Sound pressures measured with a probe‐tube microphone in front of the tympanic membrane of the guinea pig have been compared at 100‐cps intervals in the frequency range 50 to 8000 cps to the sound pressures similarity measured in a speculum coupling an acoustic source to the ear canal. Rather different functions were obtained for each of five conditions: (1) normal, (2) hair‐line slit in the tympanic membrane, (3) rupture of tympanic membrane, (4) fluid behind tympanic membrane, and (5) bulla open. The variability in each of the categories is presented. Variability was greatest in the normal ears and least in the bulla open ears.