Re: A Subwoofer Question

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

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

First can I say this doesn't belong here (oops I just did it too though...)
and please don't flame me for the business related content, I think it is
part of the answer to the question...

>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...

It also has to use a woofer *designed* to go low in a sealed (acoustic
suspension) box, this usually means very low Fs (floppy heavy cone), *long*
voice coil, medium sized magnet and putting it in the right size box.  Which
can only be calculated with Qt, Vas, etc.  (or by trial and error)

>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. 

No, it's not.  It's a technique of manipulation the LF resonance of the
system to get a lower -3dB point, ie extended low bass.  Or a horrible boom
if done poorly.

>This is usualy the only way to go with small 
>boxes where the wavelength of the lowest frequency is much larger than 
>the box. 

wavelength has nothing to do with it. (100 Hz = 11 feet, and that's not low.
..)

>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!

probably better, unless the woofer was *designed* to be in a certain size
box with a certain length and diameter vent and is in it now.

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

Absolutely agree :-)

>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! 

Agree even more

>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, 

not necessarily

>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! 

hey dude go to my web site and check out the 8" and 6" high performance
woofers I build with my bare hands!  (Fs on the 8" = 26 Hz...) http://www.
thebook.com/human-speakers

>Your ports, are they reflex (on the same side as driver) or direct 
>(behing the driver).

What or what? Alexei, where are some of these concepts being formulated?  I
think this is the stuff he's trying to avoid.  The auto sound market is
driven in a bizarre fashion, by people who don't understand technical terms 
(like "free air resonance") using them to mean something else until it
becomes a virtual feature ("free air installation")  At the frequency vents
resonate at location is irrelevant - although of course whether they connect
with the passenger space or not is! 

>How is the box mounted??

Hey Alexei I'm curious actually about some of this.  I don't mean to flame
your response but some of the things you said seemed suspect.  Would rather
be in touch direct/private email and listen & learn than hack up your post! 
No offense meant. 

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

Same reaction I had.  

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

>Alex

And I yours...

Huw Powell 
human@nh.ultranet.com
HUMAN Speakers, chief cook and bottlewasher


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...
> 



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