How to calculate mechanic power a person outputs?

[APUGYael]

Hey all,

I am doing some work to brush up my acoustics and one of the problems I came across this goes like this (translated, so sorry for potential grammar mistakes although I'll do my best to avoid them):

A singer exhales 0.1 Litre Air per second. The 'overpressure' (pressure difference, so not absolute pressure) of his lungs is 1257 Pa. On 1 meter from his mouth the soundpressurelevel is 70 dB. We assume a point source (so Q=1).
- Caculate the mechanical output power of the person.
- Calculate the acoustic output power of the person.
- Calculate the acoustic effeciency.

So, I know how to do the last 2 parts:

L_p = L_w - 10*log(Q/(4*pi*r^2)) is the equation I'll use for this problem.

r = 1, Q = 1, so: L_p = L_w - 11dB
Which means
L_w = 70 dB + 11 dB = 81 dB
L_w = 10*log(W/W_0) --> W = (10^-12)*(10^8.1) = 0.1259 mWatt
This is the acoustic ouput power. The answer sheet says this is correct.
The effeciency would simply be: (acoustic power/mechanic power)*100%.

But this is where I am stuck. How do I calculate the mechanic power with the given values?
Thanks for reading!

 

[russ_watters]

Mechanical power in airflow is just pressure times volumetric flow rate.

 

[sophiecentaur]

Bearing in mind that our strongest (leg) muscles can only manage a few hundred W mean, you can expect much much less than that from your intercostal muscles. Ball park figures can be good when trying to make estimates of this sort of thing. Then consider that an efficient loudspeaker can be deafening with a couple of W.

 

[APUGYael]

Mechanical power in airflow is just pressure times volumetric flow rate.

This is the one I needed to get the right answer it seems!
Thanks :-)