A 声波 with a short 包络 通常 with a less sharp 起（始） than a click 脉冲 is also called an 脉冲 With 频率 below about 20Hz individual oscillations may be 听 or sensed as slow pulses of 压力 variation without 音高 特征 源 Barry Truax - Handbook for Acoustic 生态学 CD-ROM Edition Cambridge Street 出版 1999 - CSR-CDR 9901