"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)

I have questioned a number of speaker builders and a couple of companies
that 'tune' speakers the answers given on what they measure with and how the
do the tests give serious rise to the claims at low frequency.

Almost any speaker system sold today claims 20Hz to 20Kz response yet this
is so far from the truth I cannot understand how they dare claim such
figures.

Almost ANY speaker system? You haven't looked at many speaker systems
if that's your claim.

Further, a specification such as "20 Hz to 20 kHz" is, in an of itself,
pretty damned meaningless. You have left out a crucial portion of the
specification, the tolerance of the amplitude response within those
limits.

I have yet to find ANY speaker system using an 8 inch driver that has the
ability to produce 40hz or lower frequencys as they all have fallen so far
down in output level as to be useless.

Determined how?

How do you measure the responese at 20Hz?,

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.

The remainder of the measurement and analysis chain has similar
properties: the primary measurement chain is DC coupled, for example.

Secondly, the size of the venue required for accurate measurements
is inversely proportional to the frequency you need to measure. Even
using techniques such as gated or windowed measurement, the distance
to the first reflection surface is a prime determinant of how low
you can measure. You want to measure 20 Hz accurately? Then you need
to find a room where the distance between the speaker/microphone and
the NEAREST surface is a minimum of 25 feet.

I have tested a few speakers and
basically I cannot get accurate (repeatable) data at much below 60Hz, even
using a borrowed shure KSM141 stereo pair in omnidirectional gives vague
results ($3000 for the mic's!)

$3000 for microphones that were NEVER designed to be used as measurement
microphones, ESPECIALLY at low frequencies. These are recording
microphones, NOT measurement microphones.

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,

That may be the conclusion you came to, but that conclusion just happens
to be quite wrong. The physical stimulus that the ear responds to as
sound are periodic pressure variations of a sufficient amplitude and
within certain frequency limits. That's it. As long as a device can
detect these pressure variations, it can be used to measure sound.

The problem with you big flat panel method is that it assumes, quite
incorrectly, that the imnpinging waves are planar: unless you are VERY
far away from the speaker, such the wavefronts are no longer spherical,
it isn't going to work. The smaller the diaphgragm of the microphone,
the less it is affected by such a problem. That's one reason why
measurement microphones have very small diaphragms: they are essentially
point transducers over a wide range of frequencies.

how one calibrates said device is open for discussion.

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

(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.

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.

Also they place a
passive radiator 1 meter infront of the driver unit and use the same method
to measure its deflection.

Well, not quite, I believe you are completely misinderstanding what they
told you. I would suggest you look up "reciprocity methods."

The apply a 'correction factor' and the produce the frequency response data
(company builds very expensive car and home audio subs!)

Hardly any correction factor needed: simply understand what's going on
and that you are simply relying on the physical first principles of
sound.

What's wrong with that?

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.