Most ordinary speakers use an electromagnet to push and pull on a cone-shaped diaphragm (see How Speakers Work for details). But there are a few other technologies on the market. The speakers you saw at your friend's house are probably electrostatic speakers.
Instead of using an electromagnet, electrostatic speakers vibrate air using a large, thin, conductive diaphragm panel suspended between two stationary conductive panels. These conductive panels are charged with electrical current from a wall outlet, creating an electrical field with a positive end and a negative end. The audio signal runs a current through the suspended diaphragm panel, rapidly switching between a positive charge and a negative charge. When the charge is positive, the panel is drawn toward the negative end of the field; when the charge is negative, it moves toward the positive end of the field.
Based on the varying electrical audio signal, the diaphragm is alternately charged with a positive current and a negative current.
In this way, the diaphragm rapidly vibrates the air in front of it. Because the panel has such a low mass, it responds very quickly and precisely to changes in the audio signal. This makes for clear, extremely accurate sound reproduction. The panel doesn't move a great distance, however, so it is not very effective at producing lower-frequency sounds. For this reason, electrostatic speakers are often paired with a woofer that boosts the low-frequency range. The other problem with electrostatic speakers is that they must be plugged into the wall and so are more difficult to place in a room.
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