Also known as Surface Transducers and other things…
https://www.sparkfun.com/products/10975
“Surface transducers give you the awesome power to turn almost any surface into a speaker. They’re essentially just a speaker except instead of a cone, the coil is attached to a pad that conducts the vibration into whatever you press it against. Hook it up to an audio source and press it against the nearest table, wall or cardboard box. You can even put it against your head and play music directly into your skull (the ultimate surround sound).”
– Sparkfun