Hearing is not a purely mechanical phenomenon, but is also a sensory and perceptual event. When a person hears something, it arrives at the ear as a mechanical sound wave traveling through the air, but within the ear it is transformed into neural actions. These nerve pulses then travel to the brain where they are perceived. Hence, it is important to take into account not just the mechanics of the environment, but also the fact that both the ear and the brain are involved in a person’s listening experience.
The inner ear does significant signal processing in converting sound waveforms into neural stimulus, so certain differences between waveforms may be imperceptible. MP3 and other audio compression techniques make use of this fact. In addition, the ear has a nonlinear response to sounds of different loudness levels. Telephone networks and audio noise reduction systems make use of this by nonlinearly compressing data samples before transmission, and then expanding them for playback. Another effect of the ear’s nonlinear response is that sounds that are close in frequency produce phantom beat notes, or intermodulation distortion products.
There are true psychoacoustic effects introduced by the brain. For example, when a person listens to crackly and needle on vinyl hiss filled records, he or she soon stops noticing the background noise and enjoys the music. A person who does this habitually appears to forget about the noise altogether, and may not be able to tell you after listening if there was noise present. This effect is called psycho acoustical masking.
The brain’s ability to perform such masking has been important for the adoption of a number of technologies, though in this age of digital signaling and high fidelity playback the effect is typically used to hide losses in compression rather than to cover up analog white noise. As another example of a psychoacoustic effect, the brain appears to use a correlative process for pattern recognition; much like is done in electronic circuits that look for signal patterns. When the threshold for acceptance of a correlative match is very low a person may perceive hearing a sought after pattern in pure noise or among sounds that are similar, as the brain fills in the rest of the pattern.
This is a psychoacoustic phantom effect. For example when a radio operator is straining to hear a weak Morse code signal in a noisy background, he or she often perceives hearing the pitch of tiny dots and dashes even when they are not present. In general, psychoacoustic phantom effects play an important role in any environment where people have heightened perceptions, such as when danger may be perceived to be near. The psychoacoustic phantom effect is conceptually distinct from hallucination, where the brain automatically generates perceptions.
Psychoacoustics is presently applied within music, where musicians and artists continue to create new auditory experiences by masking unwanted frequencies of instruments, causing other frequencies to be enhanced. Another application is in design of small or lower-quality loudspeakers, which use the phenomenon of missing fundamentals to give the effect of low frequency bass notes that the system, due to frequency limitations, cannot actually reproduce.
It is also applied within many fields from software development, where developers map proven and experimental mathematical patterns, in the design of audio systems for accurate reproduction of music in theatres and homes, as well as defense systems where scientists have experimented with limited success in creating new acoustic weapons, which emit frequencies that may impair, harm, or kill.