Nara Hahn

Identification of dynamic acoustic systems by orthogonal expansion of time-variant impulse responses

We present a system identification method for dynamic acoustic systems. The system is excited by a periodic sequence the autocorrelation function of which is an impulse train. The circularly shifted versions of such sequence form an orthogonal basis for the corresponding vector space. As each orthogonal component of the system is sequentially excited, the output sample is the expansion coefficient of the instantaneous impulse response. Unfortunately, the expansion coefficients obtained from the output signal are undersampled, and thus, the time-varying impulse response cannot be fully determined. In this study, we estimate the missing expansion coefficients by interpolation. As a result, a full set of impulse responses is obtained that describes the history of the dynamic system. The performance of the proposed method is demonstrated by simulations and real measurements.

Continuous measurement of impulse responses on a circle using a uniformly moving microphone

We propose a continuous measurement technique which can be used to capture a large number of impulse responses within short time. The response of an acoustic system is continuously captured by a moving microphone, and the instantaneous impulse responses are computed by post-processing. The time-variance due to the movement of the microphone is compensated by employing a recently proposed system identification method. In this method, each sample of the captured signal is interpreted as the orthogonal expansion coefficient of the instantaneous impulse response. The impulse responses are computed from the interpolated orthogonal coefficients. This method is applied to the measurement on a circle. Based on the modal bandwidth of the spatio-temporal impulse response, the relation among the length of the impulse response, the angular speed of the microphone, and the effective number of measurements is revealed. The presented measurement technique was used to measure a large number of room impulse responses, and the results were compared with a conventional sequential measurement technique.

Comparison of continuous measurement techniques for spatial room impulse responses

A large number of spatial room impulse responses can be measured efficiently by using a moving microphone in combination with a time-varying system identification method. The microphone moves on a predefined trajectory and captures the response of the acoustic system which is periodically excited. The instantaneous impulse responses are computed from the captured signal by taking the time-variance explicitly into account. In this paper, three different continuous measurement techniques are investigated and compared in a unified framework. It is shown that impulse response estimation constitutes a spatial interpolation process, where each method corresponds to a specific interpolation filter. In numerical simulations the performance of theses approaches are evaluated in terms of system distance and spatial bandwidth.

Synthesis of a sound field scattered by a virtual object using near-field compensated higher-order Ambisonics

For a physically accurate reproduction of a complex virtual sound field, the interaction of sound waves with objects or boundaries have to be taken into account. In this paper, we attempt to synthesize a sound field scattered by an acoustic obstacle. Based on an analytic representation of the incident and scattered sound fields, the loudspeaker driving functions are derived. We explicitly pay attention to the sound obstruction effect behind the scatterer, which is caused by the destructive interference of the incident sound field and the forward scattering. The physical properties of the presented approach are examined through numerical simulations, and the perceptual aspects are discussed in an informal listening test.

Synthesis of a spatially band-limited plane wave in the time-domain using wave field synthesis

Wave Field Synthesis (WFS) is a spatial sound reproduction technique aiming at a physically accurate reconstruction of a desired sound field within an extended listening area. It was shown in a recent study that the accuracy of the synthesized sound field can be improved in a local area by applying a spatial band-limitation to the driving function. However, the computational complexity of the frequency-domain driving function is demanding because of the involved Bessel functions. In this paper, a time-domain WFS driving function is introduced for the synthesis of a spatially band-limited plane wave. The driving function is obtained based on a time-domain representation of the sound field which is given as a superposition of plane waves with time-varying direction and amplitude. The performance of the proposed approach is evaluated by numerical simulations. Practical issues regarding the discretization of the analytic driving function and dynamic range control are discussed.

Azimuthal localization in 2.5D near-field-compensated higher order ambisonics

Sound Field Synthesis approaches aim at the reconstruction of a desired sound field in a defined target region using a distribution of loudspeakers. Near-Field Compensated Higher Order Ambisonics (NFCHOA) is a prominent example of such techniques. In practical implementations different artifacts are introduced to the synthesized sound field: spatial aliasing is caused by the non-zero distance between the loudspeakers. Modal bandwidth limitation is a well-established approach to reduce spatial aliasing in 2.5D NFCHOA, but introduces temporal and spectral impairments to the reproduced sound field which strongly depend on the relative position to the center of modal expansion. Also, the dimensionality mismatch in a 2.5D synthesis scenario results in a different amplitude decay compared to the desired sound field. Listening experiments already investigated the azimuthal localization in 2.5D NFCHOA. It is however unclear, in how far individual artifacts caused by spatial sampling, modal bandwidth limitation, a...

Acoustic Characteristics and Timbre Preferences of Korean Bells

The sounds of the Korean temple bells, that are located in the various places, were recorded and classified into two groups according to the size of bells. The sound preference was investigated with the subjective listening test on the bells of each group. And the acoustic characteristics of the bells such as the frequency, amplitude, beat period, and 20 dB decay rate of the partials was analyzed. The correlation between the acoustic parameters and timbre preference were analyzed and the acoustic characteristics of highly preferred bell sound were presented.