Spatial sound background

I had a good lunch-time talk with Anthony Hornof today regarding spatial audio.  Anthony's lab has a research-level spatialized audio setup, which he will allow us to  use if we find it useful.  Primarily I was interested in what might be feasible with off-the-shelf hardware such as we might be able to deploy in a middle-school classroom.  I wondered whether we might get benefit of spatialized sound with standard stereo speakers rather than headphones.

First, Anthony told me that high-end audiophile headphones typically use an "open air" design that does not shut out other sound in the environment.   A typical cost for a really good pair is around $300.

Anthony was skeptical of being able to achieve usable spatialization with ordinary stereo speakers.  Of the two cues our brains use to judge location of sound, only one of them would be effectively controlled with ordinary speakers --- I'll write a little more about this below.  Second, the software that is used for spatialization has been designed for controlling sound near the ear, and would not effectively utilize speakers.

Anthony sent me several papers on spatialization, but for background he particularly recommended a book by Durand Begault which is available online at  Durand Begault's publication web page  . The book can be downloaded from this link:  3D Sound for Virtual Reality and Multimedia.   Chapter 2 provides a concise summary of sound location perception.


We use two cues to judge the location of a sound.  One is the relative loudness of a sound at each ear, called "interaural intensity differences", or IID.  The other is small time differences between sound arriving at each ear, called "interaural time differences", or ITD.    These  time differences range from about half a millisecond up to 1.5 milliseconds. 

In a natural setting, we not only judge the location of a sound by differences between what is received at each ear, but we also actively enhance those differences ... if we really want to know where a particular sound is coming from, we turn our head to minimize interaural differences, whereupon our head is pointed in the direction of the sound.   When our head is still, it is difficult to distinguish whether a sound is in front or behind us, but turning our head apparently disambiguates it (Begault, page 39).    We cannot duplicate this facility with headphones or speakers, so we probably could at best simulate the location of objects left-to-right or in a half-circle centered in front of the listener.  That is the approach that Zhao, Plaisant, and Shneiderman used in a pilot study of a sonified choropleth map.  (I'll write more about the study of Zhao et al in another post.)