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#241
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Distorsion percentage, power or voltage?
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#242
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Distorsion percentage, power or voltage?
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#243
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Distorsion percentage, power or voltage?
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#244
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Distorsion percentage, power or voltage?
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#246
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Distorsion percentage, power or voltage?
(Dick Pierce) wrote in message . com...
(Stewart Pinkerton) wrote in message ... On Sat, 17 Jan 2004 22:53:10 +0000, Glenn Booth wrote: I mean, the fundaments of dB assumes that we measure a power ratio. No, it doesn't. It is simply a useful logarithmic ratio. I can't bring to mind any meaningful use of the decibel that is not derived from power measurements, including dBSPL. SPL is a pressure level, not anything to do with power per se. Well, no. 0 dB SPL is defined as 0.0002 dyne/cm^2, which is also 10^-12 watt/m^2. Oops! My apologies. I just posted that this was wrong but I didn't read it properly. I guess I stumbled upon the dyne/cm^2 part, I usually say 20 uPa. Anyway, the reference for sound pressure 20 uPa was selected such that SPL and SIL would yield the same numerical value for normal contitions. It assumes that the wave impedance is 400 Ns/m3 and that there is a plane wave. Still, IMO, the SPL is derived from the SIL, not the other way around, since the Bel is fundamentally a power ratio measure. That would also explain why the reference for sound pressure has been selected to such a strange number. |
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#247
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Distorsion percentage, power or voltage?
(Dick Pierce) wrote in message . com...
(Stewart Pinkerton) wrote in message ... On Sat, 17 Jan 2004 22:53:10 +0000, Glenn Booth wrote: I mean, the fundaments of dB assumes that we measure a power ratio. No, it doesn't. It is simply a useful logarithmic ratio. I can't bring to mind any meaningful use of the decibel that is not derived from power measurements, including dBSPL. SPL is a pressure level, not anything to do with power per se. Well, no. 0 dB SPL is defined as 0.0002 dyne/cm^2, which is also 10^-12 watt/m^2. Oops! My apologies. I just posted that this was wrong but I didn't read it properly. I guess I stumbled upon the dyne/cm^2 part, I usually say 20 uPa. Anyway, the reference for sound pressure 20 uPa was selected such that SPL and SIL would yield the same numerical value for normal contitions. It assumes that the wave impedance is 400 Ns/m3 and that there is a plane wave. Still, IMO, the SPL is derived from the SIL, not the other way around, since the Bel is fundamentally a power ratio measure. That would also explain why the reference for sound pressure has been selected to such a strange number. |
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#248
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Distorsion percentage, power or voltage?
(Dick Pierce) wrote in message . com...
(Stewart Pinkerton) wrote in message ... On Sat, 17 Jan 2004 22:53:10 +0000, Glenn Booth wrote: I mean, the fundaments of dB assumes that we measure a power ratio. No, it doesn't. It is simply a useful logarithmic ratio. I can't bring to mind any meaningful use of the decibel that is not derived from power measurements, including dBSPL. SPL is a pressure level, not anything to do with power per se. Well, no. 0 dB SPL is defined as 0.0002 dyne/cm^2, which is also 10^-12 watt/m^2. Oops! My apologies. I just posted that this was wrong but I didn't read it properly. I guess I stumbled upon the dyne/cm^2 part, I usually say 20 uPa. Anyway, the reference for sound pressure 20 uPa was selected such that SPL and SIL would yield the same numerical value for normal contitions. It assumes that the wave impedance is 400 Ns/m3 and that there is a plane wave. Still, IMO, the SPL is derived from the SIL, not the other way around, since the Bel is fundamentally a power ratio measure. That would also explain why the reference for sound pressure has been selected to such a strange number. |
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#249
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Distorsion percentage, power or voltage?
(Dick Pierce) wrote in message . com...
(Stewart Pinkerton) wrote in message ... On Sat, 17 Jan 2004 22:53:10 +0000, Glenn Booth wrote: I mean, the fundaments of dB assumes that we measure a power ratio. No, it doesn't. It is simply a useful logarithmic ratio. I can't bring to mind any meaningful use of the decibel that is not derived from power measurements, including dBSPL. SPL is a pressure level, not anything to do with power per se. Well, no. 0 dB SPL is defined as 0.0002 dyne/cm^2, which is also 10^-12 watt/m^2. Oops! My apologies. I just posted that this was wrong but I didn't read it properly. I guess I stumbled upon the dyne/cm^2 part, I usually say 20 uPa. Anyway, the reference for sound pressure 20 uPa was selected such that SPL and SIL would yield the same numerical value for normal contitions. It assumes that the wave impedance is 400 Ns/m3 and that there is a plane wave. Still, IMO, the SPL is derived from the SIL, not the other way around, since the Bel is fundamentally a power ratio measure. That would also explain why the reference for sound pressure has been selected to such a strange number. |
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#250
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Distorsion percentage, power or voltage?
chung wrote in message rvers.com...
