Psychoacoustic signal processors are a range of specific devices that exploit some of the mechanisms by which the human brain perceives sound.
One example of this would be auditory masking, where the human brain ceases to perceive part of the sound. In this case, a sound with a specific frequency response is masked by another sound with a similar frequency response, only with a higher volume. This phenomenon is commonly used for data compression of sound.
Another type of psychoacoustic processors is exciters/enhancers. These devices work on the basis of harmonic distortion and add previously non-present, higher harmonic content, calculated with an algorithm using frequencies contained in the original audio. This helps to achieve a 'brighter' sound (see Frequency response).
Other psychoacoustic processors can produce the opposite effect by adding lower harmonic content to the original audio.
Some psychoacoustic processors are called 'loudness maximizers', which make the audio sound subjectively louder without any actual change to the overall volume.
To learn more about psychoacoustic effects, please read: