For the more information about natural sounds and night skies in
the National Park Service, please visit
http://www.nature.nps.gov/sound_night/. Science of Sound Sound
is perceived by humans as an auditory sensation created by
pressure variations that move in waves through a medium such as
air or water.It is measured in terms of frequency and amplitude.
Noise is sometimes used as a synonym for sound, but there is a
subtle difference. Noise is a sound that is unwanted or
inappropriate in an environment. Frequency, sometimes referred
to as pitch, is the number of times per second that a sound
pressure wave repeats itself. A drum beat has a much lower
frequency than a whistle, and a bullfrog call has a lower
frequency than a cricket. The units of frequency are called
hertz (Hz). Humans with normal hearing can hear sounds between
20 Hz and 20,000 Hz, although we lose the highest frequencies as
we age. Frequencies above 20,000 Hz are known as ultrasound.
When your dog tilts his head to listen to some seemingly ima