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Re: [Phys-L] Acoustics: Does a speaker create a displacement antinode?(amplifier power article)



Many thanks, Bernard, Bryan, Bill, John for enlightening me on this.
-As we can see there are many factors.
-Isn't this one ripe for an AJP article?
-Or maybe an article in The Physics Teacher, given that esp. the high
school teachers need to be bringing this stuff to the attention of their
students, now disproportionately losing their hearing to ear buds and
extreme concerts. This topic could be used as a lead-in for the hearing
damage discussion.
-Not that I am up to such writing.
Bill Norwood




On Thu, Apr 10, 2014 at 9:47 AM, John Clement <clement@hal-pc.org> wrote:

Of course the actual amplifier power depends on the efficiency of the
speaker. Most speakers are not very efficient. Years ago theaters used
large horn speakers which literally could produce adequate sound in the
theater with just a small amplifier. Of course the horns were extremely
large, but were hidden behind the screen. I actually saw one in an old
theater at college. It was humungous compared to most modern speakers.
The
folded corner horn speakers are actually very efficient, but expensive to
make and must be placed in a corner. Amplifiers by contrast are cheap.
Properly vented speakers are more efficient than closed boxes. Actually a
lot of outdoor concerts are producing 120dB at the front of the audience,
not just at the front of the speaker.

So you have to look at the rated input power for the 120dB output. Also
you
need to look at the frequency response curves. The bass is what the fans
often want and that is in the less efficient range of most speakers. Any
energy argument needs to take into account the efficiency factors. There
is
also the comparison with the alternative, a councert in a hall. The hall
has to be heated or air conditioned, which is not a factor for an outdoor
concert.

John M. Clement
Houston, TX


I mean whatever comes out of he speaker that commonly gets
measured in terms of dB - say a mfr will claim that his
speaker will "put out" 120 dB I guess right at the front
plane of the speaker enclosure.

So, I would need to know for this case what amplifier power
would be required to drive a speaker at 120 dB vs. at 110 or 100 dB.

Bottom line is that I am trying to add an environmentalism
aspect to my claim that extremely loud, outdoor, band music
not only contributes to sound pollution for a neighborhood,
but it also wastes a lot more electrical energy than one might expect.


Hi Jeff,
Long shot here, but since you are looking a bit into audio:
If one increases by 10 dB the sound level out of a speaker at a
specified distance, by what factor has the amplifier
power requirement increased?
Thanks, Bill Norwood, U of MD at College Park



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