I would like to create a white noise generation box. I want it to be as sustainable as possible (within reason), as it will likely be used 24/7.
My thinking so far is to use an old, unused smartphone, and install any one of a number of white noise generation apps. I'm thinking an old smartphone with a removable battery would be best, and then remove the battery so there is no energy waste from the charging circuits. (To prevent the battery from eventually becoming waste, I'll first charge it to 50-60%, and then charge it back up to 50-60% every year or two.) Once the battery is removed, I'll plug it into AC power via its "charger", which at that point, I'll just be using as a compact AC/DC inverter.
To reduce energy consumption, I will keep the phone in airplane mode with all radios disabled (cellular/WiFi/bluetooth). The screen will stay off as well except for a moment here or there if I want to change any settings.
Is that the best solution, or are there better alternatives? If my idea is a good one, does it need any tweaking to improve it?