Svante wrote: chung wrote in message ervers.com... Svante wrote: chung wrote in message rvers.com... Svante wrote: chung wrote in message ervers.com... Svante wrote: I mean, the fundaments of dB assumes that we measure a power ratio. No such assumption. The equation for "voltage dBs" (20*log(u/uref)) is a derivation based on that p~u^2 neglecting the effects of varying load resistance. It is a definition, not a derivation. So, if it is a definition, free from association with the power ratio, why does it say "20" times the logarith of the ratio. deci would mean ten (or a tenth). It's defined in such a way so that voltage ratios in dB is consistent with power ratios in dB. Read any textbook. dB is always defined, not derived. I have read not just any, but many textbooks. And yes, dB is defined, but only once. That definition is dB = 10 * log (p/pref) You need to read more EE textbooks  From that we can DERIVE, given a constant load resistance and that p=u^2/R, that dB = 10 * log (u^2 / uref^2) = 20 * log (u/uref) Am I the only one that sees the power ratio as the logical way to define the dB. I mean, I know that deci stands for a tenth, WHY ON EARTH would we put TWENTY in the equation if we defined dBs based on a voltage ratio???? You missed what I wrote: to keep it consistent with power ratios expressed in dB's. So that a dB is a dB! Ok, I did. But if it is to be kept consistent with power ratios, isn't it then DERIVED from the power ratio? It is not a stand-alone definition, it is derived from the original definition of the dB, that applies to the power ratio. And yes, a dB is a dB. Unless the load resistance varies. |
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#251
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Distorsion percentage, power or voltage?
chung wrote in message rvers.com...
Svante wrote: chung wrote in message ervers.com... Svante wrote: chung wrote in message rvers.com... Svante wrote: chung wrote in message ervers.com... Svante wrote: I mean, the fundaments of dB assumes that we measure a power ratio. No such assumption. The equation for "voltage dBs" (20*log(u/uref)) is a derivation based on that p~u^2 neglecting the effects of varying load resistance. It is a definition, not a derivation. So, if it is a definition, free from association with the power ratio, why does it say "20" times the logarith of the ratio. deci would mean ten (or a tenth). It's defined in such a way so that voltage ratios in dB is consistent with power ratios in dB. Read any textbook. dB is always defined, not derived. I have read not just any, but many textbooks. And yes, dB is defined, but only once. That definition is dB = 10 * log (p/pref) You need to read more EE textbooks From that we can DERIVE, given a constant load resistance and that p=u^2/R, that dB = 10 * log (u^2 / uref^2) = 20 * log (u/uref) Am I the only one that sees the power ratio as the logical way to define the dB. I mean, I know that deci stands for a tenth, WHY ON EARTH would we put TWENTY in the equation if we defined dBs based on a voltage ratio???? You missed what I wrote: to keep it consistent with power ratios expressed in dB's. So that a dB is a dB! Ok, I did. But if it is to be kept consistent with power ratios, isn't it then DERIVED from the power ratio? It is not a stand-alone definition, it is derived from the original definition of the dB, that applies to the power ratio. And yes, a dB is a dB. Unless the load resistance varies. |
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#252
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Distorsion percentage, power or voltage?
chung wrote in message rvers.com...
Svante wrote: chung wrote in message ervers.com... Svante wrote: chung wrote in message rvers.com... Svante wrote: chung wrote in message ervers.com... Svante wrote: I mean, the fundaments of dB assumes that we measure a power ratio. No such assumption. The equation for "voltage dBs" (20*log(u/uref)) is a derivation based on that p~u^2 neglecting the effects of varying load resistance. It is a definition, not a derivation. So, if it is a definition, free from association with the power ratio, why does it say "20" times the logarith of the ratio. deci would mean ten (or a tenth). It's defined in such a way so that voltage ratios in dB is consistent with power ratios in dB. Read any textbook. dB is always defined, not derived. I have read not just any, but many textbooks. And yes, dB is defined, but only once. That definition is dB = 10 * log (p/pref) You need to read more EE textbooks From that we can DERIVE, given a constant load resistance and that p=u^2/R, that dB = 10 * log (u^2 / uref^2) = 20 * log (u/uref) Am I the only one that sees the power ratio as the logical way to define the dB. I mean, I know that deci stands for a tenth, WHY ON EARTH would we put TWENTY in the equation if we defined dBs based on a voltage ratio???? You missed what I wrote: to keep it consistent with power ratios expressed in dB's. So that a dB is a dB! Ok, I did. But if it is to be kept consistent with power ratios, isn't it then DERIVED from the power ratio? It is not a stand-alone definition, it is derived from the original definition of the dB, that applies to the power ratio. And yes, a dB is a dB. Unless the load resistance varies. |
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#253
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Distorsion percentage, power or voltage?
chung wrote in message rvers.com...
