Effect of Binaural Beats for Maintenance of General Anesthesia

The goal of this clinical trial is to learn about the effects of binaural beats on maintenance of general anesthesia in patients undergoing thyroidectomy without intraoperative neuromonitoring. The main question it aims to answer is: * 1) Does applying binaural beats during surgery reduce the gas anesthetics (especially sevoflurane) requirement to maintaining adequate anesthetic depth during general anesthesia? * 2) Does applying binaural beats during surgery affect intraoperative hemodynamic stability or post operative nausea and vomiting? Participants will wear headsets with a sound generator which contains music files (binaural beat file in the intervention group (BB) ; silent file in control group (C)) according to the randomization. Researchers will compare the BB and C group to see if intraoperative binaural beats reduce the requirements of sevoflurane for maintaining adequate anesthetic depth.