Capturing Manipulation and Reproduction of Sampled Acoustic Impulse Responses
We discuss the capturing manipulation and reproduction of impulse responses (IRs) of acoustic spaces. While trying to maintain the accuracy of an IR, other factors such as sound quality and musical character of sound, should also be considered. Furthermore, IRs are not limited to preserving the sound of venues but also as a tool in music production. Therefore, the IRs are converted to standard multi-channel reproduction formats, such as stereo and ITU 5.0. In order to obtain a flexible reverb tool, the IRs are manipulated to modify acoustic properties such as reverb time and inter-channel de-correlation. A new real-time audio plug-in was developed for which Irs of venues and devices were recorded worldwide. The IRs are convolved with dry audio. The plugin supports mono, stereo, and surround, at sample-rates up to 96kHz.
Ronen Ben-Hador and Itai Neoran
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Software Implementation of B-Format Encoding and Decoding
New software tools are presented, which enable a standard PC equipped with a multichannel sound board to be used both for the creation of B-format signals, and for their playback over a suitable array of loudspeakers. The encoding of B-format signals is obtained by convolution of each original soundtrack with a proper B-format impulse response, obtained from measurement in existing spaces or from computation by a room acoustics software. The decoding is made thanks to a small dedicated program, which can be set to emulate a standard Ambisonics decoder (with fixed-gain shelf filters), or a new, wider class of decoders based on the recent theory about Energetic Analysis of Sound Fields. The decoder includes convolution with inverse filters which compensate for the irregularities of the loudspeakers. The system was developed mainly as an analysis tool and as a support for subjective listening tests in the automotive field and in the room acoustics field.
Angelo Farina, Emanuele Ugolotti
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Simultaneous measurement of impulse response and distortion with a swept-sine technique
A novel measurement technique of the transfer function of weakly not-linear, approximately time-invariant systems is presented. The method is implemented with low-cost instrumentation; it is based on an exponentially-swept sine signal. It is applicable to loudspeakers and other audio components, but also to room acoustics measurements. The paper presents theoretical description of the method and experimental verification in comparison with MLS.
Angelo Farina
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Acoustic measurements in opera houses: comparison between different techniques and equipment
In room acoustics, many objective parameters to quantify subjective impressions have been introduced. These quantities can be measured by using a wide variety of powerful tools and equipment. The results can be influenced by the measurement techniques and instruments used. Furthermore, the results also depend on the measurement positions and on the condition of the hall (full, empty, etc).
The aim of this work is to define a tightly standardised measurement procedure for the collection of a complete objective description of an opera house’s acoustics. In this paper some of the results obtained by the authors after measurements made in three different halls are presented. Comparisons were made both between different hardware and software tools (real-time analyser, DAT, PC-board, source, microphones, post-processing software) and between different measurement methods (interrupted stationary noise, true-impulse, pseudo-random white noise with impulse-response deconvolution, sine sweep) as well as between different positions in the halls, with and without the presence of musicians and audience. The results have shown that the differences obtained using different measurement techniques and equipment are not of significant importance. The only effective differences were found regarding the recording techniques, as the monoaural measurements give appreciably different results from the average of left and right channel of binaural measurements. Slightly different results were also found between true impulsive sources (pistol shots, balloons) and omnidirectional (dodechaedral) loudspeakers. Attention must be paid to the signal to noise ratio, as this can influence the correct calculation of some acoustical parameters. Some differences, not as great as expected, were found in the results with and without the musicians in the orchestra shell and with and without the audience in the hall. This is probably due to the high sound absorption that is typical in Italian opera houses even without an audience. However, important differences were found in the calculation of some acoustical parameters, particularly clarity C80, by changing positions in the hall.
Patrizio Fausti, Angelo Farina
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Real-Time partitioned convolution for Ambiphonics 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. In this paper 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.
Anders Torger, Angelo Farina
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Recording concert hall acoustics for posterity
When the famous and renowned Gran Teatro La Fenice in Venice burned during the night of 29 January 1996, one of the best sounding opera houses in the world suddenly disappeared. Its sonic behaviour, however, was at least partially saved, because several acoustical measurements had been performed just two months before, employing the binaural impulse response technique.
Angelo Farina, Regev Ayalon
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