Carole -- if you'd like to read (gawd!, the thing goes on forever! must be 300+ lines. . .) a poem that over-indulges in sibilance (and just about every other poetic device you've ever heard of), read Christina Rossetti's "Goblin Market", where you'll see gems like "must she no more such succous (sic) pasture find" and "thesunset flushes/Those furthest loftiest crags". Rossetti's excesses also illustrate a point often missed in explanations of sibilance: the effect is felt in terminal "s" as well as initial or medial sound. Darren gave the key--sibilance is not peculiar to poetry; rather, it is a term used in Descriptive Linguistics pertaining to sound formation alone. (Aside: I used to amuse my Chinese students who claimed they could NOT pronounce "l", since the sound was absent in their language. I'd say, "Ok, open your mouths and begin pushing some air out in an "ahhh" sound . . good . . .now keep your mouths open, keep pushing air and press your tongue to the roof of your mouth." They instantly heard the "l" sound, and were shocked and delighted. Kinda like Dorothy opening the door to a technicolor world in T.he Wizard of Oz). So, when you work with sibilance, just listen for the hissssss. Not all "s" sounds are sibilants. For example "s" followed by a plosive generally is NOT sibilant. "Spend", for example--the sibilant sound is immediately snapped off by the plosive, and is of little or no effect.
You asked for a particularly good example of the sibilant effect? Some of you won't be surprised that I turn to Keats. The opening of "To Autumn" (IMO the greatest of his great Odes):
Season of mists and mellow fruitfulness
Close bosom friend of the maturing sun
Conspiring with him how to load and bless..........etc
I just suggested that an "s" followed by a plosive is often NOT sibilant, but in "conspiring" I would suggest it IS sibilant, because the preceding long, open "o", followed by "n", which is formed by pushing air in an "ehhh" sound, then pressing the tongue to the roof of the mouth, SLIDES very easily into the "s" and obliges you to HOLD the "s" longer than in "spend". When I'm writing, I pay no conscious attention to any of this linguistics stuff. . .but when I'm editing, I often form and pronounce words and phrases to fine-tune the FEEL of the sound.
Hope that helps a bit.
Bookmarks