Nordic Calculation Method

Each road is divided into sections with a minimum length of 5 metres. Each section is represented by a noise source in the middle of the section. The length of the section depends of the distance between the calculation point and the road. As this distance increases, the length of the section increases. When the distance between a noise source and the calculation point is longer than a certain value, the program will set the contribution from the noise source to zero. This way the calculation stops automatically. The figure shows an example of how the road is divided into sections.

A) Noise source
B) Section
C) Centre line
D) Calculation point

The initial noise emission level is calculated as a function of traffic volume, speed limit, percentage of heavy vehicles and the road segment length. In addition to this, the calculation will take into consideration a range of various corrective factors.

The amount of distance attenuation is calculated as a function of the distance between the noise source and the calculation point.

The effect of barriers and ground absorption is calculated based on the terrain conditions. The terrain conditions between each noise source and a calculation point is represented by the terrain profile along a straight line between them. This terrain profile is created from the terrain model. The terrain profile is supplemented with data from buildings and barriers when they are intersected on the line between the noise source and the calculation point.

The total terrain profile is analyzed to find a possible barrier. The barrier's placement is primarily a function of the difference of height between the terrain profile and the sight line. The distance from the road has importance as well. Distances to the barriers and barrier heights are calculated. This is the basis for calculating the barrier attenuation. From the thickness of the barrier the correction for thick barrier will be calculated.

A reflection plane will be calculated automatically from the terrain profile. The terrain profile undergoes up to four routines to find a reflection plane that is as correct as possible.

Where there are very complex terrain conditions, it may be difficult to define a reflection plane. In Novapoint Noise such reflection planes are calculated in a reasonable and systematic manner. It can not be guaranteed that the reflection plane will be 100% correct, but it is important to point out that the calculation is based on the same principles on all projects.

With barriers present, the reflection plane is defined both in front of and behind the barrier. The figure below shows a profile between a noise source/section and a calculation point. Reflection planes are inserted both in front of and behind the barrier.

Each terrain profile is also analyzed to check if the ground is hard or soft. The main principle is that the ground is chosen as ”soft” or ”hard” depending of the characteristics of the surrounding area. This is closer described in Handbook 064 Nordic calculation method for road traffic noise, pt. 2.4.1.1. The first part of each terrain profile is always considered to be the road itself and is defined as hard ground.

Correction for road gradient is calculated. The gradient is calculated when we define the centre line of the road. In this case, heights are collected from the terrain model.

We get the equivalent noise level of a calculation point by adding up each noise source's contribution logarithmically. In cases where we have defined several roads, the total contribution from each road is found, subsequently this is summed up for all the roads.

The maximum level of the calculation point becomes the highest single maximum level. This is more closely described in Handbook 064 Nordic calculation method for road traffic noise, pt. 2.7.

Routines for the calculation of sound reflection from vertical surfaces are included. Reflection from facades are calculated based on the size of the facade, orientation of the facade according to road etc. Equivalently there may be calculated reflection from sound barriers. The figure below shows an example from a noise calculation where we have included reflection both from sound barrier and facade.

The continuous line states the noise contribution directly from the noise source (the noise value without reflection) while the dotted line states the facade reflection, the dashed line states the barrier reflection and the dashdot line states reflection from both sound barrier and facade. The figure below shows the terrain profiles for the calculations shown in the figure above.

The dotted, vertical lines in the terrain profiles show the reflection from vertical surface, while the continuous, vertical line to the left in the profile shows the noise source, and the line farthest to the right in the profile shows the calculation point. In the profile “Reflection from facade” there is drawn a circle. This circle shows that the sight line from the noise source to the calculation point via the facade, is shielded by the terrain.

Non-reflecting barriers are made from special sound absorbing materials. Wooden barriers and barriers of concrete are examples of reflecting barriers.

Reflection from non-vertical surfaces (e.g. rocks) cannot be calculated directly. One may attempt to simulate the surface by inserting a reflecting barrier right in front. But such calculations have to be considered as very uncertain. If one suspects that reflection from non-vertical surfaces matters, one has to get the situation evaluated by an expert.

When we allow for reflection, the calculation takes more time than when we calculate without reflection. It is not possible to use calculated profiles again, which also may result in increased use of time if one is engaged in small adjustments. We therefore recommend to organize as much as possible without reflections and then run the calculation with reflections at the end.

Calculation of facade alleviation and indoor noise levels are not entered into this version. Calculation of indoor level requires knowledge and experience on building conditions. As a preliminary guidance to this, we refer to method and data basics in Handbook 47, Norwegian building research institute: “Isolating for outdoor noise”, 1999.

In the design situation the indoor noise level may as a first estimate be set 25 dBA lower than the noise level outside the wall (facade reflection not included).