Absolute Polarity

[PheelTheMusic]

:::Probably opening up a can of worms here :::

The science seems to say (fairly conclusively) that any given independent audio source should sound exactly the same as it's reversed polarity sibling.

However, alot of people on this board whom I highly respect strongly disagree and assert that any reproduced wave should begin in the same direction as the wave of the original acoustic source to "sound right".

I have generally been a believer in the "science" logic until last night when I did some playing and listening for myself. In a recorded vocal I could hear a clear and repeatable change in tonality between the original recorded source and a "phase" inverted version.

I am going to perform the same experiment tonight but combine the sources in a null test. I fully expect them to null. If they do, how the heck do I hear a difference???

[Ben B]

It depends on how you're doing this test. If you're just playing the track, and reversing the polarity while it's playing, you're probably not "hearing" a difference -- rather, you're experiencing a physical sensation of sudden pressure reversal in your ears.

Do this: Duplicate the track, and reverse the polarity of the duplicate. Have someone else play the files from random starting points, not telling you which "version" you're hearing. Unlikely you'll be able to reliably tell which is which under these conditions.

-Ben B

[Dan]

I think it would make a difference in how the signal combines with other signals. Are you soloing, and hearing a difference?

[ed97643]

Absolute polarity corupts absolutely!

(Sorry, couldn't help myself! : )

I'm in the camp that believes that a blind test as described above would prove that one couldn't tell the difference... but I have been wrong before...

[Bob Olhsson]

Originally Posted by PheelTheMusic

The science seems to say (fairly conclusively) that any given independent audio source should sound exactly the same as it's reversed polarity sibling.

What science is this? The Bell Labs folks found numerous physiological reasons why it should not!

[Ethan Winer]

Originally Posted by PheelTheMusic

The science seems to say (fairly conclusively) that any given independent audio source should sound exactly the same as it's reversed polarity sibling.

In my experience any audible difference is probably due to asymmetrical loudspeaker nonlinearity. That is, the speaker itself responds differently for different wave directions.

--Ethan

[narcoman]

Originally Posted by Ethan Winer

In my experience any audible difference is probably due to asymmetrical loudspeaker nonlinearity. That is, the speaker itself responds differently for different wave directions.

--Ethan

Partially, and true of ALL speakers - but also the other side of it is that transients will be forward or backward pushing , depending upon polarity. Sitting in my room (which has very good acoustics - ought to have, cost me enough) I can hear the difference quite easily.

[andy_simpson]

Originally Posted by Ethan Winer

In my experience any audible difference is probably due to asymmetrical loudspeaker nonlinearity. That is, the speaker itself responds differently for different wave directions.

--Ethan

Brass instruments often show extremely asymetrical waveforms when recorded, so in this case it could be interesting to reverse polarity.

I'm sure there were some 'listening tests' done involving a trumpet recording, but I can't remember who did them.

Andy

[matt thomas]

I was recently converted to the school who knows that absolute polarity makes a difference. I made a saw tooth wave and flipped its polarity every second or so, there was an obvious difference.

but I'm with ethan, I think its at least mostly down to speakers not reacting equally in either direction

I also think it will only be apparent for certain shaped waveforms (the sawtooth in my test was obviously an extreme example), and how apparent it is will depend on the speakers of the listener.

narco

[theblue1]

Originally Posted by PheelTheMusic

:::Probably opening up a can of worms here :::

The science seems to say (fairly conclusively) that any given independent audio source should sound exactly the same as it's reversed polarity sibling.

However, alot of people on this board whom I highly respect strongly disagree and assert that any reproduced wave should begin in the same direction as the wave of the original acoustic source to "sound right".

I have generally been a believer in the "science" logic until last night when I did some playing and listening for myself. In a recorded vocal I could hear a clear and repeatable change in tonality between the original recorded source and a "phase" inverted version.

I am going to perform the same experiment tonight but combine the sources in a null test. I fully expect them to null. If they do, how the heck do I hear a difference???

Try it with a true double blind testing methodology (a big PITA, no doubt)... that's the only way you can be sure your imagination is not playing tricks on you and that you really can tell the diff.


That said -- whether or not one can possibly tell the diff with his naked ear -- I figure there's no good reason to invert polarity on a single mic in most imaginable cases so why not capture the sound 'correctly' rather than with inverted polarity?

