it requiers subscripiton to Apple ecosystem: closed hardware iPhone, closed-source iOS, app store blessing to distribute. :)

Could you do an Android version to alleviate some of these hurdles?

P.S. Do not mean do diminish your achievement. I've tried many these apps which fed me ads or ask to subscribe, so a free alternative is very welcome! Thanks!

I use ChromaDoze from F-Droid when I am sleeping somewhere intermittently noisy and need some base noise to raise the floor. There's a couple of other ones, but I found this the most simple, you just draw a filter and click go.

There's a few on F-droid. Soothing Noise Player seemed interesting. I'm sure there's a bunch more on Google Play.

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