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Originally Posted by
kjg
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IImagine a super deluxe coffee machine in your studio.. With a control to scale coffee power/intensity, in dB!
The control would have to be stepped in 0.5 dB steps of course, for recallability, but that goes without saying :P
Lets make that .2dB increments. I'm pretty sure we can distinguish that, eh?
Quote:
Originally Posted by
Audiop
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A good paper with some examples and calculations.
I remember this article well. Allow me to quote from it because what I'm talking about is woven all through that paper:
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Two drivers connected in-parallel.
. . . . . . Each driver will generate 100dB SPL, so that total SPL of the system is now 103dB.
Notice the term "total" in that sentence. Peter, for the purposes of this discussion, loudspeaker design isn't really relevant. We could be discussing Mole farts, as long as they were the same. Ironically, that paper should give you a lot of insight to explain why you have a 6dB increase at your crossover frequency, instead of just 3dB.
Quote:
Originally Posted by
Audiop
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The author is unknown to me but the references are long time legends in the field.
And a walk down memory lane, for me. I studied with John Eargle when he taught at Eastman, (his alma mater too) and he was one of the guys (along with Melvin Sprinkle) who used to beat our heads around about exactly the decibel stuff we're discussing here. Believe me, I was just as stubborn about it then as you guys are now! I had some dealings with him later due to his association with Harman/JBL, and at AES. He passed on a couple years ago, but he's sorely missed. (Mel too).
I studied with Don Keele too, back when I first started using TDS. I was an original TEF10, 12 and 20 user and he would co-teach the TEF seminars at Crown. Gee, I hope he's still alive!
OK, I digress, on to the point . . .
Whenever we talk about the decibel in the context of acoustics, it is ALWAYS about power. The Bel is an expression of power ratio, 10:1 in fact. In order to apply it to pressure, we first have to convert to the power ratio. So:
10Log (P1/P2) gives us our basic power ratio in dB. To convert that to pressure, we simply use 20Log (P1/P2) because, as I said earlier, it takes four times as much power if you want to achieve twice as much pressure. Going from 10Log to 20Log is simply a convenient way to double your answer and make it relate to a pressure ratio. 6dB when you double the pressure instead of 3dB when you double the power.
Now we can't directly measure the sound power of things, (we can calculate it but it's awfully, awfully difficult) we instead conveniently measure the pressure, (force) at a given point in the sound field. If two devices both measure the same, the power ratio between both running and one running, is 2:1. That's the nice thing about watts, you can add them directly.
Since the decibel is not a unit value, but only an expression of ratio, and in acoustics only ever an expression of power ratio, then the only way we can express the ratio between two pressures in decibels is with a power ratio. Phew.
Now we all know from Audio 101 that when we double the power of anything, it's 3dB. Since the decibel is only ever capable of expressing a power ratio between two sound pressures, then the answer must be 3dB.
10Log (2/1) = 3.
What's confusing for many is that when we apply it to different mediums, like voltage, current, impedance or pressure, it's no longer a power ratio. It's been mutated for these mediums to express their own ratios. That is when we can use the 20Log equation, eg 10V/20V is a 6dB voltage ratio. 20uPascal/1 Pascal is a 94dB pressure ratio.
Quote:
Originally Posted by
Audiop
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If you take two identical sources, say speakers, and add them as proposed you WILL get +6dB SPL and also quadrupled (sp?) power.
Think about your statement, Peter. If I have 2 speakers each outputting 1 acoustic watt, where would I get quadruple the power from?
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Originally Posted by
Audiop
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This is really basic physics that anyone involved with audio engineering should understand IMO.
/Peter
Which is exactly why I'm trying to make the effort to help you understand it!
This isn't so much about physics as it is about convenient mathematical notation and how we name things. For example, a Pascal is a unit of sound pressure but we never use it to describe Sound Pressure Level. Likewise, Sound Intensity refers to the absolute intensity in watts per square meter whereas Sound Intensity Level is the magnitude of the sound intensity relative to a reference intensity.
OK, I'll stop there before Gearslutz crashes again! LOL.