While the Boston Museum of Science houses many wonders, an anecdotal experience at an Illusions show in their auditorium was the most memorable for me.  The auditorium seats were filled with people of all ages - babies to the elderly.  A staff member told the audience we were about to hear something...and to raise our hands when we started to hear it.  Nobody knew what sound would be played for us, but as we looked around the room, hands slowly started to raise.  Amazingly, the hands went up in chronological order - starting with the young children, and ending with the elderly.  Turns out, the sound being played was just a simple tone decreasing in frequency (from high pitch to low pitch).  

What was the point of this demonstration?  As we get older, we lose the ability to hear high tones.  The reasons for this are varied, and a current area of active research, as I'll discuss below.  But this natural phenomena is not lost on marketers.  We were told of two products being sold because of the observation that adults experience high-tone hearing loss.  The first is a high-pitch noise that can be played in locations where youth tend to loiter - a clever deterrent.  The second is a cell phone ring tone, one that students can hear but the teacher can't.  

The NIDCD estimates that approximately 15 percent (26 million) of Americans between the ages of 20 and 69 have high frequency hearing loss due to exposure to loud sounds or noise at work or in leisure activities (http://www.nidcd.nih.gov/health/statistics/quick.htm).  This suggests that within that 20-69 age group in the Museum of Science auditorium, perhaps the hands that went up last were of those exposed to loud sounds on a regular basis.

 Hearing loss is especially prevalent in people with diabetes - diabetic patients experience deafness twice as often (http://www.nidcd.nih.gov/news/releases/08/06_18_08.htm).  Other diseases and genetic predispositions may affect someone's hearing loss over their lifetime.  But when it comes to environmental variations, like what loud sounds and noises we're exposed to, there appears to be 'No single archetype for noise injury' (K. Ohlemiller, Hearing Research, 2008).  At noise intensities above 125–130 dB - the level at which hearing loss abruptly accelerates - there is clear loss of hair cells in the inner ear (more on this below), however other changes are also apparant and other mechanisms of hearing loss may be at play below that decibel level of noise. 

 Noises above 125 dB include many of our common noises and loud sounds: concerts, jet takeoffs, and loud stereos.  A more complete list of the decibel measurements for common sounds and noises can be found here: http://www.nidcd.nih.gov/health/education/teachers/common_sounds.asp.  Conventional wisdom and many lay publications advise against playing loud music through headphones, but research on what type of damage to our hearing mechanism this may cause has lead to inconclusive results.  I expect more publications will cover research in this area in the coming years, especially given the increase in sales for personal music devices like MP3 players, which are used with headphones.

So HOW do we hear??  Simply, anatomically we have the inner, middle and outer ear, with autitory nerves traveling to the midbrain and then to the auditory cortex of our brain.  The inner ear houses very important organs like the cochlea, an organ specialized for hearing.  You may have heard this term from the revolutionary cochlear implants that have restored many people's hearing.  The inner ear also contains organs for balance - therefore, ear infections may make us feel like we have a bit of hearing AND balance loss temporarily.

Inside the cochlea is the organ of Cotri, which is essentially a sensory receptor (we have many sensory receptors in our body, specialized for different things like mechanical stimulation (ie: touch) and chemical stimulation (ie: smell, taste).  In the case of the organ of Corti, this sensory receptor is comprised of fluid-containing hair cells - which move in response to sound vibrations entering our ears, and then trigger a neurological electrical signal that travels to our brain via the nervous system.  These hair cells are important in that we are born with a certain number, and disease or trauma which leads to death of hair cells will result in their irreparable loss - no scientific technology yet exists to regrow hair cells.  (Also see http://www.nidcd.nih.gov/health/hearing/noise.asp for a more detailed description of how our ears allow us to hear.)

 Sometimes just understanding the mechanisms behind our physiology may be enough to assist us in maintaining good health and well being.   For me, I try to avoid extremely loud situations- and I always wonder which high-pitch noises I'm missing.