It is required for ideal acoustic diffusers that the incident acoustic energy is scattered, without any loss of energy, in all directions with equal distributions. Quadratic-residue type diffusers were designed for this aim. But it has become clear that such type of diffusers also provide highly selective and inevitable absorption depending on both the frequency and the angle of incidence. In this article, in order to develop optimum acoustic diffusers that provide highly diffuse reflections and less absorption of sound, the possibility of realization is discussed theoretically. A measure regarding the statistics of diffusion is defined, which enables one to evaluate the performance of the diffusers. As a result, it is shown that a surface profile that satisfies the condition of optimum diffuser can be obtained without any difficulty in manufacturing. The design procedure and a few examples are presented.

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This paper investigates how to measure the degree of interaural cross correlation (ICC) as a physical measure of auditory source width (ASW), the just noticeable difference (jnd) of ASW and the estimation equation of ASW in degree for a music motif as a practical evaluation method of ASW in concert halls. Some physical measurements and psychological experiments show the following results.(1) ICC measured without Aweighting using a dummy head without ear simulators agree with spatial impression measured by Barron and Marshall which corresponds to ASW. It is termed DICC, in distinction from IACC measured with A-weighting.(2) Weber's law is applicable to the perception of ASW. The measured jnd becomes smaller as DICC becomes higher, and the Weber's ratio, ΔDICC/(1-DICC) is almost constant in the range from 0.2 to 0.3.(3) The equation and the chart to estimate ASW in degree are obtained as functions of DICC and the binaural summation of sound pressure level (BSPL). An increase of DICC by 0.1 causes a decrease of ASW by about 4° and an increase of BSPL by 1 dB causes an increase of ASW by about 1.6°. A decrease of DICC by 0.1 is equivalent to an increase of BSPL by 2.6 dB in ASW.

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In this paper, the reflection of a spherical sound wave by an infinite elastic plate is theoretically investigated. A far-field expression of the reflected field including the effects of the plate's elastic vibration is analytically derived in an explicit closed form. The energy ratio of the reflected sound wave by an infinite elastic plate to that by an infinite rigid plate is defined. It is shown that the ratio takes the same form as the energy reflection coefficient of an infinite elastic plate which has been derived for plane wave incidence by the authors. Thus, it is found that the effects of the plate's properties and the specific acoustic admittance of the plate surface appear in the same manner as seen in the case of plane wave incidence. Some numerical examples are shown, and the main features of the reflected sound field are discussed.

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The authors formulated a technical prediction model of auditory source width (ASW). By the model, ASW for any stimulus can be estimated by summing up 1/3 octave band ASW. It is necessary, however, to multiply weighting factor W by each 1/3 octave band ASW before summation. In this paper, the psychological experiment was performed to examine the validity of the model under the very fundamental condition that the number of adjacent 1/3 octave bands was two and each band had equal sound pressure levels. The results of the quantitative judgments of ASW confirmed the validity of the model and showed that the weighting factor depended on the frequency band and the subject. Roughly speaking, the weighting factor for 500 Hz band was larger than those for the other frequency bands under the given condition.

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In order to construct an evaluation method for structure-borne sound in building floor slabs, the dynamical analysis method for floor slab vibration in audio-frequency ranges and the numerical calculation method for sound radiation produced from slabs have already been developed. In the present paper, the heavy weight floor impact sound is numerically calculated by generalizing these developed methods, and the effectivity of this simulation method is shown. The vibrational characteristics of the diverse floor slabs and the stiffening effects of beams in particular are also clarified using this method

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Auditory spaciousness is primarily dependent on the strength of early lateral energy. The difference between the degree of spaciousness caused by only lateral reflections and that caused by lateral and ceiling reflections, however, has not been definitively clarified yet. In this paper, a series of psycho-acoustical experiments is performed to examine the effect of early reflections from upside on spaciousness with lateral reflections. The results show that a single reflection from upside causes a vertical spread of sound image and auditory envelopment becomes stronger as vertical component of reflected energy increases when lateral energy fraction and listening level are constant.

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The mechanism of seat-dip attenuation was examined using a simplified parallel barrier model representing rows of auditorium seats. From the results measured in the fullscale auditoria, seat-dip attenuation was thought to be caused by interference between the direct sound wave and the successive waves forming a negative-positive sequence in the impulse response immediately after the direct sound. The negative pressure wave was found to be caused by barrier (seat back) top diffraction and not to be reasonable to apply local reactive hypothesis. The measurement arrangements altering the geometry of the parallel barrier on the floor enabled to clarify the effect and cause of the positive wave. The diffraction at the barrier tops reflected off the floor, resulted in positive wave in the impulse response, caused the excessive attenuation at low frequencies interfering with the direct sound, and the arrival time of the positive wave determined the frequency of the maximum attenuation. The phase relationships depending on the diffraction angle were found to be crucial in order to explain the mechanism of the seatdip attenuation, and these can not be explained by vertical resonance alone suggested by the previous study

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The authors have indicated the possibility of auralization for a sound field consisting of only discrete reflections based on the running interaural cross-correlation (RCC) model, which is regarded as one of the mechanisms in binaural signal processing. In this paper, further experiments were performed to apply the RCC method to the auralization of a sound field which includes non-discrete reflections of 2s long, such as a concert hall. The results show that the sound image perceived in the simulated sound field by the RCC method using 10ms long temporal window with the inner ear mechanism is indistinguishable from the perceived one in the original sound field which is used in this paper. Moreover, all reflections of the impulse response must be reproduced for an auralization of a sound field which includes non-discrete (reverberant) reflections, though all reflections are not necessarily reproduced for the sound field which consists of only discrete reflections.

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