That just seems like a common sense approach. (Of course, if I was miking a drum head from both sides I would probably start off with the two mics in opposite polarity. Whether it stayed that way would depend on a number of factors, primarily the resulting sound. I would also be likely to change physical position as well -- after all, the relative distance of the two mics from the skin will have much to do with the phase relationship of their signals. And, of course, any multi-mic drum setup is a complex beast. I start in one place and work 'til I've got what I want. [If I recall correctly... ... my time in other folks' studios is long gone, for the most part.] )

[O.F.F.]

You might like to try different speakers as well.
The only speaker manufacturers to make statements about absolute polarity I know of are Tannoy and JBL. Both produce woofers which move into the box when faced with a positive going voltage rather then out. Both claim that this is to maintain 'absolute polarity'.

[Ethan Winer]

Originally Posted by andy_simpson

I'm sure there were some 'listening tests' done involving a trumpet recording, but I can't remember who did them.

There was a report a few years ago in the AES journal with the headline claiming that absolute polarity is audible. But when you read the article, and especially the conclusion, the authors admitted it was barely perceptible and only on certain types of program material.

Originally Posted by narco

I made a saw tooth wave and flipped its polarity every second or so, there was an obvious difference.

Yes, and I've done that too. But I hear no difference except at very low frequencies, where loudspeakers and headphones are not as linear. What frequency did you try?

Even if absolute polarity is barely discernible in a few situations, to me it seems a non-issue. Versus distortion or rolled-off highs etc that really do impact quality.

--Ethan

[dbbubba]

Take a kick drum, play it at a decent level where you are moving the low freq. driver a fairly significant amount and see what it sounds like.

Remember that I deal with fairly high wattage audio systems.
The one I am sitting next to at this very moment has 6Kw of power per sub cabinet.
Also, take into consideration that they drivers are arranged in a cardioid array.
This is all designed for a VERY specific response.
Change any variable and the outcome is different,

Do you think that if reverse the phase of a low freq., but steep transient attack signal that the drivers sucking IN instead of out is going to produce the same audio experience?

When dealing with a sustained and constantly shaped waveform it might be hard for any one to tell the difference without clues that anything is different.

If you take into consideration steep transient attack waveforms at substantial levels (even at commonly accepted near field levels) you WILL hear a difference.

[matt thomas]

Originally Posted by Ethan Winer

Yes, and I've done that too. But I hear no difference except at very low frequencies, where loudspeakers and headphones are not as linear. What frequency did you try?
--Ethan

Hi Ethan

I can't remember the exact frequency, I made an audio file from a sawtooth wave from a middle-of-the-keyboard note in a soft synth. Then I cut a one second bit out of it, made a copy and flipped the phase of the copy, and made a repeating two second loop of the two samples.

I was listening on my lipinski 707's, which should be very accurate speakers. There was an obvious (but hard to describe) difference, from memory it was in the mids and sounded possibly like different harmonic content. It wasn't huge, but it was definitely not an "I-think-there-might-be-a-differnce" type difference.

I would redo the test and post the sample here, but I sent my converters in to get repaired today, perhaps someone else could do it?

narco

[Francis Vaughan]

As one might gather, there is good reason for thinking either way. At reasonable levels there is no mechanism in the ear that can discern absolute polarity. It simply isn't how the ear works. So from that point of view a purist approach says that absolute polarity cannot matter. But between the sound source and your ear there can be non-linear processes that are polarity dependant. So these can change the harmonic content of the sound, and that you can hear.

As Ethan says, - speakers are asymmetric devices - just look at a cone - it isn't symmetric about the plane normal to its movement. A high energy pulse will distort the cone in a different manner moving one way to the other. But you do need a significant force to do this.

Curiously, air itself is asymmetric - trivially - if the sound is loud enough that the high pressure half of the cycle reaches 2 atmospheres the low pressure side must be a vacuum. You can't get less air than this so the air clips. (The gas laws result in asymmetric distortion long before this point - but either way we are talking sound pressure levels that would have your brains running out of your ears as a thin grey goop.)

And of course sound processing devices are asymmetric - a single ended class A device that is driven hard is intentionally so - and so will respond differently. So within a recording chain there should be no surprise that there are elements for which absolute phase matters.

The difference is however clear - you can't directly hear absolute phase - but you can hear the effects of absolute phase.

[jigsawlogic]

Well, depending on the size of her lips, the sound of suck just MIGHT be different from the sound of blow.

But what do I know? I'm married.

[Bob Olhsson]

Originally Posted by Francis Vaughan

As one might gather, there is good reason for thinking either way. At reasonable levels there is no mechanism in the ear that can discern absolute polarity. It simply isn't how the ear works. ....

Not according to the Bell Labs folks!