Is sh a word? Sure. It’s even in dictionaries. They call it an interjection, though it really functions as an imperative – when you say “Sh!” you mean ‘shut up’ or ‘be quiet’ or ‘shut your festering gob’ or ‘stop making that dreadful noise’. But we can’t say it really is an imperative, syntactically, unless there are other forms that show it to be a verb: “Will you sh?” “OK, I am sh-ing. I have sh-ed.” Doesn’t really work, at least on paper.
We could say “Will you shush, I am shushing, I have shushed,” but we all know that shush is not sh. No, shush is almost polite. There’s nothing polite about sh. It’s most often a high-handed command issued with full assumption of the right to correct and boss around. When not irritable, it is at least familiar: a winking correction to a friend or descendant.
Well, OK, I suppose that’s all more true of shh. The shorter form, sh, is quicker and less expressive, a drive-by hushing. Conversely, a more forceful one would be shhh. Or maybe sshhh? Hm. Hmm. Are those different words or are they multiple versions of the same word? Hmmm.
Well, at least hm, hmm, and hmmm are clear enough in the spelling department. We lean towards shhh over sshh by convention, but it’s all just one sound, not two; there’s no place where s ends and h starts – that would be something of a mishap. We need two letters just because we got saddled with this alphabet from a language that didn’t have the sound we spell sh at the time the alphabet was set.
Just that little movement of the tongue back towards the palate can make the difference between peccadillo and tibia (sin and shin), between drink and transport (sip and ship), between being seated and… you get the idea. The “sh” sound, which linguists render in the International Phonetic Alphabet as /ʃ/, is palatalized. While /s/ belongs to the higher-pitched “hissing” fricatives, /ʃ/ belongs to the lower-pitched “hushing” ones. Both are sibilants; both are stridents. Both are audible at some distance, but I’d say “sh” carries more.
Well, I’ll say for sure that you can hear it across a theatre. It is irrefutable evidence of the presence of someone who does not have proper respect for others or for the occasion. Seldom do I attend a live performance of music, dance, or drama without hearing someone on the far side of the place issuing a “Shh!” to some noisemaker near them. I rarely hear the person they’re addressing; the quiet speech that provokes it doesn’t carry nearly as far. One ought not to talk during performances, of course; it’s not very considerate. But far less considerate is the person who, instead of leaning forward or back and quietly saying, like a proper Canadian, “Sorry, could you please not talk?” prefers to issue a loud passing of gas through tongue and palate audible across the auditorium and almost certainly on the stage. So rude – far worse than the offence that provoked it. For heaven’s sake, shushers, do be less strident.