This paper mainly deals with the problem of sound diffraction by the slit which exists between two half infinite planes. The planes are located between a point source and a receiver in the arrangement that the edges of the planes are in parallel holding certain distance. At first, a rather complicated formula to predict the amount of diffraction effect was derived based on Keller's geometrical theory of diffraction almost exactly. Secondly an approximate formula, simpler and more practical, was derived from the above formula. Calculated values from there two formulae were almost the same. The results of calculation using the formulae agreed well with the results of calculation using a boundary element method in two dimensions, except the case where multi reflections are supposed to occur between two planes. It was also confirmed that the simpler formula can be used to predict double diffraction by two thin barriers.
An experiment was conducted concerning the effect of frequency characteristics of sound insulation on the impression of road traffic noise. Using semantic differential subjects judged the impression of stimuli which were simulated as if they were transmitted through facades having various sound transmission characteristics. The following results were found: (1) The timbre of road traffic noise was judged to be more powerful, fluctuating and unpleasant as the level of LAeq became higher. But the impression of “shrill” was not influenced by the values of LAeq. When overall values of LAeq were equal, the impression of “shrill” became stronger for the direct sounds than transmitted sounds. On the other hand, the calm impression became stronger for the sound equalized -5dB/oct. than direct sounds. The frequency characteristics of sound insulation affects the shrill impression. (2) The three factors were extracted by the factor analysis: they were “powerful” or “fluctuating, ” “metallic” and “unpleasant” factors. (3) The timbre of road traffic noise was judged to be more unpleasant as the level of LAeq, loudness level based on ISO532B (LLz) and each octave band from 2kHz to 4kHz became higher. Also the effects of closing windows (difference between 63dBA (direct sounds) to 43dBA (equalized sounds)) suggested that the level of each octave band level over 2kHz were related to the unpleasent impression. (4) It was suggested that the impression “unpleasant” could be approximately evaluated by the arithmetic average of sound pressure levels in octave bands from 500Hz to 4kHz (SIL).