UDF sine wave

enda

I am simulating a horn with a pres inlet with a half sine wave and a pressure outlet at atmospheric pressure, aft of the mouth. I am monitoring the pressure levels at the mouth but instead of the pressure dissipating after a few micro secs, I am finding that new waves are "creating" themseleves in the horn. I expect a certain amount of this due to standing waves etc. but not to this extent. Does anyone have any ideas?