Dipole woofer nonlinear distortion

[gainphile]

Hi all,

This is my measurement of the popular eminence Alpha 15A. There are few caveats: the signal is already EQ'd (meaning the driver is being pushed more below its Fs) and measurement was taken in a small room. It's surprising that given the clean and deep output which OB bass can provide the measurement shows significant distortion at the extreme lows, even for a 15" driver.

What got me interested to do these measurement was when I feed 20Hz signal to my speakers, I can hear tones ! ... now that's not right as 20Hz shouldnt be audible

Hope you can add to this 'library' measurements of other popular woofers. We NEED clean bass - measured and listened



First test, 20Hz tone. 3rd order distortion dominates and this is not good as the ear gets more sensitive (fletcher-munson curve).





Second test, 30Hz. Signal to noise ration is significantly better and 2nd harmonics is equal with 3rd harmonics.





With 40Hz signal feed, the woofer seems to be performing very well. Note that Fs of the Eminence is 41Hz.





This is the overall plot.




Learning how simple these measurement activity was, it makes me wonder why we rarely see nonlinear (and linear) distortion taken into design or review -- SL and Zaph being the few who emphasize them.

[gedlee]

My position on this is that simply measuring the THD does not mean that its audible under real world signals or in real world conditions. In most of my work I find that the signal is measurably distorted, but its not audible.

To really discuss this topic however we would need to know some more specifics about the test conditions. This is nearfield? Whats the signal level? Whats the enclosure design?

[Drew Eckhardt]

Monte Kay has woofer distortion measurements on his site, with comments on applicability for dipole applications due to mechanical and aerodynamic noises.

http://www.mfk-projects.com/woofers.htm

[BHTX]

Originally posted by gainphile
What got me interested to do these measurement was when I feed 20Hz signal to my speakers, I can hear tones ! ... now that's not right as 20Hz shouldnt be audible

I've always noticed this as well, particularly with woofers at the lowest frequencies, just as you've realized. Whether they're in boxes or not, a 20 Hz sine wave often gives a strong 40 and/or 60 Hz tone. Not completely certain about high frequencies which Gedlee has experimented with, but at very low frequencies it's VERY audible. Isobaric configuration of woofers in enclosures supposedly helps this a bit.

Originally posted by gedlee
Whats the enclosure design?

Originally posted by gainphile
OB

[gainphile]

Drew, thanks for that great link. I see similiarities between my measurements and his, so some things are right.

Earl, that was actually my experience. While had been listening for months with no complaint on LF distortion, this was an eye-opener as feeding 20Hz tone produced very audible 60Hz sound and was verified by the measurement. I suppose ear cannot discriminate absolute distortion but if theoretically we can have a 0 distortion speakers, I think these would stand out.

Imagine if manufacturers or reviewer include these data along with FR plot, it would be much easier to evaluate speakers, is it not?

Btw. the setup was nearfield measurement with OB setup, signal was equalised using SL shelving lowpass down to 11Hz. I may want to do measurements on unequalised signals later on.

[gedlee]

Originally posted by gainphile
Earl, ... but if theoretically we can have a 0 distortion speakers, I think these would stand out.

Imagine if manufacturers or reviewer include these data along with FR plot, it would be much easier to evaluate speakers, is it not?

I think that its not time well spent running after things that don't make any difference, like THD in woofers.

But to get into more detail its critical to next consider the bandwidth that the woofer will have because the greater the bandwidth the more audible the nonlinearity will be. An OB with EQ is a worst case scenario for LF distortion and I would definately NOT take a woofer in that configuration up very high in frequency.

A bandpass design, on the other hand, will have the harmonics acoustically LP filtered and the bandwidth is minimal so nonlinearity is not ever a factor. Its all about the system design as to whether or not these things are a problem.

[cuibono]

gainphile, thanks for doing that - I am planning on doing just the same with my Alpha 15s in H frames soon. Your measurements look about right to me. I would expect a lot of distortion from this driver. But in my system, it sounds pretty good, even considering our ears increased sensitivity to the higher harmonics. I'm a fan of Monte Kay's site, he has the only set of woofer distortion measurements I know about. I'm hoping to build another set of H frames soon, using the RSS315 he suggests.

[Moondog55]

Wish I had a bigger vehicle, I'd run over with my big baffles when they get finished and have you measure them, DEAF old B****r that I am I can't hear anything that low

[CLS]

Originally posted by Moondog55
I can't hear anything that low

Maybe, but you can certainly FEEL it by your whole body. That's also count in 'audio' experiences. For some, it's rather important.

[Moondog55]

OH!! I do like that feeling of having my "Liver quiver"


m\My comment was directed more at my ability to hear anything distorting or being able to hear any overtones