Low-frequency source height measurements

The Federal Highway Administration's development of a new highway noise prediction model (TNM) necessitated the collection of equivalent source height data on moving highway vehicles as a function of frequency. An original method developed by Glegg and Yoon was used in the initial collection of this data. Analysis of this data indicated the measured source height was overestimated at frequencies below 500 Hz. In order to improve the equivalent source height estimates below this frequency two alternative methods were investigated. The first method made use of the coherence function to remove noise from the autospectral density estimate for an array element through the cross spectra of two other elements. This method is called the noise extraction method and was found to be ineffective for this application. However, the second method used matched-field processing, and a significant improvement in the estimated equivalent source heights was achieved for frequencies below 500 Hz.