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White Noise Pro allows you to build your own relaxing soundscapes or listen to prearranged ones. As you've undoubtedly noted, there are a few apps like this one throughout this gallery, but they all have their own set of sounds, usability factors, and feature sets. Visit the official app link below to read more since I just recently purchased this one and have yet to give more than a couple of (satisfactory) listens.
App Store Link: White Noise Pro ($2.99)
Though it's primarily a weather/time app, Living Earth HD is a great app to keep open next to you on your iPad while doing other work. There's something considerably calming to me about the zoom-able 3D simulation of Earth, which allows you to see other parts of the world from a satellite's-eye view. Simplistic, beautiful, and relaxing.
App Store Link: Living Earth HD ($0.99)
Do you ever wish you could create the perfect thunderstorm? Well, with Storm Sim, the only thing you'll lack is real, actual rain, thunder, and lightning! Working from pre-designed templates or creating your own, you control the specific combination and volume of samples you want to play. A great app for lovers of thunderstorms!
App Store Link: Storm Sim ($0.99 + optional in-app purchases)