Natural Science Forum / Physics / Acoustics / August 2005

I meant if the results are too diferents or one can expect to get a
similar result, and in this case justify the money saved.

"Intensity" methodology is the brainchild of the end of the 20th
century. Hailed as a a know-all, catch-all, method, it has evolved into
a costly toy, in my opinion. Serious drawbacks have occurred. Right now,
I suspect that instrument manufacturers are the main supporters of the
method because of the cash flow it can generate for them.

Beyond that prejudiced opinion, I can think of only one hypothetical
advantage and one real disadvantage that I can provide to you:

The surviving advantage is that intensity sound power measurement can
hypothetically be precise in a noisy environment where it may
discriminate against unrelated noise coming from a long distance, and
where this interfering noise intensity is uniform and unidirectional.
Then the intensity algorithms and principles simply cancel that
interfering noise energy among all the intensity values integrated over
the envelope. This should leave only the sound power emerging from that
envelope caused by your device under test. This may not work well in a
standing wave field of interfering noise from other machinery, as found
in a reverberant on-site factory hall where you are obliged to measure
the noise from your device under test, and the interfering sound comes
from many directions.

A rather distressing disadvantage is that there exist upper and lower
limiting frequencies because of physical limitations of hardware
microphones. This causes the need for multiple setups to get the
required data over all frequencies of interest.

The 3740 series provides an array of techniques that function in a wide
variety of test environments. Some are more accurate than others.
ISO3745 provides the greatest precision. ISO3747 provides perhaps the
least precision when the K2 factor is large (small reverberant spaces);
good precision when K2 is small. But they all give the best estimate of
sound for the conditions under which your test must be performed. I have
often tested on-site at factories after-hours and through the quiet
night hours to get low K2 values. This is the most cost-effective
methodology, in my opinion.

Angelo Campanella

Over ten years of measurements in industrial situations, I only came
across two situations where the use of sound intensity was an advantage
and then only because of the distribution of other close by sound source
areas. On both occasions, set-up, layout and interpretation were
complicated by the source area geometry.

It was good for mapping a surface in a noisy environment but then only
if I could be confident that the general reverberant noise components
were subdued enough. The sound intensity microphone would essentially
pick up in a narrow (12 degrees) cone in front of, and behind, the
probe. Any on-axis reflections or contributions from behind had to be
carefully controlled to ensure good results.

On several other occasions it was equally as easy to use a standard
microphone in an anechoic termination i.e. a microphone inside a bucket
stuffed with rock wool.

Brian Marston

read the following conference report (BEMS 2005) for further details of
acoustic-nuclear connection

http://www.unifiedphysics.com/Medical_therapy_cyma1000_racehorse_study_june_2005.pdf

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