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Author Topic: 1 + 1=7?  (Read 9924 times)

Jonathan Johnson

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Re: 1 + 1=7?
« Reply #20 on: July 19, 2011, 01:40:41 am »

So if 2 trees fall in the forest and there is no SPL meter to measure it........
You must first define sound. Is it the vibration of a source or output transducer? Is it the vibration of air molecules sympathetic with a source vibration? Is it the vibration of an input transducer (i.e., eardrum) sympathetic with a vibration of air molecules? Or is it a graphical representation of that vibration as seen on the screen of an oscilloscope or digital recording device? Or maybe it is the motion of the needle on the SPL meter coincident with the motion of the built-in transducer.

Now this becomes important, because if I strike a drum, I perceive that the sound occurs when the drumstick contacts the drumhead. But you, watching from a mile away, perceive that the sound occurs approximately 5 seconds after the strike... the sound can be neither the striking of the drumhead nor the vibration of the eardrum, since those events are not coincidental. Further, if the drum were struck in the vacuum of space, your eardrum would not be actuated ("...nobody will hear you scream"); therefore, sound must be defined as a packet of energy within a certain band of frequencies moving through a medium.

Now my head hurts.  :o
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Dave Gunnell

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Re: 1 + 1=7?
« Reply #21 on: July 20, 2011, 02:53:41 pm »

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.

The statement above is inconsistent and misleading.  While unreferenced decibel values are not absolute but represent a ratio (i.e. "a 3 dB increase in level" or "95 dB of dynamic range"), once tied to a reference point they are absolute.  Any decibel value that is referenced with a suffix (dBu, dBV, dBm, etc) is an absolute measurement that can also be written as the corresponding voltage, power, or acoustical value.  For example, 0 dBu = 0.775 Vrms, 3 dBm = 2 mW, 0 dBV = 1 Vrms.  0 dBSPL (mentioned above) doesn't just "relate" to 0.00002 Pa--it's equal to it.

Note that most pro audio spec sheets use "referenced" dB values, such as nominal bus levels (often +4 dBu), bus insert levels (maybe -2 dBu), main output max levels (maybe +26 dBu), all of which correspond to actual absolute voltages.
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Charlie Zureki

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Re: 1 + 1=7?
« Reply #22 on: July 20, 2011, 04:34:18 pm »

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.

The statement above is inconsistent and misleading.  While unreferenced decibel values are not absolute but represent a ratio (i.e. "a 3 dB increase in level" or "95 dB of dynamic range"), once tied to a reference point they are absolute.  Any decibel value that is referenced with a suffix (dBu, dBV, dBm, etc) is an absolute measurement that can also be written as the corresponding voltage, power, or acoustical value.  For example, 0 dBu = 0.775 Vrms, 3 dBm = 2 mW, 0 dBV = 1 Vrms.  0 dBSPL (mentioned above) doesn't just "relate" to 0.00002 Pa--it's equal to it.

Note that most pro audio spec sheets use "referenced" dB values, such as nominal bus levels (often +4 dBu), bus insert levels (maybe -2 dBu), main output max levels (maybe +26 dBu), all of which correspond to actual absolute voltages.

   +1

  Hammer
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Brad Weber

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Re: 1 + 1=7?
« Reply #23 on: July 20, 2011, 04:58:03 pm »

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.

The statement above is inconsistent and misleading.  While unreferenced decibel values are not absolute but represent a ratio (i.e. "a 3 dB increase in level" or "95 dB of dynamic range"), once tied to a reference point they are absolute.
While it was not as clearly stated as it could have been, we seem to be saying the same thing, that "dB" without any qualifier represents the ratio of two values both of which have to be defined while dB(SPL) represents a sound pressure value relative to a defined standard sound pressure reference value.
 
Decibels are always relative, you don't measure dB(SPL) or dBu or whatever directly, you measure a sound pressure level, voltage or whatever is involved and the correpsonding dB value represents that measured value relative to a defined reference value.   This is done because it's probably easier to talk about 45dB(SPL) versus 85dB(SPL) than it is to discuss 0.003556559 Pa versus 0.355655882 Pa.  In fact one could probably argue that 0dB(SPL) might be more accurately stated as representing a sound pressure level of 0.00002 Pa relative to a 0.00002 Pa reference value.
 
Now that this is really off track, what I was actually trying to do, and probably did a poor job of, was to get people to think about that in the OP the relative change in dB was the same regardless of the specific numbers involved.  X+X/X=2 for any non-zero value of X.
 
0 dBSPL (mentioned above) doesn't just "relate" to 0.00002 Pa--it's equal to it.
What I said was "0dB(SPL) equates to a sound pressure level of 0.00002 Pa" while the comment "1dB(SPL) relates to 0.00002244 Pa and 7dB(SPL) to 0.000044774 Pa" seems correct as there is a direct relationship or logical connection, thus they relate.
« Last Edit: July 20, 2011, 10:54:15 pm by Brad Weber »
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Charlie Zureki

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Re: 1 + 1=7?
« Reply #24 on: July 20, 2011, 05:12:11 pm »

So if 2 trees fall in the forest and there is no SPL meter to measure it........
You must first define sound. Is it the vibration of a source or output transducer? Is it the vibration of air molecules sympathetic with a source vibration? Is it the vibration of an input transducer (i.e., eardrum) sympathetic with a vibration of air molecules? Or is it a graphical representation of that vibration as seen on the screen of an oscilloscope or digital recording device? Or maybe it is the motion of the needle on the SPL meter coincident with the motion of the built-in transducer.

