Stereo .. how do we hear in stereo?

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Stereo .. how do we hear in stereo?

Postby Hugh-AR » 05 Mar 2017 15:39

Stereo .. how do we hear in stereo?

In the early days all the music we listened to was in “mono”. One speaker, no separation between 'left' and 'right. Then they brought in "stereo". Some of the recordings they did I found a real pain, because they put the sound of instruments all the way to the left, or all the way to the right. Now I can understand the reasoning behind this. A piano coming through a left speaker in front of you would be like having the piano actually there, on the left. Then when you listen to it, you can identify the position of that piano as being on the left. But you need both ears to make this a reality. What if you are wearing headphones? You would get no sound of that piano at all in your right ear, and that experience is enough to drive you up the wall.

Putting the sound 'to the left' or 'to the right' is called a 'pan', and they basically make the sound louder for one ear than for the other, thereby fooling the brain into thinking the sound is coming from one side more than the other, thereby positioning it either to the right or to the left. On my AR organ, they never 'pan' 100% to the right or to the left, so if you are wearing headphones you can hear the sound with both ears, with one side louder than the other (and you can adjust how much 'pan' you are getting).

What has always interested me is how the brain interprets the sound you are listening to. Obviously, the ear nearest to the sound source would receive a louder signal than the other ear, as it is closer to the sound source (even though only marginally). The further away you are from a sound the quieter it becomes. And the brain would interpret this difference and 'position' where the sound source is coming from. I find this quite amazing, as there is only a difference of about 7" between your ears, so how much of a difference in volume does 7" make?

But this is not the only clue the brain gets to determine where the sound is coming from. Another is the time difference between the sound arriving at one ear and arriving at the other. For example, sound coming from the left is detected in the left ear nanoseconds before it is detected by the right ear, and vice versa (at the speed of sound, the time lapse between the sound reaching one ear and then the other is about one half of one thousandth of a second .. ie. 0.0005 secs).

Below is a YouTube video explaining what is meant by the Interaural Time Difference (ITD)

.. and this one is an example of how deliberately creating a time difference between the sound reaching each ear affects one's perception of where the sound is coming from (you may have to turn the volume of this one down a bit!). It starts with no time delay to either ear so the sound appears to come 'from the centre'. Then a delay of 0.8msec is inserted in the signal from the Left Channel, so the sound reaches the right ear first and so the sound appears to come from the right.

.. and this final one places the sound source at various points round a 360˚circle so there is a combination of 'time difference' and 'volume' for the brain to determine where the sound is coming from.

I wonder when our electronic organs/keyboards will use time difference as well as volume difference to position instruments either to the left or to the right? It would be more effective than what we have at the moment.

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Re: Stereo .. how do we hear in stereo?

Postby PeterA » 16 May 2018 18:00

It is truly amazing. Back in the late 1960's Magnavox (an upmarket sound system) produced a demo vinyl record to demonstrate their latest product and one of the tracks had moving sounds from left to right. After being brought up on mono sound, this was awesome! We tend to take stereo sound reproduction for granted today.

Human stereo vision, (the principle behind stereo vision and stereo hearing is very similar, based on reception of an object by 2 separate receivers, and then computed by the brain into one finished article) is also remarkable, considering our eyes are only 3 inches (75 mm) apart! We are able to distinguish depth several hundred yards away.

It always amazes me that birds, with eyes on either side of their head, can fly with consummate ease through tightly packed branches, without any collisions. Now what do they actually see? A chicken usually turns its head, taking a look at the seed with each eye in turn, then accurately and rapidly picking it up......without getting a headache! Fascinating.

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