Audio signals have a maximum bandwidth of about 20 kHz.

Explanation:
Audio signals are representations of sound waves. These signals are typically generated by microphones or other audio recording devices and are used for various purposes such as communication, entertainment, and information transmission. The bandwidth of an audio signal refers to the range of frequencies present in the signal.

Frequency Range:
The human hearing range is generally considered to be from 20 Hz to 20 kHz. Frequencies below 20 Hz are referred to as infrasound, while frequencies above 20 kHz are referred to as ultrasound. Therefore, audio signals are typically limited to the audible range of human hearing, which is up to 20 kHz.

Sampling and Nyquist Theorem:
Audio signals are typically digitized for processing and storage purposes. This process involves sampling the analog audio signal at regular intervals to obtain a series of digital values. According to the Nyquist-Shannon sampling theorem, in order to accurately represent a signal, the sampling rate must be at least twice the highest frequency present in the signal.

CD Quality Audio:
CD quality audio, which is commonly used for music and other high-quality audio recordings, has a sampling rate of 44.1 kHz. This sampling rate allows for accurate representation of audio signals up to half of the sampling rate, which is 22.05 kHz. However, to avoid potential issues with anti-aliasing filters and other factors, the audible range is typically limited to 20 kHz.

Practical Considerations:
In practice, limiting the audio signal bandwidth to 20 kHz is sufficient for most applications. The majority of people cannot hear frequencies above 20 kHz, and most audio equipment is designed to accommodate this limitation. Additionally, limiting the bandwidth helps to minimize the amount of data required for storage and transmission, making it more efficient.

In conclusion, audio signals have a maximum bandwidth of about 20 kHz because it encompasses the range of frequencies that can be heard by the human ear and is sufficient for most practical applications.