Jim Wrote: If the frequency deemed as being desirable of cancellation is such that after exposure there is some hearing degradation, for whatever length of time and at whatever level, the same degradation of hearing will exist after the use of any "cancellation" device. What will not be present is the "fatique factor" associated with this noise as it concerns the decoding of this frequency, the "fatique factor" on the ear is still there though. Jory Responds: Most of the studies I've read regarding the fatigue caused by continuous high levels of white noise, like the noise present on modern jet airplanes, suggest that the fatigue is due to the load placed on ones "perceptual filters" by constantly trying to pull intelligible information, like speech, out of a very high level of background noise. The brain is an amazing pattern matcher. It will try to make sense out of any sound presented, even if it's just noise. This constant processing is stressful and is believed to be the source of the fatigue one experiences on long flights. There is no evidence that ultrasonic (sound above the limit of human perception) sounds cause fatigue. There have been some studies that show near ultrasonic sounds, like the horizontal oscillator in your TV set at about 17 KHz, can make people edgy. It's also been shown that most Americans should measurable hearing loss at the horizontal refresh frequency of American TV sets (NTSC). Likewise, Europeans show the same loss at the horizontal rate of the PAL TV sets in their homes. This is a notch in hearing at a very specific frequency still within the audible range. I'm reminded of a study done in Arizona a while back. A researcher recorded the sounds of bats in the desert. Then through digital signal processing he translated the bats sounds down into audible range for humans. He was amazed to find out that the noise level in the ultrasonic range at night in the desert is quite high. Why is it that we find nights in the desert so soothing if there is all of this sound energy bombarding us all the time? It's because we can't hear it, don't perceive it, and don't process it. For us it doesn't exist. Jim Wrote: An example of this is infared light. True infared light cannot be perceived by normal visual acuity but visual acuity is effected by it nonetheless. If you point a remote control at your eye and key the control you will not be able to see the light as it strikes your retina but after you shut off the remote and close your eye you can "see" the 'burn' left by the light on your retina. The same is true of these noise "cancellation" devices and ears. Jory Responds: Have you done this experiment? I just did it with my "high power" remote. I went into the photolab (darkroom) and waited for my eyes to adjust. I then put the remote 1/2 inch away from my eye and held down the channel change button for 1 minute. I then shut my eyes. As expected I saw NOTHING. The rods and cones in the human retina respond only to certain wavelengths of light. The human eye is most sensitive to a wavelength in the green spectrum. Our visual acuity is best at this wavelength. It falls off toward blue and toward red. Infrared (or IR) literal means "below red". Below meant below human perception. Neither the cones nor rods respond to the light in the IR region. That's why we don't see it. The "burn" your writing about is not actually a burn to the retina, but a temporary shortage of a chemical called "visual purple". If you stare at a bright light for a while and then look away you will see a dark spot, until your body can make some more visual purple. If what you say about the human eye responding to IR was true, you would be blinded by walking in the kitchen when the electric range was on with the coiling glowing red. There is much more power coming off the "burner" on an electric range than one finds outdoors on a sunny day. You wouldn't believe what a stove looks like through a night vision system operating in the infrared region. You may be confusing all of this with the danger of an IR laser. Since an IR laser, by definition, is not visible, it poses a number of concerns. Since the light isn't visible, your iris will not contract if you get the IR laser in your eye. Also you will not see anything so you won't know to look away. However, the lens and the other optical components of your eye will still focus the laser light onto your retina pretty well. The shear power of the laser can then literally burn your retina. This is not merely a shortage of visual purple,but the overheating of the retinal tissue. This is why laser labs have big warning signs and why one is compelled to wear safety goggles with IR filters at all times when in the lab. It's also why laser pointers are limited to 5 mW (Class IIIa). The FDA has decided that 5 mW isn't enough to damage your eyes. The IR light from a remote control is from an IR Light Emitting Diode (LED). The light is not coherent (aligned in phase) and is not collimated so it does not pose a risk to your eyes. In a sense your ears work much like your eyes. Attached to the inside of your cochlea are little hairs called cilia. When your eardrum is moved by pressure waves arriving at your ear the motion is amplified and conducted to the fluid inside your inner ear. Each frequency of sound waves stimulates the cilia in a different region of your cochlea. The auditory nerve carries this information to your brain to make sense of it all. Noise cancellation headsets eliminate the motion of your eardrum to a very large degree over a very large dynamic range and very large frequency range. See "Tone Deaf and All Thumbs" by Frank R. Wilson for a much more complete understanding of how your ears work. Jim Wrote: Another example of this to be found, in tuning this time, is a three string unison where one string is tuned to pitch, the next string set at just slightly sharp and the last string set at slightly flat. This note when, flatness/sharpness is equal, will be perceived as if it were in
This PTG archive page provided courtesy of Moy Piano Service, LLC