"Vocal detractors" refers to people who openly express criticism or disapproval of something or someone, often in a loud or outspoken manner. Full definition
The most vocal detractors of the X1 controller can only complain about rechargeable batteries not being included in the controller. (thewirecutter.com)
He thinks that bitcoin's vocal detractors on Wall Street like JP Morgan's Jamie Dimon exhibit «old thinking» when it comes to a revolutionary new concept. (ccinews.net)
While I don't see as much value in it as some have, I also didn't find it as mindless and meritless as vocal detractors like Roger Ebert have. (dvdizzy.com)