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The sound of speaker cables

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4 minutes ago, MartinC said:

The dB values plotted must represent something.

Quote: "The ordinate, or y axis represents the voltage dropped across the cable at the relevant frequency. The dB scale is relative and is a feature of the Room Equalizer Wizard (REW) when used as a spectrum analyser, to measure voltage with respect to frequency in real time".

So unless I've missed it, it's actually meaningless. You cannot correlate voltage directly to decibels and it's hardly reasonable to show a voltage on a decibel scale without any indication of the 'conversion'.

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7 minutes ago, MartinC said:

The dB values plotted must represent something.

Yes, but if the something they represerepresent is the square root of F-all, then it isn't terribly interesting.

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Just now, Tony_J said:

Yes, but if the something they represerepresent is the square root of F-all, then it isn't terribly interesting.

Which is what I'm trying to establish. To sensibly critique the data we need to know what it actually represents.

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17 minutes ago, rabski said:

Quote: "The ordinate, or y axis represents the voltage dropped across the cable at the relevant frequency. The dB scale is relative and is a feature of the Room Equalizer Wizard (REW) when used as a spectrum analyser, to measure voltage with respect to frequency in real time".

So unless I've missed it, it's actually meaningless. You cannot correlate voltage directly to decibels and it's hardly reasonable to show a voltage on a decibel scale without any indication of the 'conversion'.

It would be sensible if, for example, what was plotted was dBC(f) = 20*log[Gs,C(f)/Ga,C(f)], where I've added the 'C' subscript to represent a specific cable. This clearly isn't what's plotted though.

I'd assumed not but just to check: you're not thinking of dB in the SPL sense are you? There's no suggestion that this is what the graph is referring to.

Edited by MartinC

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dB=20log(Vo/Vi) though I am not sure if that it is much help. 

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5 minutes ago, MartinC said:

Which is what I'm trying to establish. To sensibly critique the data we need to know what it actually represents.

In the absence of information to the contrary I'm putting my money on the sqrt of FA. :cafe:

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1 minute ago, Tony_J said:

In the absence of information to the contrary I'm putting my money on the sqrt of FA. :cafe:

Indeed, hence my first post in this thread to say that I believe what was shown in the graph was misleading... However as Max (presumably) has chosen to post to clarify what was actually done then I'd be interested to know.

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16 hours ago, MartinC said:

Indeed, hence my first post in this thread to say that I believe what was shown in the graph was misleading... However as Max (presumably) has chosen to post to clarify what was actually done then I'd be interested to know.

Yes, dB=20log(Vo/Vi). It is a standard way of expressing voltage ratios where the only interest is in the ratio, not the absolute value. It is universal in engineering. Here is a convenient calculator http://www.sengpielaudio.com/calculator-gainloss.htm  For example if you start with 1 volt and increase it by 6dB, the voltage is 2 volts. 

Speaker cables handle voltages from millivolts for very soft music to 100s of volts for very loud music. It is the ratio between the voltages is what matters.

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On 20/11/2020 at 12:04, bobovox said:

Richard, how does skin effect manifest? Is it a contributing factor in the reactance of the cable or is there more to it?

In fairness, the dummy load does attempt to model the variation in impedance of a “typical” speaker - granted different speakers will exhibit different impedance variation across the audio spectrum but it looks like a reasonable representation of a 2way speaker and it makes the test reproducible.

I am however puzzled by the characteristic impedance of the cables quoted in the paper - they are orders of magnitude higher than I would expect; typically in the bass region, you might allow 0.2 ohms for calculating an effective Qes in a box simulation. I have done impedance sweeps on a 5m length of cable, using an 8ohm resistor as a dummy load. After subtracting the impedance of the resistor (which was flat from 20Hz to 20kHz at very close to the quoted 8ohms) the impedance of the cable was pretty much flat at 0.2 ohms across the frequency range albeit with some increase in the top octave, although I doubt that it would have any audible consequence.

