The invention relates to a speaker, in particular to a flat panel speaker which ensures electric energy is converted to sound energy. The flat panel speaker comprises a diaphragm, a frame and a driver system; the diaphragm is characterized by the flexibility and is stretched by a pretightening force along at least one direction, and then is fixed on the frame and coupled with a drive system, and vibrates to sound after being excited by the drive system. The drive system at least comprises a power amplifier and an executor. The executor can directly connected to the back surface of the diaphragm to excite the diaphragm to sound, and the change of fluid pressures in a cavity at the back of the diaphragm can also be controlled to indirectly excite the diaphragm to vibrate and sound. Output sound pressure and frequency response of the speaker can be modulated by changing the pretightening force and peripheral constraint of the diaphragm; the pretightening force of the diaphragm is improved to enhance the intermediate frequency and high frequency responses of the speaker, reduce the peripheral constraint of the diaphragm and enhance the low frequency response of the speaker.