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Old 6th August 2002, 05:18 PM   #1
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Default Question about phase and chair placement

I was wondering about our ear's ability to discern phase correctly.

My first problem was with the large variations in frequencies that our magic ears can hear. A large range in frequencies neccessitates a large range in wavelengths.

Here are some numbers:
Speed of sound in air = 343m/s
Freq range = ~20-20000hz
Wavelength range = ~16m - 1.6cm

This means that if you move your listening chair a mere .8 cm forward, you've completely changed the phase of the 20khz frequencies compared to the 20hz. I know this is being picky, but this makes me question our ability to hear phase differences at all. If such a small motions of our head can make large differences in the phase of the sound we are hearing, when does it start to matter? I've never noticed a difference between leaning forward or leaning backward while listening to music, a difference of about 20 cm. This 20 cm changes the phase of 850hz by 180 degrees, while changing 1700hz by 360 degrees. You'd imagine that such a change would be noticable.

Any absolute polaritists want to speak out?

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Old 6th August 2002, 11:43 PM   #2
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This is probably one of the best questions I've read here on the forums. Very, very perceptive of you to ask it.

It's now been 6 hours since you posted it and no one has tackled it.

Before I answer it for you I'm going to hang back and see if anyone has a go at it.
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Old 7th August 2002, 12:29 AM   #3
Keld is offline Keld  Sweden
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OK I'll have a go on it

First of all the most important: whatever the frequence sound travels at the same speed. If you lean your head forward the hi and low have the the same phase diff as it will have when the sound reach the place where your head was supposed to be (uppright position) some millisec later.

do you understand ??

so the sound is the same in a relative way but you're older, shorter of breath and one soundwave closer to death

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Old 7th August 2002, 02:41 AM   #4
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[quote]so the sound is the same in a relative way but you're older, shorter of breath and one soundwave closer to death /[quote]

Too true

When I did my degree in psychology in the early 80s I was very interested in psychoaccoustics, and read many a paper on the subject. None, repeat, None, showed any proof that absolute phase is perceived by the human ear, phase differences between the two ears are only used for the localisation of sound in space, and that mostly only in two dimensions, as the human ear is geared not to notice up and down, only left and right, and peceptual cues are used to differentiate height, ie bird song is up and scratching sounds are usually on the floor, showing you have a mice infestation!

If you lean your head forward the hi and low have the the same phase diff as it will have when the sound reach the place where your head was supposed to be (uppright position) some millisec later.
Sorry Keld, but phis is correct, phase does change between positions, proportionately by the difference in frequency between the two wavelengths, this is one of the perceptual mechanisms the human brain uses to locate a sound source.

Now normally at this point I would duck and cover, but hopefully with the new regime in place, all I will get is constructive critique
Rick: Oh Cliff / Sometimes it must be difficult not to feel as if / You really are a cliff / when fascists keep trying to push you over it! / Are they the lemmings / Or are you, Cliff? / Or are you Cliff?
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Old 7th August 2002, 03:48 AM   #5
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Rod Elliot had a good series of articles about phase relationships and their audibilty. Basically, our ears care for harmonic content only, and not their components phasing, so we can only hear changes in phase when they create interference effects f.ex. (cancellations and such).

Having said that, i LIKE to have the minimum phase shift necesary on my audio circuits, even if all that is ruined by the internal crossover of my speakers. Such is life.
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Old 7th August 2002, 04:34 AM   #6
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Come on you guys. Let's get back to the original question.

Or, Is this another post that goes off on a tangents?

The answer has nothing to do with phase, phase relationships, absolute phase, frequency, the speed of sound, psychoacoustics, wavelengths or localization.

Again, phish, I must say this is a most excellent question and I really hope it will get people to think.
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Old 7th August 2002, 04:39 AM   #7
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When the hunter-gatherer came down from the trees he was always on the move. It was very important for him to know whether the thing he was hearing was moving or if he was moving and the thing was fixed in place.

You are moving, the speakers are fixed, the ear-brain knows how to deal with things when you move your head. Now if you clamped your head and had someone move the speakers...

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Old 7th August 2002, 05:08 AM   #8
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You knew my answer I don't beleive in phase audibility, so *I* don't care. Yet, i like to keep my signals as "in-phase" as posible.

As for moving the speakers, its the same deal. Your brain will make it sound like the music is moving, but it will STILL be audible as music. It might change (doppler effect, frequency response change because of room, etc) but that'd be in no way related to the phasing of the harmonic content.
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Old 7th August 2002, 07:07 AM   #9
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Hi phishead8

This is indeed a good question you asked.

The main fact is that our hearing system doesn't use phase to determine the direction of a sound source. It mainly uses the TIME DELAY between the ears to do so (I.e some other effects are also used but to a lesser extent).
Because of this, it doesen't matter that much from a first view, if you move your head back and forth while listening.

Our hearing system is performing a temporal correlation process on the lows and highs of the signals it receives. The more they correlate the more exact we can localize the source of an acoustic event.
According to John Watkinson our hearing system is able to distinguish inter earal time-differences of about 16 microseconds !
The signal localisation is performed on transients because with a steady state signal you don't have any unique time relationship between the left and the right ear anymore. If you'd use phase to detect direction by using only two receivers (in our case our ears) then the result would always be ambiguous.
Just try to to localise a sound source with sinusoidal character and steady level in a reverberant room and then try to localise somebody who is speaking in the same location. Guess which one you can do more easily ?

But there IS a relationship between phase and time as you have already noticed.
If your reproduction system has too much phase DISTORTION (i.e. the phase isnt't shifted proportional to frequency anymore) it will disturb the temporal alignment of lower- and higher frequency contents of transients (and music contains a lot of it -- at least I don't listen to steady sinusoidals but I don't know anybody else's taste so far).
To say it once again: detection of phase doesn't give you an exact direction of an acoustic event but phase distortion in a reproduction system would blurr the localization of sound sources.
This is one reason why some people prefer to listen to single broadband drivers: The absence of a crossover and the single source caracter introduce less phase distortion and give better imaging.

There was a good series of articles in Electronics World and Wireless World on that subject, written by John Watkinson. You can find some info in short form on this subject (amongst other stuff) written by him also under:

If I was unclear in any way don't be afraid to ask.
Anybody who disagress is also invited to shoot at me (In the end life would be boring if everybody always agrees to everybody).


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Old 7th August 2002, 09:23 AM   #10
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I think it was the late Neville Williams in Electronics Australia magazine who was discussing the left to right phase difference of early (then new) CD players that because of cost, only had one D/A converter. Apparently it was rapidly switched back and forth between the two channels and as a result one channel always ran slightly behind the other. The remedy he sugested was to "collectively move your [heads] to the left about an eigth of an inch!" Sounds about right for 1/44,000 of a second phase difference.

Also, once while messing around with a dc/dc converter using ferrite potcores that ran at about 3.5 kHz and as a result made quite a piercing squeal, I noticed that if I moved my head slightly it made quite a difference to the perceived loudness of the sound. If I moved around the room there were very noticeable live and dead spots. The converter ran with a square wave and so the resulting sound from the ferrite core magnetostriction would have had lots of harmonics that would have added and subtracted as the listening position was changed.

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