Parallax: Seeing and Hearing in Stereo

October 17, 2009

I was sitting in my Audio Aesthetics class last Tuesday and taking in Stan Freberg Presents The United States of America for the first time in my audio life. I was thinking about how the fidelity was truly amazing and how the stereo imaging was perfection. As I closed my eyes and visualized the orchestra playing in front of me, I began to think of how this image was being created. Small differences in loudness and time of arrival all contributing to making this recording as realistic as possible. Stereo is an illusion. And then I wondered, would this illusion still exist if I only had one ear to hear with. I realized that I had always thought of my left and right ears as the left and right side of a stereo feed. Maybe this just came from growing up listening to headphones, but either way, I had a disconnect. So I decided to test this theory. While basking in a Freberg recording, I plugged up my left ear… waited a few moments, and soon realized that my right ear does in fact hear in stereo, which must mean that my left ear does as well.

Intrigued by this, I looked online to find if there was any research on the matter. Though I was unsuccessful, I did find a page that was advertising a single stereo earbud. Seeing as the price is low, and it does not say otherwise, I assume that all this product just runs both channels into one speaker and creates a mono feed into one ear. It seems that the main use this product is for people who have hearing loss in one ear and don’t want to miss out on half of the signal and not to create an actual stereo image in one ear. Plus, who knows if that is even possible.

I thought about this more until my brother brought something else to my attention. He related my thoughts to how our eyes work. We can comprehend depth because of the differences between the way each of our eyes perceive a certain object. This is called parallax, and it allows humans, through the process of stereophsis, to perceive depth.  He said that you can still see to the left with your right eye and everything is almost as clear. But what you loose is depth to your vision. You can no longer sense depth of field. Aha! After this I went back and tested this theory on my home stereo. I plugged up one ear and stood in front of a really magnificent stereo recording. I realized that while I could distinguish between left and right, the recording had lost much of its depth. And in general, it just didn’t sound as pleasing though a single ear.

My conclusion is that having two ears is not so that we can can better perceive Left-Right or even Up-Down, but rather Near-Far. While one ear is capable of deciphering sound on the X and Y axes, two ears allows the Z axis to come into play. With two ears our brains can more accurately create a realistic image of an actual event.

Suggestion: Do the same test I did. It is interesting. Also, think about how headphones distort our hearing. Recordings are for the most part mixed on monitors to have bleed from ear to ear. Canceling this bleed changes everything.

Parallax is also an astronomy term. It used to figure out how far away certain stars are, but it is the same concept.

If July is your left ear/eye and January is your right ear/eye, stars A,B, and C might as well represent… a Jazz trio.


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