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,
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