The first question to address is whether polarity reversal is EVER audible. Many people believe it is not, but they are mistaken.
After a little practice I found that quite easy. It sounds like one tone has a higher frequency than the other. Now let's say on a particular trial A sounds higher pitched than B. Here's where it gets really interesting - my headphones (Grado's) can be rotated so that the speakers are facing outwards, away from the ears, rather than inwards. When I wear them like that, I can still hear the difference (10/10 correct on ABX), but now B sounds higher than A!This result has several implications. First of all, it proves that the difference I'm hearing is in my ears, not to anything in the playback chain like the computer or the phones. If the difference was due to distortion in the phones, for example, inverting them wouldn't change anything.
So polarity reversal is audible on certain asymmetric test tones, and the likely explanation is that the ear/brain acts to a first approximation as a half-wave rectifier.
However there are two major problems that arise the moment we consider a more realistic situation.
That would have been a post I made a couple years ago with a very similar-sounding name. Also I linked that post a couple weeks ago on Audio Asylum, thereby igniting a tinderbox of controversy between the always-easygoing Clark Johnson and, well, everybody else.
Phase correctness in mastering and playback is a matter of standard. Its so trivially easy to maintain, that there is no justification whatsoever to phase reversal in the recording chain. I consider this part of the matter irrelevant. Incorrect phase on a recording is simply huge label of disgrace for the recording company. Its a sign of negligent attitude. What quality can you expect from a recording company that can't even get its phase (wires) right?
I don't see how this is true. If even-order harmonic distortion existed in the phones, it could either increase or decrease the signal's assymmetry, depending on its orientation. Given the ear's sensitivity to assymmetry, your results can be completely explained by distortion in the playback chain.
This was the biggest counterargument to my original post - that I was able to get stunning ABX results, but it could only be reproduced at extremely loud volumes (which naturally makes distortion more likely), it was better on some headphones and sound cards and not others (possibly reflecting different levels of distortion), etc. Asymmetric limiting distortion, being a form of even-order harmonic distortion, can happen (perhaps quite often) for amplifiers and transducers. If one polarity was simply clipping more than another, than clearly it would be far more audible than if the clipping was equal in both polarities.QuoteSo polarity reversal is audible on certain asymmetric test tones, and the likely explanation is that the ear/brain acts to a first approximation as a half-wave rectifier.I don't think anybody has come forward with a physical model for the audibility in the literature; your explanation is as good as anybody's. However, I strongly suspect that the behavior exists as an assymmetric soft-limiting effect rather than a rectification. That is, one pressure polarity reproduces more weakly relative to the other pressure polarity, the stronger the pressure wave is.
QuotePhase correctness in mastering and playback is a matter of standard. Its so trivially easy to maintain, that there is no justification whatsoever to phase reversal in the recording chain. I consider this part of the matter irrelevant. Incorrect phase on a recording is simply huge label of disgrace for the recording company. Its a sign of negligent attitude. What quality can you expect from a recording company that can't even get its phase (wires) right?is just misguided - when more than two mics are involved in making a stereo recording, what does phase reversal even mean?
Another comment - it's not true that you can't reverse the polarity by moving around a source. If the source is a dipole, moving 180 degrees around it exactly inverts the polarity.
So I think these results can only be explained by polarity sensitivity in the ear. Since turning the phones around exchanges compression for rarefaction, that works as expected.
I was able to do this at medium to low volumes, making distortion less likely (and see above). Actually there are models for the ear like this in the literature - I think half-wave rectifier is more or less the standard starting point (obviously that's oversimplified, so people replace it with some smoother function that's qualitatively similar). I can dig up a web reference if you're interested.
That's true, but very few things in life seem like "true" dipoles to me. If a sound source has the same absorbtion/reflection behavior for every two opposite directions, then you're right, but very few things in reality behave like that. Off the top of my head, the only sound sources that qualify would be:Only certain kinds of speakers. They must be open-back. If multiple drivers exist, they would have to be coplanar and the drivers would have to be symmetric on that plane (ie a circle of tweeters around a single woofer). Minimal driver housing on the rear side.Percussion where only one percussive surface exists, and it's flat.Everything else - vocals, strings, etc - would have some irregular change when turned around, that won't quite be a real polarity reversal.
So I think these results can only be explained by polarity sensitivity in the ear. Since turning the phones around exchanges compression for rarefaction, that works as expected.Yeah, I think I might have not completely understood what you were doing. I can't find a particularly good counterexample that would cause the same behavior you're seeing but exist entirely in the playback-distortion domain. So I can no longer dispute this method (for now ). Very clever too.
Porcupine,For all your "I think I hear this but didn't ABX" posts, there must be more friendly forums for you! That's not a criticism - I just wonder why you enjoy the inevitable pain that comes from constantly ignoring this forum's founding principle?!As for speaker drivers out of phase - if the signal, as delivered to the cones, was out of phase between tweeter and mid, then that would cause horrible destructive interference in the cross over region. This really can't be the case, surely? I'm speaking outside of my knowledge - I did some of the theory of analogue cross overs a long long time ago, but the only ones I ever worked on were digital, so of course all drivers were very much in phase, and sometimes phase corrected.Cheers,David.
The phase of the reflected sound waves from hard surfaces and the reflection of string waves from their ends determines whether the interference of the reflected and incident waves will be constructive or destructive. For string waves at the ends of strings there is a reversal of phase and it plays an important role in producing resonance in strings. Since the reflected wave and the incident wave add to each other while moving in opposite directions, the appearance of propagation is lost and the resulting vibration is called a standing wave.When sound waves in air (pressure waves) encounter a hard surface, there is no phase change upon reflection. That is, when the high pressure part of a sound wave hits the wall, it will be reflected as a high pressure, not a reversed phase which would be a low pressure. Keep in mind that when we talk about the pressure associated with a sound wave, a positive or "high" pressure is one that is above the ambient atmospheric pressure and a negative or "low" pressure is just one that is below atmospheric pressure. A wall is described as having a higher "acoustic impedance" than the air, and when a wave encounters a medium of higher acoustic impedance there is no phase change upon reflection.On the other hand, if a sound wave in a solid strikes an air boundary, the pressure wave which reflects back into the solid from the air boundary will experience a phase reversal - a high-pressure part reflecting as a low-pressure region. That is, reflections off a lower impedance medium will be reversed in phase.
Even if there were open doorways that created reversed-polarity reflections, your ear can still hear the absolute phase of the direct initial wave.
Only with recordings made with a single mic per channel...could some semblance of polarity be preserved.
One last thing. Make sure you don't confuse "polarity inversion" with "phase." One is electronic while the other is based on "time shift." Although polarity inversion (phase flip) can correct the SYMPTOMS of phase shift, it does it by re-polorizing the signal, not by shifting it's timeline.
Hmm, I just have one questions with regards to this - on a speaker (like from an old stereo system) is the wire with the marking + or - ? One is plain and one has a black line down it. There doesn't seem to be a way to open these to see if there are any markings on the speaker, so I'm hoping there's an industry standard on this sort of thing.
Part of the problem here is no one knows what makes polarity audible - is it the transient at the start of a note, is it asymmetry during the steady-state part of a tone, is it harmonic distortion in the playback system, is it audible at all on real music? If it's the transient at the beginning of a note and you want to preserve that, then flipping the polarity switch on any mic is bad. Instead if you have a problem with phase cancellations you should either move the mic or time-shift the recording by 1/2 a period. If on the other hand it's a steady tone issue, then flipping the switch is fine.