OpenAL声音的衰减
alDistanceModel(AL_INVERSE_DISTANCE_CLAMPED);// 衰減速度,內插範圍內才有效 alSourcef(theSource, AL_ROLLOFF_FACTOR, 1.0);// 內插範圍內界,進內界之前不會衰減 alSourcef(theSource, AL_REFERENCE_DISTANCE, 5);// 內插範圍外界,出外界之後不再衰減