Recruitment in conduction deafness. Observations using a midline loudness balance test.

A midline loudness balance test was made in 10 normal ears with blocked ear canals and in 104 conductively deaf ears with loss not exceeding 45 dB. The test was made by giving the tones simultaneously in the ears at 10, 60 and 80 dB sensation levels for 500 and 2 000 Hz. In the normal group the midline balance method indicated no reduction from the threshold differences at any of the three levels whereas significant reductions occurred in all groups of conduction deafness. The occurrence of true partial recruitment in conduction deafness demonstrated by ABLB (Fowler's) test was attributed to altered middle ear mechanics, loud tones stimulating the inner ear relatively more effectively than tones near the threshold level. The midline balance method is more closely dependent upon bone conducted sounds and on the altered relative movements of the labyrinthine windows.