A high-frequency loudspeaker. The audio signal is connected to the ribbon, which is a very thin (usually aluminum) strip suspended in a magnetic field. The current in the ribbon establishes another magnetic field, causing the ribbon to move in synchrony with the input signal waveform, hence, is a direct radiator of sound. Ribbon tweeters are effective at very high frequencies, and usually require a step-down transformer because of the very low impedance of the ribbon.