Different methods exist for testing performance in background noise, using multiple loudspeakers that preserve the spatial sound field. They are the four-loudspeaker-based method as described in ETSI EG 202 396-1, higher-order ambisonics (HOA), and matrix inversion methods.
HOA optimizes the reproduced sound at a sweet spot in the centre of the array with the radius determined by a spherical microphone array, which is used to derive the spherical harmonics decomposition of the reference sound. The four-loudspeaker-based method equalizes the magnitude response at the ears of a head and torso simulator (HATS) for sound reproduction, while the matrix inversion method optimizes the local sound field around a few target positions.
This whitepaper documents an investigation contrasting the following five methods for the reproduction of background noise:
- ETSI EG 202 396-1
- Higher-order ambisonics
- Matrix inversion method
- ETSI TS 103 224
- Matrix inversion method optimized for a specific device
For each method, the quality of the reproduced sound was evaluated both objectively and subjectively, at microphones close to a device under test and at the ears of the HATS. A listening experiment evaluated the perceived quality of the sounds at points where telecommunication devices would be placed, around the head.