Akira Omoto

A study of an actively controlled noise barrier

This paper describes an application of active control of sound diffracted by a barrier. The basis of this application is the cancellation of the sound pressure at the diffraction edge, which behaves like a virtual source to the diffracted field. Several points on the edge were canceled simultaneously by several secondary sources to get a large attenuation over a wide area. While many factors appear to influence the effectiveness of active control, the points of cancellation on the diffraction edge and the arrangement of the secondary sources were studied. The optimum conditions for control were investigated using numerical simulation. These conditions were then confirmed experimentally using multichannel adaptive signal processing. It was found that control is stable and effective when the intervals of the points of cancellation on the edge are shorter than half of the wavelength, and that attenuation is greatest when the secondary sources are nearest the primary source.

Active suppression of sound diffracted by a barrier: An outdoor experiment

The active control method was used to suppress the sound diffracted by an outdoor barrier. This method operated by the cancellation of the sound pressure at the diffraction edge of the barrier, which normally behaves like the virtual source of the diffracted field. The results of two experiments are shown in this paper. In the first experiment, we employed two independent controllers that utilized multi-channel adaptive signal processing to minimize the sum of the mean square of the sound pressure at four and six points along the diffraction edge. Measurement of sound pressure levels at various distances from the barrier showed effective sound suppression, with about 6-dB excess attenuation over the barrier’s insertion loss at the receiver at a distance of 50 m. A practically realistic noise source, a fan blower, was used as a primary source in the second experiment and the moderate attenuation could be achieved at almost all the receiver points. The results obtained in these two experiments provided the ...

The effect of structured uncertainty in the acoustic plant on multichannel feedforward control systems

Algorithms used for multichannel adaptive feedforward control, such as the multiple error LMS algorithm, require a model of the cancellation paths, which a control engineer would refer to as the plant, to update the signals driving the secondary sources. Uncertainties in this plant model or in the physical plant can lead to instability or suboptimal performance. This paper describes the effect of practically realistic uncertainties in both estimated and physical plant on the stability and performance of such a control algorithm. The system used here has 32 error sensors and 16 secondary sources and is designed to control a pure tone of 88 Hz. We consider the structured uncertainties due to changes in the error sensor locations, changes in the secondary source locations, and the excitation frequency. Results of preliminary experiments and numerical simulations show that these uncertainties have very different effects on the stability and performance of the control system. The singular value decomposition of the transfer matrix is used to discuss the characteristics of each type of plant error in the principal coordinates of the system.

Comment on “A theoretical framework for quantitatively characterizing sound field diffusion based on scattering coefficient and absorption coefficient of walls” [J. Acoust. Soc. Am. 128, 1140–1148 (2010)] (L)

The relationship between the acoustic scattering characteristics of materials and the degree of diffusion in enclosed acoustic spaces has recently attracted considerable research attention. Hanyu [J. Acoust. Soc. Am. 128(3), 1140-1148 (2010)] introduced a theoretical framework, in which the diffusion time in an enclosure is expressed as a function of a materials average scattering coefficient. In this letter, a modification of this theory is proposed. The decay process of the sound energy through scattering is divided into discrete sub-processes, specifically, a purely scattering process, and alternating scattering and specular reflections. The behavior of each process is examined for different scattering coefficients.

Application of an Auditory Filter for the Evaluation of Sounds and Sound Fields

Auditory filters that simulate function of the human auditory organ are introduced for the evaluation of a sound and sound field. Three applications are discussed in this report. The first examines the similarity of sound fields. Music signals convolved with different impulse responses are compared subjectively and quantitatively. The output of the auditory filter indicated high correlations with subjective evaluation. The second example examines the decay process of the sound field, especially in rather dead conditions. Impulse responses measured in recording studios are passed through both an auditory filter and a conventional band-pass filter. Decay processes are then calculated by inverse integration. The responses after passing through the auditory filter show more stable decay curves and the obtained reverberation times are closer to the early decay time, which is closely related to human perceived reverberance. The timbres of musical instruments are used as the third example. The signals from instruments with different means of sound production (such as reed or mouthpiece) are processed by auditory filters and normal band-pass filters. Statistically-obtained characteristics such as kurtosis of the waveform again show higher correlation with subjective evaluation when employing the auditory filter. All the results shown in this report strongly suggest the superiority of an auditory filter in the qualitative evaluation of sounds and sound fields.

