well u know in spys they have these headphones and when some is far away they can here them by putting on the headphones what ever u call them
Walkie Talkie? Headset? I don't really know.
Sound or voice "Amplifiers" ??!!.
Theres so many different kinds of those....
Here i found this website, maybe it will help?
http://sciencing.com/types-listening-devices-spying-8135997.html
