posted
ok...this is going to be a really long winded post with several questions, but for those of you that can, please give me any input you have, it will be greatly appreciated....
so im trying to read up on bandpasses to learn how to calculate everything and desing them. most of the info i am going off of (as far as actual equations) is from diysubwoofers.org. i can get my numbers to match the outputs from a online calc at carstereo.com. im pretty good with excel so equations arent really that hard for me....my questions lie with all of the variables involved.
ok... so you have to choose an 'S' value. do most people choose .7? what determines on which value you should choose?
F1 is the lower 3db cutoff frequency. but what EXACTLY, in layman's terms, does that mean. i think i know but wanna here more on it.
what im really clueless on is choosing a value for Pa (gain in efficiency). someone plz explain that as thouroughly as possible.
the equations i am going off of are below. what is the difference (if any) if Fb (resonance frequency) from equation one, and Fb (tuning frequency) from equation two?
if S = 0.7, then b = 0.7206, passband ripple = 0.00 dB if S = 0.6, then b = 0.9560, passband ripple = 0.35 dB if S = 0.5, then b = 1.2712, passband ripple = 1.25 dB 4th order bandpass system with desired low frequency limit Choose a value for Fl, the lower 3dB cutoff frequency, then, Fl' = (Fl*Qts)/Fs Fh = (Fl'+b)*Fs/Qts Qbp = (Fl'*(Fl'+b))^0.5 Fb = Qbp*Fs/Qts Vf = (2*S*Qts)^2*Vas Vr = Vas/((Qbp/Qts)^2-1) Pa = -40*LOG(1/(Qbp*2*S)) where, Fh = upper -3dB cutoff frequency (Hz) Qbp = Qtc of sealed chamber Fb = resonance frequency of vented chamber(Hz) Vf = net volume of vented chamber (litres) Vr = net volume of sealed chamber (litres) Pa = gain (dB) 4th order bandpass system with desired gain Choose a value for Pa, the gain in efficiency, then, Qbp = ((10^(-Pa/40))*2*S)^-1 Fl = ((-b+(b^2+4*Qbp^2)^0.5)/2)*(Fs/Qts) Fh = Fl+(b*Fs/Qts) Fb = Qbp*Fs/Qts Vf = (2*S*Qts)^2*Vas Vr = Vas/((Qbp/Qts)^2-1) where, Fl = lower -3dB cutoff frequency (Hz) Fh = upper -3dB cutoff frequency (Hz) Qbp = Qtc of sealed chamber Fb = tuning frequency of vented chamber (Hz) Vf = net volume of vented chamber (litres) Vr = net volume of sealed chamber (litres) Pa = gain (dB)
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Electrical: 200 amp Iraggi alt 1/0 gauge NSB 90 Big 3 upgrade 5 cubic ft. box with swappable ports.. All in a '92 Accord http://www.sounddomain.com/ride/2277525 Posts: 518 | From: louisville, ky | Registered: Dec 2004
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I have no experience with bandpass but i do know that F1 -3db means exactly what it says. It's the point at which it is 3db lower then the peak frequency or at tuning, you know what i mean.
Everything else you named... I may be wrong but can't you just use box building software or is that not accurate enough?
posted
thats what i thought F1 meant, but just didnt know if there was more to it than that.
and i know there is software out there, but i dont have access to it, or the money for it, and would really just like to know the in's and out's of things.
i like knowing the nitty gritty...all the details, and im old school and like doing things myself....by hand and stuff
-------------------- (2)10" SE RE 12.1
Electrical: 200 amp Iraggi alt 1/0 gauge NSB 90 Big 3 upgrade 5 cubic ft. box with swappable ports.. All in a '92 Accord http://www.sounddomain.com/ride/2277525 Posts: 518 | From: louisville, ky | Registered: Dec 2004
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posted
in a bandpass box you're going to have a -3dB point above and below the peak/peaks. could even have one in the middle of the band. just depends how it plots out.
-------------------- World Record-Loudest SPL in my driveway Team Powermaster/XS-Power Hifonics Amps, Kicker subs-The way it should be....all were Stolen from me...boo hoo Zensky....World Domination at It's Finest www.zensky.com Posts: 3737 | From: Columbus, OH - formerly FL | Registered: Oct 2003
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posted
The -3dB frequency is the "half power" frequency which effectively means your filter is cutting out half the input power at that frequency. There will be a half power frequency both above and below the central value of the filter.
According to the S and b values you have. They are choosing S = 0.7 because it makes b ~ 0.7. I'm more familiar with calling this Q but either way it's the same thing. From what you have listed, this corresponds to a filter with zero passband ripple ( aka "Butterworth"). Pass band ripple boosts the frequency just inside the -3dB frequency. If you are cascading filters, this is useful because it will allow you to ensure the filter response is flat when it reaches a gain of 1 (which means the input signal is passed unaltered). A linear gain of 1 corresponds to a 0dB gain which is what you want in the passband of your filter.
I hope this helps.
Posts: 5 | From: Atlanta | Registered: Apr 2008
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