声音回音功能， 一．　　原理：设原语音信号为x(n),d为从声源到反射面间的往返时延，a为衰减系数（a<1）。(Sound echo function, 1. Principle: set the original speech signal as X (n), D is the round-trip delay from the sound source to the reflecting surface, and a is the attenuation coefficient (a<1).)