[Dick Pierce[_2_]]

Barkingspyder wrote:
Gary Eickmeier wrote:
The inside of the Sonotube should be layered with a couple
of inches of fiberglass (mine is held in place with 3M
spray adhesive, and the fiberglass is actually furnace
pipe wrap with the foil removed), and the idea there is for
the heating and cooling of the fiberglass under pressure
(which can occur at any frequency above the port resonance)
to help the driver to "see" a larger interior
space than is actually there.

While all the words are sorta right, they're assembled in a way
that, well, falls a wee bit short of the mark,

If you have the advantages of the curved shape and you have the
right internal volume to begin with, ( I'll be using 143 liters
with a Shiva MKII 12" woofer) what actual need if any for the
the fiberglass? As to fiberglass I would be hesitant about
using it in a ported enclosure since the notion of fiberglass
being pumped into the air from the port does not appeal to me.
It just seems to me that in 5 cu ft there is little chance of
much heat developing, so little need for cooling.

Actually, it's not about "insulating" to keep the heat down.

Sound, as it turns out when you follow it down to its physical
underpinnings, is simply a thermodynamic phenomenon. ALL sound must,
at its very root, involve changes in temperature. Take the simple
case of a moving diaphragm in air: When it moves out, it not only
compresses the air it, it also raises its temperature. And the raise
in temperature and pressure results in the energy moving outward
as the air attempts top reach thermodynamic equilibrium with the
surrounding air. And when the diaphragm, conversely, moves inward,
not only does the pressure drop, but so does the temperature.

Now, it's not that the inside of your enclosure is going to heat
up to the point of causing serious thermal problems, it's that
the pressure AND the temperature in the box is constantly going
up and down, (both by quite small amounts, to be sure).

Now, if you're box was PERFECTLY sealed, and the walls were PERFECT
thermal insulators AND PERFECTLY rigid (such that 100% the acoustic
energy generated by your driver went into increasing and decreasing
the internal air pressure (and, at the same time, the temperature),
the box would follow perfectly the ideal gas relation, which looks like

PV = nRT

where P is pressure, V is volume, n is the total amount of gas in
the box (in conventional speak, that'd be in mols), R is a nice
constant (Boltsmann's in this case) and T is temperature. Any
chink in that "perfect" enclosure (a leak, the walls are thermally
conductive or can move), and the equation starts to work differently
(R, in this case, starts changing).

The idea to the fiberglass (or ANY fibrous tangle of the
appropriate properties) is not to insulate, but to change the
operating conditions of the system. Our "perfect" enclosure
would operating under "adiabatic" conditions (constant energy).

Pick the right kind of fibrous tangle (and fiberglas, for the
restricted purposes of this explanation, is not the ideal stuff),
and what happens is that it shifts the oepration to "isothermal"
(constant temperature).

Behind the big words is a pretty simple principle: with a fibrous
tangle in place, when the pressure in the system increases, so
does the temeprature, but a portion of that heat is transfered to
the tangle. The resulting loss of temeprature results in a reduction
in pressure. By the same token, when the pressure is reduced, so
is the temeprature, but now the tangle, being every so slightly
warmer than the auir, gives up some of its heat, which raises the
air temeprature and pressure slightly.

Now, the result is that because the presence of the tangle is
modifying the thermal and thus the pressure conditions: when
you compress it, the pressure doesn go so high, for example,
the entire system behaves as if the enclosed volume was slightly
larger. Assuming a "perfect" fibrous tangle" (with an appropriate
heat capacity and surface area, neither of which fiberglas has),
it's theoretically possible to make the enclosure appear about
1.4 times bigger than it really is.

Now, put a thermometer inside your enclosure BEFORE you put
the stuffing in. Play it REALLY loud for, oh, a day. Come back
and look at the temperature.

Now, put whatever stuffing you want inside the enclosure and
put the thermometer in again, and play the SAME music just
as loud for the same time. Then look at the thermometer.

(and, to be simple, assume a sealed enclosure in both cases).

Can you guess what the difference is going to be (assuming
room temperature is the same)?

(hint: there will be little if any difference at all).

If it's necessary fine, but if not then why bother.
I thought the main reason for damping material was to help
absorb vibration and keep resonance down.

Yes, that's another good reason to have it.

The reason why it may NOT be appropriate for a reflex enclosure
is because it DOES change the effective volume.

Now, assuming your numbers are reasonably accurate, a 143 L
enclosure with an inside diameter of 19.25" will have to have
an internal length of 30", exclusive of the volume taken up by
the woofer and the port. Assuming a perfect absorber, the actual
physical length could be about 1.4 times smaller than this, or
only 12 inches.

Now, whether or not you actually use any absorbtive material is
dependent upon a number of things, most of which have to do
with the target alignment of system. A so-called Extended
Bass Shelf alignment (what I assume you mean by EBS) while it
has some bandwidth advantages, but the actual response can
be sensitive to tuning parameters (not that you or anyone might
case). The actual amount of internal stuffing really depends
upon the exact parameters of the woofer usedm the desired response
and the acceptable tolerance to misalignment. Certainly, systems
such as B4 alignments would really like to have minimal enclosure
losses: that not only includes absorbtive losses, but leakage losses
as well.

At very low frequencies, a small ampount of stuffing is unlikely
to make a large difference. But you're not only building a subwoofer
with that tube, you're building an organ pipe. And, at 30" long, it's
going to support some NASTY 1/2 wave resonances, like a big one at
justr above 200 Hz. And don't believe for a moment that your crossover
is going to help unless you have yourself a nice, sharp high-order
low-pass. Assume you cutoff is 80 Hz and 2nd order: by 200 Hz, your
drive to the woofer is down a measely 18 dB at the point where you're
near the peak in typical music energy spectrum.

Cylindrical enclosure may look neat and be easy to build and may
make you feel like it has all sorts of advantages because of its
symmetry: but that symmetry makes for very high-Q, sharp line
resonances: the worst, by the way, is a spherical enclosure. One
might argue that it is or is not mechanically the best, but
acoustically, it's a "perfect" disaster.

Lot/s of absorbtive ,aterial at one end will help. And I'd line
at least half the circumference to try and tame the higher
frequency cylinder-mode resonances, if possible.

--
+--------------------------------+
+ Dick Pierce |
+ Professional Audio Development |
+--------------------------------+