Pumping and Breathing
In studio jargon, an effect created when a compressor is rapidly compressing and releasing the sound, creating audible changes in the signal level. “Pumping” generally refers to the audible increase of sound levels after compression has taken place; “breathing” refers to a similar effect with vocals, raising the signal volume just as the vocalist is inhaling. Pumping and breathing is a sign of cheap compression or over-compression, and is usually undesirable, although some engineers and musicians use it on purpose occasionally to create a particular effect.
