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Focusworks Audio GUJ Driver Discussion

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I think you may be right about current-induced distortion (in BL or Le) for THD at high frequencies but not so much for low frequencies.  High frequencies generally involve a much higher ratio of power to stroke, so distortion related to power or current is likely to be much more important at high frequencies than low frequencies.

 

If we focus on low frequencies, then inductance shouldn't matter much if at all because it's too low, relative to Re.  I brought up the 18" vs. 21" comparison specifically to argue against current-induced BL distortion as being a big factor in the low end.  To reiterate, the 21" failed CEA testing at 12.5 Hz with 400W and 7A vs. 600W and 8A for the 18".  That is definitely not a consequence of current-induced BL distortion.

 

I do agree that voltage sensitivity is not really the right tool, but 1W power response is not exactly right either.  For motor distortion, whether it is caused by excessive stroke or excessive current, the effect of this distortion will be on the motor force and the harmonics in the motor force will be amplified relative to the fundamental according to the transfer function of motor force vs. output.  That transfer function is not the same as either the voltage sensitivity or the 1W power efficiency but it has characteristics in common with both.  For example, it rolls off at 12 dB/octave once you go low enough.  I am certain that this accounts for the rapid rise in THD with decreasing frequency below resonance, which is observed in measurements of almost every sealed system that I'm aware of.  It occurs even if you keeps excursion constant through your testing.

 

Your example of an efficient driver with rising high frequency response having lower high frequency distortion deserves more discussion.  Inductance definitely interest the picture, both as a source of distortion and a modulator of that distortion.  How much does a rising high frequency response in one driver vs. another have to do with high efficiency vs. low Le/Re ratio?  If you looked hard enough, I'm pretty sure you could find a driver with high upper frequency efficiency that still distorts badly up there because of Le problems.  Here's an idea for testing the hypothesis that distortion depends on efficiency at high frequencies:  After measuring a sealed system, add some mass to the driver(s) and re-measure.  The added mass will reduce the upper end efficiency, but will leave Le and BL completely unchanged.

 

Lastly, I *do* doubt that a weaker motor with identical geometry, coil, and soft parts will have higher distortion everywhere.  FunkAudio suggests that the opposite is true at resonance, and I agree.  Again, I believe this can be explained by the relative difference in the motor force vs. output transfer function at the fundamental and its harmonics.

 

Edit: I'll concede that there are probably cases where current-induced distortion matters at low frequencies too, but I think this will only happen when the boxes get real small.  In almost all the sealed Data-bass sub measurements, they fail CEA at power levels well below what they are able to take at higher frequencies.

 

 

This is a great observation and I just took a look at the two systems, its indeed interesting to see the 21 get edged out by the 18 and 12Hz. It almost does not make any sense because the 21 has better sensitivity, so why is it amplifying (or creating more) distortion than the 18 with lower power and lower current... hmmmmph.

 

That 3rd harmonic is really amplified over the 18" even at the lower voltage sweeps. This might be something as simple as a lead wire tapping the cone or the surround behavior but those are just guesses. This can not be explained with inductance or flux modulation since its making more distortion with less current/power.

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I just realized that SME might be supporting the myth that smaller drivers sound better  :D

 

oh shit, quick delete this thread!

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I just realized that SME might be supporting the myth that smaller drivers sound better  :D

 

oh shit, quick delete this thread!

The 21 would do better in a little bigger airspace, similarly as the 18 did if you look at the tests in small vs large box, proportionally to its cone size the 21 was in similar size as the 18's small box. Mind you the non distortion limited output is still real solid with 98db at 10hz, and in room with some room gain will increase the fundamentals output more than the harmonics thereby reducing the distortion, so usable in room output should be better. Even in the as tested box size, for ~300L I would still choose the 21 over the 18, for under 200 the 18. 

Also if you look at the long term output sweeps for 115db with them both in ~300L, the 21 was still putting out little more across the board with a tiny bit less distortion across the board

 

My thoughts on it are that it just takes a certain amount of BL to overcome the air spring of the box, determined by cone size to box size ratio(given similar excursion). If the drivers BL while linear is already closer to that minimum then it will work within the Geometric Xmax but as soon as the coil starts to leave the gap it just wont have the force to carry on and distortion will rise very fast as soon as it hits that point. If the ratio of BL vs air spring is higher to start then you will reach the minimum force required to keep moving the cone with more of the coil out of the gap, and distortion will go up slower as you get further and further out. 

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The 21 would do better in a little bigger airspace, similarly as the 18 did if you look at the tests in small vs large box, proportionally to its cone size the 21 was in similar size as the 18's small box. Mind you the non distortion limited output is still real solid with 98db at 10hz, and in room with some room gain will increase the fundamentals output more than the harmonics thereby reducing the distortion, so usable in room output should be better. Even in the as tested box size, for ~300L I would still choose the 21 over the 18, for under 200 the 18. 

Also if you look at the long term output sweeps for 115db with them both in ~300L, the 21 was still putting out little more across the board with a tiny bit less distortion across the board

 

My thoughts on it are that it just takes a certain amount of BL to overcome the air spring of the box, determined by cone size to box size ratio(given similar excursion). If the drivers BL while linear is already closer to that minimum then it will work within the Geometric Xmax but as soon as the coil starts to leave the gap it just wont have the force to carry on and distortion will rise very fast as soon as it hits that point. If the ratio of BL vs air spring is higher to start then you will reach the minimum force required to keep moving the cone with more of the coil out of the gap, and distortion will go up slower as you get further and further out. 

 

Yes, I think we're debating an interesting academic point but the 21 is by far the better option even in this case than the 18 for pretty much all cases and that is evidenced by the long term max output especially.  

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