Now this becomes important, because if I strike a drum, I perceive that the sound occurs when the drumstick contacts the drumhead. But you, watching from a mile away, perceive that the sound occurs approximately 5 seconds after the strike... the sound can be neither the striking of the drumhead nor the vibration of the eardrum, since those events are not coincidental. Further, if the drum were struck in the vacuum of space, your eardrum would not be actuated ("...nobody will hear you scream"); therefore, sound must be defined as a packet of energy within a certain band of frequencies moving through a medium.

Now my head hurts.  :o


 Hello,

  Sound is defined as a mechanical wave whose oscillation of pressure is transmitted through a solid, liquid or a gas , and, is within the frequency range and of a pressure level to be perceived by human hearing.

  And....your example "watching from a mile away....the sound can be neither  the striking of the drum head nor the vibration of the eardrum...." is wrong.  The energy being released by the striking of the drum head does not have to be simultaneous with the energy being perceived by human hearing to be defined as Sound.

  The definition of Sound is very simple and precise...you either perceive "sound" ...or you don't.

  Cheers,
  Hammer
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Bob McCarthy

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Re: 1 + 1=7?
« Reply #25 on: July 21, 2011, 08:52:20 am »

Against the advice of my doctor I will post my 2 cents worth

1) There seems to be confusion here over power and pressure.  Power addition is complex (requires two parts such as voltage AND current) and follows the 10 log equation. Therefore 2x power (10watts to 20 watts) is +3 dB.  Voltage gain goes by the 20 log formula. Therefore 2x voltage is +6dB. On the acoustic side the "P" in SPL stands for pressure - not power. Therefore 2 x pressure (like voltage) is + 6dB.  Acoustic POWER is complex: pressure AND surface area and goes by the 10 log formula - but we are talking acoustic watts here - which is of very limited practical use.
2) If you take two speakers and drive them with the same signal so that they arrive at matched pressure (SPL) and matched phase the summation at the location where this occurs will be + 6 dB.  This is true regardless of whether they are spaced a 1/4 wavelength apart or any other distance. It is pressure addition. it happens  (in the exact center) with two speakers adjacent, it happens (in the exact center) with L/R stacks that are FAR apart and it can happen when your mains meet your delays out in the parking lot - if there is a location where they are matched in level AND phase.  It is pressure addition as long as the signals are correlated and the responses matched. You can prove this to yourself in less than a minute with a pair of speakers and an FFT analyzer.
3) Non-correlated random noise sources add (statistically averaged) at a +3dB per doubling (rather than the +6dB of correlated sources). So if you measure air conditioning noise +3dB is the ticket. If you are in the music business think +6dB when you double up your sources.
4) Decibel scales are always ratios. Ratios against a fixed standard (such as 0 dB SPL mentioned previously here) or dBV (reference to 1 volt) or ratios against a variable standard such as "I turned it up 3 dB.
5) A related formula for audio signal addition that I have said for decades is: 1+1=1 (+/-1)* 

*depends on phase ;D 

Have fun

6o6
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andy craig

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Re: 1 + 1=7?
« Reply #26 on: July 22, 2011, 02:45:33 am »

  X+X/X=2 for any non-zero value of X.
 

Being something of a mathematical pedant I have to point out that X+X/X=2 only when X=1.
(X+X)/X = 2 is undeniably true for all non-zero value of X. As Laurie Anderson said so eloquently, "Let x = x".
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Keith Broughton

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Re: 1 + 1=7?
« Reply #27 on: July 22, 2011, 07:10:15 am »

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

LOL ;D
Made my day
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Charlie Zureki

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Re: 1 + 1=7?
« Reply #28 on: July 22, 2011, 07:25:14 am »

  X+X/X=2 for any non-zero value of X.
 

Being something of a mathematical pedant I have to point out that X+X/X=2 only when X=1.

   Hello,
    Really? ....don't think so....   ::)

  Hammer
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David Morison

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Re: 1 + 1=7?
« Reply #29 on: July 22, 2011, 07:50:41 am »

   Hello,
    Really? ....don't think so....   ::)

  Hammer

Technically, he's dead right.
Without the brackets, the division should be done first so for example 3 + 3/3 = 4.
The brackets force the addition to be done first, yielding the expected result (3+3)/3 = 2.

Of course, if it was written out fully rather than inline there would be less confusion:

3+3 = 2
  3

Cheers,
David (fellow pedant).
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ProSoundWeb Community

Re: 1 + 1=7?
« Reply #29 on: July 22, 2011, 07:50:41 am »


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