[Audio Empire]

On Mon, 3 Sep 2012 16:59:10 -0700, Dick Pierce wrote
(in article ):

Gary Eickmeier wrote:
How can my 901s do
such a show of dynamics? Well, most speakers have but one little 1 inch
dome
tweeter, maybe one or two midranges. I have NINE - on each box (the dust
cap
behaves like a tweeter at the highest freqs)

This is not about Bose, but rather about Mr. Eickmeiers specific,
testable technical assertion:

"the dust cap behaves like a tweeter at the highest freqs"

Simply stated, it most assuredly does NOT.

You are right. The dust cap might be one inch or less in diameter, but
the frequency response of the driver is determined by the dynamics of
the entire driver, not just its dust-cap. If the rest of the speaker
has poor response at, say, 15 KHz, then so does the dust-cap.

WE may safely assume that what Mr. Eickmeier is claiming is
that the 1" dust cap of a 4" driver has the same behaviour
as a 1" purpose-designed dome tweeter. The two differ
in profound and fundamental ways. Let's just look at a few.

Yep.

First, let's compare the voice coils. The voice coil and
former of a 4" driver is longer, thicker, has more wire on
it and thus is SUBSTANTIALLY heavier than the voice coil of
a purpose-built 1" dome tweeter. By "substantial" I assert,
having measured literally thousands of such beasts, it's
a minimum of 4 to 5 times heavier.

Second. the mechanical tolerances required of a 4" driver
are very different and result in the magnetic gap being
substantiall (by a factor of 2) wider than that of a typical
1" dome tweeter. The result is a substantially lower
magnetic reluctance and thus a higher flux density in the gap.

Third, the effective moving mass of the 4" driver is at a
minimum or order of magnitude (that's a factor of 10) higher
than that of a 1" dome tweeter. Why? Because the 1" dome
tweeter's moving mass is not encumbered with the moving mass
of the spider, the entire rest of that 4" diaphragm, the
surround, the lead-in wires, and so on.

Fourth, the radiating area of a 1" dome tweeter at 10-15 kHz
is pretty much a 1" diameter dome. That of a 4" driver at
those frequencies is substantially greater.

The point being, the claim that "the dust cap behaves like a
tweeter at the highest freqs" is unsubstantiated, technically
unsupportable specualtion.

That matches my thoughts on the subject as well. I think that Gary is
comparing his dust-cover theory to "whizzer cones" a popular concept
in the late 1950's and early 1960's. I worked at that time (as a
teenager) for a large stereo shop in Washington DC. A couple of the
test instruments we had in the shop was a Ballentine Audio generator
and (as I recall) a B&K sound level meter with an attached, calibrated
microphone (equalized flat to 20KHz). I once took an Electrovoice
"Wolverine" 12" "full-range" speaker (with whizzer cone) in a Karlson
enclosure and ran a frequency response sweep on it. EV advertised
that the whizzer cone was "good" to 13K. Well, it depends upon one's
definition of good, I suppose. If Â*9 dB at 13 KHz (relative to 1K)
is "good" Then I guess it was.

You can't get around needing a purpose-built tweeter because the
design criteria for speakers with the speed, dispersion and low
distrotion required for tweeters are pretty specialized.