RE: terminology - a somewhat complicated explanation - neolith

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RE: terminology - a somewhat complicated explanation

Posted by neolith (A) on March 25, 2014 at 20:11:02

In Reply to: terminology posted by pictureguy on March 25, 2014 at 18:42:53:

The knee is also called the cutoff frequency, corner frequency or breakpoint. This is defined as frequency when the amplitude in the stop band is 1/2 of that in the bandpass. If the impedance of a speaker is R then the output would be halved if the filter provided an impedance of R (total impedance is 2R and the voltage drops in half). Since the impedance of a capacitor is 1/(2*pi*f*C), the "knee" occurs when f = 2*pi*C*R. For an inductor the impedance is 2*pi*f*L so the knee is when f=R/(2*pi*L). Using the two definitions of R, we can derive that for a 2nd order filter, f= 1/(2*pi*sqrt(C*L).

For the MG 1.6, the HP filter has a knee at 1608 hz (R=4.5, C=22 mfd), and the for the 2nd order HP filter (C=25 mfd, L=3.5) the fc=538 hz. The crossover frequency is the point where the two filters cross and thus have the same amplitude. While this can be solved algebraically, it is a true pita and is more easily solved graphically. The value is ~580 Hz.

Hope this helps. One other thing, just to confuse you, the cutoff frequency calculated for the 2nd order filter is not the 3db point. This because the impedance of the inductor and capacitor interact. The relation between fc and f(3db) is f(3db)= fc * sqrt[2^(1/n) - 1], where n is the order of the filter. For a 1st order filter this reduces to fc=f(3db). Probably TMI :).

"Our head is round in order to allow our thoughts to change direction." Francis Picabia

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Follow Ups Full Thread

Topic - terminology - pictureguy 18:42:53 03/25/14 (10)

RE: terminology - JBen 22:27:30 03/27/14 (0)

Electrical vs. Acoustic crossover - BigguyinATL 07:56:54 03/26/14 (2)

RE: Electrical vs. Acoustic crossover - pictureguy 12:59:37 03/27/14 (1)

I apologize, I don't know how much everybody already know about this - Mart 03:02:56 03/29/14 (0)

RE: terminology - Davey 06:29:57 03/26/14 (0)

RE: terminology - Roger Gustavsson 00:50:59 03/26/14 (0)

RE: terminology - a somewhat complicated explanation - neolith 20:11:02 03/25/14 (3)

RE: terminology - a somewhat complicated explanation - Satie 20:30:42 03/25/14 (2)

RE: terminology - a somewhat complicated explanation - Davey 06:34:01 03/26/14 (1)

RE: terminology - a somewhat complicated explanation - Satie 08:57:20 03/26/14 (0)

Follow Ups

RE: terminology - a somewhat complicated explanation - neolith 20:11:02 03/25/14 (3)

RE: terminology - a somewhat complicated explanation - Satie 20:30:42 03/25/14 (2)

RE: terminology - a somewhat complicated explanation - Davey 06:34:01 03/26/14 (1)

RE: terminology - a somewhat complicated explanation - Satie 08:57:20 03/26/14 (0)

RE: terminology - a somewhat complicated explanation

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