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In many applications of acoustics and audio signal processing it is necessary to know what humans actually hear.
Sound, which consists of air pressure waves, can be accurately measured with sophisticated equipment. However, understanding how these waves are received and mapped into thoughts in the brain is not trivial. Sound is a continuous analog signal which (assuming infinitely small air molecules) can theoretically contain an infinite amount of information (there being an infinite number of frequencies, each containing both magnitude and phase information.)Recognizing features important to perception enables scientists and engineers to concentrate on audible features and ignore less important features of the involved system. It is important to note that the question of what humans hear is not only a physiological question of features of the ear but very much also a psychological issue.
The human ear can usually hear sounds in the range 20 Hz to 20 kHz. With age, the range decreases, especially at the upper limit. Lower frequencies cannot be heard but loud sounds can be felt on the skin.
Frequency resolution of the ear is, in the middle range, about 2 Hz. That is, changes in pitch larger than 2 Hz can be perceived. However, even smaller pitch differences can be perceived through other means. For example, the interference of two pitches can often be heard as a the (low-)frequency difference pitch. This effect is called beating.
The intensity range of audible sounds is enormous. The lower limit of audibility is defined to 0 dB, but the upper limit is not as clearly defined. The upper limit is more a question of the limit where the ear will be physically harmed (see also hearing disability). This limit depends also on the time exposed to the sound. Sometimes, the ear can be exposed to short periods of sounds of 120 dB without harm, but long times of 80 dB sounds will harm the ear.
A more rigorous exploration of the lower limits of audibility determines that the minimum threshold for which a sound can be heard is frequency dependent. By measuring this minimum intensity for testing 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 1kHz and 5kHz, though the threshold changes with age, with older ears showing decreased sensitivity above 2kHz.
The ATH is the lowest of the equal-loudness contours. Equal-loudness contours indicate the sound pressure level (dB), over the range of audible frequencies, which 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 chamberAn anechoic chamber is a room that is isolated from external sound or electromagnetic radiation sources, sometimes using sound proofing, and prevents the reflection of wave phenomena ( reverberation). Anechoic chambers are widely used for measuring the ac. The Robinson-Dadson curves were standardized as ISO 226 in 19861986 is a common year starting on Wednesday. Events January January 1 Spain and Portugal enter the European Community January 1 Aruba gains increased autonomy from the Netherlands and is separated from the Netherlands Antilles. January 9 After losing a pa. In 20032003 is a common year starting on Wednesday (link will take you to calendar), and also: The International Year of Freshwater The European Disability Year Summary Perhaps the defining global event of the year 2003 was the Invasion of Iraq launched by the U, ISO 226 was revised using data collected from 12 international studies.
The human hearing is basically a spectral analyzer, that is, the ear resolves the spectral content of the pressure wave without respect to the phaseThe phase of a wave relates the position of a feature, typically a peak or a trough of the waveform, to that same feature in another part of the waveform (or, which amounts to the same, on a second waveform). The phase may be measured as a time, distance, of the signal. In practice, though, some phase information can be perceived. Inter-aural (i.e. between ears) phase difference is a notable exception by providing a significant part of the directional sensation of soundSound localization is a listener's ability to identify the location of origin of a detected sound or the methods in acoustical engineering to simulate the placement of an auditory cue in a virtual 3D space (see binaural recording). There are two general m. The filtering effects of head-related transfer functionThe head-related transfer function (HRTF describes how a given sound wave input (parameterized as frequency and source location) is filtered by the diffraction and reflection properties of the head, pinna, and torso, before the sound reaches the transducts provide another important directional cue.
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