An unprecedented window. (Natalia Lebedinskaia/Moment/Getty Images) It's hard to imagine pregnancy care without the ultrasound. Since the 1950s, this incredible technology has provided an essential ...

A normal human hears sound in the frequency range 20 hertz (Hz) to 20 kilohertz, and the normal loudness, measured in decibels (dB), is between 30 and 70 db. Higher volumes, e.g. 85 dB and above, can ...