How is sound measured in decibels?

Sound is measured in decibels by comparing the sound pressure level to a standardized reference level, typically set at 20 micropascals in air, which is considered the threshold of hearing for the average human ear. This logarithmic measurement effectively represents the intensity of a sound relative to a reference point, allowing for a more manageable scale of perception of sound intensity. This method is crucial because the human perception of sound is nonlinear; the decibel scale compresses a wide range of sound pressures into a more usable form, making it easier to understand and communicate the loudness of sounds encountered in everyday life.

The other options do not accurately describe how sound is measured in decibels. Measuring the length of sound waves, calculating the frequency of sound, or recording sound energy over time do not provide a direct measurement of loudness or intensity as defined by the decibel scale.