Psychoacoustics is the branch of psychophysics involving the scientific study of the perception of sound by the human auditory system. It is the branch of science studying the psychological responses associated with sound, including noise, speech, and music. Psychoacoustics is an interdisciplinary field including psychology, acoustics, electronic engineering, physics, biology, physiology, and computer science.
Background Hearing is not a purely mechanical phenomenon of wave propagation, but is also a sensory and perceptual event. When a person hears something, that something arrives at the ear as a mechanical sound wave traveling through the air, but within the ear it is transformed into neural action potentials. These nerve pulses then travel to the brain where they are perceived. Hence, in many problems in acoustics, such as for audio processing, it is advantageous 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, for example, does significant signal processing in converting sound waveforms into neural stimuli, this processing renders certain differences between waveforms imperceptible. Data compression techniques, such as MP3, make use of this fact. In addition, the ear has a nonlinear response to sounds of different intensity levels; this nonlinear response is called loudness. Telephone networks and audio noise reduction systems make use of this fact 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 at least five features to identify effective psychoacoustic practices: Loudness (the measures of perceived volume), Roughness (sensory dissonance), Sharpness (spectral distribution), Tonalness (the ratio of tonal spectral peaks), and Spaciousness (to predict perceived Spaciousness). Another procedure for recognizing music genres or recommending music is to remove a wide range of objective features that are not entirely related to human perception. However, there are some low level features that aren't related to human/physical perception, but can improve the discovery of psychoacoustics. The first one, Root Mean Square (RMS), is another way how sound can be measured, specifically loudness. RMS is a significant process of measurement because it helps people monitor volume. Spectral Rolloff helps guide the frequency to a balance. Spectral Flatness is considered to outline how loud or quite a noise range. Lastly, Inter Channel Cross Correlation estimates the relationship between how one ear perceived sound in relation to the other ear.
Limits of perception
The human ear can nominally hear sounds in the range 20 to 20000 Hz. The upper limit tends to decrease with age; most adults are unable to hear above 16000 Hz. Under ideal laboratory conditions, the lowest frequency that has been identified as a musical tone is 12 Hz. Tones between 4 and 16 Hz can be perceived via the body's sense of touch. Human perception of audio signal time separation has been measured to be less than 10 μs. This does not mean that frequencies above 100 kHz (1/10 μs) are audible, but that time discrimination is not directly coupled with frequency range. Frequency resolution of the ear is about 3.6 Hz within the octave of 1000–2000 Hz That is, changes in pitch larger than 3.6 Hz can be perceived in a clinical setting. However, even smaller pitch differences can be perceived through other means. For example, the interference of two pitches can often be heard as a repetitive variation in the volume of the tone. This amplitude modulation occurs with a frequency equal to the difference in frequencies of the two tones and is known as beating. The semitone scale used in Western musical notation is not a linear frequency scale but logarithmic. Other scales have been derived directly from experiments on human hearing perception, such as the mel scale and Bark scale (these are used in studying perception, but not usually in musical composition), and these are approximately logarithmic in frequency at the high-frequency end, but nearly linear at the low-frequency end. The intensity range of audible sounds is enormous. Human eardrums are sensitive to variations in sound pressure and can detect pressure changes from as small as a few micropascals (μPa) to greater than 100 kPa. For this reason, sound pressure level is also measured logarithmically, with all pressures referenced to 20 μPa (or 1.97385×10−10 atm). The lower limit of audibility is therefore defined as 0 dB, but the upper limit is not as clearly defined. The upper limit is more a question of the potential to cause noise-induced hearing loss. A more rigorous exploration of the lower limits of audibility determines that the minimum threshold at which a sound can be heard is frequency dependent. By measuring this minimum intensity for test tones of various frequencies, a frequency-dependent absolute threshold of hearing (ATH) curve may be derived. Typically, the ear shows a peak of sensitivity (i.e., its lowest ATH) between 1–5 kHz, though the threshold changes with age, with older ears showing decreased sensitivity above 2 kHz. The ATH is the lowest of the equal-loudness contours. Equal-loudness contours indicate the sound pressure level (dB SPL), over the range of audible frequencies that are perceived as being of equal loudness. Equal-loudness contours were first measured by Fletcher and Munson at Bell Labs in 1933 using pure tones reproduced via headphones, and the data they collected are called Fletcher–Munson curves. Because subjective loudness was difficult to measure, the Fletcher–Munson curves were averaged over many subjects. Robinson and Dadson refined the process in 1956 to obtain a new set of equal-loudness curves for a frontal sound source measured in an anechoic chamber. The Robinson-Dadson curves were standardized as ISO 226 in 1986. In 2003, ISO 226 was revised using data collected from 12 international studies.
Sound localization
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