It's been said that by the time she was with Big Brother and the Holding Company, Janis' voice had already passed it's prime.
Anyone that says this clearly knows nothing of what they are talking about. For starters, Big Brother & The Holding Company was Joplins first band, having joining them in 1966 and lasting until the end of 1968. If there was any waning of her voice, it occurred during the period when she recruited The Full-Tilt Boogie Band for backup on the recording of Pearl.
While Pearl became Joplins most commercially successful effort (in which she would not live to enjoy), the vocals were much more mellow than the whiskey growls and shouts that were prevalent during the period with Big Brother, and later with the Kozmic Blues Band in 1969.