Spatial Sound Encoding Including Near Field Effect: Introducing Distance Coding Filters and a Viable, New Ambisonic Format

Higher Order Ambisonics have been increasingly investigated in the past years, and found promising as a rational, scalable and flexible way to encode, transmit and render 3D sound fields. Nevertheless, studies concerning virtual source imaging or natural 3D sound encoding mainly focussed on the directional encoding of plane waves, and neglected the near field effect of finite distance sources though it's present in any ordinary sound field. This paper highlights that with near field, the infinite bass-boost affecting ambisonic components makes the currently accepted format unviable. By introducing from the encoding stage a near field compensation of reproduction loudspeakers, a viable, modified ambisonic format is defined, distance-coding filters are designed, and higher order ambisonic recording and synthesis become practicable.