From: kmr4@po.CWRU.edu (Keith M Ryan)
Newsgroups: rec.audio.car
Subject: Re: capacitors
Date: 11 Nov 92 02:50:46 GMT
Organization: Case Western Reserve University
NNTP-Posting-Host: b64746.student.cwru.edu

In article <1992Nov10.235109.15048@scott.skidmore.edu> jreiser@scott.skidmore.edu (Jason Reiser... Asleep) writes:
>> }	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
>> 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).
>
>While this is a correct explaination of a crossover's function, I felt
>it necessary to point one thing out... That is, that -3db is often
>spoken of as the point at which the sound level has dropped in a way
>similar to dropping the power output from the amps to 1/2.  I have
>never seen a crossover of any type with a slope of -3db.  A first
>order (the simplest I know of) has a slope of -6db, and increasing
>orders follow... (2nd order -12db, 3rd order -18db, 4th order -24db,
>etc.).

	Essentually correct. The correct units for a crossover slope is 
db's/octave. The most common passive ones are 6 and 12 db/oct. Many people 
drop the "per octave", as they assume other's will understand. Higher order 
crossover's are a little tricky to make. That is, most people design cross 
overs using the speakers DC resistance. However, every speaker has a 
complex impedence, with elements of inductance, etc. The net effect from 
these deviations from the ideal speaker are compounded in the higher order 
speakers. Also, there is less "free-play" between matching speakers, etc. 
	Now, in modern DSP, it is possible to have incredible crossover 
points, slopes, virtually ( no pun intended ) no phase lag, etc. 

>Since someone who knows more about this than I will likely read this,
>I'll just add a quick question here... that is, when a passive
>crossover is said to be at say 100Hz (low pass) at -12db/octave,
>then...
>	Will the lowest frequency whose sound is produced at full
>volume (no attenuation) be 100Hz?
>
>OR
>	Will the note at 100Hz be produced with -3db attenuation?

	Correct. In the idealized model, with two speakers, say a woofer 
with a low pass, and a midrange with a high pass with the same slopw and 
the same freqency at 3db's down, they should combine to have a flat response 
at that point. However, this very rarely works out.

>I don't know that a steeper slope is necessarily "better", just
>steeper and greater attenuation in a narrow span of frequencies.
>Other factors generally come into play as far as what is better,
>including phase and timing.
>-24db/octave is just a fourth order crossover... not common, though
>not exotic or rare at all.

	Essentually correct, to my knowledge. There are so many factors in 
determining which slope is the best one. There is a small sect of British 
manufactors which build 3/4/5 way speakers which only have 6db slopes. They 
feel the phase response of the 6db/oct filters allow better imaging and 
sonics. I personally, find some truth in this. For a three way system, 
tweeter/woofer/subwoofer, I prefer a 6 db/oct high and low pass filters for 
the woofer, 6 db/oct high pass for the tweeter. This seems to be better 
suited for imaging. However, it demands that one has more robust speakers.
However, since the subwoofers ruin the sound staging, if they produce any 
sound above 100 hz or so, I prefer an 18 or 24 db/oct lowpass crossover for 
the subwoofer ( usually around 60 Hz or so ) To counter act the midbass hump 
found in most cars, I have the high pass of the woofer around 120-150 hz or 
so. This creates a slight trough in the speakers outputs, forming a total 
flatter and smoother response. The actual values depends on the car and the 
placement.

	This only shows, that you can have to take the accoustics of the car 
into effect!


, I usually prefer the 6 db 
slopes for the midrange to tweeter crossover ( providing the tweeter is 
robust ) to allow a smoother transition. However, I find a shallow slope for 
the subwoofers to destroy the imaging, hence, I prefer the 24 db/oct 
crossovers. A