05 July 2011

Why Are Certain Sounds Annoying?

I'm currently reading a fascinating book, "Annoying: The Science of What Bugs Us" by Joe Palca and Flora Lichtman. The two science journalists take a look at the literature in various disciplines (psychology, evolutionary biology, anthropology and others) to try to discover why certain things drive us crazy.

Of course, I was drawn to the chapter on annoying sounds, with no surprise that fingernails on a chalkboard clocked in as the number one annoyance as compared to others sounds. Various researchers have attempted to discover what it is about this particular sound that makes it so irritating. First, they filtered the high frequencies of the fingernail sound (their hypothesis was that high-pitched, "screechy" frequencies might be to blame). Surprisingly this only muffled the sound but did nothing to make it more pleasant. When they filtered the frequencies from 500Hz to 2000Hz, that did the trick.

A few reasons why this is important:
  • The human ear can detect frequencies between 2000Hz and 5000Hz at lower volumes than other frequencies, which is good because this is where most of the important sounds of speech fall.
  • The ear is most sensitive to noise loss between 2000Hz and 5000Hz, so one hypothesis is that this is ear preservation in action. The most sensible way to protect hearing is to develop an aversion to damaging sounds.
  • The natural resonant frequency of the average adult ear canal is around 3000Hz.
  • Interestingly, the researchers found that the sound with the most amount of energy at 3000Hz is a human scream (men also scream at 3000Hz, since they break into falsetto). Babies cry at this frequency, and alarms are also often at 3000Hz.
So the sound that we can detect at the greatest distance is a scream, and when the energy the human ear is most sensitive to is removed from the fingernail sound, it's easier to ignore.

Another factor that may contribute to the fingernail sound being annoying is that it is more rough (a subjective perception of rapid amplitude modulation). Basically, the fingernail grabs the surface of the slate, and as you continue to move your hand down it's stuck and then suddenly will slip and jump to the next position. This produces a highly unpredictable, varied sound. The roughness is what seems to make people cringe the most.

Just some things to think about next time you hear that annoying screech!

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