Should speakers be connected in series or parallel?

So I'm following an instructable for making credit card speakers using piezo speakers https://www.instructables.com/id/Ultra-Thin-Credit-Card-Speaker./ . In their instructable they only use one speaker so there's no need to wire it in series or parallel. I have 3 speakers. Will the speakers work in both series and parallel? What are the pros and cons of both?

lemonie8 years ago
Usually best in parallel, you'll get a weaker sound in series due to the combined resistance of the piezos. Also, these sound better with the black plastic cavities left on (that's what they're there for)

L