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How our ears really work - Part I




The latest research on hearing shows that our hearing mechanism acts differently from the way our doctors and audio engineers have been trained. The hearing model they use is a frequency domain model. That is, they characterize hearing by our response to a sequence of pure tones. We’re told that we can hear pure tones from 20 Hertz to 20,000 Hertz and that we are sensitive to frequency domain distortions like harmonic and intermodulation but that we are not sensitive to phase or time distortions.  

Our hearing is far more complex than that and our tests on audio components have convinced me that the ear is exquisitely sensitive to the time domain (and the frequency domain) and another domain that I’ll discuss later.  Let me illustrate with an example. 

I work in a room with a wooden ceiling, which is also the floor of the room above.  All too often I’m startled by someone dropping a heavy object on the floor above and it scares me every time - but just for a fraction of a second or so. Over the years I’ve developed the habit of analyzing how my body reacts to the sound. I‘ve not been able to get used to it and my reaction goes through the same stages every time: 

First - my shoulder and neck muscles tighten in reaction to the sharp, threatening sound

Second - I determine the direction of the sound (in my case from above)

Third - I analyze the makeup of the sharp sound (a dropped stapler)

Fourth - I’m ready to run - away from the threat - if I think this “sound” is out to get me. but since, by now (a fraction of a second into the event) I know it’s that stapler again, I don’t run.

Fifth - I experience an adrenaline rush  

Keep in mind that all five stages are happening in sequence and in less than a second. Now here’s the really interesting part. I’ve reacted and directionalized the sound before I even know what the sound is! My body has reacted to the sound as though it were a threat before the more rational part of my brain has figured out what the threat is. It’s like the ear has a fast connection to the primitive fight- or- flight more ancient part of the brain and a slower parallel connection to the newer and more rational part of the brain that tries to make sense of the sound by thoroughly analyzing it’s complex frequency and time relationships.  
 

This primitive part of the brain is what most likely evolved to warn or alert us of threats like a tree falling (a sudden loud snap) or a saber tooth sneaking up (the soft sounds of paws on leaves). This sense works night and day and does not need line of sight. So it must have played a pretty important role in our early survival and evolution and hence its prominence in the early part of our brain. 

I think music imparts excitement to us through its sequence of continually changing patterns of leading edge sound events that trigger the ancient threat-warning fight-or-flight or alert response. I’ve therefore come to think of our response to music as having three basic processing axes: Time, Frequency and Excitement (alert or threat). Our first response to sound is along the Excitement axis (think of it as pre-processing) while the somewhat slower but nevertheless parallel processes of analyzing time and frequency components of the incoming sound completes our sense of a musical experience. This is why we say leading edge fidelity is essential to recreating the excitement of live music. 

The next time you’re startled by a singular loud sound recall as soon as you can what you experienced during that fraction of a second. You don’t need any test equipment only your ears and a desire to find out more about how they work.   

In part 2 of this series I’ll discuss some of the latest hearing research and what it means to reproducing sound realistically. Here’s a sample of what I’ll be covering: 

Our notion that we hear from 20 to 20 kilohertz is shattered by this paper’s findings - J Neurophysiol 83: 3548-3558, 2000; 0022-3077/00 $5.00 (my thanks to Dave Schwartz of the Gotham City Audio Society Gotham City Audio Society for this reference). This study shows that we react to recordings of complex musical instruments differently when the bandwidth of the playback system is 100 KHz vs. 20KHz. In other words, we can hear sound components at least as high as 100KHz (or maybe we should say we are sensitive to time changes in the five microsecond range) in the context of humans playing man made music on acoustic instruments!