Physics Forums

Physics Forums (http://www.physicsforums.com/index.php)
-   Classical Physics (http://www.physicsforums.com/forumdisplay.php?f=61)
-   -   Desciribing intensity of human scream? (http://www.physicsforums.com/showthread.php?t=665363)

utkarsh5 Jan19-13 05:02 PM

desciribing intensity of human scream?
 
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.

Simon Bridge Jan19-13 06:26 PM

Re: desciribing intensity of human scream?
 
Quote:

in the following article,the person who answered the question did not take [area of mouth opening] into account
It appears the author did - from the article cited:
Quote:

80 decibels produces .001 watts of energy(sic)
... the intensity from the sound would be in watts per square meter.

80dB would be about 1x10-4Wm-2 ... so he seems to have used an area of about 10m2 ... 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

AlephZero Jan19-13 06:33 PM

Re: desciribing intensity of human scream?
 
"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.

Simon Bridge Jan19-13 07:31 PM

Re: desciribing intensity of human scream?
 
@AlephZero: yes, I noticed that too.
The author could just be using a different scale to me.

utkarsh5 Jan20-13 11:58 AM

Re: desciribing intensity of human scream?
 
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


All times are GMT -5. The time now is 08:55 PM.

Powered by vBulletin Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
© 2014 Physics Forums