A horn speaker is a louspeaker which uses a horn to increase the overall efficiency of the driving element, typically a diaphragm driven by an electromagnet. The horn itself is a passive component and does not amplify the sound from the driving element as such, but rather improves the coupling efficiency between the speaker driver and the air. The horn can be thought of as an "acoustic transformer" that provides impedance matching between the relatively dense diaphragm material (typically made of paper or, more recently, more exotic materials such as aluminum, titanium, or mylar) and the air (which has a very low density).