In order to determine the influence that a vehicle’s shadow effect can have in the effectiveness of an emergency vehicle siren noise, acoustical field-testing must first be carried out. For this study, a large empty parking lot was used. Ambient noise and weather conditions were recorded to ensure appropriate noise measurement conditions. The shadow vehicle (barrier) used was a 2006 Toyota Sienna while a 2010 Ford Focus was used as the receiver vehicle. The field testing was composed of two different scenarios; on-axis and off-axis position testing. On-axis, also known as in line or parallel testing, represented the scenario where the siren device is located behind the receiver vehicle and the shadow vehicle is positioned just behind the receiver vehicle. This portion of the experiment was conducted to simulate a traffic situation where an emergency vehicle is travelling on a road behind and in the same travelling direction as the shadow and receiver vehicles. The objective was to determine the role that a vehicle as a barrier plays in a parallel driving situation on siren effectiveness. This test was conducted for two different scenarios, where the siren is positioned at two different distances from the vehicles, but the vehicles still remain the same distance from one another. The purpose of testing at two different distances was to determine to what extent the distance between the source and receiver plays in siren effectiveness when a shadowing vehicle is present. Figure 1 shows the two different positions. For the testing without the shadow vehicle, the van was simply removed from the testing environment completely.