Author Topic: Power in vs. power out  (Read 4739 times)

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Offline James

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Power in vs. power out
« on: September 18, 2009, 12:22:18 am »
Did you know....

a speaker that has a rated SPL of 90dB @ 1W @ 1M is considered relatively efficient. Right?

Do you know what that means?

Look at it like this...

If a speaker was 100% efficient, that is to say that 100% of the electrical energy going in to the terminals came out as sound energy, the speaker would be producing 120dB from 1 watt of energy in, measured 1 meter away from the front of the speaker.

The difference between 90dB and 120dB is -30dB.

Every 3dB is a power of 2, so -30dB is 1/1024.

That means that from the electrical energy going it to the terminals of a 90dB SPL speaker, only 1/1024 or about 0.1% of that energy actually makes it out as sound! The rest is heat.

That's pretty crappy. Huh?

James.  :)
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Offline drlava

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Re: Power in vs. power out
« Reply #1 on: September 18, 2009, 01:42:14 am »
even worse than my blue dpss laser which is 0.4% efficient.  However it can't hold a candle to an argon at about 0.02% efficiency :)

Offline BlinkenLights

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Re: Power in vs. power out
« Reply #2 on: September 18, 2009, 02:09:06 am »
lasers holding candles... thats funny

Offline Fanny Pack

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Re: Power in vs. power out
« Reply #3 on: September 18, 2009, 06:52:56 am »
I don't think the rest is all heat.  Some of it is.  Speakers don't make sound.  They move air.  The sound is a result of the receiver and there is atmosphere in between where most of the energy is lost.  I think that comes into play.

Offline James

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Re: Power in vs. power out
« Reply #4 on: September 18, 2009, 02:31:49 pm »
That is correct. And if the vibrations don't move your ear drum, then they are eventually absorbed into the air and other surrounding materials as friction. In other words...... HEAT!

Just about any kind of energy, when used, eventually becomes heat.

Newton had a lot to say about all that.

James.  :)
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Offline Fanny Pack

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Re: Power in vs. power out
« Reply #5 on: September 18, 2009, 05:51:04 pm »
Yes, you are right.  I guess what I meant is that it isn't the speaker coil that is generating all of the heat.  In a vacuum, there would be no sound.  And, the speaker would behave much differently due to no air resistance.  So, ,,, well, no point here.  Just rambling.

Offline James

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Re: Power in vs. power out
« Reply #6 on: September 18, 2009, 06:26:14 pm »
Actually, the air mass is part of the effect, but a smaller part than you might think. The spring mechanism of the cone suspension and the mass of the cone play a MUCH larger role. However, the volume of air inside the box, behind the speaker is important, not for its mass, but for its volume. A volume of air is also a spring because it can be compressed and decompressed.

The interface between a woofer cone and the air in to which it is supposed to launch an acoustic wave is very miss-matched.

That is to say, that the motor of the woofer, the magnet and coil, is very strong compared to the task of moving the tiny amount of air that is in front of the cone. It would be much better if a speaker had a huge moving surface that was evenly driven, all over, by much less force. This is why some people like electrostatic or planar dynamic speakers. But these methods are more expensive to manufacture and even WAY less efficient than cone speakers.

This is also why I like LOTS of little speakers instead of just a few bigger ones.

The best solution for any loud speaker system is a cone with a big horn on it!

The horn is an acoustic transformer. Just like an electric transformer can convert high voltage at low current into low voltage at high current, so a horn can transform high pressure at small surface area into low pressure at high surface area. The ratio of thrasformer effect comes from the area of the speaker opening compaired to the area of the mouth of the horn. Unfortunately, one horn is only effective for about 2.5 to 3 octaves. So multiple horns are needed to cover the whole 10 octaves. Horn dimensions are relative to acoustic wave lengths as well, so true first-octave horns are bigger than some houses!

There are some really great sounding horn speakers out there that approach 20% efficiency!

The horns I used in one of my designs put out 107dB @ 1W @ 1M. 107 - 120 = -13dB. That's about 1/20 or 5%. Not too bad!

James.  :)
« Last Edit: September 19, 2009, 03:56:14 pm by James »
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