"The Flash" wrote in message ...
I have had no luck in measuring speaker response at low frequency's, I
can
measure from ~60Hz to 20Khz with near 100% repeatability (How accurate
is
questionable)


You measure it with microphones and ancillary equipment that is up
to the task. For myself, I have a number of Bruel & Kjaer, ACO,
GR and pther microphones that have verified flat response to well
below 20 Hz. Some of the B&K 1/2" capsules, for example, are within
+-1/2 dB from approximately 3 Hz to 30 kHz and above.

Can you give a recommend model of Bruel that you like?

Bruel & Kjaer makes a wide variety of laboratory microphones that
are intended for different purposes. They range in size from 1"
diameter to 1/8", there are microphones intended for pressure,
random incidence and such, carrier models for extremely low
frequency and so on. I might usggest a visit to the B&K, ACO
pacific and other websites for more information (q.v., bkhome.com).

As to what Bruel & Kjaer microphone I "like," well, no insult
intended, but it isn't a matter of what I "like," it's a matter
of what is best suited for the task at hand based on technical
considerations. I have a number of 4133 and 4134 that I use for common
frequency response measurements.

Please note that the price of these microphone is NOT cheap. You will
find that by the time you are done with the impdeance converter/
preamplifier, powersupply and capsule, you're already up in the
$2000 range.

I have come to the conclusion that you are
really measuring air displacement at anything under 30Hz and a flat
panel
with a transducer is the only repeatable method of measuring the output,

One calibrates it by throwing it in the nearest landfill and going
out an learning the proper ways of measuring acoustic phenomenon.

You comments are most welcome but not vey helpful, I seem to be getting the
feeling you are giving me the 'I've be doing it for decade and got really
expensive gear, so my methods, results and opinions are gospel - do it my
way or be burned at the stake as a heratic'

Hold on there, bucko, YOU'RE the one who came riding into town
yelling at everyone about how all this 20-20 kHz is bogus and
declared the whole world of speaker measurement bogus and then
declared a VERY well understood measurement paradigm as "novel."

One starts to wonder if perhaps you are one of the 'Experts' that 'corrects'
the measurments to what you 'know' is correct. Your attitude in your
responses seems to show this blinded mentality.

Sir, you make assumptions out of tyhin air. Never once did I
say ANYTHING about correcting measurements, I talked about
making measurements correctly. I suggest it's time you get
down off your mighty steed and start learning instead of
holding forth on topics about which you state quite clearly
you know little about.

(Oh one firm told me that they use a laser to measure the low frequency
of
their speakers, check this out for novel! They place a small piece of
reflective foil on the base driver, and shine a laser beam on it, they
then
apply signal and measure via 'laser' the deflection,

Well, gee golly, since it can be shown on physical first principles
that the requirement for a constant sound pressure level (that would
mean flat frequency response) from a piston radiator is simply a
displacement which goes as the reciprocal of the square of frequency,
then if you know the displacement, which you can measure with a pretty
high degree of accuracy, then you can, over the piston range of the
driver, DIRECTLY and UNAMBIGUOUSLY determine the total acoustic power
as:

Pa = p/(2 pi c) * (Sd w^2 X)^2

where

p = density of air, typ. 1.18 kg/m^3
c = velocity of sound, typ 343 m/s
Sd = emissive area of the diaphragm in m^2
w = radian frequency
X = displacement of the diaphragm, in m.

the method is HARDLY novel at all, as it is well understood and utilized
in the field. If provides, for example, a means of measuring acoustical
power output without the confounding innaccuracies of microphones, rooms
and such, though the microphone innaccuracies are not a problem if you
use proper microphones to begin with.

I do undestand the theory, and I expect it would work well if the speaker
was moving in piston motion. (An some of the car sub woofers with ridged
aluminium cones would no doubt at low frequency)

As ALL speaker cones do in the piston region of operation, below
a frequency which is roughly determined when the wavelength reaches
the circumference of the driver. For an 8" driver, that frequency
is roughly 600 Hz, for example.

However typical stereo speakers using either doped paper or pp cones won't,
as you know the LF breakup happens quite early on these type of drivers due
to the tradeoff of trying to give a wide response range with only 2 or 3
drives in most cases and stiff cone suspension.

My humble suggestion, sir, is that you're experience in this area
is not congruent with the rather large amount of data that is known
and has been known for quite some time in this realm. The operation
of drivers operating in the piston region has been very well studied,
not by car audio or home audio speaker makers, most of whom,
ESPECIALLY those in the car audio business, are clueless and work
more on legenbd, rumor and unfounded assumption, but by the likes of
MacClachlan, Beranek, Thiele, SMall, and many others. Some of these
researchers did their work 60 years ago, and developed information
that much of the speaker industry has yet to clue themselves in on.

SHould you want a comprehensive set of references, I can start
supplying such should you desire.

More to your notion that it is "novel," you might want to modify that
opinion when you discover the technique is described in nearly every
text on acoustics.

-Noted and yes I have seen it mention and was taught such things, but it was
always theory and I had not heard of it done on audio speakers (however I do
know it is done on building structures to measure characteristic)

Well, again, you seem to be unfamiliar with the vast amount of
literature on the topic. Indeed, far back in the 1970's, there is
an extensive amount of literature in the technical press describing
the characterisation of loudspeakers via such techniques. Anyone
familiar with the theory is likely to be aware of these articles.

I do know that laser scanning of drive to measure the breakup is done, but
this was not what they were doing (So they said)

Compare that to using recording microphones whose measurement capabilities
are entirely unknown, in a room of unknown characteristics, using unknown
poorly calibrated and undoubtedly poorly controlled techniques by someone
who has little or know experience in measurement and acoustics...

I'd not bet good money on getting ANY reliable data out of the latter.

Well I have to say getting reliable data from 'experts' is not very
successful!

Not when you come charging in challenging a tpoic with which you
are unfamiliar. You made some smeasurements and you ASSUMED without
ANY supporting evidence that your methods have any validity.

The apparently very narrow topic of measurement of acoustical
phenomenon is, in fact, a very large, complex and quite fascinating
one. You've not even touched on 1% of it. It's not something you're
going to learn about via an Usenet article or two. You've not even
touched on the issues of boundary conditions and nearfield vs farfield
and diffraction effects, an entire sub-topic on suitable stimuli,
of MLS vs impulse vs gated sine vs swept sine vs narrow band noise
vs wide band noise vs averaged narrow-band analysis and so one and
so forth. None of that.

Despite that, you seem to have the attitude that you have just
discovered some vast conspiracy or fraud, indeed the very title
of your post states that. Yet, you clearly state that you know
little or nothing about the basic principles involved.

You claim to "understand the theory" that in the pistonic region
of operation, there is a direct causal link between driver excursion,
frequency and sound pressure level. If you are "familiar" with it as
you claim, you coudn't possibly find the method "novel." It's
a fundamental property that is taight in the very early stages
of acoustical principles. All due respect, you don't even use the
lexicon of the subject in a way that suggests you have any familiarity
with the topic.

You want some information? Fine, ask the questions. Don't come
charging in like some expert, which you are not. The experts here
are happy to answer your questions. Accusations such as "20hz to
20Khz, yea right!" are going to be dismissed as uninformed rantings
that they are.