Editorial

Editorial for January 2010


Published on January 1, 2010






Our January 2010 drawing/giveaway is for the Roku HD-XR Player, going to two lucky AUDIOPHILE AUDITION readers early next month. The digital multimedia receiver was already a good way to add Netflix and many other streaming Net capabilities to any SD or HDTV. But now it has new content channels – including many in HD – and faster Wi-Fi connectivity. We reviewed it here, and the press in general has raved about it.  To be considered for the drawing, you must Go Here to Register. Remember: we need your address in order to ship.

Last month our drawing/giveaway was for three complete sets of the 30-CD Sacred Music Box set from Harmonia mundi. It features some of today’s finest artists at the peak of their talents. 111 cornerstone works of western sacred music are featured, from earliest Christian chants to Bernstein’s Mass, from the gems of the Baroque to Bach chorales.  The three lucky winners will be receiving their 30-CD sets shortly. They are:

Brian Trout, Camp Verde, AZ; Steve Graham, Buffalo, NY; & Raymond Burnham, San Leandro, CA.  Congrats to all!


   Guest Editorial – How We Hear
by Gary Leonard Koh
 
[Originally published in "Absolute Fidelity”]

20/20 vision is supposed to be perfect eyesight, and 20Hz to 20kHz is supposed to be perfect hearing. However, as perfect eyesight doesn’t mean that you see everything, perfect hearing doesn’t mean that you hear everything.

The ear, can detect the faintest audible sounds that impart no more energy at the ear-drum than thermal noise 4 zJ (zepto-joule – 4 x 10-21 that is twenty-one zeroes after the decimal point!!) and yet has a dynamic range of over 7 orders of magnitude – from 0dB-140dB. In order to properly design playback systems, we need an understanding of how and what we hear… at least so that we know what is important and the priorities when we have to make trade-offs. For example, is 20Hz to 20kHz +/- 3dB good enough?

The basic principle of how we hear is actually very simple. Sound waves are collected by the external ear, and funneled down the ear canal to vibrate the eardrum. Within the middle ear, the ear bones mechanically carry the vibration of the eardrum to the cochlea.

Coiled around the inside of the cochlea, the Organ of Corti contains hair cells that convert the sound waves into electrical signals, which are transmitted by the auditory nerve to the brain where it is interpreted.

There are two types of hair cells – inner hair cells and outer hair cells. These cells are called hair cells because they grow stereocilia (or hairs), and it is the bending of these stereocilia that create the electrical signals detected by the auditory nerve. Sounds simple enough!

Until 20 years ago, it was thought that the cochlea was a passive receptor of sounds. Now, it is well known that we actually have “active hearing”.

Otoacoustic emissions. It was discovered in the late 70’s(ref 1) that sound is generated by the inner ear. It “sings” to help with the detection and resolution of incoming sound(ref 2). This would be equivalent to light coming out of our eyes to help us see.

Cochlear amplification. The outer hair cell can be made to elongate and shorten by electrical stimulation. Now, the outer hair cells are known to be an active amplifier that refines the sensitivity and frequency selectivity of the mechanical vibrations of the cochlea. This contributes to the huge dynamic range of the ear – with a sensitivity range of 140dB.

Non-linear critical underdamping. In the past, we thought that the hair cells in the Organ of Corti acted like a bank of harmonic oscillators. Now, we realize that the cochlea responds primarily to formant frequencies, extracting the defining features of the input sound. Two important things: response amplitude equalization and tone-to-tone suppression. However, latest research has shown that the amplification from the electromotile response of the hair cells is a Hopf bifurcation. Hence, while the steady-state sine-wave sensitivity of the ear is known to be only up to 20kHz, the theoretical sensitivity of the formant frequency analysis of the human ear extends to over 100kHz!!

Time dimension analysis. We always thought that the ear was a one-dimensional measurement. However, we now know that it analyses incoming soundwaves in time, and does its own fast Fourier analysis. The eardrum and middle ear changes the impulse and stretches it out in time like an oscilloscope, not a frequency analyzer.


This makes the ear capable of discriminating small differences in the structure of sounds, and their attacks. In particular, pulsed sounds and the formants, much better than any device so far invented.

The cochlea acts as a time-dimension frequency analyzer that exhibits resonant vibrations at characteristic frequencies that vary with position along the basilar membrane.


This “unraveling” of sound in time has significant implications to how we hear. This unraveling is able to detect minute differences in phase of the sound between the two ears, resulting in exquisite sensitivity to imaging.

This is simple experiment to do. Locate a piece of music that has good imaging in space (e.g. Track 2: Peace on Earth, FIM SuperSound III, First Impressions Music with the shaker going from low left to high right).

Listen to it and make slight changes to the volume balance. Note that this does not affect this image (unless you have a really badly implemented volume control in your pre-amp). However, moving one loudspeaker forwards and backwards by as little as ¼ inch changes the image of this shaker.

Unlike the eye and optic nerve, which carries a measurable and recognizable signal from the retina to the brain, how our brain interprets sound is still not well known. The auditory nerves disappear into six separate areas of the brain, and our understanding of how our mind puts the whole sound together is not well advanced.

In a future article, we will attempt to explore what we hear – the waveforms of music, speech, and why it is so difficult to reproduce.
 
References:
Kemp, D.T.; Stimulated acoustic emissions from within the human auditory system.
Journal of the Acoustical Society of America, 1978.
Duke, T.; Vilfan, A.; Andor, D.; Julicher, F.; Prost, J.; Camalet, S.; Active detection of sound in the inner ear
Andor, D.; Duke, T.; Simha, A.; Julicher, F.; Wave Propagation by Critical Wave Oscillators, Auditory Mechanisms, World Scientific, 1999.
E. de Boer, Auditory physics. Physical principles in Hearing Theory, 1980.
How the Ear Works – Nature’s Solutions for Listening 


EDITORIAL
 
AUDIOPHILE AUDITION began in 1985 as a weekly national radio series hosted by John Sunier, which aired for 13 1/2 years on up to 200 public radio and commercial stations coast to coast. In September 1998 its site for programming information was expanded to the present Internet publication.
 
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