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One important technical data in audio equipments is the dynamic range. If the dynamic range is too small you either get very noisy signals or are in great danger of running the system to distortion.
Dynamic range is the ratio of maximum undistorted signal (full-scale or onset of clipping) to residual noise (noise floor). Dynamic range in generally measured in decibels (dB).
According to the international standard IEC 268, the dynamic range of any kind of professional audio equipment is calculated as the difference between the total noise floor (measured in dB(A)) and the equivalent sound pressure level (measured in dB) where a certain amount of total harmonic distortion appears.
The manufacturer can choose the amount of distortion he wants to specify the SPL at, as long as the THD appears in the printed literature. Normally that level is around 1%.
For comparing equipments it is sometime quite annoying that manufacturer sets the THD himself and can measure the noise floor differently.
In microphones there is sometimes a term "headroom" also mentioned. The range from the equivalent SPL to the clipping point of the microphone is called the headroom of the microphone.