What does the decibel (dB) unit express?

The decibel (dB) unit is a logarithmic measure that expresses the ratio between two values, typically related to sound pressure levels. In the context of acoustics, it provides a way to quantify sound intensity levels by comparing a given sound pressure level to a reference pressure level. This is crucial because sound intensity can vary widely, and using a logarithmic scale helps represent these differences more manageably.

The use of a ratio allows for a meaningful expression of our perceptions of loudness, given that our ears perceive sound intensity in a logarithmic manner rather than linearly. For example, an increase of 10 dB represents a tenfold increase in sound intensity, which aligns with how humans perceive changes in loudness.

The other options focus on different aspects of sound. A fixed measure of sound intensity does not capture the proportional nature of the decibel, while the amplitude of sound waves refers to the physical characteristics of sound itself rather than its representation via dB. Similarly, frequency pertains to how many cycles of a wave occur in a second, which also falls outside the scope of what the decibel specifically measures. Hence, option B accurately captures the essence of what the decibel unit represents in acoustics.