Extending woofer f3 point?

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Extending f3 point at expense of sensitivity? Here's a strange woofer crossover, from an 1981 Infinity RSII speaker, claiming to be an impedance equalized? 'dual woofer' system; woofers are some 'crappy' 4ohm Tonegen polypropylene 10", separated 1.1 cu ft per woofer, and they claim response down to 36Hz (-2dB):
 

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The lower woofer's crossover has an upper cutoff frequency of 37 Hz. That means they add a second woofer where the first one starts to roll off. So they do in fact trade efficiency for bass extension (what in fact EVERY approach does if the cutoff frequency is lowered using a box of the same size).

Another principle that was used by Infinity is the "Watkins Woofer". This is a woofer with two voice coils driven by a circuit like the one above. This is nothing else than handing a speaker problem over to the amp which now has to be able to deliver huge amounts of current at the lower end.
Ininity speakers in particular are infamous for this.

Regards

Charles
 
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