Svante wrote: chung wrote in message ervers.com... Svante wrote: chung wrote in message rvers.com... Svante wrote: chung wrote in message ervers.com... Svante wrote: I mean, the fundaments of dB assumes that we measure a power ratio. No such assumption. The equation for "voltage dBs" (20*log(u/uref)) is a derivation based on that p~u^2 neglecting the effects of varying load resistance. It is a definition, not a derivation. So, if it is a definition, free from association with the power ratio, why does it say "20" times the logarith of the ratio. deci would mean ten (or a tenth). It's defined in such a way so that voltage ratios in dB is consistent with power ratios in dB. Read any textbook. dB is always defined, not derived. I have read not just any, but many textbooks. And yes, dB is defined, but only once. That definition is dB = 10 * log (p/pref) You need to read more EE textbooks From that we can DERIVE, given a constant load resistance and that p=u^2/R, that dB = 10 * log (u^2 / uref^2) = 20 * log (u/uref) Am I the only one that sees the power ratio as the logical way to define the dB. I mean, I know that deci stands for a tenth, WHY ON EARTH would we put TWENTY in the equation if we defined dBs based on a voltage ratio???? You missed what I wrote: to keep it consistent with power ratios expressed in dB's. So that a dB is a dB! Ok, I did. But if it is to be kept consistent with power ratios, isn't it then DERIVED from the power ratio? It is not a stand-alone definition, it is derived from the original definition of the dB, that applies to the power ratio. And yes, a dB is a dB. Unless the load resistance varies. |
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#254
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Distorsion percentage, power or voltage?
(John Fields) wrote in message ...
On 18 Jan 2004 14:54:25 -0800, (Svante) wrote: Glenn Booth wrote in message ... Hi, In message , Stewart Pinkerton writes I am a Scot, but I'm 56 years old, and I was brought up with feet and inches, and with pounds, shillings and pence. Yes, the metric system is simpler, but this doesn't affect how we *think*. I am 6 feet 3 inches tall, I know how tall is someone who is 6 feet tall, but I have no idea how tall is someone who is 1.83 metres tall................. I'm similar, but not a Scot :-) We bought new weighing scales that only work in kilos, and I'm tempted to throw them at the wall. I can only think in stones, despite knowing the conversion factor very well. I think I'll be that way until I die. It makes me laugh when the large print on milk bottles says "1.136 litres", with "2 pints" written underneath in a minuscule typeface. It's so obviously designed for people who think in imperial measures, but the law says it has to be sold in metric. Slightly more on topic, I understand there is a move afoot to replace the decibel with the 'neper' (an SI unit). I hope it doesn't catch on; I'm only just beginning to get a proper understanding of the old system. I have hesitated to bring the Neper into this discussion, but as you do I'll bring the following up: While the dB is fundamentally defined as describing a POWER (or intensity)ratio, the neper is defined as describing a VOLTAGE (or pressure/velocity/current) ratio. Further more, it uses the natural logarithm. So, while the dB is defined in it most pure :-) form as dB = 10 * log10 (p/pref) the Neper is defined as: Np = ln (U/Uref) On the page http://physics.nist.gov/cuu/Units/outside.html , however, both dB and Np are listed as being "outside" the metric system, but accepted for use with tha metric system. I also hope that the neper never will be a success. Possibly this has to do with me having ten fingers, rather than 2.71. --- The neper is already a success, and has been used for years and years in telecommunications. For a quick peek, http://www.sizes.com/units/neper.htm Possibly the reason you'd like for it not to have become a success is because you're already having a lot of trouble sorting out n(dB) = 20 log10 from n(dB) = 10 log10, and being confronted with having to sort out n(Np) = logn A/Aref from n(Np) = 0.5 logn P/Pref is 2.713... times more daunting?^) I think my reason is rather that i know that 20 dB is 10 times the voltage and 100 times the power. Nice and even numbers. But I guess it is like the americans still thinking in feet and inches. So, in telecommunications, is there an example that the neper simplifies things, compared to the decibel? |
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#255
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Distorsion percentage, power or voltage?
(John Fields) wrote in message ...
