Sound Localization Discussion

[BassAwdyO]

I'm pretty new to diyaudio.com, but thought it was about time i started my own thread. I've browsed quite a few threads and havent found one truely dedicated to sound localization yet so i thought i might start one. Here's what i know. Sound localization occurs due to a likely 2 things. Intensity differences between ears, and phase differences. well intensity is a for sure, i havent exactly read anything that talked about phase though. Lower frequencys i dont believe can be localized audibly at all, but perhaps there is some physical localization. if bass gets intense enough you can feel it and perhaps where the force is coming from? I believe the secrets of the ear are truely what will lead us on to making better sound reproduction equipment. I've searched around a bit on the internet about the subject and havent exactly found an abundance of information, but i did once read a bit of a study done by the air force that assesed localization of sounds while wearing protective ear muffs. The air force has an awesome sound lab from what i saw. They had a geodesic sphere with like 4" bose full range speakers at each of the many many verticies. the whole thing was located in a anechoic chamber. It seemed very cool. a True version of surround sound perhaps. what does anyone know about localization?



I like to figure stuff out

[sreten]

As food for thought consider binaural recordings over headphones.

I disliked these intensely because everything seemed to be
coming from behind me, even a recording of a plane passing
overhead changed direction over my head.

I think subtle head movement in real life quickly sorts
out back to front along with of course visual cueing.

sreten.

[phase_accurate]

I think subtle head movement in real life quickly sorts

Additionally there is the HRTF (head reletad tranfer function) that helps with sound localistion, somrthing that is out of service when headphones are worn.

Sound localisation was discussed in a great series of articles in EW+WW, written by John Watkinson. The name of the series was "Stereo from all angles".
Some of the thoughts and conclusions can be found at :
http://www.celticaudio.com/

A common misunderstanding is that the ear uses phase-differences to determine the direction of a sound source. It is in fact time-delay that is used for that purpose. They are however related to each other. But one has to bear in mind that it is exactly possible to calculate phase difference for a given frequency and time-delay but the result will be ambiguous when the calculation of time-delay from a phase-difference is attempted.


Regards

Charles

[kspv]

I remember reading from Yakov Perelman's "Physics For Fun" in my childhood, that there is something about moving one's head in the direction of the sound, and sometimes one may actually get acoustically disoriented by moving his head, i.e., the source suddenly can no longer be pin-pointed. Yakov Perelman did this experiment with a chirping cricket. Anybody remembers that experiment?

[Svante]

Originally posted by phase_accurate



A common misunderstanding is that the ear uses phase-differences to determine the direction of a sound source. It is in fact time-delay that is used for that purpose. They are however related to each other. But one has to bear in mind that it is exactly possible to calculate phase difference for a given frequency and time-delay but the result will be ambiguous when the calculation of time-delay from a phase-difference is attempted.

So, if there is only one frequency played, and there is a phase difference between the ears, how does the brain figure out the time delay? I mean, the only thing that is available at the ears is the phase difference (for the stationary sinusoid).

[rakeshln]

Diffraction is one more factor.
At very low frequencies the sound waves diffract around our head so we cannot locate the sound source. This leads to near equal intensities at both ears.

Path length difference is not a factor for localization. Difference in path lengths lead to phase differences which become apparent as the frequency rises(wavelength reduces). Added to this is the diffraction reduction at mid and highs around our head which leads to intensity difference.

A 35ms time lag can be perceived by human ears according to Haas effect. It means a path length difference of abt 35 feet. Our ears are just abt 6-8 inches apart, i suppose.

[phase_accurate]

A 35ms time lag can be perceived by human ears according to Haas effect. It means a path length difference of abt 35 feet. Our ears are just abt 6-8 inches apart, i suppose.

The Haas effect is dealing with the perception and localisation of delayed sound sources, it has nothing to do with the inter-aural delay.

The accuray of interaural-perception is in the order of 16 us !!!

So, if there is only one frequency played, and there is a phase difference between the ears, how does the brain figure out the time delay? I mean, the only thing that is available at the ears is the phase difference (for the stationary sinusoid).

Some good thoughts. If you think a little further then you might come to a conclusion what implication this has upon spatial reception.
Just a hint: Ever tried to localise a continuous sinusoidal sound source and compared this to the localisation properties of human speech ?

Regards

Charles

[sreten]

Originally posted by Svante


So, if there is only one frequency played, and there is a phase difference between the ears, how does the brain figure out the time delay? I mean, the only thing that is available at the ears is the phase difference (for the stationary sinusoid).

The answer is not very well at all. Localisation of undistorted
single frequencies is very poor, the ear does work on time
differences not phase. The sound of a match being struck
for example is extremely easy to loacate, due to a rich
spectrum and consistent time delay between the ears.

If interested read about autocorrelation functions, as far
as I understand it this is a good analogy for starters as
to how the brain processes timing information, and a
pointer as to why the ear canal is a tranmission line.

sreten.

[phase_accurate]

The candidate has 100 points !

The ear does indeed use correlation between the different spectral parts of an acoustical event in order to achieve it's high resolution regarding the measurement of interaural time-delay.

It is also the starting transient that we use to perceive direction. So the START of a sinusoid would give us some hint about the direction of the source while a stationary sinusoid would give us the least information one could think of.
Ever tried to locate a sinusoid soundsource in a room ? This gets even more difficult in a reverberant environment, while it is a fairly easy task when the source is human speech (having a lot of transients over a wide spectrum) for instance, even in a heavily reverberant environment.

The correlation function has additional effect: Small sources (the match is a very good example) generate every spectral part at places that are closely located in space. Therfore all initial spectral content is spaced closely as well in terms of arrival time. Take a large instrument like a double-bass and things look different.
We actually use this temporal distribution of the initial spectral content (amongst others like loudness, fundamental frequency etc) to determine the size of a sound source.
Therefore the acoustic image of sound reproduction is blurred if insufficient care of transient reproduction is taken.

Regards

Charles

[geolemon]

Delay is the more correct term to use in this case, not phase, although the two are unfortunately used interchangably.

You can speak of delay with respect to a pathlength difference scenario, because all frequencies will be delayed in arrival time by whatever the speed of sound dictates, given a particular distance.

Or, in phase terms...
All frequencies travel at the speed of sound - regardless of wavelength.
180 degrees of a high-frequency wave (1/2 of a wave) would take less time to pass than 180 degrees of a low-frequency wave.

This thread is interesting, in that it's come down to all the dynamics that the ear uses to localize sound...
It's not just "phase or amplitude", but all the things related to the geometry of our own ears, and a lifetime of our brain taking in sounds, processing them, and learning about them.

There's a very interesting article that I read a while back, a "loudspeaker imaging theorum" that really seemed to nail it from the standpoint of recordings...
...basically, that since the ear used so many low level cues... even lower level cues than one might imagine - that it was important to record at this fine level of detail, play back at that fine level of detail, and have audio gear that could reproduce that fine level of detail... to really get that "is it live... or is it memorex?" effect that we're obviously trying to get to, here.

The article seems very much on-topic with respect to what we are talking about here, albeit with that slightly different bent:
http://diyaudiocorner.tripod.com/imaging.htm