A speaker includes a case, a dome, a diaphragm, a first voice coil, a second voice coil, a first pole plate, a second pole plate, a first magnet, a second magnet and a yoke. The case is supported on the frame to form a receiving room therebetween. The yoke includes a bottom portion and a side portion perpendicularly extending upward from the center of the bottom portion. A first magnetic gap is formed between the inner wall of the side portion and the outer wall of the magnet for receiving the first voice coil, and a second magnetic gap is formed between the outer wall of the side portion and the inner wall of the second magnet for receiving the second voice coil.