On 4/3/2014 9:40 AM, William Sommerwerck wrote:
"Bill Gill" wrote in message ...

In one respect you are right +1 dB is just detectable.
That is how it was originally defined.

It was /never/ defined that way.

The bel is the logarithm (to the base 10) of a power ratio. Decibels are
ten times that. That a decibel just happens to be the smallest change in
level that can be easily detected is total coincidence.
The original work on which the system was based was done
by seeing what was the smallest increase in sound level that
could be detected by the human ear.

quote (from Wikipedia)
The decibel originates from methods used to quantify reductions in
audio levels in telephone circuits. These losses were originally
measured in units of Miles of Standard Cable (MSC), where 1 MSC
corresponded to the loss of power over a 1 mile (approximately 1.6 km)
length of standard telephone cable at a frequency of 5000 radians per
second (795.8 Hz), and roughly matched the smallest attenuation
detectable to the average listener. Standard telephone cable was
defined as "a cable having uniformly distributed resistance of 88 ohms
per loop mile and uniformly distributed shunt capacitance of .054
microfarad per mile" (approximately 19 gauge).[4]

The transmission unit (TU) was devised by engineers of the Bell
Telephone Laboratories in the 1920s to replace the MSC. 1 TU was
defined as ten times the base-10 logarithm of the ratio of measured
power to a reference power level.[5] The definitions were conveniently
chosen such that 1 TU approximately equaled 1 MSC (specifically, 1.056
TU = 1 MSC).[6] In 1928, the Bell system renamed the TU the decibel.[7]
Along with the decibel, the Bell System defined the bel, the base-10
logarithm of the power ratio, in honor of their founder and
telecommunications pioneer Alexander Graham Bell.[8] The bel is seldom
used, as the decibel was the proposed working unit.[9]


Bill