The Etymotic ER-3C Earphones are basically updated and improved on the ER-3A inserts and have incorporated "touch-proof" electrical connection for safety. They have a smooth frequency response and provide 30dB of isolation between ears, which reduces the need for masking. Compared to supra-aural headphones, they reduce test/retest variability and eliminate test errors due to collapsed ear canals. They may be calibrated in a 2-cc coupler, occluded ear simulator, or a 0.4-cc coupler.