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An effective program for external noise reduction of vehicles was firstly established in Brazil by an agreement between ANFAVEA and CETESB in the beginning of last decade. IBAMA worked this proposition and published in 1993 the first CONAMA Resolution, including the pass-by noise limits and the deadlines to satisfy the new sound pressure levels. Since then, CONAMA is responsible for this subject. In 2001 lower levels were imposed, similar to those existing in Europe, with 2006 as the deadline for all vehicle families. The paper describes and gives and overview of the procedures, the acoustic technologies and improvements developed to achieve, in two steps, the new limits, mainly for commercial vehicles, like heavy trucks {from 92 dB(A) to 80 dB(A)} and passenger cars {from 84 dB(A) to 74 dB(A)}. We enumerate also the non-acoustic problems solved, for instance the thermal problems, due to the acoustic barriers applied to reduce pass-by noise.

Since the Weber-Fechner Law (1860) until 1950 there was no trustful method to calculate Loudness of complex sounds. At that time, ISO proposed three weighting curves, A, B and C based on rough approximations of the isophonic curves, 40, 70 and 100 phons. It was supposed to be a temporary suggestion. Curves B and C were abandoned, but A weighting survives until today! In 1957, Stevens and Zwicker presented two independent methods to obtain Loudness in sones, based on the new Stevens' Power Law. In 1965, Kryter introduced Noisiness in noys. Stevens in 1975 presented Perceived Magnitude as an improvement of Loudness calculation. In these acoustic parameters, the sound pressure levels per frequency band are transformed into acoustic sensation levels, and through the sensation spectrum the magnitude of the overall sensation is calculated. The A, B and C curves, for low, moderate and high levels do not follow this concept.

The paper looks for the degree of linear correlation of some acoustic parameters when their results are compared taking into consideration the internal noise of a set of passengers cars and another set of commercial vehicles. The acoustic parameters dealing with the overall noise (Loudness, dB(A) and dB(B)) and those related to the high frequency portion of the spectrum (Articulation Index and Sharpness) are chosen. The sound pressure levels in the passenger compartment are measured according to ISO 5128. The parameters are calculated in some specific velocities. A least square fit to a straight line to each pair of the acoustic parameters is applied. The results are discussed taking into consideration the degree of correlation trying to investigate how dependent or independent the parameters are. In addition it is possible to see the fluctuation range of these parameters concerning each set of vehicles.

The paper describes the acoustic development effected in one middle-class commercial vehicle with the purpose of achieving the external sound pressure levels dictated by new Brazilian legislation. The philosophy adopted as well as the mandatory modifications of the truck to fulfill the noise limits are showed together with the non-acoustic characteristics. Besides the acoustic treatment of some specific sources, it was introduced an acoustic barrier, usually called capsule or shield to reduce the noise emission of the engine-transmission assembly working by the mass law principle and sound absorption.

The paper looks for the degree of linear correlation of some acoustic parameters concerning Speech Intelligibility in vehicles. The Articulation Index (AI) was originally a criterion to characterize the influence of parasite noise on the Intelligibility of a conversation in the design of speech communication systems. Introduced in the automobile acoustics, it is more and more commonly used by vehicles manufacturers to estimate the middle and high frequency content of spectra noise inside various types of vehicles driven under several running conditions. The correlation with Speech Intelligibility, measured through subjective measurements is well known. Presently we have more sophisticated parameters like STI, RASTI, SII… directed mainly to architectural acoustics and sometimes used in vehicle acoustics. They require specific hardwares and softwares and are more complicated to deal with and often are called machine measures of Speech Intelligibility.

This paper uses sets of experimental data, concerning commercial vehicles, to study the correlation between pass by and stationary noises through measurements executed according to external noise standards. We consider two sets of experimental data: before the new legislation (higher levels) and lower levels satisfying the new limits in Brazil. In addition we calculate the Loudness in sones, from the octave band noise spectrum, and compare the results with the usual evaluation in dB(A), with the purpose of emphasizing the qualification besides the quantification aspects and promoting conditions for people understanding. Besides the overall noise, we show the bandwidth contribution for each parameter and analyse the spectrum profile associated to the “A” weighting curve applications. The discussions, conclusions and recommendations are supported by regression curves and graphics taken from the measurements effectuated involving the chosen parameters.