Prediction of road traffic noise in the vicinity of a road tunnel entrance has been one of the most difficult problems. The built-up noise in the tunnel radiates towards outside the entrance. A practical calculation model for this noise radiation is developed in this paper, which is based upon a sound energy balance inside the tunnel. Two imaginary sources are assumed. One is a point source which represents a direct sound from a vehicle in the tunnel. The geometrical location of the imaginary point source is determined by the position of the vehicle and by a new parameter “a” that is related to the acoustical properties of the inside walls. The second is a surface source which represents residual sound with multiple reflection between the walls of the tunnel. This source is specified at the tunnel entrance. Results of a sound level (LAeq) measurement at a highway tunnel shows good agreement with an accuracy of less than 3dB between the calculated and the measured. It is shown that this model is applicable to road traffic noise simulation at a tunnel with a noise control design by absorptive treatment on the walls.
In the surface intensity measurements, a microphone and an accelerometer are used to measure surface acoustical pressure and normal velocity of vibrating structure. Therefore, an efficient technique for setting two different sensors on vibrating structure is required to measure the surface intensity at multiple points. In this study, we developed a sensor-probe whose microphone was mounted on an accelerometer in order to simplify setting the sensors on vibrating structure. In the experiment, the intensity was simultaneously measured at multiple points of a vibrating structure using a sensor-probe array. The result showed that a newly developed technique can estimate the intensity radiated from a steel sheet where an exciting point had been traveling. The measurement error is also caused by instrumental phase mismatch. We calibrated the phase characteristics of the microphone in sound fields near vibrating surfaces. The correction coefficients were determined by comparing the phase characteristics of the microphone and the vibrating surface of a piezo-electric shaker. The intensity measurements were carried out with and without phase correction of the instrumentation. The result showed that the effect of the phase correction was necessary in the higher frequency range above 2kHz.