Some years ago, a young female friend of mine was involved in a survey of corncrakes (which produce a distinctive grating sound)on flood meadows of the River Shannon about ten miles south of where I lived. One afternoon she was talking to an old farmer , as they stood on the riverbank. He was lamenting the loss of the bird from the area - "Haven't heard one for years...". She could hear FIVE as they spoke. He wasn't deaf in general, but was obviously losing the higher frequencies.

Regards

p.s. He was also prescient, I'm afraid. The species is nearly gone from the area at this stage.