Answer: a vacuum- cleaner running.
The frequency at which the sound waves resonate or vibrate is the pitch of the sound. Pitch is a perceptual property of sound. It allows us to judge the quality of sound as high or low. A vacuum- cleaner running is an example of low pitch because it is an electrical instrument, which is used for cleaning purpose in household and offices. The sound of vacuum cleaner remains same during the working process. It's pitch remains low and constant through out the working process. On the other hand, the pitch of bird, a lawn mower runner and a dog barking usually remains high.