On 18 Jan 2004 14:54:25 -0800, (Svante) wrote: Glenn Booth wrote in message ... Hi, In message , Stewart Pinkerton writes I am a Scot, but I'm 56 years old, and I was brought up with feet and inches, and with pounds, shillings and pence. Yes, the metric system is simpler, but this doesn't affect how we *think*. I am 6 feet 3 inches tall, I know how tall is someone who is 6 feet tall, but I have no idea how tall is someone who is 1.83 metres tall................. I'm similar, but not a Scot :-) We bought new weighing scales that only work in kilos, and I'm tempted to throw them at the wall. I can only think in stones, despite knowing the conversion factor very well. I think I'll be that way until I die. It makes me laugh when the large print on milk bottles says "1.136 litres", with "2 pints" written underneath in a minuscule typeface. It's so obviously designed for people who think in imperial measures, but the law says it has to be sold in metric. Slightly more on topic, I understand there is a move afoot to replace the decibel with the 'neper' (an SI unit). I hope it doesn't catch on; I'm only just beginning to get a proper understanding of the old system. I have hesitated to bring the Neper into this discussion, but as you do I'll bring the following up: While the dB is fundamentally defined as describing a POWER (or intensity)ratio, the neper is defined as describing a VOLTAGE (or pressure/velocity/current) ratio. Further more, it uses the natural logarithm. So, while the dB is defined in it most pure :-) form as dB = 10 * log10 (p/pref) the Neper is defined as: Np = ln (U/Uref) On the page http://physics.nist.gov/cuu/Units/outside.html , however, both dB and Np are listed as being "outside" the metric system, but accepted for use with tha metric system. I also hope that the neper never will be a success. Possibly this has to do with me having ten fingers, rather than 2.71. --- The neper is already a success, and has been used for years and years in telecommunications. For a quick peek, http://www.sizes.com/units/neper.htm Possibly the reason you'd like for it not to have become a success is because you're already having a lot of trouble sorting out n(dB) = 20 log10 from n(dB) = 10 log10, and being confronted with having to sort out n(Np) = logn A/Aref from n(Np) = 0.5 logn P/Pref is 2.713... times more daunting?^) I think my reason is rather that i know that 20 dB is 10 times the voltage and 100 times the power. Nice and even numbers. But I guess it is like the americans still thinking in feet and inches. So, in telecommunications, is there an example that the neper simplifies things, compared to the decibel? |
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#256
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Distorsion percentage, power or voltage?
(John Fields) wrote in message ...
On 18 Jan 2004 14:54:25 -0800, (Svante) wrote: Glenn Booth wrote in message ... Hi, In message , Stewart Pinkerton writes I am a Scot, but I'm 56 years old, and I was brought up with feet and inches, and with pounds, shillings and pence. Yes, the metric system is simpler, but this doesn't affect how we *think*. I am 6 feet 3 inches tall, I know how tall is someone who is 6 feet tall, but I have no idea how tall is someone who is 1.83 metres tall................. I'm similar, but not a Scot :-) We bought new weighing scales that only work in kilos, and I'm tempted to throw them at the wall. I can only think in stones, despite knowing the conversion factor very well. I think I'll be that way until I die. It makes me laugh when the large print on milk bottles says "1.136 litres", with "2 pints" written underneath in a minuscule typeface. It's so obviously designed for people who think in imperial measures, but the law says it has to be sold in metric. Slightly more on topic, I understand there is a move afoot to replace the decibel with the 'neper' (an SI unit). I hope it doesn't catch on; I'm only just beginning to get a proper understanding of the old system. I have hesitated to bring the Neper into this discussion, but as you do I'll bring the following up: While the dB is fundamentally defined as describing a POWER (or intensity)ratio, the neper is defined as describing a VOLTAGE (or pressure/velocity/current) ratio. Further more, it uses the natural logarithm. So, while the dB is defined in it most pure :-) form as dB = 10 * log10 (p/pref) the Neper is defined as: Np = ln (U/Uref) On the page http://physics.nist.gov/cuu/Units/outside.html , however, both dB and Np are listed as being "outside" the metric system, but accepted for use with tha metric system. I also hope that the neper never will be a success. Possibly this has to do with me having ten fingers, rather than 2.71. --- The neper is already a success, and has been used for years and years in telecommunications. For a quick peek, http://www.sizes.com/units/neper.htm Possibly the reason you'd like for it not to have become a success is because you're already having a lot of trouble sorting out n(dB) = 20 log10 from n(dB) = 10 log10, and being confronted with having to sort out n(Np) = logn A/Aref from n(Np) = 0.5 logn P/Pref is 2.713... times more daunting?^) I think my reason is rather that i know that 20 dB is 10 times the voltage and 100 times the power. Nice and even numbers. But I guess it is like the americans still thinking in feet and inches. So, in telecommunications, is there an example that the neper simplifies things, compared to the decibel? |
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#257
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Distorsion percentage, power or voltage?
(John Fields) wrote in message ...
