An 回声 is a 重复 or a 分音 重复 of a 声音 due to 反射 混响 is also 反射 声音 but in this 案例 separate repetitions of the 原始/原创 声音 are 不 distinguishable For a 重复 to be 清晰（独特） from the 原始/原创 it must 出现 at least 50 ms afterwards without being masked by either the 原始/原创 信号 or other 声音 In practice an 回声 is more likely to be 可听 after a 100 ms 延迟 Very short 延迟 cause an 图像（描绘） 移位/偏移 sideways and 音 colouration due to 定相 longer 延迟 contribute to a 空间 印象 of 混响 and stronger 延迟 greater than 50 ms disturb the 声音 图像（描绘） and are 感知（认为） as 回声 化约 from Barry Truax - Handbook for Acoustic 生态学 CD-ROM Edition Cambridge Street 出版 1999 - CSR-CDR 9901