Audio frequency

Audio refers to sound that can be perceived by hearing.

Human hearing frequency range

The audio frequency range is about 20Hz to 20000Khz, for very young people who have the most excellent hearing. In general, most people hear up to 15 or 16 kHz but it is reduced in time.
When the audio passed through a transducer such as a speaker, stereo headphones or a headset, this signal produces audible sounds.

Sensing frequency as well

Although it can be seen in a graph, how low frequencies can be detectable by the ear, sound in the lowest audio range may also be perceived, or felt, if it is loud enough. When the sound is under the audio range, it may only be felt as vibrations. Therefore, we may relate to very low frequencies as not only audio, but also with a sense of vibrations.
A good example is a large pipe organ, where the lowest notes go below 20Hz. It is a certain experience to listen to such an organ, and to also feel the sound.

In order to possess full frequency response, one might argue that a sound needs to encompass also the inaudible, yet perceptible frequencies.
Because low frequencies can also be sensed, this "full" frequency response can only be perceived with the use of speakers, and not headphones. This circumstance is rarely, or if ever, mentioned in when audio of headphones is reviewed.

Audio spectrum

The range of audio frequencies is called the audio spectrum, and we might think of an audio spectrum analyzer, that measures and monitors an image of the entire audio spectrum at the same time.