re what is zero db in article sehari vincent iastate edu sehari iastate edu babak sehari writes in c true aa ms uky edu msunde mik uky edu mark underwood writes i am somewhat familiar with the db measurements as they apply to electrical circuits i e it is the gain of for example an amplifier measured on a logarithmic scale however this requires that you have a reference value i e the ouput is db e g with respect to the input signal stuff deleted what you actually talking about here is dbm and not db however the terms are used loosely by most people dbm is power with respact to mw whereas db is a ratio now like these two english statement i am doing well i am doing good similarly people usually use db for dbm another common mistake is spelling db instead of db as you did in your article see the b is for bell company the mother of at t and should be capitalized with highest regards babak sehari good gravy decibels are all ratios the question that remains in any ratio is the reference unit used sometimes this will be a reference power such as milliwatt given a certain circuit impedance which should also be included in the fine print or known like ohms in an rf circuit of that impedance leading to an accepted notation of dbm maybe it might be dbv disregarding the impedance of the circuit and power developed using volt as reference amplitude rather than reference power or it might have an arbitrary or omitted reference that is not included in the notation leading to just plain db so look at it this way db has an implied reference while notation such as dbm has an explicit reference for power db log p measured p reference for amplitude db log a measured a reference b is for bel which is a standard term for a log ratio to the base named after alexander graham bell a deci bel is of a bel it has nothing to do with the bell telephone company except for the common founder s name the small d large b is per si notation convention i don t know anyone that s been crucified for messing it up common references for audio are dbm milliwatt across ohms dbv volt vu a zero on the vu meter dbm pro gear line level vu dbv consumer gear line level often times a power amp vu meter will be aligned using the rated power of the amp as the db point it is all done to whatever reference is reasonable for the application or moment note that in a circuit with a given and maybe unknown linear impedance if the amplitude goes up so many decibels the power will also increase the same amount this proof can be done with the above two identities and ohm s law dave medin phone w ssd networking h intergraph corp m s gd internet dtmedin catbyte b ingr com huntsville al uucp uunet ingr b catbyte dtmedin everywhere you look at least around my office the opinions expressed here are mine or those of my machine
