Projection at extended Sound Sources

The automatic division of an extended sound source into smaller partial areas depending on the distance of the receiver (see Sound Rays at Line and Area Sources) is not sufficient to avoid errors in the calculation of sound pressure levels.

Calculation of the area source „soccer field“ with two sound rays

Let us take the example of a soccer field as in the upper figure. For a noise calculation at the remote receiver point IP CadnaA would split the soccer field into two partial areas and would therefore take into account two sound rays. This is correct for the shown situation. Now a shielding club house with a height of 8 m is located in the position shown in the following figure.

Soccer field with partially shielding club house

In the calculation of both displayed sound rays, screening and therefore a lower sound pressure level occurs in each case. But, in fact, not all partial areas, which are included in the calculation with the paths of the two rays, are shielded by the building. The resulting sound pressure level would be too low in the present example.

CadnaA avoids this error by using a projection method.

With that, in the first step the area sources will be divided into shielded and non-shielded areas and in the second step subdivided according to the relation of area size to receiver distance. In this way, in the present example, 12 partial areas are created: altogether, nevertheless, because of the large distance from the receiver (340 m), 8 areas are not shielded and 4 areas are shielded.

Two step division according to shielding objects in the path of rays and the distance of the receiver

In the present example the calculation results in 44 dB(A) of free sound propagation, 39 dB(A) in the calculation of the situation with the club house without using the projection method and 42 dB(A) with correct division resulting from the use of the projection method.

The next figure shows the projection method with a road.

Projection method for a road

Both axis of the outer road lanes - that is the margin of the road stripe in the CadnaA presentation - are first divided according to the shielding objects between receiver and road. Then a subdivision occurs according to the distance criteria.

Projection method for reflecting sound parts

Much more complicated are the relations in the projection of the reflecting sound parts dealt with in the next chapter. The figure 3 above shows the construction of both sections for the axis of the outer road lanes which are necessary because of the reflecting building. The same method - graphically hardly to explain - will also be applied by CadnaA for the multiple reflections dealt with further on.

Although the projection method naturally extends the calculation considerably it should always be switched on for a detailed calculation like „acoustical zoom“. After all, even in the simple situation 12 paths of rays instead of 2 must be calculated.