There's nothing too complicated about the engineering of a headphone hat. The hat can come in baseball styles for regular wear and warmer styles for winter weather. Like a normal set of headphones, there will be two speakers either connected across the top of your head by some sort of band, or secured to the sides of the hat. The speakers are connected by a cable that leads to a universal plug. This plug should fit into most headphone jacks, so you can use your hat with your MP3 player, your cell phone, your computer and so on.
Most styles allow you the convenience of being able to remove the headphones from the hat. This way, you can throw your hat in the washing machine, and you'll have a spare pair of headphones if you need them. Because of this ease, if you lose the headphones, you may be able to replace them with a pair similar in size and design.
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