure. The enclosure can be reduced in size.
The bandwidth of the filter is simply the difference between the upper and lower cutoff frequencies.
The shape factor is the ratio of bandwidths measured using two different attenuation values to determine the cutoff frequency, e.g., a shape factor of 2:1 at 30/3 dB means the bandwidth measured between frequencies at 30 dB attenuation is twice that measured between frequencies at 3 dB attenuation.
Using a 5000 Hz second order high pass filter on a tweeter or tweeters, at 2500 Hz very high quality reproduction is obtained. The circuit is a high Q low-frequency one with a small capacitance, which is realized by applying an output capacitance multiplier to the circuit.