What if Fs=Fb in ported box??? - diyAudio
 What if Fs=Fb in ported box???
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 11th December 2011, 03:20 PM #1 diyAudio Member   Join Date: Jul 2010 What if Fs=Fb in ported box??? Well we know that in free air at Fs= impedance =max. I have 15 inch paradimg drivers with Fs=18Hz , can i tune it to 18 Hz. currently using them in 2.5 Cuft sealed enclosure, they need good boot at LF to give 20ish output. So the plan to go ported around 6-7 cuft . Would it then have max or min impedance or current draw ? Any draw back of doing it or should it tune it lower 13-15 Hz??? thanks
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Join Date: Jun 2002
Location: USA, MN
Quote:
 Originally Posted by muddasirwaheedmalik So the plan to go ported around 6-7 cuft .
A good starting point is that given by small-margolis in 1981:
Vb=Vas*20*Qts^3.3
Fb=0.42*Fs*Qts^-0.96
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diyAudio Member

Join Date: Feb 2008
Location: Sheffield
Quote:
 Originally Posted by muddasirwaheedmalik Well we know that in free air at Fs= impedance =max. I have 15 inch paradimg drivers with Fs=18Hz , can i tune it to 18 Hz. currently using them in 2.5 Cuft sealed enclosure, they need good boot at LF to give 20ish output. So the plan to go ported around 6-7 cuft . Would it then have max or min impedance or current draw ? Any draw back of doing it or should it tune it lower 13-15 Hz??? thanks
If you tune Fb to Fs, the impedance peaks (approx 1/3 oct each side IIRC) will be of equal magnitude. Tune lower, and the peak above Fb will be greater in magnitude than the one below Fb. Note that at Fb, impedance is at a minimum.

All of this is pretty much irrelevant if you're using a decent solid state amplifier - they'll drive any weird impedance curve you like.

Chris

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