In CadnaA, an acoustic screen can be modeled on both sides according to its absorption coefficient or the dB value of reflection losses (RL attribute). The maximum RL value that can be introduced is limited to 20 dB.

In view of the following formula extracted from the CadnaA reference manual, which relates to reflection losses and the coefficient of reflection, the reason for the limit of 20 dB of R.

If the Alpha variable is isolated and a value of RL = 20 dB is applied, it results in a value of Alpha = 0.99 (which can have a value between 0 and 1). If a value of RL = 40 dB would give a value of Alpha = 0.9999, virtually the same as with RL = 20 dB. That is why it has that limitation. It should be borne in mind that an absorption coefficient of 1 means that the surface absorbs all incident sound energy.