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Ambiance is like many of the other soundscape creation apps I've listed in this gallery, but with over 2000 free sounds that are exclusive to Ambiance, there are plenty of unique soundscapes awaiting your creation. Play just one sound or layer multiple ones -- all with individual volume selectors. This was the first app of its kind that I ever downloaded, and despite others I've downloaded since, the exclusive sounds belonging to this app are great to return to again and again.
App Store Link: Ambiance ($2.99 + optional in-app purchases)