## Health and Medicine Question #134

Jonathan Wever, a 15 year old male from the Internet asks on February 16, 2000,

What is the maximum number of decibels one ear can hear?

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### Barry Shell answered on February 16, 2000

The lowest sound a human ear can hear is a pressure energy of 1*10-12 W/m2, that is one thousand billionth of a watt per square meter. This is called O decibels or 0db. It goes up from there. Normal talking is 60db. Pain is felt at about 130db. Instant rupture of the eardrum is at 160db. Check out the chart at http://www.abelard.org/hear/hear.htm for more detail.

### Barry Shell answered on December 16, 2003

Cathy Gulli asks: I've heard or read somewhere that the human ear can only recognize some 300 different sounds, which is why often we confuse one voice for another over the telephone or one singer for another. Is this true? If so, why?

No. The human ear can hear about 400,000 different sounds and it's extremely sensitive. The problem you cite is more likely due to the very low fidelity of telephone sound. In order to maximize the number of phone calls carried over a limited number of telephone lines, all phone conversations are reduced down in sound content to the lowest possible amount that is still intelligible. The phone company does this by digitizing the voice and then stuffing it into little packets. This is all reconstructed at the other end. They use an 8KHz sampling rate, which means they sample the sound 8000 times a second. You get about 256 different possible sound levels this way. To put it in perspective, for CD quality sound the sampling rate is about 44KHz and this gives about 65,000 different sound levels. Good enough for most voice and music (but still far from the 400,000 we are capable of and why a real concert sounds way better than a CD recording). The lower rate for phone calls is good enough to understand what the person is saying but not good enough to recognize their voice perfectly, and certainly not good enough for music.

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