https://phys.org/news/2018-06-feature-apple-airpods-impromptu-aids.html

Through iOS 12, AirPods can exploit a "Live Listen" Apple-developed assistive hearing technology that has been available since 2014 on an iPhone, iPad or iPod touch with compatible, third-party Made For iPhone hearing aids or cochlear implants.

To use the feature with your AirPods, pair them per usual through Bluetooth on your iOS 12 device. Then, in Settings go to Control Center, tap Customize Controls and then Hearing. After placing the AirPods in your ears, bring up Control Center on your phone, tap Hearing—it's the onscreen button with an ear—and then tap Live Listen.