Angelo Farina

Real-time partitioned convolution for Ambiophonics surround sound

Ambiophonics, as one of the most realistic music reproduction methods, requires multi-channel convolution with very long impulse responses for creation of believable reverberation. It is shown how the heavy processing task required for a real-time Ambiophonics system, or similar convolution-intensive system, can be handled by a low-cost personal computer, by means of partitioned convolution. The only theoretical advantage of partitioned convolution is that it provides low input/output delay. However, since the intensive part of it is very easily made optimal for the target platform, which normally is not the case for the standard overlap-and-save algorithm, it often provides the fastest convolution as well.

Acoustic quality of theatres: correlations between experimental measures and subjective evaluations

Abstract This paper deals with the psychoacoustic correlations between physical parameters and subjective aspects of perception of music in theatres. Many acoustic parameters were experimentally measured in eight Italian theatres and halls, including the Milanese Teatro Alla Scala, the Teatro Comunale in Bologna and the Teatro Comunale in Florence. A questionnaire was distributed, gathering almost 200 responses from well-known musicians (such as Riccardo Chailly, Severino Gazzelloni, Claudio Scimone and Uto Ughi). The results were compared and statistically analysed. Some interesting linear correlations were found between the physical acoustic parameters and the subjective evaluations. Finally, a short questionnaire, suitable for further psychoacoustic analysis was obtained.

Implementation of real-time partitioned convolution on a DSP board

Convolution using very long filters is required in order to achieve realistic artificial reverberation or spatial effects. Unfortunately, DSP (digital signal processor) platforms have limited computational power (compared with a modern PC) and consequently it is not possible to design very long filters based on typical time-domain, direct-form algorithms, i.e., FIR or IIR structures. To perform this task, other algorithms are necessary. The implementation of a real-time partitioned convolution algorithm on a DSP platform is demonstrated. In this manner, efficient convolution with long impulse responses is attained, with the advantage of low input/output delay.

Pyramid Tracing vs. Ray Tracing for the simulation of sound propagation in large rooms.

The aim of this paper is to introduce a new computational model (RAMSETE) for the simulation of sound propagation in large rooms; the model can easily be adapted to work outdoor, and can consider diffraction effects around screen edges and sound paths passing through (light) panels. However, this paper focuses on room acoustics, and particularly on rooms with non-Sabinian behaviour. In fact, the Pyramid Tracing algorithm does not involve an hybrid computation scheme, with a reverberant tail superposed to the deterministic early reflections estimate, as it is common with other diverging beam tracers (cone tracers, gaussian beam tracers, etc.). This make it possible to study also sound fields characterised by double-slope sound decays, inside spaces with not comparable dimensions and inhomogeneous sound absorption. It is well known that the same capabilities were already present in the (original) Ray Tracing scheme, but requiring much longer computation time. In fact, a correct Ray Tracing implementation can be considered as the reference standard for any (faster) numerical code based on the Geometrical Acoustics assumptions. After a brief introduction to some important details of the two algorithms, the results obtained in three cases are presented. The first is a typical Sabinian room (a reverberating chamber), the second is the coupling of two rooms with different average absorption (a theatre with its stage), the third is a typical industrial building (having an height very little compared to other dimensions) with nonuniform sound absorption (baffles under the ceiling). The results show how the Pyramid Tracing can give results very similar to the original Ray Tracing, provided that a proper adjustment of the parameters is performed. On the other hand, the magnitude of the errors that can be done with improper parameter settings is delimited and discussed.

Analysis of ill-conditioning of multi-channel deconvolution problems

Deconvolution of single- and multichannel systems is often an ill-conditioned problem whose exact solution boosts certain frequency bands excessively. Frequency-dependent regularisation can used to prevent this by attenuating sharp peaks in the magnitude response of the optimal filters. A z-domain analysis demonstrates that frequency-dependent regularisation works by pushing the poles of an ideal optimal solution away from the unit circle.