On 18 Jan 2004 14:54:25 -0800, (Svante) wrote: Glenn Booth wrote in message ... Hi, In message , Stewart Pinkerton writes I am a Scot, but I'm 56 years old, and I was brought up with feet and inches, and with pounds, shillings and pence. Yes, the metric system is simpler, but this doesn't affect how we *think*. I am 6 feet 3 inches tall, I know how tall is someone who is 6 feet tall, but I have no idea how tall is someone who is 1.83 metres tall................. I'm similar, but not a Scot :-) We bought new weighing scales that only work in kilos, and I'm tempted to throw them at the wall. I can only think in stones, despite knowing the conversion factor very well. I think I'll be that way until I die. It makes me laugh when the large print on milk bottles says "1.136 litres", with "2 pints" written underneath in a minuscule typeface. It's so obviously designed for people who think in imperial measures, but the law says it has to be sold in metric. Slightly more on topic, I understand there is a move afoot to replace the decibel with the 'neper' (an SI unit). I hope it doesn't catch on; I'm only just beginning to get a proper understanding of the old system. I have hesitated to bring the Neper into this discussion, but as you do I'll bring the following up: While the dB is fundamentally defined as describing a POWER (or intensity)ratio, the neper is defined as describing a VOLTAGE (or pressure/velocity/current) ratio. Further more, it uses the natural logarithm. So, while the dB is defined in it most pure :-) form as dB = 10 * log10 (p/pref) the Neper is defined as: Np = ln (U/Uref) On the page http://physics.nist.gov/cuu/Units/outside.html , however, both dB and Np are listed as being "outside" the metric system, but accepted for use with tha metric system. I also hope that the neper never will be a success. Possibly this has to do with me having ten fingers, rather than 2.71. --- The neper is already a success, and has been used for years and years in telecommunications. For a quick peek, http://www.sizes.com/units/neper.htm Possibly the reason you'd like for it not to have become a success is because you're already having a lot of trouble sorting out n(dB) = 20 log10 from n(dB) = 10 log10, and being confronted with having to sort out n(Np) = logn A/Aref from n(Np) = 0.5 logn P/Pref is 2.713... times more daunting?^) I think my reason is rather that i know that 20 dB is 10 times the voltage and 100 times the power. Nice and even numbers. But I guess it is like the americans still thinking in feet and inches. So, in telecommunications, is there an example that the neper simplifies things, compared to the decibel? |
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#258
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Distorsion percentage, power or voltage?
Glenn Booth wrote in message ...
Hi, In message , Svante writes My answer to this is that the original definition is for the power ratio, and the logarithm of that power ratio was taken as a BEL. The deci was introduced, just as for the decimeter, and we ended up with a TEN before the log. To measure a power level difference by means of voltages, given constant load resistance, we would have to take the log of the SQUARE of the voltage ratio, since power is proportional to voltage squared. Simple math makes us then realise that we can skip the square if we put TWENTY before the log instead. That was my reasoning also. The factor of 2 is only necessary to account for the squared term in the relationship between power and voltage (or their equivalents). However, having checked a few links from Google, it seems far from clear - there are many conflicting opinions. For example: http://www.madengineer.com/blunders/decibels.htm Claims the decibel was originally defined to relate pressures. http://www.sizes.com/units/decibel.htm Claims that the decibel originated to relate powers. Using dB for power relationships seems mathematically clear and intuitive - the maths needs to be massaged in order to compare voltages, for example. The same goes for sound power, and sound pressure (pressure being the mechanical analog of voltage). So in my mind there is no doubt that the original (deci-)bel definition is for a power ratio, and that the equation for a voltage ratio is derived from that. It does seem logical; unfortunately, I can't find any definitive reference. To be honest, neither can I. I think I'll let it rest with this. Just like some people are convinced there is a God and don't need proof... :-) |
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#259
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Distorsion percentage, power or voltage?
Glenn Booth wrote in message ...
Hi, In message , Svante writes My answer to this is that the original definition is for the power ratio, and the logarithm of that power ratio was taken as a BEL. The deci was introduced, just as for the decimeter, and we ended up with a TEN before the log. To measure a power level difference by means of voltages, given constant load resistance, we would have to take the log of the SQUARE of the voltage ratio, since power is proportional to voltage squared. Simple math makes us then realise that we can skip the square if we put TWENTY before the log instead. That was my reasoning also. The factor of 2 is only necessary to account for the squared term in the relationship between power and voltage (or their equivalents). However, having checked a few links from Google, it seems far from clear - there are many conflicting opinions. For example: http://www.madengineer.com/blunders/decibels.htm Claims the decibel was originally defined to relate pressures. http://www.sizes.com/units/decibel.htm Claims that the decibel originated to relate powers. Using dB for power relationships seems mathematically clear and intuitive - the maths needs to be massaged in order to compare voltages, for example. The same goes for sound power, and sound pressure (pressure being the mechanical analog of voltage). So in my mind there is no doubt that the original (deci-)bel definition is for a power ratio, and that the equation for a voltage ratio is derived from that. It does seem logical; unfortunately, I can't find any definitive reference. To be honest, neither can I. I think I'll let it rest with this. Just like some people are convinced there is a God and don't need proof... :-) |
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#260
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Distorsion percentage, power or voltage?
Glenn Booth wrote in message ...
