title: Planar Wave 

Test case description: This setup investigates the acoustic behaviour, solving the Acoustic equations. 
In this testcase cosinus waves in x directions is prescribed at initial condition. The simulation
domain is defined as a cube with periodic boundaries.

**Features used**

1. Projection: fpt

2. Polynomial representation: Q

3. Filtering: -- 

4. Timestepping: explicitRungeKutta, 4 steps 

5. Boundary conditions: slipwall 

6. Others: 
   - Derived quantity: Momentum
