My parents used to listen to the Carpenters. I was a hard rock/heavy metal guy back in the 80s, so I kept the fact that I liked their music well hidden.
British guitarist analyses Karen Carpenter youtube says her voice was incredibly accurate, live or in studio. He or one of the comments notes that they tried adding a second track of her voice with a slight delay. They couldn’t do it because the waves cancelled each other, they matched so well.