In the measurement of room impulse response, the synchronous averaging technique and such new methods as the MLS and the swept-sine methods are being widely used to improve the signal-to-noise ratio. In actual measurement conditions, however, the air in a room is continuously moving and the temperature is changing to some degree. The measured value of the reverberation time in such a room tends to be shorter at higher frequencies when applying the synchronous averaging. Therefore, the assumption of a time invariant has to be carefully considered, and, on this point, some research has been conducted to date. We also have reported various research results concerning the impulse response measurement under the time-variance conditions. In this paper, the permissible number of synchronous averaging times for reverberation measurement is studied through some field experiments. In each field, many time impulse response measurements were taken between a fixed pair of sound source and receiving positions by the swept-sine method, without averaging. After the measurements, the characteristics and the extent of the time-variance under measuring were estimated by a short-term running cross-correlation function between each impulse response. The influence of the time variance on the synchronous averaging result was studied based on the estimated time variance.
In order to propose an effective method using smartphones (including tablets) for acoustics education in introductory courses for architectural and environmental acoustics for architectural studies, the authors have examined some applications which work on smartphones. As the first step applications measuring sound level and spectrum at reasonable prices are chosen and their precisions have been verified. Results showed that the most iOS devices have somewhat reasonable precision, e.g., for SPL measurement it is similar to Class 2 sound level meter, though some Android devices give lower precision. Then the authors introduced these tools to students and encouraged them to use these tools to produce a noise map (with dB(A) only and with both dB(A) and sound spectra). Even though these are only simple tools, they allow students to understand the relationship between their sensation and physical values. As further studies, the authors also tried to use some more applications which enable students to measure more advanced physical values, such as Band Levels, Leq, Impulse Responses, Reverberation Times etc. Some examples of the measurement results of them are also presented.
The measurement of room impulse response is made under the assumption that the sound propagation system is time invariant. Actually, however, the air in a room is usually moving and the temperature is changing to a greater or lesser extent. In order to examine the influence of such time variance on the measurement of room impulse response, experimental investigations were performed in a reverberation room in which the air was excited by fans and in a concert hall in which the air-conditioning system was operated. Impulse response measurements were performed by the maximum-length sequence (MLS) method and the sweep pulse (SP) method and these results were compared. From the results, it has been found that the reverberation decay tends to become steeper by repeating the impulse response measurement to get high signal-to-noise ratio and the SP method is more robust than the MLS method against the influence of time variance of the atmospheric condition in a room.
In order to investigate the effect of hall response on players, field measurements on the stage of a concert hall and laboratory experiment using digital simulation technique were performed. In the field experiment, the subject, a professional violinist, was asked to play and to make comments on her acoustical impression of five points on the stage. As a physical measurement, impulse responses were obtained at the same points by using omni-directional loudspeakers as a sound source and an omni-directional microphone and directional microphones as receivers. As a result, it has been found that not only the strength of the early reflections but also their direction influences the subject’s impression. In the laboratory experiment (anechoic chamber), the sound field was modeled and synthesized by using a 13 channel reproduction system; ambient reverberation judged as being natural was provided by simple digital reverberators and different strength and direction of early reflections were obtained by real-time convolvers. For a constant value of reverberation, several conditions with a different level and direction of the early reflections were created. For each condition, the violin player was asked to make similar judgments as in the field experiment. The results of two experiments were examined.
In order to examine the accuracy of acoustic scale modeling, the experimental results of a 1/10-scale model study performed for acoustical design of a concert hall and those measured in the full-scale hall after the construction are compared. In the scale model study, the impulse responses were measured by the direct method using a spark discharge source and monaural and binaural microphone systems. For the measurement of the binaural impulse response, a 1/10-scale dummy head system was made on a trial basis. In the real hall, impulse responses were measured by the sweep-pulse method using a dodecahedral omnidirectional loudspeaker and monaural and binaural microphone systems. The monaural impulse responses were measured at the corresponding points in the scale model and the real hall, and such quantities as D50, C80, and Ts were obtained and compared. The binaural impulse responses measured in the model and real hall were convolved with the same sources of a variety of music and presented to the subjects through a transaural reproduction system using two loudspeakers. Through these quantitative and subjected investigations, a fairly good correspondence has been found between the scale model and the real hall.
