Equivalent Input Noise is a standardized specification used to measure the quietness of a preamplifier, allowing engineers to compare the noise performance of different devices regardless of their gain settings. Since amplifying a signal also amplifies the inherent noise of the circuit, simply measuring the noise at the output is misleading; a preamp set to high gain will always seem noisier than one set to low gain. To solve this, the EIN calculation takes the noise measured at the output and subtracts the amount of gain applied, effectively referring the noise back to the input stage. This results in a theoretical value usually expressed in negative dBu. A lower number indicates a quieter preamp, which is vital when recording quiet sources with low-output microphones like ribbon mics. For context, a measurement around -130 dBu is near the theoretical limit of physics dictated by thermal noise.