Acoustic Power Question

[Superfry]

This is probably an easy question for anyone who deals with acoustics regularly..
I'm trying to estimate the amount of power required to drive a speaker(s) which will produce 80dBA at 7 meters - radiating in every direction.
Seems easy enough to calculate the acoustic power from a point source using the inverse square law - comes out to 0.062W for 80dB @ 7 meters.
Even with a really low efficiency speaker (say 5-10%), I know it is going to take much more power to reproduce that amount of sound from an amp/speaker.
What am I missing here?

 

[bklauba]

Nothing really. "Typical" speakers have efficiency ratings in the 88 - 100 dB/1 watt/1 meter, so your spec does not seem hard to achieve.

BK

 

[Rob45]

1/16 of a watt?
Yep, that's what my calculations show: speakers aren't really all that efficient.
And, a little sound goes a long way.

 

[MikeyP]

And of course, over a certain range of frequencies, the radiation from the speaker is hemispherical (if it is mounted in a cabinet) so you get even more bang for your buck if you are standing in front of it.

M

--
Dr Michael F Platten