It may sound a bit strange, considering their different genders and genres, but Martin offered a good explanation when speaking with The Guardian for an extensive interview.
“She can turn anything into gold with that voice,” he said. “Here’s the thing: if you speak to a good singing teacher about great opera singers, they will talk about consistency of tone. Or there’s a book by Alfred Tomatis about why some people like certain people’s voices and other people hate them, like Bob Dylan or me or whoever, some people say, ‘Oh, I hate that voice’, so there’s a thing about people’s frequency responses — what they are pleased by. Rihanna has this thick tone, so it’s very hard to annoy anybody.”
Martin went on to compare Rihanna’s voice to a “beautifully squeezed tube of toothpaste,” which is a bit of an odd analogy but one we can all surely relate to on some level.
“When you think of Rihanna’s voice you think of this whole, rich thing, solid like a tree trunk, and Drake is pretty similar,” he went on. “But Rihanna’s voice is just delicious for your ear. Sinatra had the same thing; anything he sang sounded pleasing to most people.”
Read the whole interview here.