I've often thought an online hearing test could be a great service. Also a great business. It would probably have to come with a headset that can be sent.

It would allow people to check their hearing periodically to note the difference. To compare ears. To change range of hearing. etc.

Here's someone doing some work in this direction. They have an unscientific list of tones that go from 8Hz all the way up to 22,000Hz. It’s fairly common for people who are over 25 years of age to not be able to hear above 15Hz, so this will help you find out where your high frequency hearing cuts off.