Hi, In message , Svante writes My answer to this is that the original definition is for the power ratio, and the logarithm of that power ratio was taken as a BEL. The deci was introduced, just as for the decimeter, and we ended up with a TEN before the log. To measure a power level difference by means of voltages, given constant load resistance, we would have to take the log of the SQUARE of the voltage ratio, since power is proportional to voltage squared. Simple math makes us then realise that we can skip the square if we put TWENTY before the log instead. That was my reasoning also. The factor of 2 is only necessary to account for the squared term in the relationship between power and voltage (or their equivalents). However, having checked a few links from Google, it seems far from clear - there are many conflicting opinions. For example: http://www.madengineer.com/blunders/decibels.htm Claims the decibel was originally defined to relate pressures. http://www.sizes.com/units/decibel.htm Claims that the decibel originated to relate powers. Using dB for power relationships seems mathematically clear and intuitive - the maths needs to be massaged in order to compare voltages, for example. The same goes for sound power, and sound pressure (pressure being the mechanical analog of voltage). So in my mind there is no doubt that the original (deci-)bel definition is for a power ratio, and that the equation for a voltage ratio is derived from that. It does seem logical; unfortunately, I can't find any definitive reference. To be honest, neither can I. I think I'll let it rest with this. Just like some people are convinced there is a God and don't need proof... :-) |
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#261
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Distorsion percentage, power or voltage?
Glenn Booth wrote in message ...
Hi, In message , Svante writes My answer to this is that the original definition is for the power ratio, and the logarithm of that power ratio was taken as a BEL. The deci was introduced, just as for the decimeter, and we ended up with a TEN before the log. To measure a power level difference by means of voltages, given constant load resistance, we would have to take the log of the SQUARE of the voltage ratio, since power is proportional to voltage squared. Simple math makes us then realise that we can skip the square if we put TWENTY before the log instead. That was my reasoning also. The factor of 2 is only necessary to account for the squared term in the relationship between power and voltage (or their equivalents). However, having checked a few links from Google, it seems far from clear - there are many conflicting opinions. For example: http://www.madengineer.com/blunders/decibels.htm Claims the decibel was originally defined to relate pressures. http://www.sizes.com/units/decibel.htm Claims that the decibel originated to relate powers. Using dB for power relationships seems mathematically clear and intuitive - the maths needs to be massaged in order to compare voltages, for example. The same goes for sound power, and sound pressure (pressure being the mechanical analog of voltage). So in my mind there is no doubt that the original (deci-)bel definition is for a power ratio, and that the equation for a voltage ratio is derived from that. It does seem logical; unfortunately, I can't find any definitive reference. To be honest, neither can I. I think I'll let it rest with this. Just like some people are convinced there is a God and don't need proof... :-) |
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#262
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Distorsion percentage, power or voltage?
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#263
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Distorsion percentage, power or voltage?
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#264
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Distorsion percentage, power or voltage?
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#266
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Distorsion percentage, power or voltage?
On 19 Jan 2004 00:54:22 -0800, (Svante)
wrote: Ok, I did. But if it is to be kept consistent with power ratios, isn't it then DERIVED from the power ratio? It is not a stand-alone definition, it is derived from the original definition of the dB, that applies to the power ratio. And yes, a dB is a dB. Unless the load resistance varies. --- Rather than one being derived from the other, they are merely different ways of looking at the same thing. In the beginning, when the Bel was introduced in order to define a unit which described a difference in loudness of a factor of two, it was discovered that in order for one sound (and let's leave the psychoacoustics out of it) to appear twice as loud as another, ten times the power had to be radiated toward the listener. The fallout from this was that, since P = EČ/R, in order to describe current or voltage ratios using the 'Bel', the logarithm of the voltage or current ratio had to be doubled. Here's a nice link: http://www.safetyline.wa.gov.au/inst...e54/l54_03.asp -- John Fields |
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#267
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Distorsion percentage, power or voltage?
On 19 Jan 2004 00:54:22 -0800, (Svante)
wrote: Ok, I did. But if it is to be kept consistent with power ratios, isn't it then DERIVED from the power ratio? It is not a stand-alone definition, it is derived from the original definition of the dB, that applies to the power ratio. And yes, a dB is a dB. Unless the load resistance varies. --- Rather than one being derived from the other, they are merely different ways of looking at the same thing. In the beginning, when the Bel was introduced in order to define a unit which described a difference in loudness of a factor of two, it was discovered that in order for one sound (and let's leave the psychoacoustics out of it) to appear twice as loud as another, ten times the power had to be radiated toward the listener. The fallout from this was that, since P = EČ/R, in order to describe current or voltage ratios using the 'Bel', the logarithm of the voltage or current ratio had to be doubled. Here's a nice link: http://www.safetyline.wa.gov.au/inst...e54/l54_03.asp -- John Fields |
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#268
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Distorsion percentage, power or voltage?
