A method of narrow-band digital signal processing for the measurement of transfer characteristics of the system with long reverberation is described. At first the effects of reverberation on the measurement of transfer characteristics are discussed. The experimental results indicate that it is necessary to utilize the weighted time window, such as Hanning window, whose length is much longer than reverberation time. However the maximum window length of FFT processor is often too short when the acoustic system has long reverberation. The proposed method solves the problem of the limited window length by using the characteristics of FFT and the sampling theorem for band-limited signal. The method of processing is described in detail. The digital filter for obtaining the band-limited signal is also discussed. From the experimental results it is found that the proposed method is very useful for the accurate measurement of the transfer characteristics.
A smallsized digitalized measuring instrument is described by which the reverberation time of auditoria and studios can be simply measured in the field. A calculating process of the instrument is based on Schroeder's "integrated impulse method", and the fundamental principle is the replacement of visual linear approximation of logarithmic decay curves with the method of least squares. Reading out procedure of wave forms of tone burst signals in all frequency ranges from ROM is adopted. Accuracy of calculation is improved and measuring times are considerable shortened by complete digitalization and automatic measuring operations. Measured results are displayed on a videomonitor or the like. By this measuring instrument, it is possible to measure not only reverberation time but also each room acoustic parameter such as the ratio of early to reverberant sound energy and "center time "of pulse response.
The propagation characteristics of solid-borne sound in building structures are studied by field measurements on three standing buildings and scale model experiments using a simplified building structure. Both in the field measurements and the scale model experiments, vibration acceleration levels(V. A. L. )at a number of widely distributed points in the structures are measured as one point of the structure is excited. From the results of the scale model experiments, it is revealed that the relative V. A. L. at any point is determined by its distance(r)from the excitation point via the shortest route in the structure, regardless of the propagating direction. It is assumed that the variation of V. A. L. (L_r)in buildings can be expressed as, L_r=P_0-Nlog r-Mr(P_0 depends on the strength of excitation)and the value N and M are examined from the results of measurements. From the results of the scale model experiments, the value of N is determined to be 20, regardless of frequency. Using this value, the M value in each octave band is determined from the results of the measurements taken on the standing buildings. The result shows that the M value is approximately proportional to the square root of the frequency.
Some experimental results on the sound reinforcement system design for multipurpose halls are presented, as musical entertainments fully using the electroacoustic equipments are increasing currently. The experiments have been made through the sound reinforcement system designs for the World Popular Song Festival annually held at the Nippon Budokan Hall. For the purpose of optimum loudspeaker system design, comparison between the distributed and the concentrated layouts of loudspeakers is made. Following informations concerned with the reinforced sound field are obtained. (1)Average SPL of reinforced sound is 91 to 92dB. (2)Average spectrum of reinforced sound is measured and dynamic range is approximately 40dB. (3)The peak factor of RMS output voltage of power amplifier ranges from 20 to 25dB, and the total power output corresponding to the peak value required is estimated to be approximately 10kW in the hall concerned. These feature the reinforced sound of latest popular music and will provide the necessary information on the sound reinforcement system design hereafter.
Reflection characteristics of a sound absorbing wedge are related to acoustical properties of material as well as wedge dimensions. However, there are no reliable values of the acoustical properties of materials, so wedge design problems have been experimentally solved in each case. In this paper, the values of acoustical constants of glass wool such as flow resistance, effective density and effective volume elasticity are investigated and it is confirmed that the most important factor is the flow resistance. Next, the wedge design including selections of material and wedge dimensions are studied by the computer simulation whose results agree well with experimental results, and the useful results about the influence of base length, air space and wedge angle on the absorbing characteristics of wedge are obtained. Among these results, the most interesting one is that the wedge having satisfactory characteristics can be made by choosing the optimum base length corresponding to the value of flow resistance.