I need to have my device be able to produce sounds of at least two different frequencies, as loud as possible at around 3/5V. So far it seems like the best thing to go for would be a piezo speaker but the cylindrical case ones are still a little bulky for my needs. I've found some flat piezo speakers too which would be better but they seem to need casing built for them anyway to make them as loud as they can get. Is there any clever acoustical tricks or devices I should be aware of while investigating this issue?
I'm looking for a loudness of around 80dBA @ 1ft (90dBA @ 10cm) and a size of <10mm for all dimensions.