Rational Acoustics



Ferrit37
August 31st, 2010, 05:23 PM
Hey Guys,
I've recently been mulling this over :confused:

ABSOLUTE POLARITY - when instruments produce compression waves, loudspeakers must speak likewise. (notice I didn't say phase :rolleyes: )

With our measurement systems its reasonably easy to verify this, does anyone practice this? Does it make a difference?

I know we spend a lot of time getting "everything to play-well-together" subs/mains/delays etc, should this be the final step?

Arthur Skudra
September 1st, 2010, 12:15 AM
Ok, major can of worms opened! I guess you can be 50% right, 50% wrong! :p There is the AES26 standard, not sure if everyone follows it! In section 10.1.3 Multiple-transducer loudspeakers:
"NOTE The characterization of polarity in multiple-transducer loudspeakers can be complex at best, and indefinable at worst."

The ProAud listserve had an extensive discussion about this last April (Audibility of absolute polarity), and to summarize a few points I found interesting (though more from a studio perspective):
1. Under some circumstances, a polarity reversal can be heard when the entire system is switched, but coming to a consensus on which way is right is a different matter, especially when it comes to music reproduction! Some say it doesn't matter.
2. Very few loudspeaker systems are absolutely linear in it's response, eg. it's not equally easy to push the cone vs pulling the cone, unless we're using flat electrostatics for our speakers...
3. The diffuse field in the room may "cloud" our perception on what polarity is correct. Unless you mix in an anechoic chamber...
4. The nature of some musical instruments makes getting the right polarity questionable. Take for instance the grand piano. Sound coming above the strings and reflected off the lid may have a different relationship versus sound coming underneath the piano, reflected off the floor. We listen to the combined response of the two. Which is correct, and at which octave? A drum may be easier to get the "right" polarity, but even then, what if we double mic the snare and invert the polarity of one? Which is the correct polarity configuration?
5. Consider the asymmetrical nature of our hearing! Stanley Lipshitz did some study on this, he equates our hearing mechanism to a half-wave rectifier!

I love what one person said in the thread... "Most speakers suck, and most music blows" :D

Certainly worth a double blind test using both speech and music to test whether we can tell whether the entire system is in polarity or not under controlled conditions! Perhaps with speech, yes, but with music, probably not.

Ferrit37
September 1st, 2010, 02:35 AM
Hey Arthur,
Yep I've been double blind testing (ABX) for a couple of days now.

1. assymetric test signals on headphones easy, not so easy on speakers (probably not linear enough)

2. music tests on headphones, probable, on speakers- inconclusive

3. I'm confused and the dogs might bite me if I continue (the wifes already left) :)

I'm sure we've got bigger fish to fry but interesting nontheless

Dr. J
September 1st, 2010, 12:59 PM
I am waay under experienced for this but I know in principal that my system needs to be in polarity (+). I do know that some horns are reversed because it takes less delay to align them than it does with the horns wired correctly.

BUT -- Can a room change the polarity (acoustically in some way) within a verified known "In-polarity" system?

Just a crazy thought.....

Ferrit37
September 1st, 2010, 01:26 PM
hey Dr J,
It's all about direct sound/first arrivals.

quote: A frequent argument to justify why phase distortion is insignificant for material recorded and/or reproduced in a reverberant environment is that reflections cause gross, position-sensitive phase distortion themselves. Although this is true, it is also true that the first-arrival direct sound is not subject to these distortions, and directional and other analyses are determined during the first few milliseconds after its arrival, before the pre-dominant reverberation's arrival. Lipshitz et al. do not believe that the reverberation effects render phase linearity irrelevant, and there exists confirmatory evidence."
from Daisuke Koya "Aural phase distortion detection", masters dissertation, May 2000

Dr. J
September 1st, 2010, 04:37 PM
Awesome! Thanks Ferrit!