We've asked Google what changed to allow Sound Amplifier on older phones and will let you know if it can elaborate. The initial version required Android Pie's Dynamic Processing Effect.

The premise remains the same. The app can customize frequencies to boost important sounds while filtering out noise, including at different levels for each ear. It's not a full substitute for a hearing aid, but it could help you overcome a noisy street corner or ensure that you hear a quiet radio. And importantly, it's easily accessible -- it's using tech you already have, and you don't need to dive into your phone's settings.

Update 7/24 5:15PM ET: Google said it's still using the Dynamic Processing Effect on devices that aren't running Pie, but it's now including a local version of DPE as well as the Android Oboe library.