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Not just the 3dB point but the shape of the rolloff shoulder or the Q.
Every time you cascade, the Q decreases meaning shoulder gets more
rounded. In the pic the top one is a 4-pole Butterworth and the bottom
one is a 4-pole Linkwitz-Riley formed by cascading a pair of 2-pole
Butterworths. Note that Fc, the cut frequency, doesn't change even
though the LR is down 6dB. Fc is defined as the point where the filter
has done half its phase shift, in this case 180 degrees for a 4-pole
filter. I'll send the phase and group delay pics in separate postings to
stay under the size limit.
--
Dennis
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