A "vocal critic" is someone who expresses strong and outspoken disapproval or criticism of something or someone, usually using their voice or words to convey their opinion. Full definition
Ironically, he has been one of the most vocal critics of Silver's handling of sex - harassment claims. (nypost.com)
As vocal critics of the government, the commissioners worked consistently to reform and improve the code on which the commission was based. (thecanadianencyclopedia.com)
They're a sector of the real estate industry that has very vocal critics, as well as fans. (nreionline.com)