3D Sound Characterisation in Theatres Employing Microphone Arrays

The analysis of the sound field’s 3D properties has been strongly improved in recent years, after spatial properties of sound propagation have been acknowledged to be important during the design or correction of theatres and auditoria. Besides, a proper assessment of spatial accuracy is requested for 3D sound reproduction systems, initially designed for acoustical virtual reality and now also employed in the entertainment/cinema industry (Immsound, Auro-3D, NHK 22.2). Often only monoaural or binaural measurements are performed by means of omni-directional microphones and dummy heads, although international standards like ISO 3382/1:2009 also define some “truly spatial” parameters such as JLF and JLFC. The two latter parameters are derived from measurements made with a pressure velocity (p/v) microphone, but this is still a 2-channel measurement. 3D Impulse Responses (4-channel B-format) have for many years been measured and employed for sound reproduction. Recently, higher-order 3D Impulse Responses have been measurable thanks to the availability of compact microphone arrays employing a much larger number of transducers. In this paper, two procedures for measuring and analysing the complete spatial sound information are presented, which are aimed at creating easy-to-understand images and videos showing the direction-of-arrival of the room reflections. The description of these techniques is emphasised. Furthermore, results are illustrated of experiments conducted with the novel methodologies in different rooms, including some ancient theatres.

MEASURING IMPULSE RESPONSES CONTAINING COMPLETE SPATIAL INFORMATION

Traditional impulse response measurements did capture limited spatial information. Often just omnidirectional sources and microphones are employed. In some cases it was attempted to get more spatial information employing directive transdudcers: known examples are binaural microphones, figure‐of‐8 microphones, and directive loudspeakers. However, these approaches are not scientifically based and do not provide an easy way to process and visualize the spatial information. On the other side, psychoacoustics studies demonstrated that ‘‘spatial hearing’’ is one of the dominant factors for the acoustic quality of rooms, particularly for theatres and concert halls. Of consequence, it is necessarily to reformulate the problem entirely, describing the transfer function between a source and a receiver as a time/space filter. This requires us to ‘‘sample’’ the impulse response not only in time, but also in space. This is possible employing spherical harmonics for describing, with a predefined accuracy, the directivi...

Acoustic characterisation of “virtual” musical instruments: Using MLS technique on ancient violins*

Abstract In this paper the realisation of “virtual” musical instruments, in which the instruments are treated as linear systems characterised by their impulse response, are analysed. The impulse response is treated as a numerical filter, which is convolved with the force excitation signal coming from the chords, producing a signal containing all the acoustic characteristics of the instruments, and avoiding all nonacoustic phenomena. The aim of this work is multiple: the “virtual” instruments can be used in subjective listening tests for the evaluation of the “sound quality” of different instruments, as reported in this paper. Other possible uses are for the (real or virtual) restoration of ancient instruments, and for preliminary listening tests on newly designed instruments. Various measurement techniques of the impulse response have been tested, employing different transducers and numerical analysis. For validating the accuracy of the new technique, a subjective listening test has been conducted. Some o...

Reliability of scale-model researches: a concert hall case

Abstract To verify the reliability of acoustical tests on concert halls performed by means of scale models, a quality evaluation has been carried out on a recently constructed concert hall in accordance with the Ando methodology. A scale model has been made of the same concert hall in which the effects of some modifications of the ambient geometry were examined both from the objective standpoint and from the viewpoint of the subjective acoustical quality. This paper shows the results obtained in the above-mentioned research.

On the effects of pre-processing of impulse responses in the evaluation of acoustic parameters on room acoustics

The evaluation of room acoustics characteristics in rooms has been thoroughly described in several papers since 1960s. Moreover, the ISO 3382 standard describes several acoustic parameters and their measurements. However, there are only a few information about the methods of pre-processing the impulse responses that are required before calculating those acoustic parameters. In this paper, the main processing methods (based on Luneby, Chu, and Hirata methods) are analyzed. Moreover, they are compared with the Schroeder (background integrated) methods. In a further step, these methods are applied in some acoustic measurements employed in some opera houses in Italy. Finally, after a full discussion about the uncertainties that is beyond these methods, the acoustic parameters are compared with the JND that is actually accepted in the evaluation of the mono-aural, binaural, and spatial acoustic parameters.