From: bell@hops.larc.nasa.gov (John Bell)
Newsgroups: rec.audio.car
Subject: Re: subwoofers/boxes
Date: 31 Dec 1992 11:56:03 GMT
Organization: NASA Langley Research Center
NNTP-Posting-Host: hops.larc.nasa.gov

I tried to email the box tuning article to Brian, but it bounced :( So, here
it is. Those of you who don't want it, just hit 'N' now...
By the way, credit goes to Cal Demaine for the article.

-----------------------------------------------------------------
Ok, due to the high demand, I'm posting a method for verifying the tuning of
your fourth order subwoofer enclosure.  By fourth order, I mean a single
enclosed volume with a driver and a port both radiating out.  I don't mean
sealed enclosures or band-pass enclosures.  Yes, this information will work
for fourth order enclosures using an isobarik arrangement.

Time needed: About 1 hour

Equipment Needed:
	1 fourth order loudspeaker enclosure
	1 frequency generator (10Hz-100Hz will suffice)
	1 multimeter
	1 amplifier
	1 1K resistor
	1 Great Outdoors

Set-Up:
	1.  Go outside.  Point the loudspeaker straight up.
	2.  Feed the amplifier from the frequency generator, being sure to
	    select sine wave output!
	3.  Set up the following circuit, not minding the crappy text
	    representation.  Don't use any passive or active crossovers!

                              !----------!     (My first text circuit!
	    ---/\/\/\---------!          /      I'm so proud!)
      Amp     resistor        !  box    < 
			      !          \
	----------------------!          !
			      !        ---	
			      !        ---
			      !----------!

	4.  Now, moving from about 10Hz to about 100Hz, making the following
	    measurements:

	    Frequency ! Resistor Voltage ! Box Voltage

	    Be sure to keep the amp signal to about 1-2 volts P-P.  Actually
	    we need an undistorted sine wave output for both the driving
	    voltage and current, but unless you have a 'scope to check it,
	    just use 1-2 volts.

	5.  Once you've done the measurements, go inside and fire up a
	    graphing spreadsheet like Excel (tm) and enter in all the
	    data you've gathered.

	6.  Next, calculate the box/resistor resistance ratio by dividing the
	    Box voltage by the Resistor voltage.  Do this for every measured
	    frequency.

        7.  Graph this box/resistor resistance ratio vs. frequency.

	8.  The first of the two peaks occurs at the frequency Fl.  The second
	    peak occurs at Fh.  The lowest point between these two peaks is
	    Fb.  Record all three of these frequencies.

	9. Calculate the resonance frequency of the system with the following:

		Fsb= (Fl*Fh)/Fb

	   This is the effective frequency at which your box is tuned.

	10. Calculate the compliance ratio (a) from:

		a= (Fh+Fb)(Fh-Fb)(Fb+Fl)(Fb-Fl)/((Fh^2)*(Fl^2))

	11. The effective volume inside the box is:

		Vb= Vas/a

	    This is the volume your driver 'sees' in your box.  Vas is the
	    driver's equivalent compliance volume.

	12. Cry, because your box isn't anything like you designed it.    

That's it.  There are actually a few more technicalities involved, but we don't
need to worry about them for our purposes.

Now, all of you are screaming that you don't have a HPxxxxA frequency
generator.  Stop your wining, because for $25 you can order one.  What was
that? A $25 frequency generator!?  Are you kidding me?


Of course I am.


However, for $25 you can order a compact disc with pure sinewaves from 10-100Hz
recorded on it.  Track 10 is 10Hz, track 11 is 11Hz... etc.  Very handy for
this and other stupid subwoofer experiments.  Anyway, you can order it from:

	Autosound 2000

	California Office - (209) 465-3450

	North Carolina Office - (919) 570 0341


Now, for kicks, try the same experiment with the subwoofer in your car.
Compare results.  Better yet, post results to rec.audio.car.  And
definitely mail them to me.

  Questions? Comments? Corrections?

    - Cal

-- 
  - Cal Demaine                   | The university may own my ideas and opinions
    (demaine@bode.ee.ualberta.ca) | but don't think for a moment they share them