One technical thing to listen for when working out an impression is the pitch of the voice. This refers to the physical rate of vibration of the sound, whether of a high frequency or low. Do you need to understand wave forms and other scientific basics to have an opinion about this? No.
Basically you want to know if it is a higher register voice, medium or low.
And that of course is relative to your instrument; are you a flute, a cello or a bassoon? Or the airhorn on the QE2?
It’s going to be easier for you to duplicate voices that are in your natural range, because you will already share the general rate of vibration. Can you change your own voice to sound lower and higher? With practice, you absolutely can. The great stage and film actor Lawrence Olivier, who was a consummate voice artist, master of dialects and vocal characterizations, yet, like me, never guested on The Simpsons, wrote in his autobiography that he was able to very causatively deepen his natural voice with exercises and a lot of practice, to play Othello, since he felt the character required a considerably deeper sound than his own natural timbre.
Conversely, there’s always helium. Or that other stuff that is heavier than air. Are they bad for you? I have no idea. But there must be a good reason people don’t breathe in a lot of stuff other than air, unless they are doing deep dives on wrecks in the ocean.
Often the pitch of a celebrity’s voice is one of the unusual or stand-out characteristics that “Place” the persona for an impressionist; such as in the case of Truman Capote, Barry White, or Curly from the Three Stooges.
Should a performer eschew an impression that he finds to be beyond his range? Well, you have to know what “eschew” means first of all.
(“Avoid”. Eschew means avoid. Why didn’t I just say “avoid”? Because this is MY BOOK.)
So, should a performer eschew an impression that is beyond his range? Not necessarily. Why? Because the most important thing is not the technical rendition. It is the… anyone? Anyone?
That’s right the… (writing on chalkboard) VIEWPOINT.
But, if you get the pitch really close, the audience will have a greater agreement with your rendition, and they just MIGHT want to buy you a drink after the show. Or give you some helium.