In this paper we describe a fast and precise method of estimating a correlation matrix and its application. The estimation of correlation matrices is widely used in array signal processing. The estimation is commonly carried out by averaging input signals using a fixed-length time window. To achieve high performance, the window length should be set at the optimum value depending on the acoustical environment, such as the signal-to-noise ratio. However, in dynamically changing environments it is difficult to set a fixed window length because the optimum value also changes dynamically. To solve this problem, we propose an optimally controlled recursive average (OCRA) method that can control the window length adaptively. To evaluate our OCRA method, we applied it to geometric source separation (GSS), which is a sound source separation method suitable for real-time systems. Experimental results showed that the proposed method improved sound source separation.
In this paper, we verify the performance of multiple-channel active noise control systems using the simultaneous equations method. This method has the advantage that it can automatically recover the noise reduction effect degraded by acoustic path changes. The method is hence applied to single-channel systems, and also it is shown that the method results in better performances than the filtered-x algorithm. In this paper, we extend the method to multiple-channel active noise control systems and verify the performance of the extended method by using impulse responses measured with an experimental system. The extended method is applied to a 1-2-2 system, consisting of one noise detection microphone, two loudspeakers and two error microphones, and to a 2-2-2 system including two noise detection microphones, resulting in noise reductions of 14 and 18 dB, respectively. This result demonstrates that the simultaneous equations method well works also for multiple-channel systems.
For environmental noise prediction, it is practicable to use meteorological data available from local meteorological observatories. However, these observations have limitations induced by the methods of measuring and data processing. Usually only mean meteorological values averaged over one 10 min period every hour are calculated. To apply these mean meteorological variables to noise propagation appropriately, we need to investigate the characteristics of both acoustic and meteorological parameters within the 10 min period. We made simultaneous measurements of both parameters over flat grass-covered ground and estimated effective sound speed profiles by similarity theory, using the meteorological data measured under conditions similar to those at local observatories. The changes in sound pressure level in periods around sunrise and sunset were similar and were smaller than those around culmination, in which fluctuations of approximately 20 dB were measured at higher frequencies at a distance of 100 m. Noise predictions by the parabolic equation method and sound speed profiles determined from instantaneous meteorological variations generally agreed with the measurements except in the time period around culmination. When we used 10 min mean meteorological values in combination with the parabolic equation method, we obtained reasonable agreement with the measurements at middle frequencies in time periods around sunrise and sunset.
We investigated the effect of a competing noise source on the intelligibility of target speech in various acoustic environments. To spatialize these sources in virtual acoustic space, we used head-related transfer functions (HRTFs) measured using the KEMAR dummy head, as well as HRTFs measured individually for each subject. We also compared the intelligibility when these sound sources were generated from loudspeakers located at the target and competing source positions in real acoustic space. The speech intelligibility of the target speech was evaluated using the Japanese diagnostic rhyme test (DRT). The target speech was placed directly in front of the listener, and a single competing source was placed on the same horizontal plane at various azimuths and distances. Individual HRTFs showed slightly better intelligibility than those with KEMAR HRTFs, but the difference was small in most cases. Intelligibility in real acoustic space still outperformed both types of HRTFs, especially when the competing sound source was closer to the listener than the target speech and the competing source was located at 180°. However, this difference was small in most of the other competing source locations tested.
The audibility of pure tones presented against typical domestic sounds was investigated in a psychoacoustic experiment conducted with young and older listeners. The sound pressure levels of pure tones were varied at several signal-to-noise ratios to find a range of auditory signals that are audible and comfortably loud in noisy conditions. Ratings of the listeners were analyzed in terms of A-weighted sound pressure levels and 1/3 octave band levels of the target tones and background noises. The results revealed that both listener groups assigned similar ratings to various combinations of pure tones and domestic sounds. However, when the tone frequency was 2,000 Hz or higher, older listeners needed a higher tone level to attain a certain level of audibility. On the basis of the results, the authors propose sound-level ranges of auditory signals for consumer products intended for users of various ages and for users who might have age-related hearing loss.