Don, N8DE wrote, in part:
> All measurements in 'db' ..really dB ... are comparisons ...
> to some 'standard'.
> 20 dB means NOTHING ... 20 dB above/below/gain/loss
> is meaningfull.
Explanation:
The Bell Labs numbers in my post are related to the following
standard. This standard of air pressure was defined by them
as being about the threshold of human hearing (remember
they were designing telephone systems). They defined,
based upon their founders name, Bell, that 0 dB (1/10th Bell =
dB) of sound pressure level (dBSPL) would be equal to 0.0002
dynes/cubic centimeter.
That is why Don pointed out that the B must be capitalized;
it stands for Alexander Graham's last name, Bell, hi.
Sorry, I have lost the Bell System
Technical Journal specific reference, probably can be
found on the Internet somewhere, hi. Bell Labs also
listed that the dBspl level of the "rustle of leaves" was
about 10 dBspl above that defined threshold of human
hearing.
The entire Bell Telephone system and all other communications
systrems continues to this day to make power and pressure
level measurements in relative dBs. And as Don so correctly
says, to be meaningful, you have to know the 0 dBxxx
reference level. For example, in radio we often use the
dBm to measure power levels. The defining power level
is that 1 milliwatt of power dissipated in a 50 ohm resistor,
is a power of 0 dBm (50 ohms at least for most of our
gear and amateur usage). In telecommunications there
are many sorts of dBxxx scales; for example, the dBrnC0,
which is the power level referenced to the 0 carrier level,
if I recall correctly, and there are many more. But all
have a base reference which must be known for the
numbers to have meaning.
So, the dB sound pressure levels of my previous post all
relate to the sound pressure level that the Bell Labs
decided was the threshold of average human hearing,
some 10 dB below the sound presuure level of the quiet
rustle of leaves on a Fall day, hi.
Hope that gives a more complete explanation.
73, Jim KH7M
|