@Rouben Rostamian  Thank you very much for telling me this, I would of spent ages trying to work it out and wasted alot of time. Thank you

@Kitonum Thank you very much for this, it is very helpful. Can I also ask, this is when the two sides at an angle are not fixed, is there a way to make them a fixed boundary condition. Basically I want to imitate what happens when a sound wave goes into a space like this and where it reflects to. This triangle is trying to simulate part of an anechoic chamber and how their triangular shaped walls reflect/absorb the sound. Thank you.