Did you know that when a dog's sense of hearing fully develops, it can hear about four times the distance of a human with normal hearing? How is this possible?! Well, it actually has a lot to do with physics.....
Humans can typically hear sounds from within the frequencies of 20 Hz and 20,000 Hz, meaning they usually can't hear any sounds below the frequency of 20 Hz or anything above the frequency 20,000 Hz. Frequencies that are out of the range of human hearing are untrasonic (cyclic sound pressure with a frequency greater than the upper limit of human hearing) On the other hand, dogs can typically hear sounds with frequencies approximately 40Hz to 60,000 Hz. Frequency in this case is the number of sound pressure level vibrations (sonic waves) per second.
Although a dog's hearing capability is dependant on its breed, there are some distinct reasons why dogs hear better than humans. First, when dogs hear a noise, they move their ears towards the sound in order to maximize the reception (they have at least 18 muscles in their ear that allows the ear to tilt and rotate). By doing this the sound waves are more easily directed into the ear.
This may be why vacuum cleaners often scare dogs away. The vaccuum, and other sounds that are loud to humans often emit high frequency tones that scare away dogs. This video below further discusses frequency and the range of sound humans can hear vs. dogs.