Sound Source Localization
In the field of psychoacoustics, the ability of the human auditory system (two ears and brain !) to identify the direction from which a sound is emanating is called sound source localization. Humans can locate this sound source in space with extreme precision – within 2 degrees in the horizontal plane. This remarkable feat is accomplished by the brain’s ability to process the binaural signal coming from the ears.
Neuroscientists believe that sound localization in the horizontal plane relies on two “cues”: the sound amplitude (loudness) difference between the two ears (inter-aural level difference ILD), and the time difference (delay) of sound reaching each ear (inter-aural time difference ITD). The brain uses both cues to localize sound sources. While variations of either can evoke a perceived direction change, ITDs and ILDs are not completely independent of each other. There exists a natural cohesive spatial relationship between ITDs and ILDs . For example, suppose you have a single speaker off to your left side. The sound (wave) coming from the speaker would reach your left ear sooner and be louder than the sound that reaches your right ear. Your brain compares these differences and interprets where the sound is coming from.
Our stereo music systems employ two sources of sound: a left speaker and a right speaker. The stereo field is the spatial “width” between the two speakers. For a listener sitting in front of and centered between the speakers, as shown in the figure above, sound emanating from the left speaker reaches your left ear (“direct” red line) and your right ear (“crosstalk” green line). Same thing for the right speaker and your two ears. There is a summation of the direct and crosstalk signals at each ear. Our perception of a single source of sound, a “phantom image”, coming from somewhere between the speakers will be based on the inter-aural level difference (ILD). For example, if identical signals at the same volume level are played from the two speakers, the listener will perceive that the source of the sound is located exactly midway between the speakers, as there is no level difference between the ears. This situation is indicated in the figure above by the centered source (blue musical notes).
This is the concept of “panning” (placing) a musical track in the stereo field. By adjusting the relative volume levels of the track signal sent to the Left and Right channels of the Main stereo output of the mixer console, we can move the location of that track’s sound in a continuous fashion from Left speaker all the way to Right speaker. As an example, a “hard-panned-Left” track will have volume level in the Left speaker only – there is zero volume level in the Right speaker. Consequently, the image location of that instrument in the stereo field is at the left speaker.
At this point in the discussion, I want to mention briefly the idea of “pushing” the sound image beyond the speakers, i.e., perceiving the sound location as being further out on the sides, past the locations of the speakers. You may want to do this to “widen” the stereo field. As will be discussed in the post on Mid-Side processing, this can be done ! For example, say a signal is already panned completely to the Left speaker (so there is zero signal output in the Right speaker.) A small, phase-inverted version of this signal can be placed in the Right speaker. Upon summation of the waves coming from both speakers at both ears, there is a differing amount of destructive interference caused by the phase-inverted signal at the two ear locations. This in turn leads to a greater inter-aural level difference (ILD) and the perception that the sound is coming from a direction even further to the left of the left speaker ! In addition to using Mid-Side processing in an Imager plug-in, there are specialized plug-ins that use this phase-inverted signal technique to widen the stereo field, such as the Audec Extra Pan plug-in.
This “outside-the-speakers” technique, however attractive, must be used with caution, since playing around with phasing can lead to considerable problems, such as sound drop-outs. So, heed the warning.
In the next two posts, I’ll discuss in detail imaging in the stereo field and utilizing Mid-Side processing.