Following up elog 217, I want to make sure that the 2 BRD model is correct. Here is a comparison of the model with the 2 identical BRDs compared to the simple model with 1 BRD (the mass is twice the mass of an individual BRD but the Q is unchanged). In both cases the BRDs are tuned at the exact resonant frequency of the BS mode.
We can see that the models are perfectly overlapping. Hence, the 2 BRD model is in agreement with the 1 BRD model (see figure 1)
For the Roll mode, the poles and zeros of the 2 BRD model are:
The poles and zeros of the 1 BRD model are:
If the frequencies of the BRDs are symetrically detuned from the BS resonant frequency in the 2 BRD model, the poles and zeros are:
For 1 % detuning:
For 0.1% detuning :
The resulting transfer function is presented in figure 2.