Re: A Subwoofer Question

[human <human@nh.ultranet.com>]

-- [ From: human * EMC.Ver #2.5.02 ] --

First, tiny Audi content... I like my Audi better than my friend's old man
zneB (190e?).  It doesn't rock back and forth while doing the big ess curves
on Route 1 north of Boston.  I can accelerate through them in the coupe.  
(end of Audi content) While listening to loud, clear music....

Subject: A Subwoofer Question

        I have a 10" sub hooked up in the trunk. It doesn't bump, rattle
windows, etc.. It just sounds good. Great in fact!  > Question 1: then why
do you want to change it? My current dilemma, is that I'm not sure I like
the way it is set up. It will work Ported, sealed, bandpass, maybe free air.
. Basically, whatever. 
        Currently, I have it in a 1.75 ft^3 box ( a bit large.. ) >Doesn't
sound too large.  What is the Qt, Vas, Fs of the woofer?
 driven by a 120w (bridged) amp. I have a passive crossover at ~80-100 Hz.
>How many poles? (chokes/capacitors doing the filtering)
 Then, I have the rears driven by the same amp, >two more channels I presume
?
 with about 18 ohms of resistance per speaker, >no line level gain control? 

 and they are crossed over about 100 or 200 Hz.  >Ditto previous crossover
question.  Also why such uncertainty as to freq.? The sub is where the
action is though. I have ports in the box, but the ports are stuffed,

>stuffed does not equal sealed.  Try screwing 1/2" plywood over them with a
gasket.  As you say, >sealed usually sounds better.  Not many people really
have the knowledge and resources to >effectively tune a speaker/cabinet/vent
system to get flat results.  Listen to most Auto "boomboxes"!  
 so the box functions as a sealed box. I like the sealed box sound better,
it will function at a lower freq., and has tighter bass. My question is -
What will happen if I go ahead and port the box for 10, 15, or 20 Hz? 

> this is not technically valid.  If you say, tune a vent resonance to 15 Hz
, you will get a peak at 15 Hz that rolls off at roughly 18 dB/Octave above
and below that freq, and "blends" with the drivers direct output eventually
at what, 40-60 Hz depending on driver resonance.  Not flat at all, and
pointless because there's no recorded music down there to play.  Some movie
soundtracks (SFX) go below 20 Hz, for rumbling earthquakes & stuff, but you
probably aren't setting up a home theatre in your car...are you?

>Technically, you want to tune the vent resonance to approx. 1/3 octave
below the 3dB cutoff if the box were sealed, giving you about an extra 1/2
octave of *flat* bass response, which then rolls off at 18 dB/octave below
that.  As opposed to a sealed box which rolls off at only 6 dB/oct below the
cutoff.

Many "designs" nowadays only use the vent output, perhaps using two
different resonances to get an octave or so of really low or really
offensive output.  This doesn't have much to do with good sound and nothing
to do with great sound.  Which is what it sounds like you want (great sound)

How will the driver function? Will it be louder at the higher freq's? >no
 More boomy? 
>probably
 Just plain louder, but have the same accuracy? >no

 I think that porting it for 15 Hz sounds like a good idea, but I do not
know how it will function compared to the sealed box. Any ideas? >well,
don't do that.  & how would you be measuring the port resonance?
>How is the woofer coupled to the passenger space, BTW?

Brooks

>sorry more questions than answers, Brooks.  I hope I've helped a bit,
though.  I build speakers for a living and constantly have to fend off guys
who want more "boom" and "punch" in their car/truck ....usually for $100 or
less....

>I prefer music reproduced accurately and cleanly.  Maybe it costs me some
business (prostitution?) but I have to do what I have to do.  If you want to
try to list the details I've asked for I will try to info you more
completely, though I suspect your system is already giving you the most it's
capable of.

Be seeing you Huw Powell 82 coupe 210,000 &c, 500 w sufficient but not
necessary...