This troublesome pair originated from the common misconception that there is such a word as stentorous, which is something to do with loud speaking or breathing. I’ve seen and heard this used, but in fact, it’s not a word at all! The word people are looking for there is stentorious, and the word they are probably being affected by when they think about it is stertorous.
Let’s sort out these two words that do actually exist, then.
Stentorious (or, indeed stentorian, which is listed in the Oxford Concise Dictionary at the expense of stentorious, but I reckon I’ve never come across) is used to describe a person’s voice, or the sound they emit, as being loud and powerful. Stentorious and stentorian are both listed in the Oxford Dictionaries online resource (which you can find here). I suppose stentorian might be marginally easier to spell, but in this case it seems a bit odd to have two words meaning exactly the same, even though I usually like that kind of thing.
Stertorous sounds pretty similar AND means a pretty similar (though not exactly matching) thing – it describes laboured and noisy breathing. I do like this distinction and think it’s important.
So stentorious or stentorian for vocal expression, stertorous for breathing, and stentorous for neither of them, because it doesn’t exist (but I bet a lot of people look it up!.
Do comment if you found this article because you looked up stentorous! Thank you!