Re: A Subwoofer Question

[Alexei M Voloshin <Alexei.M.Voloshin-1@tc.umn.edu>]

I built a box for my friends two 12'' JBLs GT. It's sealed and goes down 
to about 15 Hz. However, I have a bit of background in physics and sound 
design. In order for a small sealed box to sound great and go down low 
enough it has to be presicely built includeing the metrials, sealing, 
glue used etc...
What most manufacturers and car intallers do now is port the box. Sure it 
doesn't give you the same quality, but it's easier to built and the 
quality is acceptible to most people. Porting is needed to relieve the 
pressure on the back wall. This is usualy the only way to go with small 
boxes where the wavelength of the lowest frequency is much larger than 
the box. Bigger boxes can be designed to avoid porting and thus produce 
better sound. 
You can not simply plug up the holes or put some board over them!!! Sure 
it'll sound, but how well I'm not sure!

Remember, sound design is not just a driver in box! It is a tradition and 
science! 

IF you design or have somebody design a sealed box, you'll hear the 
difference.

Most likely though, the sound quality will be limited by the sub! 


What brand is you sub?? What is the max power rating on the sub??
What other speakers do you have and how you power them??
What is the limiting fequency range component of your system??
Is there a crossover network??
What is the power feed and are your wires coaxial or normal?? 

Not knowing all that I can tell you that generally:


 Ported will sound a tad more boomy, 
but that really depends on your sub. 10'' is a tad big for really precise 
sound. 8'' high performance woofers work best for clear sound, but the 
difference depends on the box design! 
Your ports, are they reflex (on the same side as driver) or direct 
(behing the driver). How is the box mounted??

God, all these questions pop into my head when I try to think about it!
It is a science indeed!

I'm interested in hearing more about your system!!

Alex




On Mon, 12 Aug 1996, human wrote:

> -- [ 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...
>