A 62-year-old man presents with progressive bilateral hearing loss over 10 years. Audiometry shows bone conduction thresholds within normal limits but air conduction thresholds elevated across all frequencies with an air-bone gap of 35 dB. A 58-year-old woman with similar age of onset shows bilateral high-frequency sensorineural hearing loss (4–8 kHz) with air and bone conduction equally affected and no air-bone gap. Which audiometric finding best distinguishes conductive from sensorineural hearing loss in these two patients?