That's funny - I came on here to ask for any tips on training an older dog to come to a whistle.
Ddog (12) has exactly the same problem - he's not completely deaf, but definitely getting harder of hearing. If he's paying attention he hears me call, but if he's got his head in a bush / distracted by something I have to yell my head off to get his attention.
I'd say with him its about 30% selective deafness (why is she calling me? the road isn't for another 100 yards, there's no need for me to stop now . . .) and 70% genuinely not hearing.
So I did think that a whistle would a) be more piercing, and b) stop me sounding like a pillock having to shout full volume across the fields.
My old boy is completely deaf now at 16, and if he's out of my eye sight I stamp my feet on the floor, this gets him to look up and then he ambles back over. Not really sure why, maybe he can feel vibrations/see me jumping about like a dickhead he doesn't go far anyway so can usually just grab him if need be!