Adjusting a Hearing Aid Device on Shabbos

“What? What?” I cannot hear you darling, said Bubby to hear 7-year-old grandson Shmuel at the Friday night seuda, “Can you say your d’var Torah a bit louder?”

So Shmuel tried saying his vort louder, but from his Bubby's facial expression, it was clear that it did not help.

Then, her grandson Chaim had an idea: “Bubby - I see that there is a little knob on your hearing aid that adjusts its volume. See if turning it up will help."

“What?!” wondered Bubby as she raised her eyebrows, “You want me to adjust the volume?! A hearing aid is an electronic device! I cannot adjust its volume on Shabbos!"

"But you're not really turning anything on or off by just adjusting the volume," said Chaim.

Question: Who is right - Bubby or Chaim?

(The "Hints & Answers" section is on page 4)