A lot of people get wrong impression from Japanese 'A' Movies. Those AV ladies make a lot of noise while having sex. Actually it is very easy to tell the difference in between a real and fake orgasm; during orgasm, the pressure comes from lady's sexual organ is much much stronger than "假高潮". There is no way for a lady to manually control the pressure to that level.
Also, during the peak of sexual excitation, the lower part of lady's body will have uncontrolled movements which should be in sync with the noise come from the lady's mouth. A lot of ladies do not even know this when having "假高潮".