Insertion loss: Train and light-vehicle horns and railroad-crossing sound levels.

Abstract

Studies and analyses of sound levels inside road vehicles frequently require estimation of the sound levels of moving or changing positions of the source (such as a train horn) and receiver (road vehicle). The study presented here focuses on two of the components of the sound transmission from the source to the interior of an automobile. One part of this paper is the development of a method to predict the attenuation of the sound level over the path from the source to the vehicle. The method is based on the classical sound decay equation (including variable directivity and theoretical 6-dB drop-off per doubling of distance) but is modified to accommodate different drop-off rates including experimentally measured values. An example using a typical train horn sound power level is provided. The second portion of the sound path covered in this paper is the attenuation of the sound level due to transmission through vehicle bodies and relates to annoyance of vehicle exterior noise. Insertion loss values and insertion loss spectra are measured for seven different light vehicles. An unusual property of insertion loss spectra is observed and studied. It is shown that direct subtraction of measured band-filtered levels can provide misleading overall levels. A method of correction of the spectrum is presented.