Achim Kuntz

Listening room compensation for wave field synthesis

Common room compensation algorithms are capable of dereverberating the listening room at some discrete points only. Outside these equalization points the sound quality is often even worse compared to the unequalized case. As wave field synthesis in principle allows to control the wave field within the listening area it can also be used to compensate for the reflections caused by the listening room in the complete listening area. We present a novel approach to listening room compensation which is based upon the theory of wave field synthesis and that results in a large compensated area.

Sound Field Synthesis

Conventional multichannel audio reproduction systems for entertainment or communication are not capable of immersing a large number of listeners in a well defined sound field. A novel technique for this purpose is the so-called wave field synthesis. It is based on the principles of wave physics and suitable for an implementation with current multichannel audio hard- and software components. A multiple number of fixed or moving sound sources from a real or virtual acoustic scene is reproduced in a listening area of arbitrary size. The listeners are not restricted in number, position, or activity and are not required to wear headphones. A successful implementation of wave field synthesis systems requires to address also spatial aliasing and the compensation of non-ideal properties of loudspeakers and of listening rooms.

A Dedicated Decorrelator for Parametric Spatial Coding of Applause-like Audio Signals

Parametric spatial coding of multichannel content has acquired widespread use in today’s audio codecs to enable efficient operation in the medium bitrate range. However, applause signals are still challenging to code with good perceptual quality using these techniques. To obtain a sufficiently low side information bitrate, the spatial parameter update rate is usually chosen in the range of several tens of milliseconds which is too low to restore a convincing spatial listener envelopment of applause-like signals. Moreover, state of the art decorrelators that are mandatory in parametric multichannel decoders inevitably deteriorate the signal quality of transient sound events, like handclaps, by dispersion. Therefore, we propose a novel decorrelation and parametrization technique for efficient coding of applause sounds: transient events are separated from the core decoder output and a dedicated decorrelator algorithm distributes the transients in the spatial image according to parametric guiding information transmitted in the bitstream. Listening tests show a substantial improvement in subjective quality compared to standard methods.

Room acoustic analysis: Complementing 2D measurements by 3D simulations

This paper demonstrates the analysis of complex acoustic environments using high-resolution circular measurements and complementary 3D simulations. The possibilities and limitations of room acoustic analysis using either measurements or simulations are discussed. An exemplary study is presented on the sound propagation in a complex shaped room: The analysis based on multichannel room impulse responses measured on a large circular aperture is introduced as a starting point of the room acoustic investigation. It is shown how the extrapolation of the measured data provides some insight into the sound propagation but fails to explain sound field characteristics caused by the complex 3D shape of the acoustic environment. In particular the individual 3D acoustic paths of higher order reflections cannot be derived from measurements alone. This paper demonstrates how they can be identified by complementary 3D simulations. It is elaborated that in the proposed joint application of 2D measurements and 3D simulations both techniques complement each other such that their limitations can be removed.

Wave field analysis using multiple radii measurements

Wave field analysis of stationary sound fields can be performed with high spatial resolution when sequential measurements on a circle are employed. Applying a circular harmonics decomposition to wideband audio data requires a careful setup of the measurement process to avoid noise amplification at certain frequencies. To this end, measurements with several microphones at multiple radii are considered here. Three mathematically well-founded strategies to combine their measurement signals are compared regarding the resulting noise amplification.