A hertz is a unit used to measure frequency. One hertz is equal to one cycle per second. Hertz is commonly used to describe the range of hearing of animals and humans. Dogs can hear between 60 hertz and 40 kilohertz, while a human can typically only hear between 20 hertz and 20 kilohertz. This indicates that dogs can hear significantly higher frequencies than humans.

Each note of sound that is heard can be measured in hertz. The hertz measurement of a sound will indicate to which species the sound is audible. An example of this is a dog whistle. A dog whistle typically has a frequency between 23 kilohertz and 54 kilohertz; while this is out of the range of human hearing, it is well within the range for canine hearing. For this reason, a dog whistle will be perfectly audible to dogs, while humans listening to the same whistle will hear nothing at all.