Cochlear acoustic emissions were recorded with a sensitive microphone from within the ear canal of normal hearing subjects. Frequency spectra of these acoustic emissions were obtained with two different procedures: real-time recording or calculation of the spectrum of time-averaged emissions. The two procedures give different input–output curves for click-evoked acoustic emissions. A phase-locking mechanism is proposed to explain this difference. It is shown that the real-time spectrum recording procedure can be used to measure tuning curves or to study the distortion product 2f1–f2. Experimental results indicate that sharply tuned emission generators are present in the human cochlea.