decibels

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hi, I was told in BRS Physiology that the equation for decibels is dB = 20 log (P/P0), but I was told in physics undergrad that dB = 10 log (I/I0); what is the difference between these two equations?
thanks for your help

 

[creamyBokah]

Use the BRS formula most of the time. The "I" that is used in the physics formula is for sound intensity (or power) and is related to sound some distance squared away from something else or another ... bottom line, sound intensity is pretty useless in the real world and not even really measurable. The "P" used in the BRS formula is for sound pressure and the square of it is related to the sound intensity so there is a log formula that reduces it down to that BRS formula you see, namely, dB = 20 log (P/P0)

hi, I was told in BRS Physiology that the equation for decibels is dB = 20 log (P/P0), but I was told in physics undergrad that dB = 10 log (I/I0); what is the difference between these two equations?
thanks for your help

 

[MilkmanAl]

Awwww...I didn't get to feel special. 😛 Creamy's correct. Remember that exponents in logs can be brought out of the log function as coefficients.

 

[lildave2586]

Do they really put this crap on the test?

 

[MilkmanAl]

Doubtful, but judging by the results of the math-ish questions on UWorld, I hope my entire test is biostats. Hellooooo 300. You guys don't like math at all, and it shows. 😛

 

[creamyBokah]

Sorry if I did a crappy job explaining it because I don't remember the details but what Milkman said is right, you can pull the square out of the log function as a coefficient so you have:

2 * (that physics equation but now with pressure instead of intensity)

which is equivalent to:

2 * 10 log (P/Po) = 20 log (P/Po)

Awwww...I didn't get to feel special. 😛 Creamy's correct. Remember that exponents in logs can be brought out of the log function as coefficients.