At any point in a transmission system, the ratio of the circuit noise at that point to an arbitrary level chosen as a reference.

The circuit noise level is usually expressed in dBrn0, signifying the reading of a circuit noise meter, or in dBa0, signifying circuit noise meter reading adjusted to represent an interfering effect under specified conditions.

This article incorporates public domain material from . General Services Administration. Archived from on 2022-01-22. (in support of MIL-STD-188).