Hearing

I could try to explain how hearing works, but I could not do better than this video by Crash Course:

Quick facts

  • The ear’s malleus, incus and stapes (aka the hammer, anvil and stirrup) are the smallest bones in the human body – all three can fit together on a penny.
  • Your internal ear is the size of a pea but contains more than 20,000 sensor “hairs.”
  • Hearing can be damaged permanently from a single incident of exposure to extremely loud noise.
  • The ear continues to hear sounds, even while you sleep.
  • A healthy young person can hear all sound frequencies from approximately 20 to 20,000 hertz.
  • The maximum range of human hearing is about 15 to about 18,000 cycles per second.
  • Humans can hear an impulse about 20 microseconds long, although some studies suggest that some musicians can hear even shorter impulses.
  • Some blind people have learned how to “see” using a form of echolocation.

Sound direction
The pinna, the outer part of the ear and catches sound waves, but it has another critical function. This structure of the pinna (curves and position) helps determine the direction of a sound. Sound coming from behind bounces off the pinna differently than that coming from in front, which alters the pattern of sound waves. The brain recognizes the pattern differences and determines direction of the sound. In addition, the distance between your ears can help you determine direction, and it all happens very quickly:

  • Sound travels at the speed of 1,116 feet per second (761.2 mph) at sea level,
  • The average human head is 6 to 7 inches wide,
  • So hearing can detect the difference of half a thousandth of a second (about 500 microseconds)

Hearing loss
We need to remember that in nature the loudest sound a Human is likely to hear is thunder; our distant ancestors didn’t go to rock concerts, work in loud factories or set off fireworks. So it should not be a surprise that about 20 percent of Americans, 48 million, report some degree of hearing loss. Noise-induced hearing loss (NIHL) may happen slowly or suddenly (depending on the decibel level of sounds exposed to). The issue is aggravated by the fact that you usually can’t tell when your hearing is being damaged by loud sounds.

The effects of gunfire or explosions are immediately evident, but exposer to everyday noises, such as listening to very loud music often seem innocuous when they are not. Researchers estimated that up to 17% of teenagers have NIHL in one or both ears.

decibels

Tinnitus
Tinnitus (ringing/buzzing in ears) affects ~10% of Americans and about 90% of cases occur with an underlying hearing loss. Tinnitus is cause by genetic factors, noise, trauma, Ototoxic medications, infections, and aging.

Web Applications
I could find no usability information on safe sound levels for Web based video or audio. I’ve often noticed that many videos on the Web start at full volume. I’d suggest 50% is a much better percentage – then let the listener decide if they want it louder.

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