4. Investigation of the accuracy of the measure of Leq(20 s) and
the measure of the energy mean emission level
In the FHWA model, each vehicle is presumed to be an acoustical
monopole or single point source which moves along the traffic
lane. One advantage of Leq(20 s) model is that the measurements of
Leq(20 s) do not need the assumption that the sound source must
be the point source, which makes the value obtained by in-site
measurement accurate [12].
To determine whether the hypothesis of monopole source in
FHWA model is reasonable, a series of real-time in-site measurements
of Leq(5 s) and the maximum emission levels of individual
light, medium and heavy vehicles was performed on roadsides.
(The energy superposition of the maximum emission levels of a
great number of same-type vehicles formed the energy mean
emission level, ðLeqÞEi). Leq(5 s) was also calculated using Eqs. (15)
and (16). When Eqs. (15) and (16) were employed to calculate
Leq(5 s), ðL0ÞEi was replaced by simultaneous measurements of
the maximum emission levels of individual vehicles on roadsides.
The measured Leq(5 s) was compared with that calculated from
the simultaneous measured maximum emission levels of individual
vehicles. The magnitude of the differences could accordingly
reflect whether the hypothesis of monopole source in the FHWA
model was reasonable. In this study, Leq(5 s) was measured instead
of Leq(20 s) for convenience and simplicity. Although the differences
between the measured Leq(5 s) and the calculations from
the simultaneous measured maximum emission levels of individual
vehicles could not elucidate the differences between Leq(20 s)
and ðL0ÞEi, the results could reflect which hypothesis, monopole
source or dipole source was close to reality.
The result obtained from Eq. (6) with the hypothesis of monopole
source was