Final answer:
The best operating level for audio devices is measured in decibels, with the term 'decibel' being used to describe the intensity, and indirectly the perceived loudness, of a sound wave.
Step-by-step explanation:
The best operating level for audio devices is measured in terms of decibels (dB), which is a unit used to express the intensity of a sound wave. Amplitude is related to the volume or loudness of a sound and can be visualized as the height of the wave from peak to trough. However, the preferred or optimal sound level for listening depends on both the context and the content being listened to. Normal human speech is typically around 60 dB, and the softest sound a human can hear, known as the threshold of hearing, is set at 0 dB.