In phonetics, /s/ and /k/ are surd sounds because they are produced without vocal cord vibration.

surd 来自拉丁语 surdus（聋的、无声的），在语音学中特指"清音"，即发音时声带不振动的辅音。与之相对的是"浊音"(voiced sounds)，发音时声带会振动。