hear oneself think, can't
Be unable to concentrate because there is too much noise. For example, There was so much noise from the jackhammers we couldn't hear ourselves think. [Early 1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.