Current Research

Multi-channel equalization

Unlike traditional equalization schemes that aim at calculating filters for each individual loudspeaker without reference to the reproduction context (i.e. position and properties of the virtual sources), the multi-channel equalization scheme proposed here aims at controlling the sound field produced by the entire array of loudspeakers with reference to a given virtual source.
To do so, each loudspeaker is measured along an array of microphones. Sound field control is achieved by iteratively calculating a set of filters that will, once applied to the loudspeakers, minimize the quadratic difference between the sound field produced by the loudspeaker array and a given target for all of the microphone positions. The target is calculated for each microphone position by applying the laws of propagation between the virtual source being synthesized and the microphone array. A filter data base containing all of the virtual sources one wants to synthesize can then be put together, to be accessed in real time during playback.

Above Left: Characterization of a WFS reproduction setup made of 4 adjacent MAPS (Multi Actuator Panels) forming a 32 transducer array in the Espace de Projection at IRCAM.

Above Right: Top view of the measurement configuration. The black dots correspond to 32 loudspeakers; the red circles correspond to 48 omnidirectional microphones; the blue circle represents the target virtual source situated 3m behind the loudspeaker array.

PLANE WAVE

Before Equalization Procedure

After Equalization Procedure

POINT SOURCE

Before Equalization Procedure

After Equalization Procedure

Above: Frequency responses recorded along the microphone array for two different virtual sources

Room Compensation

(Under Construction)

Synthesizing Source Directivity

The multichannel equalization process can be adapted to the reproduction of directive sources by specifying the spatial distribution of the impulse response corresponding to a given directivity in the reproduction field. Using a cylindrical decomposition of directivity, it is possible to calculate the set of filters for a subset of harmonics for each source position. These filters are obtained in an off-line process and then stored in a data base that can be accessed according to the virtual position and cylindrical harmonics order. A given directivity can thus be recomposed by applying the coefficients of the cylindrical harmonics decomposition to the calculated filters.

Frequency response (right) and low-pass filtered impulse response (left) of a lateral dipole synthesized by WFS using multichannel equalization on the MAP array.

Frequency response (right) and low-pass filtered impulse response (left) of a 5th order cylindrical harmonic synthesized by WFS using multichannel equalization on the MAP array.

Room Acoustics Team

WAVE FIELD SYNTHESIS @IRCAM