如何通过归零java中的fft数据来过滤音频（android）

I'm working on an audio based application for android platform.in this code first i record my voice in wav format and then i get fft(in fftbegir method) and now i want to filter my voice in 0-4khz with zeroing fft data and then perform ifft but when i play new wav file i can hear very bad quality sound with lots of noise .the fft class is here I'm working on an audio based application for a