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(Stewart Pinkerton)
wrote:

On Fri, 03 Oct 2003 14:41:15 GMT, (ludovic
mirabel) wrote:

What I wanted to and obviously failed to communicate was that with
very little expense and skill one can get extraordinary bass
performance. The cylindrical lines I set up are amazingly efficient.

No, they're not. Let's knock this one on the head right away. Even at
their very best, true transmission lines *cannot* be more efficient
than an infinite baffle. I canna' change the laws o' physics, cap'n.

I had to equalise downwards BOTH sides. The left "only" 16db. down at
80, 50, 40 and 30 Hz. The right 30 db down *over two octaves*. And the
bass is clean incomparably better sounding than what came from the
closed boxes, properly Thiele measured for my 12" woofers.
If all these cylindrical tubes do is go hunting for "room nodes"
of that magnitude on both sides over two octaves then good hunting.
Everyone should have nodes like that.

Consider the above, and then consider what *must* be happening in your
room. Also, please note that even in 1/8 space, you *cannot* get more
than 18dB of room gain, and 12dB is a much more likely maximum. I
canna' change the laws o' physics, cap'n.
--

Stewart Pinkerton | Music is Art - Audio is Engineering

The actual amount of cabin gain is 12 dB per octave as frequency falls below
that lowest axial room mode.

For example I can measure 30 dB of cabin gain in my Corvette at 8 Hz. But the
lowest axial mode in the subcompact car occurs at 60 Hz and the whole cabin is
sealed tightly with little low frequency absorption.

In a 2136 ft3 listening room (stick frame/ drywall) some room gain was evident
beginning near 25 Hz. In my current 7600 ft3 room (stick frame/drywall
construction; adjacent co-joined spaces) there appear to be a very small effect
starting at 17 Hz.

However to attain these benefits the woofer/subwoofer(s) must have the
dispacement to take advantage of cabin gain. Thus a long stroke 10-inch woofer
is a 1-ft3 sealed enclosure can produce 120 dB at from 12 to 62 Hz in a
Corvette. To get equivalent SPL in a 7600 ft3 room requires 8 long stroke (23.4
mm Xmax) 15-inch woofers and a 5000 watt amplifier.

Low frequency absorption can be a good thing at low frequencies (knocks down
the room mode peaks) but it also tends to offset cabin gain at very low
frequencies, meaning that woofer displacement and amplifier power are required
to compensate.