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With all the equations that have already been presented, the question naturally arises, ?Which one should I choose?? Notice that the coefficients a1, a2, and a3 are not simply related to the parameters of the system response. Certain combinations of parameters may indeed invalidate one or more of the alignments because they cannot realize the necessary coefficients. With this in mind, general guidelines have been developed to guide the selection of the appropriate alignment. This is very useful if one is designing an enclosure to suit a particular transducer that cannot be changed.
The general guideline for the Butterworth alignment focuses on QL and QTS. Since the three coefficients a1, a2, and a3 are a function of QL, QTS, h, and ?, fixing one of these parameters yields three equations that uniquely determine the other three. In the case where a particular transducer is already given, QTS is essentially fixed. If the desired parameters of the enclosure are already known, then QL is a better starting point.
In the case that the rigid requirements of the Butterworth alignment cannot be satisfied, the quasi-Butterworth alignment is often applied when QTS is not large enough.. The addition of another parameter, B, allows more flexibility in the design.
For QTS values that are too large for the Butterworth alignment, the Chebyshev alignment is typically chosen. However, the steep transition of the Chebyshev alignment may also be utilized to attempt to extend the bass response of the loudspeaker in the case where the transducer properties can be changed.
In addition to these three popular alignments, research continues in the area of developing new algorithms that can manipulate the low-frequency response of the bass-reflex enclosure. For example, a 5th order quasi-Butterworth alignment has been developed [6]. Another example [7] applies root-locus techniques to achieve results. In the modern age of high-powered computing, other researchers have focused their efforts in creating computerized optimization algorithms that can be modified to achieve a flatter response with sharp roll-off or introduce quasi-ripples which provide a boost in sub-bass frequencies [8].
