NoiseMap Forums
NoiseMap Software => Support => Topic started by: roger on November 19, 2014, 10:44:37 AM
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There are a huge number of options with SiteNoise Calculations. Can you give any advice on how to choose the right ones?
The options in SiteNoise tend to reflect its development since 1989. SiteNoise has always tried to remain backwards-compatible, so that people using old models can still get 'historic' answers.
This is why there are options to use or ignore the angle of view correction for haul roads, and slant distance for distance calculations. These weren't in the early version of the BS, but should be used now for correctness. Similarly, the simple 0/5/10 barrier correction was in the early BS because without a calculator and a 3-d model, it was difficult to use a path-difference method.
Unfortunately, the new BS barrier calculation method needs not only the path difference, but also the source spectrum, which is often not available even now. If you don't have the source spectrum, you will have to fall back on the 0/5/10 or CRTN methods, noting that CRTN can give a lot more than 10 dB of screening and is fine if you are dealing with engine-driven plant.
However, if you have a spectrum, you can use the BS5228:2009 barrier correction calculation, which is likely to give you a bit more attenuation for engine-type sources than the CRTN method. This may be partly due to the fact that CRTN was devised for dealing with the attenuation of L10 from line sources.
The soft ground correction is slightly more tricky. BS5228-1:2009 requires a correction for the average height of propagation and source-receiver distance. The CRTN method also requires this, but give higher values of correction (lower noise levels). At 600 m, for an average height of propagation of 2 m, BS5228 give a soft ground correction of about 6 dB, as does CONCAWE (which does not take average height of propagation into account), but CRTN gives 9 dB. It may be noted that where there is screening, this will often be more than 9 dB, so this predominates and the value of soft ground correction is not relevant.
Atmospheric absorption is not explicitly included in the BS or CRTN methods (although it may be implicit in the distance attenuation calculation). It isn't usually significant for engine-type sources at closer distances, but it is required for wind farm calculations. It is applied to the spectrum if you have one, or to the A-weighted value (using the values for 250 Hz) if you don't.
Note that CRTN and BS5228 were set up to calculate for typical downwind propagation conditions, so there is no additional correction for wind direction.
People commonly use the CRTN method for barrier and soft ground if they don't have a spectrum and they use the BS 5228:1997 (equivalent to BS5228:2009 method) if they do have a spectrum. It may be confusing to mix part BS and part CRTN as it is possible there could be some co-dependency between the various adjustments.
The User Manual has more detail on the calculation method but ultimately, the user must rely on their own expertise.