What is a common characteristic of audio frequency signals?

Audio frequency signals are specifically characterized by their ability to be heard by humans. The range of audio frequencies is typically defined as being from 20 Hz to 20 kHz, which encompasses the spectrum that the average human ear can detect. This range includes the lowest pitches that can be heard, such as bass sounds, and extends up to the highest pitches, such as treble sounds.

Understanding this range is vital in various fields, including audio engineering, music production, and telecommunications, as it influences how audio equipment is designed and how sound is transmitted and reproduced. The other options do not accurately describe audio frequency signals; for instance, audio signals encompass frequencies that are within the range of human hearing, not below it or excessive to radio transmission limitations.