A vibration that recurring at regular intervals has its own "frequency". Frequency indicates how FREQUENT the cycle of motion is happening within a fixed unit of time. Vibration with no periodic activity has no frequency.
The measuring units of frequency are Hertz (Hz). One Hertz equals one full cycle per second. For example, when a honey bee makes one full motion of its wings in one second, the frequency of this movement is 1Hz. If the same bee would swing its wings 230 times per second, that means it raised the frequency from 1Hz to 230Hz. Luckily for the bee, along with increasing the vibrations, it would also raise itself up in the air!
Sound is a form of vibrational energy that needs to meet a few conditions in order to occur. The first condition is the presence of a medium through which mechanical vibrations could pass (gases, liquids, solids, and plasma). Also, for human hearing, the sound is determined as a very specific range of vibrations. The motion of physical matter starts to be audible when its frequency is higher than 20Hz. Vibrations with fewer than 20 cycles per second are called "infrasound". Infrasound isn't audible, although, if the impulse is strong enough, it can create a physical sensation, such as tickling.
The human ear can register up to 16000 – 20000 vibrations per second. Frequencies above the hearing range are called "ultrasound". The ultrasound isn't audible, but could affect the hearing and physical tissue.
It is a fact that all sounds are affecting our physical tissue. In other words, we hear with the whole body. Pressure waves created by the vibrating matter are being partially reflected and absorbed by our bodies.
*The sinuous wave on this diagram indicates pressure waves propagated by sound. The higher the frequency, the denser is the wave.
* The dynamic diagrams above show the distribution of sound. The molecules of the medium are being compressed and expanded with every passing wave.