Anyone familiar with deafness, particularly in older dogs?

I believe my dog is going deaf. Either that or he has developed a severe case of ODD late in life.

Over the past few weeks I have noticed that he doesn’t respond to verbal cues like he used to. He also startles easier when sleeping or not paying attention. It may have been gradual and I just didn’t notice but I think it is more likely something that has happened fairly suddenly. But it is not total, it seems like he hears sometimes, some things, but nothing I can identify as a constant. Sometimes he hears a clap, sometimes not. This morning I spoke in a deeper voice and he cocked his head like he heard but most of the time I get no response from this.
So, anyone, any insight into this? Have you had to deal with this in the past? Would a dog whistle get his attention and help keep him from danger? I have restricted his time in the front yard because I don’t know if he can hear traffic, not that it is that heavy.
If you are partially deaf and lost your hearing over time, can you tell me what he is experiencing?

I have a vet appointment next week but wanted some input beforehand. Thanks to all in advance.