A 3D stereo sound system

The human hearing system uses several methods to determine from which direction a particular sound is coming. Our hearing system is binaural, and these methods include relative phase shift for low frequency sounds, relative intensity for sounds in the voice range, and relative time of arrival for sounds having fast rise times and high frequency components. The outer ear plays a significant role in the determination of the direction. The outer ear being defined as the ear canal, the concha, and the pinna. The pinna has an effect on the spectrum of sound reaching the eardrum and the concha has an effect on the frequency at which the ear canal is resonant. Together they control the spectral shape of the sound reaching the eardrum. This effectively controls the frequency response of the auditory system. Due to the complex shapes of the pinna and the concha, sound impinging on this area is subject to reflection, reinforcement and cancellation at various frequencies. Acting rather like a multiple filter, emphasizing some frequencies and attenuating others whilst letting some go through with no change. The response changes with both azimuth and elevation, and together with our binaural capabilities helps us determine whether a sound is coming from up, down, left, right, ahead or behind. By the use of head related transfer functions (HRTF) the sound retrieval system (SRS) 3D process can take a stereo or mono input signal and create a new signal which has a 3D sound image with the correct spatial cues and a very broad centre, creating a more natural sound.