Calculate the current through a 500W,230V heater. Which fuse would you use for this appliance?

Sounds propagates through some kind of medium. usually it's air, but it goes just as well through solid objects like walls, windows etc. a soundwave will compress the (air)molecules in front of it, and if this increased density wave hits your ears, you register it as sound. solid objects like walls are a lot denser than air, this means that they will damp a wave, but also carry it easier.

Perpendicular to the wave motion 

Th answer is 3200 m/s