Dear Bempp users,

I would to simulate the radiation of an unbaffled rectangular thin plate under motion below 1Khz. To do so, I defined a velocity field on both rectangle faces, as follow:

```
plate = bempp.api.shapes.cuboid(length=(Lx, Ly, Lz), origin=(0, 0, 0), h=0.05)
@bem.complex_callable
def calc(r, n, domain_index, result):
xs, ys, zs = r
if n[2]:
result[0] = VelocityField(xs, ys)
if n[2]<0 :
result[0] *= -1
else :
result[0] = 0
```

The plate lies parallel in the xy plane, so when the z-normal is negative, then the velocity field is multiplied by -1 to shift the phase of the plate face underneath.

Then, the Helmotz integral equation is solved.

I am wondering if this the proper way to define an unbaffled plate and its velocity field.

Regards,

Mathieu