My WIP is an urban fantasy that features some merfolk coming on land and as I was researching the ocean and sea life I read that sound travels faster underwater than on land. So that got me thinking if a species was adapted to hear sound down there, when they came up to dry land would the sound be difficult for them to hear because it moves slower through the air or because they're used to louder sound in the ocean, they're hearing would be sensitive and the constant noise of the surface urban areas would be difficult for them to bear? Or would they not be bothered at all? Thanks in advance for tips and thoughts.
Being part human in the traditional merfolk sense, I would say no. They would have evolved in the dual environment and adapted the sensory perception to process both. But they're your merfolk, so you can do whatever you want. Maybe you have one stubborn merfolk who refuses to visit land, then has to, and has trouble adapting.
Do they change when they go on dry land? Like tails turn into legs for instance? If that happens, then most likely other things would transform as well, like the hearing method (and the breathing method). But yeah, it's up to you how to handle this stuff.
Sidenote, Merfolk are my least favorite race in Magic. Everyone is unblockable, or islandwalk, or curious, or buffing the rest of the tribe... royal pain in the nuts to deal with.
If you are going to go with the hearing issue, then consider the corrilary of vocal range. Listen to dolphins in and out of water their range is more toward the higher end of what humans hear.
I think you have to ask what their dominant sense would be. Light doesn't travel as well in water as in air, whereas sound travels better (try submerging your head in a bath and knocking on the side). Sharks, for example, have a very good sense of hearing, but not very good eyesight (including no colour vision). I suspect merfolk would be similar (unless magic makes them have more human-like senses on land).
