From: dpierce@cartchunk.org   
      
   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.   
   > ...   
   > As I see it, there's a few advantages for porting --   
   >   
   >    -- better low frequency response   
      
   Only if the drivers and enclosure together are suitable for   
   a ported design.   
      
   >    -- make it more efficient -- use less power for the same volume   
   >       (the power limited is limited, so this is a concern.)   
      
   Porting an enclosure WILL NOT make the system any more   
   efficient. Not in the least. Ported systems CAN be more   
   efficient, not because they have ports, but because they   
   have drivers suited for the enclosure that result in   
   electromechanical parameters that mke the DRIVERS more   
   efficient to begin with. A more efficient driver designed   
   for a prted enclosure will result in a system with   
   exactly the same reference efficieny whether the port is   
   there or not.   
      
   >    -- 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.   
      
   Wrong.   
      
   > 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.   
      
   Wrong.   
      
   > 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.   
      
   You simply cannot just take a pair of speakers, jam them   
   in a box, stick a port in it, and expect 1) for it to work   
   reasonably well, 2) for it to have "better low frequency   
   response and 3) more efficiency. It simply does not work   
   that way.   
      
   A speaker is a SYSTEM composed of drivers, enclosures and   
   amplifiers.   
      
   If I were hired to do such a job, the first thing I'd   
   do is sit down and try to come to an agreement about   
   what constitutes "efficient" and "good low frequency   
   response". The third factor is enclosure size, but you've   
   already constrained that, so the only two variables you   
   have left are efficiency and low frequency cutoff. Things   
   like XMax play no role at this stage: it's on;y relevant   
   in determining the MAXIMUM output.   
      
   And the conversation would go something like this: you   
   have presented me with a MAXIMUM enclosure size of 0.25   
   cubic feet, and maybe more like .15 cubic feet considering   
   the batteries. That's all of 5 liters. And you're going to   
   put TWO drivers in there, meaning the effective volume is   
   half that, 2.5 liters. And, at best, you're talking 5"   
   drivers. I'd stay with 4"   
      
   Now, the classic Thiele-Small efficiency/bandwidth/size   
   rule takes over and constrains reality. SP you have 2.5   
   liters per driver. The MOST your are likely to find in   
   the efficiency for a 4" driver is on the order of 89dB   
   SPL @1W/1m. That means, the absolute BEST you can expect   
   for a low-frequency cutoff is about 90 Hz. And that's   
   only with a driver PERFECTLY suited to the application.   
   Not just any 'ol driver. It would have to have T/S   
   parameters like and Fs of 90 Hz, Vas of 2.6 L,   
   Qt of around 0.4, and that would require a cone mass   
   of nearly 12 grams with magnet Bl product of 7 N/A.   
   That's a REAL tough driver to build.   
      
   And yes, then you'd stick two of them in the box, and   
   you'd stick two ports in, let's say you made them out   
   of 1" PVC, each about 2 7/8" long.   
      
   It's doable, but not the way you want to go.   
      
   Then again, just sticking what you have in the box   
   may well be good enough for a bike. That's for   
   you to decide.   
      
   --   
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   +         Dick Pierce            |   
   + Professional Audio Development |   
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