Re: [Phys-l] decibels

[brian whatcott <betwys1@sbcglobal.net>]

On 8/28/2011 2:59 PM, Kyle Forinash wrote:
> Hi;
>
> Decibels for acoustic applications are always positive numbers,
> basically because the log of the ratio is intensity (W/m^2) to a very
> small reference intensity (10^-12 W/m^2) and ambient sound is always
> greater than this reference so the ln is positive.
>
> I'm a little confused about what ratio of reference power or voltage is
> used in electronics/engineering (where the dB scale is usually a set of
> negative numbers). In particular, electrical engineers seem to come up
> with this negative scale even when the circuit is measuring input from a
> microphone. Can anyone point me to a reference that makes the connection
> between the acoustic scale and the electrical engineering scale?
>
> kyle
No reference immediately on hand - but in engineering usage, the wide 
ranging dB
 scale is always specified by reference to some standard - a popular 
one is xxx dBm

Where dBm means "with reference to one milliwatt (the m)

It is incorrect usage in that milieu to specify no reference value.

Brian W