The following table of accuracy is taken from ISO9613-2 based on tests without screening or reflections

It is essential to consider that modelling is only ever an estimate and real world measurements may differ greatly.

For more information about the calculations and their limitations, refer to ISO9613 parts I and II.

Screens and vertical edge diffraction

Complex screens and multiple vertical edges

One large difficulty in noise mapping / sound modelling is vertical edge diffraction which can create a very complex situation.

As complex ray tracing is beyond the scope of this model, vertical edge diffraction is only considered for simple situations where the screen intersects the direct path and there is an unobscured path around it.

This means that the diffraction around screens with a degree of complexity, such as multi-faceted buildings with an unusual shape, will not always be properly represented in the model.

Vertical edges must be shadowing

ISO9613-2 considers the effect of screens that are along the source to receiver path even when the source is still in sight of the receiver, for example an observer looking over the top of a wall.

This model only accounts for such a situation along the top edges. For vertical edges, the attenuating effect of a screen is only considered when you are in the shadow of the barrier.

Therefore, a tall vertical screen that stands to one side of the source to observer eye-line would have an attenuating effect, however this would not represented in the model until the observer has moved behind the screen.

Inspect the sound paths

We recommend you use a ray receiver to find these situations and decide yourself the importance of these diffracted levels. Read here for more information on using the ray receiver