B&W Bass alignment filter till t ex B&W 801 Matrix Aktivt delningsfilter för optimal frekvensgång m.m.
B&W: "This external filter gives the possibility of extending the response down to 19Hz (-3dB) with a sixth-order Butterworth alignment, and also filtering out sub-sonic frequencies which may give rise to excessive cone excursion and intermodulation distortion." http://www.bwspeakers.com/downloadFile/spe...ES_2_MANUAL.pdf
Stereophile: "Additionally, but using a sixth-order Butterworth alignment through the addition of an outboard equalizer, they have been able to achieve extraordinary low-frequency response (-6dB at 17.5Hz) without compromising bass attack and clarity. Although the speaker can operate without this optional equalizer (thereby effectively representing a fourth-order Bessel filter with a -9dB point at 19Hz), the addition of this device clearly enhances its overall musical accuracy." http://www.stereophile.com/floorloudspeakers/506/index.html