  Click-evoked otoacoustic emissions (CEOAEs) are clinically used as an
objective way to infer whether cochlear functions are normal. However, because
the sound pressure level of CEOAEs is typically much lower than the background
noise, it usually takes hundreds, if not thousands of repetitions to estimate
the signal with sufficient accuracy. In this paper, we propose to improve the
signal-to-noise ratio (SNR) of CEOAE signals within limited measurement time by
optimal shrinkage (OS) in two different settings: the covariance-based OS (cOS)
and the singular value decomposition (SVD)-based OS (sOS). By simulation and
analyzing human CEOAE data, the cOS consistently reduced the noise and enhanced
the SNR by 1 to 2 dB from a baseline method (BM) that is based on calculating
the median. The sOS achieved an SNR enhancement of 2 to 3 dB in simulation, and
demonstrated capability to enhance the SNR in real recordings when the SNR
achieved by the BM was below 0 dB. An appealing property of OS is that it
produces an estimate of every individual column of the signal matrix. This
property makes it possible to investigate CEOAE dynamics across a longer period
of time when the cochlear conditions are not strictly stationary.
