thank you very much for your help. However I still have some questions which I hope u can help me with.
First, let me explain briefly how I ran my simulation.
Steady state simulation using realizable k-e model, enhanced wall treatment, viscous heating. Ideal gas law for air density, polynomial for cp, thermal conductivity and viscosity. Operating pressure 1 atm, operating density 0 kg/m┬│. Constant mass flow rate for mass-flow-inlet(the only realworld value that I have). 0 pa gauge pressure at pressure outlet. Then run until it converge, or to a very low value.
Transient state simulation. Everything remained the same other than pulsating mass flow rate using transient table. Time step size 0.0002s. Save data every 5 time step, so every 0.001s, which FFT up to 500 Hz (0.5/0.001s).
Here is my transient mass flow rate. From here, I created a transient table and it has a period of 38.5Hz or 0.026s.
Here are my results from CFD-Post. Series 1 is from outlet, series 2 is from inlet.

Why is the SPL value below 40 Hz missing from the graph? Is this a problem of my periodic transient table?
Do I need to make any changes to the pressure value from simulation? Or I can use the pressure value computed and perfom FFT directly? 140 dB seems unrealistics.
How do I increase the resolution of my SPL chart? It seems as if only one value is calculated every 50 Hz. Does accounting more period/cycle/"exhaust" improve the accuracy of SPL chart?
Any suggestion or improvement on my simulation step is very much welcomed. Thank you.