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Author Topic: 1 + 1=7?  (Read 9964 times)

Ivan Beaver

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Re: 1 + 1=7?
« Reply #10 on: July 17, 2011, 04:58:27 pm »

Here's another :

If speaker produces 100dB @ 1m, does it produce 112dB @ 10" ???
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Just sayin'
 ;)
The answer to that is "it depends".  Specifically on the freq and the size of the device.  The problem with measuring loudspeakers up close is that they can give very "false" readings due to the size of the cabinet and the actual distance from the radiating devices.

If the devices are a far bit back in the cabinet, then measuring up close is really wrong in terms of distance from the radiating device.

That is why loudspeakers should be measured at a decent distance ( a couple of meters at a minimum-and preferably 10M away for subs), and then back calculated to the 1 meter standard.

If done at a distance, then the numbers will "stand true" for various distances away from the cabinet-except those up close-at which there are no listeners.

The whole idea  of providing specs is to provide some useful information to the user so they can determine if the product is right for their particular needs.  Numbers that have no real useful meaning do no provide that information.
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Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!

Mark Hadman

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Re: 1 + 1=7?
« Reply #11 on: July 17, 2011, 08:40:42 pm »


But here is part that just seems "odd".  If each loudspeaker is producing 1dB, then when you add another one producing 1dB, you get 7dB.


Speakers do not produce decibels.
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Ivan Beaver

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Re: 1 + 1=7?
« Reply #12 on: July 17, 2011, 09:38:43 pm »

Speakers do not produce decibels.
OK to be exact- it could be dB SPL A or C weighting-fast or slow or impulse response does not matter in this case, but only when you have an input signal.
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Ivan Beaver
Danley Sound Labs

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Jonathan Johnson

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Re: 1 + 1=7?
« Reply #13 on: July 17, 2011, 10:40:12 pm »

Speakers do not produce decibels.
OK to be exact- it could be dB SPL A or C weighting-fast or slow or impulse response does not matter in this case, but only when you have an input signal.

Because 0 != 0. In decibel measurements, zero is a reference point that does not equal zero output. The measured dB is a ratio relative to the zero reference point rather than a discrete sum. If zero dB equaled zero output, then adding 3 dB to zero would still equal zero.
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George Friedman-Jimenez

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Re: 1 + 1=7?
« Reply #14 on: July 18, 2011, 12:43:14 am »

That is right, zero dB SPL does not mean no sound. Zero is not absolute zero, it is just a point on a continuum of increasing SPL. It just means a sound too soft for most people to hear. So I think Mac is right in his first post, and 1 dB SPL from one speaker plus 1 dB SPL from a second speaker WILL give you 7 dB SPL from both. Everyone knows 1+1=2 and 100+100=200 by the same simple arithmetic logic. You could have said 100 + 100 = 106? and it would have looked just as strange on the face of it to people used to looking at simple arithmetic, but the math is correct so the weird looking equation is correct. No paradox at all.
« Last Edit: July 18, 2011, 12:49:21 am by George Friedman-Jimenez »
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Brad Weber

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Re: 1 + 1=7?
« Reply #15 on: July 18, 2011, 07:53:24 am »

Decibel values are not absolute values, they are relative values.  Decibels involve a ratio of one value to another and dB(SPL) is 20log(sound pressure/reference pressure of 0.00002 Pa).  Thus 0dB(SPL) equates to a sound pressure level of 0.00002 Pa rather than to 0 Pa while 1dB(SPL) relates to 0.00002244 Pa and 7dB(SPL) to 0.000044774 Pa.
« Last Edit: July 18, 2011, 11:55:43 am by Brad Weber »
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David Allred

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Re: 1 + 1=7?
« Reply #16 on: July 18, 2011, 09:05:49 am »

That is right, zero dB SPL does not mean no sound. Zero is not absolute zero, it is just a point on a continuum of increasing SPL. It just means a sound too soft for most people to hear. So I think Mac is right in his first post, and 1 dB SPL from one speaker plus 1 dB SPL from a second speaker WILL give you 7 dB SPL from both. Everyone knows 1+1=2 and 100+100=200 by the same simple arithmetic logic. You could have said 100 + 100 = 106? and it would have looked just as strange on the face of it to people used to looking at simple arithmetic, but the math is correct so the weird looking equation is correct. No paradox at all.

Also looks "less wrong" if in the first example it had been "101 + 101 = 107" and "1 + 1 = 7".  (At least in my brain.)

David
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Mac Kerr

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Re: 1 + 1=7?
« Reply #17 on: July 18, 2011, 09:10:21 am »

Also looks "less wrong" if in the first example it had been "101 + 101 = 107" and "1 + 1 = 7".  (At least in my brain.)

David

I think that was Ivan's tongue in cheek point. He is quite aware of what the term dB means,

Mac
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Marty McCann

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Re: 1 + 1=7?
« Reply #18 on: July 18, 2011, 03:21:39 pm »

Far be it for me to correct the esteemed members of this forum . . . however let me try an shine a little light here.

First of all for two source (acoustical or electrical) to sum to where the summation measures +6 dB greater, they must be coincident.  So two full range loudspeaker will only sum coincidentally, IF for those frequencies that are less than 1/4 wavelength of the distance between centers.

For those frequencies that are NOT coincident (i.e. greater than 1/4 wl), they only sum to +3 dB.

Regarding the Math, yes I believe that Ivan is doing a tongue he cheek job on some, and forgot the:   :P , as you simply can NOT add decibels as they involve base 10 number exponents. 

Now you can Add the exponents, which is the same a multiplication.  Let the Math impaired think about that,  :o yes and subtracting exponents is the same as division.
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James A. Griffin

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Re: 1 + 1=7?
« Reply #19 on: July 18, 2011, 06:58:27 pm »

So if 2 trees fall in the forest and there is no SPL meter to measure it........
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ProSoundWeb Community

Re: 1 + 1=7?
« Reply #19 on: July 18, 2011, 06:58:27 pm »


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