Senator Palpatine has a fairly normal voice, but Darth Sidious has a deep raspy voice, even before his disfiguration by his own force lightning. Is there an in-universe explanation for this? (Canon preferred, but a Legends explanation would be interesting too.) Anakin/Vader's voice changes as well, but it may be just his injuries and the suit -- or is it analogous? Dooku's voice didn't change, but then again I'm not sure we ever saw him in a non-Sith "disguise".
A good example of the change is when Mace Windu comes to arrest Palpatine: