I found Stan Jakubas article, Standard Measure: A Guide to SI Usage for the Reluctant (SN, September 2001), to be very interesting. However, there
is a small correction to be made. On page 22 in the caption to
the table, Selected Prefixes and Their Meaning, there is the
phrase: with the non-SI unit bel as in decibel (dB)). The
phrase non-SI unit should actually say non-unit since the
bel is a dimensionless quantity. The bel is really just a
method of notation that signals when data is being presented as
a logarithm of a ratio. The ratio consists of a data value compared
to a reference value. To use the bel in a meaningful way, the
reference value must be stated because the bel is used in several
engineering disciplines. For example, in acoustics, sound pressure
is presented using a reference of 20 mPa (an SI unit) and sound
power is presented using a reference of 10-12 W (also an SI unit).
In electrical engineering a common reference unit is volts (another
SI unit). Because the reference values can be in SI units, the
bel is not in conflict with SI.
So why use the bel? The bel has been used in some scientific
and engineering disciplines to make it easier to deal with data
values that span many orders of magnitude. Historically, the deci-
prefix was added so that whole numbers would generally be used
rather than fractions. For example, the nominal threshold of human
hearing is 20 mPa and the threshold of pain is around 100 Pa (approximately
50 million times higher than threshold). In decibel notation
this range is represented as 0 dB to 140 dB (re: 20 mPa). Since
the bel is merely a method of notation and not a unit, it can
be neither SI nor non-SI. When you see the decibel in use, check
the reference value to see how it fits into SI.
Richard J. Fridrich
General Motors Corporation
Member of E33 on Environmental Acoustics
At the risk of committing heresy, I would like to offer two thoughts
about the article Standard Measure, in which support is given
for the conversion to the metric system.
Having worked as an engineer with both metric and English systems
for years, including several wonderful years in southern Switzerland;
and having achieved an age where many of my opinions based on
emotion seem to fading, I would offer a couple of points. First,
neither system seems meaningfully better than the other. If it
were, the supremacy of one would have been driven by market, technical
or social forces some time ago.
Second, the most important real world point to be made for the
English system may just be the other side of the very argument
that has been made against it: the ease of working with multiplication
and division by ten. I have seen that very ease cause order-of-magnitude
errors that would be instantly caught in the English system because
of the obviously outlandish result. Put another way, it is easier
to use, but easier to err.
(received via e-mail)
Mr. Fridrich is correct in his explanation of the bel except in
his denial of its being a unit. Expressing a ratio and being a
unit is not unique to the bel. For example, the radian is a unit
expressing a ratio, albeit not a logarithmic one in this case.
As a matter of fact, many believe, and this author also, that
there will be an SI unit for the ratio, the neper. Unlike the
bell the neper is coherent with SI.
As for Mr. Friars letter, I believe he is missing a major point:
The error would be instantly recognized by any person in any system
who has the feel for the number in question. Examples of errors
of one or two orders of magnitude are common in either system
by people not familiar with the numbers. The error Mr. Friar refers
to would be just as instantly recognized by the metric person
who has that feel as by an English person who has that feel.
Mr. Friars issue is not related to a system. What is related
to a system is the fact that a simpler system leads to fewer errors
I rest my pro-metric case.
The official policy of both the NIST Global Standards Program and ASTM is to support a transition to SI (metric) units
Who inserted the photograph of a non-metric scale on page 2 of
the August issue of SN, thereby misrepresenting the policy of
both NIST and ASTM?
Why was a global standard millimeter scale not used?
Eugene A. Mechtly
ASTM Committee E43
on SI Practice
Sometimes, pleased to find artwork that adequately illustrates
the gist of a news item, we miss the detailsand, as they say,
the devil is in the details. Thank you for pointing this out,
and our apologies. (Editor)