Calculation Methods

NoiseMap incorporates general-purpose and UK specific calculation methods that can be used for most types of open-air noise propagation.  It is not intended for noise within buildings.

SiteNoise

This is a general-purpose calculation method that can be used for stationary point sources, moving point sources and line sources of noise. It incorporates the methodology set out in BS5228-1:2009 Code of practice for noise and vibration control on construction and open sites – Part 1: Noise, and methodology set out in ISO 9613 Attenuation of sound during propagation outdoors, Part 1 calculation of the absorption of sound by the atmosphere and Part 2 General method of calculation.

BS5228 methodology
This is a general-purpose method of predicting noise based on A-weighted sound levels. This has the benefit of simplicity: source levels are easier to determine and require less information about the source, receiver and propagation path. It is robust, well-proven and widely used for typical construction sources such as those with diesel engines, but may have less accuracy for sources with a different spectrum, such as a wind turbine. Furthermore, the A-weighted level is less helpful where the noise criterion is set in some other index such as the NR (Noise Rating) level.

Barrier calculations
BS5228 offers ‘a more accurate’ calculation of barrier attenuation ‘if the octave band sound levels’ are known. To this end, the BS provides tables of sound level data giving octave band sound levels from 63 Hz to 8 kHz. Note that SiteNoise also requires a level for 31.5 Hz. Enter a small value (say 1 dB) if you do not have a level for this octave. If you do not have any octave band data, you should calculate barrier attenuation using either the CRTN option or the ‘simple’ option which are described below.

Ground attenuation
BS5228 does not provide guidance on the frequency effects of ground attenuation. SiteNoise offers various options for ground attenuation, but these are applied to the A-weighted value, or uniformly across the spectrum where octave calculations are required. The IoA guidance on Wind Farms advises that minimal ground absorption should be included in calculations, as this is not usually a significant effect in such cases.

Atmospheric absorption
BS5228 does not deal with atmospheric absorption, which is highly dependent on frequency but can be significant beyond a few hundred metres. Since the Institute of Acoustics Guidance on Wind Farm calculation requires Atmospheric Absorption to be taken into account, using the method set out in ISO 9613, this is provided as a SiteNoise calculation option. This requires octave band source information. If you do not have octave band levels, you can undertake an A-weighted calculation, in which case SiteNoise will apply the atmospheric absorption for 250 Hz to the A-weighted calculation.

Note that the atmospheric absorption depends on the air temperature and relative humidity, which you set under the Calculation Parameters. Typical values (recommended for Wind Farms in the UK) are 10 oC and 70 % RH.

SiteNoise Calculation options

SiteNoise provides options to:

  • Calculate A-weighted level from A-weighted source values;
  • Calculate A-weighted level from octave band source spectra;
  • Calculate octave band levels from octave band source spectra.

You select the desired option when you run the calculation. Certain options require octave band source levels to be given, as shown below.

Options available with A-weighted source levels

If you only have the A-weighted source levels, you can use the following calculation options:

  • BS5228 LAeq calculation method
  • CRTN Barrier procedure
  • Simple 0/5/10 barrier procedure
  • ISO 9613 atmospheric absorption (applies the 250 Hz band attenuation to the A-weighted value);

Options available with Octave band spectra

If you have the Octave Band source spectra, you can use any of the above options, plus

  • BS5228 LAeq (Octaves) calculation method (calculates each octave band level and the A-weighted level)
  • BS5228 Barrier procedure
  • ISO 9613 atmospheric absorption (applies the relevant attenuation to each octave band)

Calculation checks

You do not need to give full spectra for every item of plant as long as that plant is not used in a calculation requiring the spectra. NoiseMap checks that appropriate source levels are available prior to calculation and will report any problems.

WIND DIRECTION

Wind direction has a major effect on sound propagation: it can increase sound levels downwind of a source by maybe 5 dB, and reduce them upwind of a source by maybe 12 dB, compared with still air. However, experience shows that attempts to include wind speed and direction as a parameter can actually reduce the accuracy of prediction. This is because the wind vector and associated atmospheric turbulence are always fluctuating. BS5228: 1997/2009 and SiteNoise, like most prediction techniques, are therefore validated against a typical downwind condition, where noise levels are slightly elevated relative to still air. The method is validated against measurements taken over sufficient time to average short-term fluctuations. Consequently, the model does not require information on wind or other meteorological effects.

