Natural Science Forum / Physics / Acoustics / August 2007

Both methods are active and current standards.  FYI: The C384 standard
was most recently updated; hence the "-04" designation.  The E1050
standard was last updated in '98.

They are two different methods of getting the same general results.
The E1050 method utilizes a two-microphone technique, whereas the C384
method, I believe, employs the more "traditional" standing-wave
technique.

There is a good treatment of both methods in the Cox & D'Antonio book,
"Acoustic Absorbers and Diffusers".

As for the calibration (or reference?) sample, I guess I'm not
convinced you need such a thing.  While I have heard of reference
materials used in round-robin testing (mentioned above) of certified
reverb chambers, I have not heard of a similar thing for impedence
tubes.  That's not to say it doesn't exist - I just can't see the
point.  The chamber method does not provide a finite range of
absorption coefficients like the impedence tube does, so round-robin
testing for the former is a requirement to make sure that the various
different chambers are getting the same general results for the same
type and amount of material.  (The fact that they often aren't even
close to the same results is another story...)

Contrastingly, the impendence tube can only provide absorption
coefficients between 0.0 and 1.0.  Additionally, no material will
yield a 1.0 result using either method (C384 or E1050).  Since either
method provides a no-sample versus sample relative absorption, you
shouldn't need to calibrate it (or reference it) to anything.  As Ang
mentioned above, the impedence tube (either method) is absolute, to a
degree.

Is this wrong?

All the best,

Savant