You are confusing resistance with impedance.

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18 hours ago, Speedskater said:

Note that the characteristic impedance is vastly different than the nominal 1 megahertz and up Radio Frequency Characteristic Impedance.

In short, the used the wrong formula for determine audio band characteristic impedance.

Transmission line theory starts with DC see attached. 

The application of transmission line theory is mandatory for electrical distribution at 50 and 60Hz. Attached is a video of the spark discharge of a transmission line as the stored energy in the electric and magnetic fields is quenched.

Here is a good intro to transmission lines 

Characteristic impedance at DC.pdf

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18 minutes ago, Townshend Audio said:

Yes, dB=20log(Vo/Vi). It is a standard way of expressing voltage ratios where the only interest is in the ratio, not the absolute value. It is universal in engineering. Here is a convenient calculator http://www.sengpielaudio.com/calculator-gainloss.htm  For example if you start with 1 volt and increase it by 6dB, the voltage is 2 volts. 

Speaker cables handle voltages from millivolts for very soft music to 100s of volts for very loud music. It is the ratio between the voltages is what matters.

With respect, your experiment claims to measure the difference between speaker cables expressed as the difference in the voltage drop along those cables. This is not a ratio and it would seem perfectly reasonable to assume that as this is the specific difference you claim has an influence on the sound. then the different voltage drops between the different cables is the vital piece of information.

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1 hour ago, Townshend Audio said:

You are confusing resistance with impedance.

No, DATS measures impedance and displays the variation in magnitude and phase from 20Hz to 20kHz. The points is that this exercise showed that there was little variation In the impedance of my 5m length of speaker cable across the audio spectrum and was close to the DC resistance at all frequencies of interest. Therefore, I conclude that the speaker cable cannot have significant effect on the tonality of my speakers.

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1 hour ago, Townshend Audio said:

Yes, dB=20log(Vo/Vi). It is a standard way of expressing voltage ratios where the only interest is in the ratio, not the absolute value. It is universal in engineering. Here is a convenient calculator http://www.sengpielaudio.com/calculator-gainloss.htm  For example if you start with 1 volt and increase it by 6dB, the voltage is 2 volts. 

Speaker cables handle voltages from millivolts for very soft music to 100s of volts for very loud music. It is the ratio between the voltages is what matters.

I'm sorry but you haven't answered my question there. As you'll have seen from my other posts I am familiar with the general equation for expressing voltage ratios in dB. What I was looking for was the specific equation used to calculate what is in your graph. To me the logical thing to be plotted would be dBC(f) = 20*log[Gs,C(f)/Ga,C(f)], where I've added the 'C' subscript to represent a specific cable. Surely this can't actually be what's plotted though as the voltage drops can't possibly be as large as your graph would indicate. 

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On 20/11/2020 at 07:04, bobovox said:

In fairness, the dummy load does attempt to model the variation in impedance of a “typical” speaker - granted different speakers will exhibit different impedance variation across the audio spectrum but it looks like a reasonable representation of a 2way speaker and it makes the test reproducible.I am however puzzled by the characteristic impedance of the cables quoted in the paper - they are orders of magnitude higher than I would expect; typically in the bass region, you might allow 0.2 ohms for calculating an effective Qes in a box simulation.

The 0.2 Ohm or less is the end-to-end resistance or impedance of the conductors. The Radio Frequency Characteristic Impedance, is almost theoretical, it's the conductor to conductor impedance of the cable at radio frequency. Once calculated, a resistor of that value can be placed at the far end of a short cable and the cable will appear infinity long.

Above a megahertz, it works well, but at audio frequencies and cables of reasonable lengths it gets real messy. Oh, a century ago, radio broadcast networks developed ways to send radio shows from city to city on telephone wires. But for audio cables less than a mile long, it's not a factor.

By the way, for reasonable speaker cables, skin effect does not enter the picture. It gets swamped by other factors.

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