SOFT GROUND

Sound attenuates more rapidly when the intervening ground is acoustically soft, in other words, porous to airflow. Although different types of ground cover may have different porosity, SiteNoise only recognises two ground types – hard and soft. Hard ground is impervious, such as concrete, compacted earth or water. All other surfaces, including loose gravel, trees, and grassland, are porous and therefore acoustically soft.

SiteNoise contains three different soft ground correction algorithms, described mathematically later in this chapter. The algorithms give different results, as illustrated in the figure below. Note that all of these algorithms apply to the A-weighted level. When calculating octave spectra, the same correction is applied to each frequency.

  • BS5228: 1997/2009 – SiteNoise applies the correction to static, mobile and haul road sources. This depends on the average height of propagation between the source and the receiver, which SiteNoise calculates from the ground model. It is therefore necessary to have ground contour information between the source and the receiver. This need not be greatly detailed, as long as the top and bottom of significant ground features are included.
  • CRTN 1988 – Calculation of Road Traffic Noise contains a methodology for predicting soft ground attenuation, which is stated to be accurate to 300 m from the source, and less accurate at greater distances. Like BS 5228, the attenuation depends on the average height of propagation, so an accurate ground model is required.
  • CONCAWE – As implemented in SiteNoise, a fixed rate of attenuation with distance is assumed. It is not dependent on average height of propagation or sound spectrum and does not require a ground model.

 

BARRIER ATTENUATION

SiteNoise provides three methods:

    • 0/5/10 – This is a simple method, giving zero screening when the source is visible, 5 dB when it is just screened, and 10 dB when it is well-screened. Although simple, it is based on observations and is satisfactory when height data is poor.
    • CRTN – uses the barrier attenuation curve from ‘Calculation of Road Traffic Noise’. Does not require a knowledge of the spectrum of the sound source, but assumes that it is not too dissimilar to that of road vehicles.
    • BS5228: 1997/2009 – requires the octave spectrum of the sound source. If you don’t have a spectrum, use CRTN or simple method. The curves only cover the octave frequency bands of 125 Hz to 2000 Hz. If you enter values for other octaves in the range 31.5 Hz to 8000 Hz, the 125 Hz attenuation will be applied to lower frequencies and the 2000 Hz attenuation will be applied to the higher frequencies.

Where there is more than one barrier between the source and the receiver, the screening correction used is the correction for the single most effective barrier between the source and the receiver.

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RoadNoise Calculation Methods

The Road calculation options only calculate for highway-type road sources. Rail and Site sources have their own calculation procedures. For RoadNoise, the calculation will depend on the periods covered by the traffic data – which can be 18-hour, day/evening night or 24 by 1-hour periods. Using this data, the calculation options are:

  • RoadNoise CRTN 2005: This calculates L10 using the standard CRTN calculation method and then applies adjustments to obtain Lden, Ld, Le, Ln, Leq 16-hour and Lden(ROI) (Method A or B). The Lden calculation and its components are calculated according to the method advised by Defra/TRL in 2005.
  • RoadNoise CRTN 2003: This calculates the same parameters as RoadNoise 2005, but using the slightly different method for converting L10 to Lden originally advised by Defra/TRL in 2003. It is retained for backward compatibility but should not normally be used.
  • RoadNoise Leq – the Noise Advisory Council LAeq method. This method does not give the same results for Leq as the 2005 method and is no longer widely used.

RoadNoise CRTN 2005 method is recommended in normal cases, as this also includes the CRTN L10 method. NoiseMap automatically chooses between Defra/ TRL Methods 1, 2 or 3 according to the traffic data you have provided.

Hint: Validity of Traffic Data

Note that CRTN states the following range of validity for traffic data: Percentage of heavy vehicles 0 to 80 %; Traffic Speed 20 to 130 km/h; Flow rate 1000 to 120,000 vehicles per 18 hours. BUT the TRL conversion method 2 requires percentage of heavy vehicles to be greater than zero. Note that NoiseMap uses a traffic speed of 20 km/h if a lower speed is encountered, to avoid misbehaviour of the prediction formula.

