IDENTIFICATION OF NOISE SOURCES THAT INFLUENCE DISTORTION-PRODUCT OTOACOUSTIC EMISSION MEASUREMENTS IN HUMAN NEONATES

Objective: The objective of this study was to identify individual sour ces of noise and their contribution to the overall noise that influenc es valid measurement of otoacoustic emissions in neonates. The hypothe sis was that careful selection of eliciting signals and signal process ing parameters, unique analysis of measured results, and control of ce rtain subject characteristics would allow isolation of these individua l noise sources and determine their relative influence. Design: Elicit ing signal parameters were optimized and held constant to minimize equ ipment noise. Analysis of noise floors in relation to signal level was used to identify equipment-related noise associated with changes in s ignal parameters. Analysis of noise floor distributions was used to de termine whether environmental noise entered the measurements via inade quate coupling of the probe to the ear. The acoustic characteristics o f the middle ear were varied via subject selection to determine the in fluence of middle-ear characteristics on noise floor levels. Results: The two sources of noise associated with the measurement equipment nee d not contribute to the noise floor for biologically relevant otoacous tic emissions measurements (eliciting signal levels between 30 and 75 dB SPL). Of the two pathways identified for environmental noise, the p athway resulting from an inadequate seal between the probe and the ear canal can be eliminated. One of the two sources of noise related to t he subject, noise resulting from biologic activity unrelated to the ea r can be minimized. However, the remaining factor, the status of the m iddle ear, has been shown to contribute as much as 6 dB to the overall noise floor. Conclusions: Careful selection of signal parameters and additional data analyses and procedural variables can isolate or contr ol several sources of noise that influence distortion product otoacous tic emission measurements in neonates. Tight coupling between the prob e unit and the external ear canal should be maintained for all measure ments. Middle ear abnormalities can increase noise floors up to 6 dB.