Summary This study will address the following important issues in research of noise-induced cochlear synaptopathy (NICS) in humans: (1) whether synaptopathy occurs in noise-exposed workers; (2) if so, how to measure cochlear synaptopathy in humans; and (3) the association between the type of noise exposure and NICS. This will be addressed as follows: (1) Collecting a large number of human subjects with well-documented noise exposure data from diverse industrial settings. This will consist of the following: i) digitally record complete shift-long temporal waveforms of the noise that individual workers are exposed to (8-hour) in a variety of heavy industries; ii) obtain audiograms from workers exposed to each of the specific noise environments; iii) screen subjects using high-frequency audiometry and Distortion product optoacoustic emission to confirm that the selected subjects have normal hearing thresholds and normal cochlear functions. (2) Conducting three supra-threshold measures for detecting cochlear synaptopathy in selected subjects. Four hypotheses related to NICS in humans and the relationship between the type of noise exposure and NICS will be tested using these three measures. The proposed study will be the first large investigation of synaptopathy in noise-exposed workers. The data collection and measurement techniques outlined in this proposal are very innovative and will help us to explore NICS in noise-exposed workers. Over the proposed 5-year course of this research 11 groups of subjects with 100/group will be collected and investigated. The success of the project will be extremely useful to understand how to measure synaptopathy in humans and apply it to develop a more accurate hearing protection protocol.