The effect of structured uncertainty in multichannel feedforward control systems

The algorithms used for adaptive feedforward control often require a model of the acoustic plant being controlled to update the signals driving the secondary sources. This paper describes the effect of practically realistic plant estimation errors on the stability and performance of such a control algorithm. We have considered the structured plant errors due to changes in the error sensors location, changes in the secondary sources location and drift in the excitation frequency. These errors have very different effects on the stability and performance of the control system. The characteristics of each types of plant error are examined by singular value decomposition of the transfer matrix.

Evaluating acoustical features of 3D sound rendering systems by using a visualizing tool of sound intensities

The authors have developed a simple measurement/analyzing tool which visualizes 3D acoustical properties of sound by using sound intensity information. The tool, VSV (Virtual Source Visualizer), has already being used for practical acoustic design works, and the authors have also begun to use it for evaluating acoustical features of 3D sound rendering systems. On the audio industries, various 3D sound reproduction systems have already been commercialized. Therefore, further skill is now required to design acoustic specifications of their production studios. From this point of view, development of a simple method to obtain relationship between loudspeaker positions and source locations rendered by a 3D reproduction system would be helpful. This research introduces analyzing flows of the measurement tool briefly, and shows measured results of physical and/or phantom sound locations detected from sound intensity information in some different 3D reproduction environments including commercial audio strategies ...

Visualization and Evaluation of Reflections inside an Enclosed Space Using Sound Intensity Measurement

To examine the relationship between surface scattering and the properties of a sound field, a visualization technique based on the sound intensity measurement is introduced. Measurement of the impulse responses at four closely located points yields the intensity response in three orthogonal directions. Dominant peaks in the intensity response could be recognized as the dominant reflections from the certain room surface. Our method projects such information onto the spherical panoramic photo of the field. In addition to such qualitative visualization, quantitative examination is also tried. We have introduced here the index called uniformity of arrival directions, UAD, as one measure of sound field. The UAD is defined by the temporal changes of uniformity of arriving directions of reflections and is calculated by the intensity levels observed at equally divided solid angles on the surface of an imaginary sphere around the measurement point.

Introduction of auditory filter into evaluation of characteristics of small enclosures

Physical measures, which are useful for evaluating small enclosures such as studios and car cabins, are examined. Especially, auditory filtering using Zwicker’s loudness model is introduced for calculating the measures. Most current measures used in evaluating the large sound field such as concert halls are based on measured or calculated impulse responses. Instead, the music signals of some genres are used in this study. Using the auditory model, the amount of information can be reduced, even when complicated signals are used as stimuli: the signals are convertible to information in 25‐bark bands. Correspondence of such physical quantities and subjective judgments are considered from the viewpoint of sound‐field similarity. Subjects are asked to judge the similarity of two successive sounds whose frequency characteristics are modified artificially using the equalizer. The analysis results obtained using the auditory filter were compared with the subjective evaluation. Results indicate that analysis using...

Visualization methods of direct and early reflection sounds in small enclosures

Many parameters exist for evaluating large sound fields such as concert halls. However, it is difficult to apply those parameters for evaluation of a small room such as a recording studio because of their different sound fields. Widely useful common parameters have not been established. Moreover, early reflections are important in small rooms for determining spatial acoustic impressions. Therefore, various methods that visualize spatial acoustic information obtained by early reflection in rooms are proposed. For this study, sound fields (a music studio and a filmmaking studio) were measured using three kinds of different techniques: instantaneous intensity, mean intensity, and a sphere‐baffled microphone array. This report compares the information of sound source directions obtained using these methods. Results show that every method can estimate the position of sound sources and important reflections with high accuracy. In the future, we shall propose a method that visualizes spatial acoustic information...