Audible fluctuations in the noise level of a signal caused by poorly adjusted or unsuitable noise reduction systems which produce a variable changing program level and/or noise floor. Also called pumping, noise pumping or breathing. Pumping is caused by the action of a compressor, occurring when one loud sound source causes severe gain reduction in the compressor. With each loud sound, the level of the other instruments will decrease sharply. Pumping occurs during program material. Breathing, on the other hand, occurs when the program stops long enough for the compressor to cease its gain reduction, suddenly boosting the noise floor of the program. Quantization noise can also exhibit breathing. See also compander.