On 19 Jan 2004 00:54:22 -0800, (Svante)
wrote: Ok, I did. But if it is to be kept consistent with power ratios, isn't it then DERIVED from the power ratio? It is not a stand-alone definition, it is derived from the original definition of the dB, that applies to the power ratio. And yes, a dB is a dB. Unless the load resistance varies. --- Rather than one being derived from the other, they are merely different ways of looking at the same thing. In the beginning, when the Bel was introduced in order to define a unit which described a difference in loudness of a factor of two, it was discovered that in order for one sound (and let's leave the psychoacoustics out of it) to appear twice as loud as another, ten times the power had to be radiated toward the listener. The fallout from this was that, since P = EČ/R, in order to describe current or voltage ratios using the 'Bel', the logarithm of the voltage or current ratio had to be doubled. Here's a nice link: http://www.safetyline.wa.gov.au/inst...e54/l54_03.asp -- John Fields |
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#269
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Distorsion percentage, power or voltage?
On 19 Jan 2004 00:54:22 -0800, (Svante)
wrote: Ok, I did. But if it is to be kept consistent with power ratios, isn't it then DERIVED from the power ratio? It is not a stand-alone definition, it is derived from the original definition of the dB, that applies to the power ratio. And yes, a dB is a dB. Unless the load resistance varies. --- Rather than one being derived from the other, they are merely different ways of looking at the same thing. In the beginning, when the Bel was introduced in order to define a unit which described a difference in loudness of a factor of two, it was discovered that in order for one sound (and let's leave the psychoacoustics out of it) to appear twice as loud as another, ten times the power had to be radiated toward the listener. The fallout from this was that, since P = EČ/R, in order to describe current or voltage ratios using the 'Bel', the logarithm of the voltage or current ratio had to be doubled. Here's a nice link: http://www.safetyline.wa.gov.au/inst...e54/l54_03.asp -- John Fields |
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#271
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Distorsion percentage, power or voltage?
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#272
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Distorsion percentage, power or voltage?
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#273
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Distorsion percentage, power or voltage?
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#274
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Distorsion percentage, power or voltage?
John Fields wrote in message . ..
On 19 Jan 2004 00:54:22 -0800, (Svante) wrote: Ok, I did. But if it is to be kept consistent with power ratios, isn't it then DERIVED from the power ratio? It is not a stand-alone definition, it is derived from the original definition of the dB, that applies to the power ratio. And yes, a dB is a dB. Unless the load resistance varies. --- Rather than one being derived from the other, they are merely different ways of looking at the same thing. But, you must admit that the power definition is more straightforward than the voltage definition? But OK, I can live with that it can be seen as two way of looking at things. In the beginning, when the Bel was introduced in order to define a unit which described a difference in loudness of a factor of two, it was discovered that in order for one sound (and let's leave the psychoacoustics out of it) to appear twice as loud as another, ten times the power had to be radiated toward the listener. You can't say that you leave psychoacoustics out of it if you say that a sound appears twice as load as another. That IS psychoacoustics! The fallout from this was that, since P = EČ/R, in order to describe current or voltage ratios using the 'Bel', the logarithm of the voltage or current ratio had to be doubled. Here's a nice link: http://www.safetyline.wa.gov.au/inst...e54/l54_03.asp A crucial line on this page is: I = 0.0024 * p^2 This relation fixes the two references (for SP and SI) to oneanother. I usually write this as p^2/(rho0 * c) where rho0 is the density of air and c is the speed of sound, and rho0*c equals the "wave impedance". If the wave impedance assumed to be 400 Ns/m3 the two levels (SIL and SPL) are identical. Otherwise there is a (mostly tiny) difference. |
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#275
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Distorsion percentage, power or voltage?
John Fields wrote in message . ..
On 19 Jan 2004 00:54:22 -0800, (Svante) wrote: Ok, I did. But if it is to be kept consistent with power ratios, isn't it then DERIVED from the power ratio? It is not a stand-alone definition, it is derived from the original definition of the dB, that applies to the power ratio. And yes, a dB is a dB. Unless the load resistance varies. --- Rather than one being derived from the other, they are merely different ways of looking at the same thing. But, you must admit that the power definition is more straightforward than the voltage definition? But OK, I can live with that it can be seen as two way of looking at things. In the beginning, when the Bel was introduced in order to define a unit which described a difference in loudness of a factor of two, it was discovered that in order for one sound (and let's leave the psychoacoustics out of it) to appear twice as loud as another, ten times the power had to be radiated toward the listener. You can't say that you leave psychoacoustics out of it if you say that a sound appears twice as load as another. That IS psychoacoustics! The fallout from this was that, since P = EČ/R, in order to describe current or voltage ratios using the 'Bel', the logarithm of the voltage or current ratio had to be doubled. Here's a nice link: http://www.safetyline.wa.gov.au/inst...e54/l54_03.asp A crucial line on this page is: I = 0.0024 * p^2 This relation fixes the two references (for SP and SI) to oneanother. I usually write this as p^2/(rho0 * c) where rho0 is the density of air and c is the speed of sound, and rho0*c equals the "wave impedance". If the wave impedance assumed to be 400 Ns/m3 the two levels (SIL and SPL) are identical. Otherwise there is a (mostly tiny) difference. |
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#276
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Distorsion percentage, power or voltage?
