The best alternatives to Ritmica are Brain.fm, Timetaco, and Pomodoro Time Manager. If these 3 options don't work for you, we've listed a few more alternatives below.
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Soundescape is a free web application for ambient background sounds. If you struggle to concentrate while working from home, this tool is for you! Audio is rendered in 3D, and you'll never hear the exact same combination of audio twice, as everything is generated randomly.
Tinnire is a generative music service made by one guy with a passion for a sound. The main goal of Tinnire is to influence your mood with music. Make you more concentrated or more relaxed. Put you in the state of mind you wish.
Concentration is tough with an absence of sound. Ambients reintroduces the quiet, persistent background noise of the world with audio tracks meant to soothe without distraction.