So, I was in the military like 10 years ago, going through AIT. I had some buddies wanting to try some kind of experiment with me back then that had to deal with sounds... I'm trying to figure out the name of this thing that my friends back then used on me. From what I recall, it basically ran through headphones and allowed ambient noise to still seep into the sound of whatever was being listened to but dampened the sound and made it echo, and I remember... Maybe remember anyways, that each side of the headphones played at different frequencies or hertz. I'm just wanting to remember the name of this thing that they found back then since I'm running into brick walls when I try to search up the keywords for this.