Hughes Aircraft, a division of General Motors Corp., developed a car- stereo enhancement system, known as the Sound Retrieval System (SRS). This system was later incorporated in stereo TVs. Sony Corporation obtained the license for the SRS technology, and since then incorporated it into nearly a dozen new stereo TV models.
The significant advantage of this system is that it can give true 3-D sound from only two speakers, unlike other systems requiring an array of speakers. The principle of the SRS system Hum ans perceives the direction of sound by at least three different means. In case of lowfrequencies we detect the relative phase of sounds between 20 Hz } and 200 Hz. For midrange sounds (300 Hz to 4000 Hz), we detect their relative intensity.
Which means that sound coming from one side is louder to the nearest ear, and softer to the other ear. For higher frequency sounds—those hav- ing a fast rise time we judge direction by the relative time of arrival. Those sounds reach the closest ear sooner than they reach the farther one. There is, however, a fourth factor that governs the way in which we judge where sounds originate. This has to do with the way our hearing system’s frequency response varies.
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The outer ear, known as the pinna, has an effect on the spectrum of the sound reaching the eardrum, while the concha (the section that leads tothe ear canal) has an effect on the frequency at which the ear canal is resonant.
Together, these two parts of the ear control the spectral shape (frequency response) of the sounds reaching the eardrum. In other words, the system func- tions as a multiple filter, emphasizing some frequencies, attenuating others, and letting some through without any change at all.
The ear’s frequency response changes with both azimuth and elevation, and together with our binaural (two ear) capabilities they help us determine whether a sound is coming from above, left, right, ahead, or behind. Human hearing system response to frontal sounds, sounds from the sides and from behind do not have same frequency response.
Omnidirectional microphones used for recordings have a flat frequency response from sounds coming from all directions. Cardioid, or directional microphones, has flat response to sounds coming from the sides and from the front, but are dead to rear sounds.
So, during playback, if sounds that originally came from the side are reproduced by speakers located ‘up front’, those sounds are heard with an incorrect spectral response. The result is a spatial distortion of the sound field, and one is prevented from beating the proper spatial cues of what was originally performed.