From: gilchr@ee.ualberta.ca (Andrew Gilchrist)
Subject: Re: capacitors
Organization: University Of Alberta, Edmonton Canada
Date: Wed, 11 Nov 1992 09:35:39 GMT

jsc52962@uxa.cso.uiuc.edu (Jeffrey S. Curtis) writes:

>In article <1doqcvINN8js@bigboote.WPI.EDU> walkerr@bigwpi.WPI.EDU (Robert A Walker) writes:
>}In article <gilchr.721389283@ee.ualberta.ca> gilchr@ee.ualberta.ca (Andrew Gilchrist) writes:
>}>That gives your -3dB points at 100Hz and 200Hz respectively (approx.)
>}
>}	Please excuse my ignorance, but what exactly does -3db mean, what does
>}-12db mean, which one is better, or is it a matter of personal preference,
>}I know what a db is but i though that the more decibles (sp?) the louder it
>}was,  am i lost or what?  

>The -3dB refers to the "cutoff rate" of the crossover.  It's the amount
No. This is not what it means.  Please read my article (dB explained).

>of filtering per octave that the crossover performs.  The crossover can't
>actually just block *everything* below a certain point - it just slowly
>cuts it off.  A 3 decibals-per-octave slope is not very effective - it
>means that one octave below your crossover point, the music in that
>frequency range will sound half as loud as at the crossover point itself
>(well, other factors like apparent relative dB @ frequency notwithstanding).

You are not totally off base.  If a filter has an attenuation of -3dB/octave
the power delivered to the load will be 1/2 of that delivered the
previous octave (for frequencies below the cutoff point).  However,
a filter with such a small rolloff would be difficult to design. A single
pole transfer function rolls off at 6dB/octave.  This is generally the
minimum possible.  Also note that this is ULTIMATE rolloff.  It is not
generally accurate around the crossover point (cutoff frequency).

-12dB/octave implies a double pole transfer function... meaning that
ultimate rolloff will be 12dB/octave (40dB/decade).  i.e. if our cutoff
is at 1kHz, then we can be pretty sure the response at 10 Hz will be
40dB below the response at 100 Hz.  Also, the response at 10 Hz will be
about 80dB below the max response.  The response at the cutoff point,
by definition is 3dB below the maximum response.

>Better crossovers - ones that include not just a single capacitor; rather,
>a collection of capacitors and inductors - will give you better slopes,
>sometimes up to -18dB/octave.  I think I might have seen one once that
>offered -24dB/octave, but I might have been dreaming at the time.

Passive crossovers aren't good for more than 6dB/octave.  If you
go past that, the rolloff at the -3dB point gets flatter and flatter.
The inductors in these passive crossovers usually serve to flatten the
resistance vs. frequency curve for the speaker as a whole. i.e.
an inductor is inserted inline with the midrange with a -3dB point at the
same place as the tweeter/capacitor.  Thus, the overall impedance of
the network is kept at 8 ohms (or whatever the drivers are). 
Active crossovers (electronic) can give excellent rolloff upwards of
18dB/octave.  I wouldn't really recommend exceeding this though, as
transient response goes to hell.

>Jeff

Nice try, though.

Andrew
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