noise floor
Background noise is the sum of all noise sources and other unwanted signals.
Like background noise, noise floor can consist of self-noise and ambient noise, electrical noise that can be generated by active and passive electronic components, or noise from magnetic materials.
In high-quality audio systems, the noise floor is usually -100 dBu or higher. The total dynamic range of a system is usually defined as the amplitude difference between the noise floor and the highest signal that can be transmitted without distortion.