Distortion-product otoacoustic emission measured with continuously varying stimulus level

Distortion-product otoacoustic emissions (DPOAE) are measured by stimulating the ear with two simultaneous tones. A novel method for measuring DPOAEs has been developed in which the tone levels vary continuously instead of in discrete steps. Varying the tone levels continuously may offer advantages for characterizing DPOAE level as a function of stimulus level. For equivalent primary levels, DPOAE levels measured with the continuous-level method were the same as levels obtained with the discrete-level method, thus validating the new method. Continuous-level measurements were used to determine the optimal L ₁ for each L₂ in individual subjects (N = 20) at f ₂ = 1, 2, 4, and 8 kHz by using a Lissajous path that covered a wide range of stimulus levels. The optimal L₁ (defined as the L ₁ that resulted in the largest DPOAE for each L₂) varied across subjects and across frequency. The optimal difference between L ₁ and L₂ decreased with increasing L₂ at all frequencies, and increased with frequency when L₂ was low. When the optimal L₁ was determined individually for each ear, the DPOAE levels were larger and less variable than those obtained using the equation for L ₁ suggested by Kummer et al. [J. Acoust. Soc. Am. 103, 3431-3444 (1998)].