|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 by
using two drivers in an isobaric configuration.
4th order bandpass systems usually demonstrate better power handling characteristics than the other main systems considered here. Its transient response is second only to the sealed enclosure systems, making it a good choice for subwoofer applications.
As all of the output of the 4th order bandpass system is via the port, the largest port diameter possible for the enclosure should be used in order to minimize port noises. The ports should be flared whenever possible, for the same reasons.
The 4th order bandpass system rarely exhibits a perfect bandpass response - there is usually some out-of-band noise present in its output. A simple notch filter can be used to reduce this noise if it is audible. Alternatively, a low-pass filter can be used in series with the driver, but the in-band response of the system may also be affected if this approach is taken.
4th Order Bandpass projects on the internet