-september.org> 42b792fd   
   From: zekor@comcast.net   
      
   Doug  wrote:   
   > I'm making a stereo that is to be mounted on the back of my bike.   
   > It's in a box that has internal dimensions of about 12" x 6" x 6" and   
   > inside this you'll find a T-amp (it's rated at 100 watts/channel, but   
   > I'm not going to power it anywhere near that high) and two 7 Ah lead   
   > acid batteries.  Input will come from an iPod that's outside the box.   
   >   
   > It uses two Polk Audio db651 6.5" car speakers, one on either side.   
   >   
   > I know it's not a good speaker design, but sound quality is one of the   
   > lesser considerations (weight, size, battery life are also important)   
   > -- and really, it's not that bad.   
   >   
   > But I'm wondering if I should port it?   
   >   
   > As I see it, there's a few advantages for porting --   
   >   
   >    -- better low frequency response   
   >   
   >    -- make it more efficient -- use less power for the same volume   
   >       (the power limited is limited, so this is a concern.)   
   >   
   >    -- if the batteries do vent some hydrogen when charged, it gives it   
   >       an easy place for it to escape.  It also lets the amp cool a bit   
   >       easier, though it's so efficient and the power relatively low so   
   >       I don't think that's a big concern.   
   >   
   > But the stuff on calculating port size is quite confusing.  I imagine   
   > I could calculate the frequency of the box (though the batteries and   
   > amp inside the box are likely to confuse that calculation) and guess   
   > at the Xmax values and go from there.   
   >   
   > I could just guess, put in a port that's about as large as I have room   
   > for, though if I understand it correctly, I want to tune the   
   > diameter/length so that the wave coming out of the port is in phase   
   > with the wave coming out of the front of the speaker -- so it *does*   
   > matter.  But wouldn't this calculation only be valid for a specific   
   > frequency?   
   >   
   > The calculators I find online talk about calculating things for   
   > woofers and subwoofers.  I guess the main speaker *is* a woofer.  But   
   > when the calculators as for a frequency ... is it the Helmholtz   
   > resonator frequency for the cabinet I should use?   
   >   
   > Maybe I just need to bite the bullet and find a book on speaker design   
   > -- since it seems that that's exactly where I'm headed.   
   >   
   > --   
   > Doug McLaren, dougmc@frenzied.us   
      
   I would suggest getting a woofer that actually has specs, and use only one,   
   or preferably isobaric, using two front to front, which work better in   
   small enclosures, each having a qts something like .6 sealed box, and use   
   separate tweeters.   
      
   But, you just will not get low bass outdoors, especially using a small   
   configuration, and I don't know if the above is worth the trouble.   
      
   Greg   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)