«The average adult human cannot hear sounds above 20,000 Hertz (Hz), although young children can hear higher. (Hertz is a measure of the frequency of a sound, and the higher the frequency, the higher pitched the sound.) Dogs, on the other hand, can hear sounds as high as 47,000 to 65,000 Hz. These are sounds far too high-pitched for us.
In addition, at high frequencies dogs can detect much softer sounds than we can. The loudness or intensity of a sound is measured in decibels (dB) with 0 dB being the average intensity of a sound that can just barely be heard by a young human. So, sounds too quiet for humans to hear are given a negative decibel rating.» -A kennel association