Audio Systems: Small audio.

Question
What is the smallest speaker you've heard of? Any that are waterproof for underwater listening?

Answer
The speaker drivers inside your smartphone are very small.

There is no limit to how small they may become.  After all they are merely a specialized transducer element.  There are micro motors now that crawl inside a human blood vein listening for sounds of the lung, heart and blood circulating system.  These tiny microphones are just a reverse transduction of a speaker.

How small?  I can imagine the voice coil or electromechanical transducer can go as low as a few microns in size.  More practically speaking the speaker in a cell phone or smartphone is smaller that your fingernail.

Using them in Sonobouy applications is very attractive.

May I ask what your application is?