Week 4 Workshop: Prediction of Road Traffic Noise.
WHEN YOU HAVE COMPLETED THIS WORKSHOP YOU SHOULD:
- know what road traffic and road geometry factors affect traffic noise levels;
- be able to predict LA10 noise levels from road traffic using either:
- graphical procedures;
- a computer (using the Noisebar program)
- WHY PREDICT ROAD TRAFFIC NOISE LEVELS?
While measurement of noise levels will nearly always be the most desirable method of ascertaining a noise environment, there are many situations, often the most important situations, where prediction is essential, e.g.
- where a new roadway is to be constructed through an existing urban area;
- where urban development and roadways are being planned concurrently;
- where there is an existing traffic noise problem and we wish to evaluate various strategies (say, the erection of barriers) which could ameliorate the problem;
- where noise levels need to be evaluated for future traffic conditions.
- WHAT ARE THE VARIABLES THAT AFFECT ROAD TRAFFIC NOISE LEVELS?
A stream of vehicles on a roadway can be regarded acoustically as a distributed line source. The noise level at a point beside the stream is then dependent on the strength of the source, the path length from source to receiver, and any excess attenuation during propagation. The variables likely to affect source strength and propagation are listed below.
- Source strength variables:
- number of vehicles on the road
- acoustic emission strength of individual vehicles:
- vehicle speed
- vehicle type and condition
- roadway factors:
- roadway grade
- roadway surface (and tyre tread)
- wet or dry surface
- driving conditions (e.g. free flow, acceleration etc.)
- Propagation variables:
- ground cover
- height of noise emissions from vehicles
- height of propagation above ground surface
- screenings (i.e. barriers)
- meteorological conditions
The effect of most of these variables on noise levels is obvious, but two of them – ground cover and height of propagation – require further comment.
Sound heard by a receiver near the ground will consist of both direct sound from the source and sound reflected from the ground surface (see notes on Outdoor Propagation in lectures from Week 2, Reading 4 on Noise Propagation). If the ground surface is “soft” (e.g. grass), then destructive interference occurs between the sound from these two paths, resulting in attenuation of levels heard by the receiver. If the ground surface is “hard” (e.g. bitumen, water, etc.), no such interference occurs. This “ground effect” attenuation depends both on the ground surface cover and the height of propagation above the ground surface. However, in the cases where a barrier is interposed between source and receiver, the reflected path from the ground is lost and no ground effect attenuation occurs, irrespective of the type of ground cover.
- THE “CALCULATION OF ROAD TRAFFIC NOISE” (CoRTN) MODEL
The Calculation of Road Traffic Noise model was developed in the UK and has been validated for Australian conditions. It allows the prediction of noise levels from road traffic.
GREAT BRITAIN DEPARTMENT OF TRANSPORT (1988) Calculation of Road Traffic Noise, HMSO.
SAMUELS S E AND SAUNDERS R E (1982) The Australian performance of the UK DoE traffic noise prediction method. Proc. Australian Road Research Board Conference 11 (6), 30-44.
This section describes the algorithms that form the CoRTN modelling procedure.
The CoRTN model requires that all source roadways likely to contribute to noise levels at the prediction site be divided into straight segments such that noise generation characteristics, and propagation conditions from source to receiver, are constant for all points along the segment. With the source and propagation variables known for each segment, the noise level can be calculated at the prediction point for an “infinite roadway” passing through the segment. This “infinite roadway” level is then corrected depending on the horizontal angle subtended at the receiver by the segment. The sound energy contribution from each segment is then summed to predict the total A-weighted L10 at the prediction site. The following is a précis of the prediction model. The complete procedure must be consulted before accurate predictions are attempted.
- Segment the roadway as described above.
- For each segment, calculate the base noise level for the given total traffic flow (q veh/h). Use the following equations to calculate L10,1 h or L10,18 h where q = vehicles/hour; Q = vehicles/18 hour:
LA10,1 h = 42.2 + 10 log10 q
LA10,18 h = 29.1 + 10 log10 Q
- Calculate the correction for the percentage of heavy vehicles in the traffic stream (p = percentage of vehicles whose tare weight is greater than 1500 kg) and the average speed of the vehicles (V km/h).
Add this correction to the base noise level. Use the figure below or the equation of the curves which are:
Correction = 33 log10 (V + 40 + 500/V) + 10 log10 (1 + 5p/V) – 68.8