How is an audio signal typically defined?

An audio signal is typically defined as having a frequency range that is perceptible to the average human ear, which is approximately from 20 Hz to 20,000 Hz (20 kHz). The correct choice correctly identifies this by encompassing the lower end of the range (20 Hz) and setting an upper boundary that is generally accepted for sounds that elicit a response in human hearing.

The frequency range indicated in the correct answer effectively captures most sounds that can be heard by people, including speech and a wide variety of musical tones. Frequencies lower than 20 Hz are often referred to as infrasound, while frequencies above 20,000 Hz are categorized as ultrasound, both of which fall outside the typical human auditory capacity. Thus, the defining range for audio signals is critical for understanding sound reproduction and its applications in electronics and audio engineering.