A 72-year-old man presents with a 5-year history of progressive hearing loss. His wife reports he frequently says "I can hear you but I can't understand you," especially in noisy environments like restaurants. Otoscopy is normal. Pure-tone audiometry shows the pattern marked **B** in the diagram. Tympanometry is normal with no air-bone gap. Which of the following best explains his disproportionate difficulty with speech discrimination in noise despite relatively preserved low-frequency hearing?