John Fields wrote in message . ..
On 19 Jan 2004 00:54:22 -0800, (Svante) wrote: Ok, I did. But if it is to be kept consistent with power ratios, isn't it then DERIVED from the power ratio? It is not a stand-alone definition, it is derived from the original definition of the dB, that applies to the power ratio. And yes, a dB is a dB. Unless the load resistance varies. --- Rather than one being derived from the other, they are merely different ways of looking at the same thing. But, you must admit that the power definition is more straightforward than the voltage definition? But OK, I can live with that it can be seen as two way of looking at things. In the beginning, when the Bel was introduced in order to define a unit which described a difference in loudness of a factor of two, it was discovered that in order for one sound (and let's leave the psychoacoustics out of it) to appear twice as loud as another, ten times the power had to be radiated toward the listener. You can't say that you leave psychoacoustics out of it if you say that a sound appears twice as load as another. That IS psychoacoustics! The fallout from this was that, since P = EČ/R, in order to describe current or voltage ratios using the 'Bel', the logarithm of the voltage or current ratio had to be doubled. Here's a nice link: http://www.safetyline.wa.gov.au/inst...e54/l54_03.asp A crucial line on this page is: I = 0.0024 * p^2 This relation fixes the two references (for SP and SI) to oneanother. I usually write this as p^2/(rho0 * c) where rho0 is the density of air and c is the speed of sound, and rho0*c equals the "wave impedance". If the wave impedance assumed to be 400 Ns/m3 the two levels (SIL and SPL) are identical. Otherwise there is a (mostly tiny) difference. |
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#277
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Distorsion percentage, power or voltage?
John Fields wrote in message . ..
On 19 Jan 2004 00:54:22 -0800, (Svante) wrote: Ok, I did. But if it is to be kept consistent with power ratios, isn't it then DERIVED from the power ratio? It is not a stand-alone definition, it is derived from the original definition of the dB, that applies to the power ratio. And yes, a dB is a dB. Unless the load resistance varies. --- Rather than one being derived from the other, they are merely different ways of looking at the same thing. But, you must admit that the power definition is more straightforward than the voltage definition? But OK, I can live with that it can be seen as two way of looking at things. In the beginning, when the Bel was introduced in order to define a unit which described a difference in loudness of a factor of two, it was discovered that in order for one sound (and let's leave the psychoacoustics out of it) to appear twice as loud as another, ten times the power had to be radiated toward the listener. You can't say that you leave psychoacoustics out of it if you say that a sound appears twice as load as another. That IS psychoacoustics! The fallout from this was that, since P = EČ/R, in order to describe current or voltage ratios using the 'Bel', the logarithm of the voltage or current ratio had to be doubled. Here's a nice link: http://www.safetyline.wa.gov.au/inst...e54/l54_03.asp A crucial line on this page is: I = 0.0024 * p^2 This relation fixes the two references (for SP and SI) to oneanother. I usually write this as p^2/(rho0 * c) where rho0 is the density of air and c is the speed of sound, and rho0*c equals the "wave impedance". If the wave impedance assumed to be 400 Ns/m3 the two levels (SIL and SPL) are identical. Otherwise there is a (mostly tiny) difference. |
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#278
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Distorsion percentage, power or voltage?
(Stewart Pinkerton) wrote in message ...
On 19 Jan 2004 00:49:24 -0800, (Svante) wrote: Still, IMO, the SPL is derived from the SIL, not the other way around, since the Bel is fundamentally a power ratio measure. NO, it was *not* originally a power ratio measure. *Fundamentally*, it is a measure of the difference in sound pressures which we perceive as a doubling of loudness. Ok, I cannot give the reference to the original or fundamental definition of the dB, and unless you can, we should settle with that we think differently. |
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#279
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Distorsion percentage, power or voltage?
(Stewart Pinkerton) wrote in message ...
On 19 Jan 2004 00:49:24 -0800, (Svante) wrote: Still, IMO, the SPL is derived from the SIL, not the other way around, since the Bel is fundamentally a power ratio measure. NO, it was *not* originally a power ratio measure. *Fundamentally*, it is a measure of the difference in sound pressures which we perceive as a doubling of loudness. Ok, I cannot give the reference to the original or fundamental definition of the dB, and unless you can, we should settle with that we think differently. |
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#280
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Distorsion percentage, power or voltage?
(Stewart Pinkerton) wrote in message ...
On 19 Jan 2004 00:49:24 -0800, (Svante) wrote: Still, IMO, the SPL is derived from the SIL, not the other way around, since the Bel is fundamentally a power ratio measure. NO, it was *not* originally a power ratio measure. *Fundamentally*, it is a measure of the difference in sound pressures which we perceive as a doubling of loudness. Ok, I cannot give the reference to the original or fundamental definition of the dB, and unless you can, we should settle with that we think differently. |
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