L10 to LAeq conversion methods

CRTN calculates the LA10 (1-hour) and LA10 (18-hour) indexes, whereas the EU’s Environmental Noise Directive and other assessment procedures now use the LAeq index evaluated over various periods, along with Lden, Lnight and the separate Ld, Le and Ln indexes. The LAeq is closely correlated to the LA10 in the case of road traffic noise, and so it has been recommended by Defra to obtain LAeq-based indexes from LA10 values. Defra issued two conversion methods, one in 2003 and one in 2005.

2003 method

The 2003 conversion method gives three methods, namely:

Method 1 requires 24 x 1-hour traffic flows;

Method 2 requires traffic flows for three periods, day, evening and night and also the 18-hour flow. Note that the 18-hour period is not the sum of the day and evening periods.

Method 3 uses the18-hour traffic flows.

Formulae are given for converting the LA10 levels for each period into Ld/e/n using the appropriate formula, then all the segment contributions are combined.

2005 method

The 2005 method differs from the 2003 method in certain details.

  • Method 3 (18-hour LA10 to Ld/e/n) is the preferred method;
  • LA10 values for all non-motorway segments are combined and converted to Ld/e/n, noise from all motorway segments are combined and converted to Ld/e/n and these totals summed to give the noise level at the receptor.

In other words, the 2003 method converts the noise contribution from each segment to Ld/e/n, whilst the 2005 method gets the total noise level in LA10 and then converts it. Because the conversion formula is linear in terms of noise levels whilst the combination formula is logarithmic, this can result in slightly different final noise levels. The change was made because otherwise the noise level depends on the segmentation of the roads and certain software (not NoiseMap) divided roads into more segments than recommended by CRTN.

Choice of method

The TRL report PR/SE/451/02 (2002) that defines the conversion methods states that Method 1 is the preferred method. However, this assumes that you have accurate counts of hourly traffic flow, percentage of heavy vehicles and speed.  These are rarely available. In practice, Method 3 seems acceptable for most cases, but where the night-time traffic flows do not fit the usual pattern, then one of the other methods should be better. If you are dealing with a ‘managed motorway’ there is likely to be a detailed traffic flow model that can provide these for you.

It would therefore seem advisable to use the 2005 method with 18-hour flows, unless there is a specific need to investigate the variation of noise levels throughout the day, or where traffic flows differ very significantly from the typical – especially with exceptionally high night-time flows.

Note that you do not specifically select Method 1, 2 or 3 – NoiseMap uses the method appropriate for the traffic flows supplied.

Lnight

DMRB states that “Lnight values calculated using the conversion formula are façade values and therefore 2.5 dB should be subtracted.” This is true if you have chosen to calculate for individual façade receivers, but not true if you select free-field receivers or are plotting noise contours. In such cases, do not subtract 2.5 dB.

 

NRA/RoI Method

The National Roads Authority of the Republic of Ireland publishes Guidelines for the treatment of Noise and Vibration in National Road Schemes. The current version (published 25th October 2004) is based on the use of CRTN with conversion formulae to give Lden, which is the noise index used by NRA.

The Guidelines give two methods, A and B, for calculation of Lden, with method A (the preferred method) requiring the calculation of 24 1-hour L10 values from 24 1-hour traffic flows, and method B requiring the calculation of the L10 (18-hour) from the 18-hour traffic flow. The Guidelines give formulae for converting these values to Lden. WS Atkins has produced a research paper which shows that when the NRA’s standard diurnal traffic flow profile is used, Methods A and B produce very slightly different results, but the Method B results can be converted to Method A results by the application of a simple formula, to give an identical result within calculation tolerances. This means that it is unnecessary to enter the 24 1-hour traffic flows in order to use Method A, as long as the standard traffic profile is applicable (which is true in almost every situation).

DMRB 2008 Additional procedures

The Design Manual for Roads and Bridges advises on additions to the procedures in CRTN 1988 to deal with situations not covered by the original procedures. These are activated by RoadNoise when DMRB 2008 Procedures is selected in the Calculation Parameters settings. These procedures also apply to later versions of DMRB.

In summary, choosing this option:

  • Limits soft ground attenuation to 600 m;
  • Calculates the cut-off distance for reflections from a formula.

Other provisions within the DMRB additional procedures serve to clarify how low-noise road surfaces and absorbent barriers should be treated. Features already within NoiseMap allow this advice to be implemented and you do not need to invoke the DMRB 2008 option in order to use them.

