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The threshold of the acoustic stapedius reflex in man was investigated by means of changes in acoustic impedance of the ear. The contralateral ear was stimulated by using bands of noise and complex tones with various bandwidths. The experiments showed that the reflex threshold, expressed in decibels re 2.10 -5 N/m •', is almost constant for bandwidths less than a specific value and designated as "critical bandwidth." When the bandwidth was further increased, a decrease in the reflex threshold at approximately 3-6 dB/oct was observed. Assuming that the stapedius reflex mechanism is loudness governed in persons with normal hearing, these findings confirm the existence of a critical bandwidth in loudness summation. In addition, the findings indicate that the basis for the critical-band mechanism is located in the peripheral part of the auditory system, probably in the cochlea.