Real-time 3D sound playback system (RT3D)
How does a person sense a direction of sound ?
A person uses both right and left ears to listen to the sound. Simply he or she might find out that it is from the right direction if the sound reaching the right ear is louder than the left ear. Or, if the sound reaching the right ear is preceding the left ear, they feel it is also from the right direction. Therefore, if sound levels at the right and left ears (the difference is called Internal Level Difference: ILD) or sound arrival times (the difference is called Internal Time Difference: ITD) can be changed artificially, the sound source position can be located anywhere you like.
Sound level differs Arrival time differs
Although, only by changing ILD and ITD, they cannot recognize forward and backward direction, or upward and downward direction. Because the sound source is at the line symmetric position and the ILDs and ITDs are equal. How complicated our ears are!
HRTF (Head-Related Transfer Function) shows
characteristic from a sound source at various angles and height to an ear, and it
is paired with one HRTF for the right ear and another HRTF for the left ear. HRTF
contains ILD and ITD information as well. Furthermore, it contains
characteristics of our head and ears' shape, so we can recognize a direction even at a
line symmetric position. Convolve any sound you like with some HRTF and listen
the convolved sound with headphones, the sound seems to come with some angles and
height virtually. The sound convolved with HRTF of some angles (called
Binaural Recording Sound) can be heard arriving
from the specific direction of HRTF.
In the case of using stereo speakers to play a binaural sound source, the space between speakers and ears becomes a problem. While playing, unlike with headphones, the sound from the right side speaker would reach the left ear not only the right ear, and vice versa. This is called "cross-talk". In addition, the sound from the speakers would be distorted through the air space. It turns out that this method does not work out.
It is a cross-talk canceller to vanish
the cross-talk and the space characteristic which distort the sound.
Last update : 2010/06/19