The use of adequately designed second-order high-pass filters, such as Butterworth (B), Chebychev (C) and Sub-Chebychev (SC) alignments, to improve the low-frequency response of fourth-order loudspeaker systems is well known. Characteristics of the filter and enclosure are derived from the poles of the desired transfer function. In the case of quasi-Butterworth 5 (QB5) alignments, which actually are sixth-order alignments, this procedure leads to very complex equation systems. Because these were difficult to solve, they have generally been disregarded. In this paper, a successful alternative method is introduced for calculating QB5 alignments parameters, and some comparisons with analogous B, C, and SC alignments are given. Systems using QB5 alignments are characterized by a small diaphragm displacement, a small enclosure and a high power handling capacity.
Kreutz, Joseph; Panzer, Joerg: Derivation of the Quasi-Butterworth 5 Alignments; JAES Vol 42 No 5 May 1994
The paper is available at the Electronic Library of the AES.