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 16th April 2003, 07:16 PM #1 diyAudio Member   Join Date: Oct 2002 Location: Cape Town About dB's I am bussy with a corespondence course in electronics and got the following question in an assignment. It sound a bit wrong to me (the uninformed one) and want to know if it make sense and what would the answer be? I don't grasp the dB thing yet, so please excuse silly questions (Maybe somebody can give me a "dB for Dummy's" explanation. ) Q: Determine the dB level from a given sound power of 15dB, and a reference sound power of 20dB. (Don't you measure sound power in Watts?) Thanx for all help. __________________ "Be carefull who's advice you buy, but be patient with those who supply it." From Baz Luhrmann's - Everybody’s Free (To Wear Sunscreen)
 16th April 2003, 07:32 PM #2 diyAudio Moderator     Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1 I think the text writer needs the "dB For Dummies" course more than you do. Or at least a writing course. Sound power is given in watts OR in dB referenced to some standard power level. Here's the entire course: For power ratios, dB = 10 log (P1/P2) Since power = V^2/R, and log (x^y) = y log x, then for a voltage ratio, dB = 20 log (V1/V2) That's all there is, there ain't no more. If the dB quoted in the question are both referenced to the same standard power level, the answer is -5 dB. __________________ "Cities are globs. Globs are bubbling collections. Bubbling collections are hurricanes of the dead and the living all blasting subliminal stories lost in the nether of breezes."
 16th April 2003, 07:39 PM #3 diyAudio Moderator Emeritus     Join Date: Jun 2002 Location: U.K. dB's are a logarithmic scale. They are not absolute, and can be referenced to any point. In this case they have told you the reference point (20dB). The magic formulae I keep in my head are: 10*Log (P1/P2) (for power) and 20*Log (V1/V2) for voltage (and other non squared values) In other words, the dB figuer is describing the ratio between the 2 voltages, powers, or whatever... You'll need to do some substitution... Cheers,

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