Road Surface Correction

(DMRB Para A4.16 – A4.30

Where low-noise surfaces are being used, DMRB states that the correction is to be calculated from measurements of the “Road Surface Influence”, and will therefore be particular to each road surface.

NoiseMap’s Road Segment properties window already allows a surface correction to be entered manually. The user needs to follow the guidance in DMRB to work out the correction to be applied.

DMRB advises that in some cases a low-noise surface correction may only apply at speeds at or above 75 km/h. NoiseMap now offers this option. When this option is selected, then a surface correction of ‑1 dB(A) is applied below this speed.

Note that when this option is used, and the speed is not marked as ‘adjusted for gradient’, then speed will vary according to gradient, traffic composition, etc. This can mean a sudden change in surface correction if the speed crosses the 75 km/h boundary.

Distance correction beyond 300 m

(DMRB Para A4.31)

The Hard Ground correction can be extrapolated to any distance as it is simply related to geometrical spreading. The normal operation of NoiseMap is not changed by this.

The Soft Ground correction is calculated from the CRTN formula for distances up to 600 m. The 600-m value is then applied to greater propagation distances.

Sound absorptive barriers and retained cuts

CRTN 88 provides advice on sound absorptive surfaces in the case of retained cuts or parallel reflective barriers, but it does not deal with sound absorptive barriers explicitly.

For retained cuts with absorptive walls, the adjustment sets CRTN parameter Δ5 = 0.

DMRB para A4.32 and A4.33 states that where the barriers or cutting walls are absorptive, this will reduce reflection effects, but no allowance is given in CRTN to allow for the effect of absorption to be calculated. The wording of DMRB implies that when a barrier is absorptive, this will remove some but not all of the reflection effect. It advises that when doing a calculation for an absorptive barrier, all the reflection should be removed, but it should be noted that this might overestimate the effectiveness of the absorptive barrier.

NoiseMap (from version 5.0.27) does not apply the ‘opposite barrier’ reflection when the barrier is marked as absorbent in its properties window. Prior to this version, the DMRB 2008 option must be selected, otherwise ‘opposite barrier’ reflections will still occur.

Note that reflections will still occur from building outlines, as these cannot be made absorptive.

Cut-off distance for reflections

NoiseMap has always allowed the user to set the distance (COR parameter) beyond which reflections from opposite barriers/façades are insignificant.

DMRB Para A4.34 now provides a formula for adjusting the cut-off distance during the calculation to take account of the distance of the receiver from the road and the distance of the opposite façade from the road, as set out in DMRB, as follows:

(a) if d < 12 m and D ≤ 20 m, then apply opposite façade correction as per present system,

[ie if d<12 m then COR=20m]

(b) if 12 m < d ≤ 300 m, then cut-off distance increases:

COR = 10^(0.825 +0.4*log10(d+3.5))

[d = horiz dist between receiver and kerb, DRC; D = dist between source line and opposite façade]

Variable Speeds

The modelling of roads with a variable speed limit is undertaken as normal, with any predicted changes in average traffic speeds, flow and composition being taken into account in the input parameters to the noise calculations.

CRTN gives charts for basic noise level based on hourly flow and on 18-hour flow, stating in para 13.2 that where hourly traffic flows are available the value of L10 (18-hour) should be determined from the 18 one-hour values.

Hourly traffic flows are being provided for variable speed schemes. NoiseMap now implements the hourly calculation method which allows users to be able to deal properly with variable speeds.

Hourly calculations do not require the DMRB 2008 option to be selected. They will be done automatically when the appropriate traffic data is supplied.

RailNoise Calculation Methods

There are three options for RailNoise calculations

  • CRN Leq – calculation of the Leq index in accordance with ‘Calculation of Railway Noise 1995’ and subsequent addenda;
  • TNPM Lmax– calculation of the Lmax index in accordance with the TNPM system used for the Channel Tunnel Rail Link
  • TNPM LAeq calculation of the Leq index in accordance with the TNPM system used for the Channel Tunnel Rail Link.

The TNPM methods are specialist procedures and full details are given in a separate manual available from NoiseMap Ltd.

The Leq index uses the measurement time to be set via the Parameter> Edit Calculation Parameter dialogue. The same dialogue is used to set up the cut-off distances, angle of view and calculation precision.