A tweeter or treble speaker is a special type of loudspeaker (usually dome, inverse dome or horn-type) that is designed to produce high audio frequencies, typically from 2,000 to 20,000 Hz. The name is derived from the high pitched sounds made by some birds (tweets), especially in contrast to the low woofs made by many dogs, after which low-frequency drivers are named (woofers).

Nearly all tweeters are electrodynamic drivers using a voice coil suspended within a fixed magnetic field. These designs operate by applying current from the output of an amplifier circuit to a coil of wire called a voice coil. The voice coil produces a varying magnetic field, which works against the fixed magnetic field of a permanent magnet around which the cylindrical voice coil is suspended, forcing the voice coil and the diaphragm attached to it to move. This mechanical movement resembles the waveform of the electronic signal supplied from the amplifier's output to the voice coil. Since the coil is attached to a diaphragm, the vibratory motion of the voice coil transmits to the diaphragm; the diaphragm in turn vibrates the air, thus creating air motions or audio waves, which is heard as high sounds.

Modern tweeters are typically different from older tweeters, which were usually small versions of woofers. As tweeter technology has advanced, different design applications have become popular. Many soft dome tweeter diaphragms are thermoformed from polyester film, or silk or polyester fabric that has been impregnated with a polymer resin. Hard dome tweeters are usually made of aluminium, aluminium-magnesium alloys, or titanium.

Tweeters are intended to convert an electrical signal into mechanical air movement with nothing added or subtracted, but the process is imperfect, and real-world tweeters involve tradeoffs. Among the challenges in tweeter design and manufacture are: providing adequate damping, to stop the dome's motion rapidly when the signal ends; ensuring suspension linearity, allowing high output at the low end of its frequency range; ensuring freedom from contact with the magnet assembly, keeping the dome centered as it moves; and providing adequate power handling without adding excessive mass. Tweeters contribute to a well-balanced and rich audio experience by focusing on the higher sound frequencies.

Tweeters can also work in collaboration with the woofers that are responsible for generating the low frequencies or bass.

Some tweeters sit outside the main enclosure in their own semi-independent unit, such as "super tweeters". The latter plugs in and swivels to adjust the soundfield depending on listener position and user preference. The separation from the baffle is considered to be optimal under the theory that the smallest baffle possible is optimal for tweeters.

Most tweeters are designed to reproduce frequencies up to the formally defined upper limit of the human hearing range (typically listed as 20 kHz); some operate at frequencies up to approximately in between 2 kHz to 20 kHz. Tweeters with a greater upper range have been designed for psychoacoustic testing, for extended-range digital audio such as Super Audio CD intended for audiophiles, for biologists performing research on animal response to sounds, and for ambient sound systems in zoos. Ribbon tweeters have been made that can reproduce 80 kHz and even 100 kHz.

All dome materials have advantages and disadvantages. Three properties designers look for in domes are low mass, high stiffness and good damping. Celestion were the first manufacturers to fabricate dome tweeters out of a metal, copper. Nowadays other metals such as aluminium, titanium, magnesium, and beryllium, as well as various alloys thereof, are used, being both light and stiff but having low damping; their resonant modes occur above 20 kHz. More exotic materials, such as synthetic diamond, are also being used for their extreme stiffness. Polyethylene terephthalate film and woven silk suffer less ringing, but are not nearly as stiff, which can limit their very high frequency output.