Can the noise level form the exhaust be higher then the rest of the generating set without affecting the overall noise level?
Yes it can. If the limit is the average sound level 1 metre from the Generating Set as a whole, then a higher sound level from one relatively small source may not significantly affect the overall (average) level.
The easiest way to explain this is by considering the sound power level (Lw = acoustic energy emitted) of the Generating Set and exhaust, rather than the sound pressure level (Lp = what we hear/ measure at a specific location).
The sound power level of a source is obtained by adding 10 x Log10 (Measurement surface area) to the average sound pressure level of the measurement surface. For a Generating Set of dimensions 4 m long x 2 m wide x 2 m high, the measurement surface 1 metre from the Generating Set is a cuboid 6m long x 4 m wide x 3 m high, with a surface area of 84 m2. So, the sound power level for this Generating Set equating to an average sound pressure level of 75 dBA at 1 m is given by 75 + 10 x Log10 (84) which equals 94 dBA.
For an exhaust pipe (of a few inches diameter) projecting slightly from a Generating Set, this relatively small source acts as a ‘point source’ so that the area 1m from the end is approximately a hemisphere of radius 1m, with a difference of 8 dB between the average sound pressure level over the surface of the hemisphere and the corresponding sound power level emitted from the exhaust. This means that if the sound pressure level 1 m from the exhaust is 80 dBA, the corresponding sound power level from the emitted from exhaust is 88 dBA.
If the Generating Set itself produces an average sound pressure level of 74 dBA at 1 m (equating to a sound power level of 93 dBA) and the exhaust produces 80 dBA at 1 m (equating to a sound power level of 88 dBA) the resultant overall sound power level is 94 dBA (due to the logarithmic addition of sound levels). As noted above this equates to an average level of 75 dBA at a distance of 1 m from the Generating Set (including exhaust) as a whole.
Additional Information – relevant formulae and Standards
Lw = Lp + 10 Log10 (A)
where
Lw – sound power level of the source being measured
Lp – average sound pressure level over the measurement surface
A – Measurement surface area (m2)
LpTot = 10xLog10 (10Lp1/10 + 10Lp2/10 + …)
where
LpTot = total sound pressure level emitted by the combined sources
Lp1 – sound pressure level emitted by source 1
Lp2 – sound pressure level emitted by source 2, …
LpAv = 10xLog10 ((10Lp1/10 + 10Lp2/10 + 10Lpn/10) / n)
where
LpAv = total sound pressure level emitted by the combined sources
Lp1 – sound pressure level emitted by source 1
Lp2 – sound pressure level emitted by source 2, … Lpn
n – number of sources
Relevant Standards relating to this type of acoustic measurement/ assessment procedure include:
ISO 8528-10:1998 Reciprocating internal combustion engine driven alternating current generating sets — Part 10: Measurement of airborne noise by the enveloping surface method
ISO 3744:2010 Acoustics — Determination of sound power levels and sound energy levels of noise sources using sound pressure — Engineering methods for an essentially free field over a reflecting plane
ISO 3745:2012 Acoustics — Determination of sound power levels and sound energy levels of noise sources using sound pressure — Precision methods for anechoic rooms and hemi-anechoic rooms
ISO 3746:2010 Acoustics — Determination of sound power levels and sound energy levels of noise sources using sound pressure — Survey method using an enveloping measurement surface over a reflecting plane.