Q: What is the softest sound humans can hear?

A: That depends on our age and on the frequency, or pitch, of the sound, says Kimberly Smuda, director of the Audiology Clinic in UW-Madison's department of communicative disorders.

Middle-range frequencies - about 1,000 hertz, or cycles per second - are easiest to hear, she says, and it is most likely not a coincidence that the major sounds of speech are in this range.

On the version of the familiar decibel measurement system that measures sound pressure, zero decibels is defined as the softest sound a healthy young person can hear on average. On the loud end, 120 decibels - think jet engine at 250 yards or a concert by The Who at 100 feet - is the threshold of pain.

On the quiet end, someone breathing quietly nearby registers about 10 decibels - more than twice the sound pressure of zero decibels. A cat purring on the next couch is also around 10 decibels. The wind rustling through trees is likely to be in the range around 20 decibels, Smuda adds.

But how "loud" is zero decibels? It's about the sound of a mosquito flying, 10 feet away.

- Produced in cooperation with University Communications

Curious? Look for Curiosities on Thursdays in the Local section. Send questions to: justaskus@madison.com; 608-252- 6192; Just Ask Us, P.O. Box 8058, Madison, WI 53708.