效應：Echo（回聲）
圖1 [1]
圖註：
來源：
https://encyclopedia.thefreedictionary.com/Echo+(phenomenon)
In audio signal processing and acoustics, echo is a reflection of sound that arrives at the listener with a delay after the direct sound. The delay is proportional to the distance of the reflecting surface from the source and the listener. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room and an empty room. A true echo is a single reflection of the sound source.
在音頻信號處理和聲學中，回聲是聲音的反射，在原聲之後延遲到達聽者。延遲與反射面的距離成正比。典型的例子是井底、建築物或封閉房間和空房間的牆壁產生的回聲。真正的回聲是聲源的單一反射。
The word echo derives from the Greek ἠχώ (ēchō),[2] itself from ἦχος (ēchos), "sound".[3] Echo in the folk story of Greek is a mountain nymph whose ability to speak was cursed, only able to repeat the last words anyone spoke to her. Some animals use echo for location sensing and navigation, such as cetaceans (dolphins and whales) and bats.
echo一詞來源於希臘語ἠχώ（ēchō）[2]，本身來源於ἦχος（ēchos），「聲音」[3]。希臘民間故事中的echo是一個山上的仙女，她的說話能力受到詛咒，只能重複任何人對她說的最後一句話。有些動物使用回聲進行位置感應和導航，例如鯨類（海豚和鯨魚）和蝙蝠。
Acoustic waves akola are reflected by walls or other hard surfaces, such as mountains and privacy fences. The reason of reflection may be explained as a discontinuity in the propagation medium. This can be heard when the reflection returns with sufficient magnitude and delay to be perceived distinctly. When sound, or the echo itself, is reflected multiple times from multiple surfaces, the echo is characterized as a reverberation.
聲波會被牆壁或其他堅硬的表面反射，比如山脈和建築牆壁。反射的原因可以解釋為傳播介質中的不連續性。當（聲波的）反射以足夠的幅度和延遲返回時，它可以清楚地（被人）感知到。當聲音或回聲本身從多個表面多次反射時，回聲的特徵是混響。
The human ear cannot distinguish echo from the original direct sound if the delay is less than 1/15 of a second. The velocity of sound in dry air is approximately 343 m/s at a temperature of 25 °C. Therefore, the reflecting object must be more than 17.2m from the sound source for echo to be perceived by a person located at the source. When a sound produces an echo in two seconds, the reflecting object is 343m away. In nature, canyon walls or rock cliffs facing water are the most common natural settings for hearing echoes. The strength of echo is frequently measured in dB sound pressure level (SPL) relative to the directly transmitted wave. Echoes may be desirable (as in sonar) or undesirable (as in telephone systems).
如果延遲小於1/15秒，人耳就無法區分回聲和原始直達聲。在25°C的溫度下，乾燥空氣中的聲速約為343 m/s。因此，反射物體必須距離聲源17.2 m以上，位於聲源處的人才能感知回聲。當一個聲音在兩秒鐘內產生回聲時，反射的物體就在343米之外。在自然界中，面對水的峽谷壁或巖石懸崖是最常見的聽到回聲的自然環境。回聲強度通常以相對於直達波的dB聲壓級（SPL）來測量。回聲可能是理想的（如聲納），也可能是不理想的（如電話系統）。
來源：
https://baike.baidu.com/item/%E5%9B%9E%E5%A3%B0/3002545#viewPageContent
聲波在傳播過程中，碰到大的反射面（如建築物的牆壁、大山裡面等）在界面將發生反射，人們把能夠與原聲區分開的反射聲波叫做回聲。
參考文獻：
https://encyclopedia.thefreedictionary.com/Echo+(phenomenon)
ἠχώ, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
ἦχος, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
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