A compressor is a device used to control and reduce the dynamic range of a signal in a controlled manner -> uncompressed audio signal -> if it exceeds a threshold level compressor turns on -> reduces output of the signal above the threshold according to the ratio that is set e.g. a 2:1 ratio means for every 2 db above the threshold only 1 db is allowed through, 4:1 means for every 4 above the threshold 1 is returned and so on -> some compressors allow you to add make-up gain to the returned signal, bringing all aspects of the signal up by the same amount. this means even teh noise in the signal gets raised by the same amount -> compressed audio signal attack and release times allow you to control how quickly the compressor turns and off; i.e. how long after the threshold is crossed does the compressor start to compress the signal and how long after the signal falls below the threshold does the compressor stop compressing the signal short values generally tend to make the compressor "pump", you can hear it turning on and off, longer times tend to make the effect less noticeable and more musical