聲音本身
這一支就是畫面本身，還沒有任何用途加上去。
noun · B2
Sound whose pitch is higher than human ears can pick up — above about twenty thousand vibrations a second — so it travels through air, water and solid things without anyone hearing it.
超音波
核心畫面
One picture holds this word together: a sound that is really there but sits above the line where our hearing stops. Because it is so high, it travels in a narrow beam and bounces back off whatever it meets. Everything else the word names is a use people found for that: catch the echo and you get a picture of the inside of a body; send in more of it and it can work on the tissue itself.
這個字可以想成一種「越過我們聽力那條線」的聲音。它確實存在，只是我們聽不到。因為頻率很高，它走得很直，碰到東西會乾淨…更多收起
這個字可以想成一種「越過我們聽力那條線」的聲音。它確實存在，只是我們聽不到。因為頻率很高，它走得很直，碰到東西會乾淨地彈回來。這個字的其他用法，都是人把這個特點拿來用的結果。接住回音，就能看見身體裡面；送進去多一點，就能對組織本身做事。所以先看見這個畫面，後面兩種用法就不用分開背。From the word’s history
Wiktionary gives the word as ultra- + sound: ultra- marks going beyond a limit, and here the limit is human hearing.
字源資料把這個字拆成 ultra- 加 sound。ultra- 表示越過某條界線，而這裡被越過的界線，就是人耳聽得到的範圍。
Wiktionary entry for ultrasound, supplied in packet (source_ref kaikki:3f6f69a190036ae4:865718381)
Three branches
拿來看：醫院的超音波
回音乾淨，就能還原成畫面——聲音被當成眼睛用。
noun · B2
A medical test that sends very high sound into the body and turns what comes back into a picture on a screen, so a doctor can see an organ, an injury, or a baby before birth; also one such test.
超音波檢查
拿來醫：治療用的超音波
同一種聲音不只帶回訊息，也能把能量送進組織裡。
noun · C1
Very high sound used as a treatment rather than to make a picture: the energy is sent into a sore muscle, joint or other tissue to act on it and help it get better.
超音波治療