This fell to 68 dB(A), 69 dB(A) and 64 dB(A) at 10 m and although the localizer siren had less overall intensity, and a greater low frequency attenuation, high frequency penetration was much better. Inside a target vehicle at 5 m, sound pressure level was 71 dB(A), 72 dB(A) and 69 dB(A) for the yelp, wail and localizable sounds respectively. This approximates the exponential decay of the source signal that would be expected of any sound in a free field. Free field sound pressure levels at 1 m in front of the ambulance were 110 dB(A) for all three sirens, falling to 83 dB(A) at 30 m. This produced the proportional transit time, where a lower score was a faster than average journey and a higher score was a slower than average journey. Since road sections differed in length, the mean transit time for each section was calculated and each data point was then expressed as a percentage of this mean.
The reliability of the measurements was judged to be within six video frames or 0.24 s. The researcher watched each video tape in its entirety and recorded the time taken to negotiate each defined road section in the emergency run, without knowledge of which siren was being used. Three types of road (urban single lane carriageways dual carriageways junction complexes) were represented by five road sections each.
Times between defined start and finish points (usually a bridge or a road marking) on 15 standard and commonly used road sections were measured using the clock on the JVC SR-S368E video player. Since there was a wide variety of routes and destinations, which were unpredictable beforehand, upon completion of the data collection it was necessary to identify specific road sections over which transit times could be consistently measured. Transit times were measured as they are the primary method by which services are compared. The ambulance crew members were at liberty to withdraw from the study at any time and to revert back from the new siren system to the existing system at any time, had they felt it necessary. It was ensured that the crew members were not disrupted by the research or the presence of the equipment and that patient anonymity was preserved.
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