The two hearing tests were performed with the object of discussion of the applicability of Noise and Number Index (NNI) to the subjective rating of traffic noise in order to estimate the annoyance of traffic noise and aircraft noise by the same scale. In this case the NNI is calculated by using the number of traffic noise peaks heard while the NNI of the aircraft noise is calculated by using the number of flyovers. The noise samples were the traffic noise recorded at the side of the traffic road (Fig. 1a) and the series of aircraft noises recorded at the field in the vicinity of the airport (Fig. 1b & Table 5). Both noises were reproduced at the various NNI values (Table 2 & 6) in the testing rooms (Fig. 2 & 6). The subjects rated the annoyance of these noises on the six categories rating scale (Table 4) while calculating the simple arithmetical tasks (Table 3 & 7). Fig. 3 shows the results obtained from the hearing tests. The conclusions are as follows. 1) The annoyance of the traffic noise can be estimated by the NNI. In this case N is not the number of vehicles but the peaks of the traffic noise heard. 2) The NNI of the traffic noise is larger than that of the aircraft noise at the same degree of annoyance, and the difference is 6. 5 NNI, therefore the comparison between the annoyance of the traffic and the aircraft noise can be made by the NNI considering this difference in the case of the traffic conditions shown in Table 1. 3) The difference of the effects due to the traffic noise and the aircraft noise on the arithmetical tasks was hardly seen.
As the method of measuring the sound absorption coeffient of acoustic materials, we have mainly used the reverberation chamber method for the random incidence and the standing wave tube method for the normal incidence. As to the method of measuring the acoustical characteristics of a material for oblique incidence, numerous investigations have been developed by many authors from different viewpoints. However, a particular room is always required in those methods except the tube method. so that the measurements will be much influenced by the room characteristics and an on-the-spot measurement cannot be made. Furthermore, the estimation of sound absorption coefficient is more laborious in the above methods than in a new method, which will be discussed in this paper. For these reasons, the new method of obtaining the sound absorption coefficient is proposed here, in which it is possible to obtain the coefficient through an on-the-spot measurement (field measurement) that has been considered very difficult. This paper describes that for tone bursts of duration of 10 milliseconds the reflection from a sample can be obtained by combining the outputs from two undirectional microphones through a phase inverter (see Fig. 2) and by comparing each amplitude of some intermediate cycles with that of the corresponding cycle of the direct sound, which is measured separately, we can estimate the sound pressure reflection coefficient, from which the absorption coefficient is obtained. The results for several kinds of samples which were obtained by this method agree with those from the tube method within a few percent at several test frequencies from 250 Hz to 4 kHz (see Figs. 10, 11, 12), and the result for a cloth with air space behind it is also consistent with the absorption coefficient already known (see Fig. 13). Further the reflection coefficient of a rigid square panel obtained by this method agrees well with the coefficient calculated from an approximate formula based on the Fraunhofer diffraction after Maekawa and Sakurai. It is also found that a diffraction phenomenon doesn't appear before the reflection from the panel edge arrives at the receiving point. This method of measurement may be conveniently applied not only to the ordinary measurement of the absorption characteristics of materials, but also to the study of room acoustics through an on-the-spot measurement.