i was trying to verify the '8 years 7 months 6 days' myth/fact that states that you would have you scream for that much time to produce enough energy to warm up a cup of coffee. i assume that an average human screams at 80 dB.if i have to calculate the power,do i have to take the area of mouth opening into account?i think i do,but in the following article,the person who answered the question did not take it into account(or as it seems). http://www.physicscentral.com/explore/poster-coffee.cfm please answer.any help would be greatly appreciated.
It appears the author did - from the article cited: ... the intensity from the sound would be in watts per square meter. 80dB would be about 1x10^{-4}Wm^{-2} ... so he seems to have used an area of about 10m^{2} ... which seems a little large. Perhaps the author has a big mouth? The whole thing is suspect isn't it - I'm sure I can make myself heard in the front rows of a rock concert - well, by yelling in someone's ear. If I yell at a cup of coffee the same intensity that is more like 0.1Wm^{-2}. Figure a mouth area of about a square decimeter (0.1m x 0.1m = 0.01m^{-2}) and that's 0.001W directed at the cup. So the final figure is about right. ref. http://www.physicsclassroom.com/class/sound/u11l2b.cfm
"Decibels" are used to measure serveral different quantities. Your link seems to be talking about "sound power level" (SPL) whcih measures the total acoustic power that the sound source (i.e. the scream) generates. That is not the same as "how loud the scream sounds" which depends how far away from the source you are, and the fact that the sound may be directional. 1 dB SPL is arbitrarily defined as ##10^{-12}## watts of acoustic power, so 80 dB SPL = 0.0001 W Assuming 80db is the right SPL, your link gives the time if you can somehow capture all the power from the scream and use it to heat the coffee. Edit: I was writing this when Simon Birdge posted, but note that SPL is measured in watts, not in watts/meter^2. The OP's link isn't very clear exactly what it means by "decibels", though.
thanks for the replies!what confused me was exactly that the author had not used watts/(meter^2),and had used just watts.that was pretty strange.anyways,thanks for the clarification! :D