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Technical Note 16
"Alignments" and Eminence Designer 1

 
Back in the distant past before handheld calculators had been invented and when computers were huge and contained vacuum tubes there were slide rules. In fact, most calculations were performed with slide rules and there were whole classes in college devoted to the instruction in their use. What do slide rules have to do with "alignments"? If you have ever looked at the math it takes to calculate the response of a vented box you will appreciate what engineers had to go through to design a vented box system with a slide rule. A simpler method of describing the response of a vented box was needed.

It was during this time in the early 1960s that a bright Australian engineer, A. Neville Thiele, presented a method of characterizing the response curve shapes of vented boxes that compared them to filters. For example, a vented box with a response curve that matched the response shape of a fourth-order Butterworth filter was said to have a "B4 alignment". A vented box with a fourth-order Chebychev response shape was said to have a "C4 alignment" and so on. Thiele grouped many of the useful "alignments" into a table along with relevant tuning ratios and compliance ratios so that designers could quickly estimate the response shape of a vented box and thereby save huge amounts of time. This simplification of the design process was applauded by the myriad of designers who secretly hated to use their slide rules.

What does this mean to us today? Do we still need "alignments"? In our opinion, No! With today's relatively cheap personal computers that can run circles around yesterday's mainframes we can now directly calculate the response of almost any vented box. In a matter of seconds and with just a few key strokes and a few mouse clicks we can view the amplitude response, maximum acoustic power, maximum electrical input power, cone displacement, vent air velocity, system impedance response, phase response and group delay of a vented box system. We no longer need to be limited by the artificial boundaries of filter descriptions because we can see the response in the graph on the screen and quickly decide if it goes as low as we need, is as flat as we want, etc. For this reason, Eminence Designer does not use "alignments".
 

"Suggested" Vented Box
Eminence Designer provides a "Suggest" button for vented box designs. When this button is clicked, a "Design Priority" window will open so that you can control the way Eminence Designer makes a suggestion. Choices include "high output", "high fidelity", "extended bass" and "desired F3". These options are described in Eminence Designer's on-screen user manual.

The "high fidelity" option attempts to create a "maximally flat" alignment. The maximally flat response is most closely achieved with a B4 alignment. However, a true B4 alignment is only possible when the driver has a total Q (Qts) of approximately 0.4. What if the driver's Qts is lower or higher? Generally, a QB3 (quasi third-order Butterworth) alignment is used for lower Qts values and a C4 alignment is used for higher values. Eminence Designer's "high fidelity" option follows this QB3–B4–C4 progression. Depending on how extreme the driver Qts value is, Eminence Designer's "high fidelity" suggestion may or may not be truly maximally flat. If it is not, feel free to experiment with the box volume (Vb) and tuning frequency (Fb) until a desired response shape is achieved.

 


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