The minimum separation-distance criteria were based on Caltrans' California REMEL study.(24)
In the Caltrans study, the following assumptions were made: (1) the vehicle behaves as a point source, i.e., spherical divergence is assumed; and (2) there is no ground attenuation of the emission level. In addition, the ambient level was at least 10 dB less than the LAFmx of the observed vehicle.
In general, when a vehicle approaches a measurement microphone at a constant speed, the observed sound level at the microphone is related to the vehicle position as follows:
L2 = L1 - 20 × Log10 √ ( Δ X2 + D2) / D
|where:||L2 is the contribution to the measured emission level of the subject vehicle, Vehicle 1 at X1, due to a subsequent vehicle, Vehicle 2, at X2;|
|L1 is the contribution to the measured emission level of the subject vehicle, Vehicle 1, due entirely to Vehicle 1 at X1;|
|Δ X is the distance between X1 and X2, or the minimum separation distance to be determined; and|
|D is the distance from the microphone to X1, or 15 m in this Case.|
If other vehicles are in proximity of the subject vehicle to be measured, the measured sound level at the microphone for the subject vehicle may increase due to contamination. A maximum of 0.5 dB contamination is considered allowable.
Based on the 0.5-dB criterion, the next step is to determine the associated separation-distance criteria. Potential sources of contamination include contamination due to ambient noise, as well as contamination due to other vehicles in proximity of the subject vehicle (See Figure 20).
The maximum contamination due to ambient noise was determined to be 0.4 dB, assuming the ambient level is 10 dB less than the LAFmx of observed vehicles. Consequently, a maximum 0.1-dB contamination due to subsequent vehicles, based on the 0.5-dB contamination criterion, is allowed.
To ensure no more than 0.1-dB contamination due to subsequent vehicles, it was determined that the emission level due to a subsequent vehicle, Vehicle 2 in the case of Figure 20, must be at least 15.9 dB below that of the subject vehicle, Vehicle 1. The next step was to determine the separation distance associated with the 15.9-dB requirement.
Using the above equation and substituting the following values:
|L2||= LAFmx - 15.9|
|D||= 15 m,|
Δ X was solved for.
Figure 20. Minimum separation distance between two similar vehicles.
For REMELs measured at 15 m (50 ft), a minimum separation distance of 93.9 m (308 ft) between similar vehicles was required to ensure that the total contamination was not greater than 0.5 dB. For automobiles in the vicinity of heavy trucks, a minimum separation distance of 300.2 m (985 ft) between the automobile and heavy truck was required, assuming a heavy truck is 10 dB louder than an automobile at comparable speeds.