Moving slower than the speed of sound — about 1,235 kilometres an hour in air at sea level. It is an adjective, and you use it mainly about aircraft, bullets and other fast-moving things: a subsonic jet, a subsonic missile.

Moving slower than the speed of sound —指的是比音速慢— about 1,235 kilometres an hour大約是每小時1,235公里 in air at sea level在海平面的空氣中。It is an adjective,這是個形容詞 and you use it mainly about aircraft, bullets and other fast-moving things:主要用在飛機、子彈這類快速移動的東西上: a subsonic jet, a subsonic missilesubsonic jet（次音速噴射機）、subsonic missile（次音速飛彈）。

Almost every passenger plane you will ever ride in is subsonic, so in practice the word is used to mark the contrast with the rare things that are not — supersonic ones. That is why you seldom hear it on its own; it turns up where the two are being set against each other, in aviation, military and engineering talk.

Almost every passenger plane you will ever ride in is subsonic,你搭過的客機幾乎都是次音速的 so in practice the word is used to mark the contrast所以實務上這個詞是用來做對比 with the rare things that are not —跟少數不是次音速的東西— supersonic ones也就是超音速的東西。That is why you seldom hear it on its own;這就是為什麼你很少單獨聽到這個詞; it turns up where the two are being set against each other,它通常出現在把兩者拿來對照的場合 in aviation, military and engineering talk像是航空、軍事和工程領域的討論。

There is a second, quieter use among shooters and sound engineers: subsonic ammunition is made to fly slower than sound so that it does not make the sharp crack that a supersonic bullet makes.

in among shooters and sound engineers,在射手和音效工程師之間 There is還有 a second, quieter use:第二種比較少人知道的用法: subsonic ammunitionsubsonic ammunition（次音速彈藥） is made to fly slower than sound造出來就是要飛得比音速慢 so that it does not make the sharp crack這樣才不會發出尖銳的爆裂聲 that a supersonic bullet makes那種超音速子彈才會有的。

Note also that people sometimes say 'subsonic' when they mean 'sound too low for people to hear' — a very deep bass note. Sound engineers do use it that way, but the careful word for that is 'infrasonic', and a purist will correct you.

Note also that另外要注意 people sometimes say 'subsonic'有時候大家說 'subsonic' when they mean 'sound too low for people to hear' —其實是想表達「低到人耳聽不到的聲音」— a very deep bass note也就是非常低沉的低音。Sound engineers do use it that way,音效工程師確實會這樣用 but the careful word for that is 'infrasonic',但比較嚴謹的說法是 'infrasonic' and a purist will correct you講究的人會糾正你。