Tweeters
A Tweeter is a loudspeaker designed to produce high frequencies, typically from around 2,000 Hz to 20,000 Hz or 20KHz. Some tweeters can manage response up to 45 kHz.
The name is derived from the high pitched sounds made by some birds, 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 an amplifier to a coil.
The electrified voice coil produces a varying magnetic field which works against the fixed magnetic field, forcing the voice coil and the diaphragm attached to it, to move. Since the coil is attached to a diaphragm, its motions become those of the diaphragm creating air motions which we hear 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 which have been impregnated with a polymer resin. Hard Dome Tweeters commonly employ 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 trade-offs.
Among the challenges in tweeter design and manufacture are
- Providing adequate damping to stop the dome's motion rapidly when the signal ends
- To allow 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 designed for sound reinforcement and musical instrument applications are broadly similar to High Fidelity tweeters, though they're usually not referred to as tweeters, but as "high frequency drivers".
Key design requirement differences are -
- Mountings built for repeated shipping and handling
- Drivers often mounted to horn structures to provide for higher sound levels and greater control of sound dispersion
- More robust voice coils to withstand the higher power levels typically encountered
High frequency drivers in PA horns are often referred to as compression drivers from the mode of acoustic coupling between the driver diaphragm and the horn throat.
Various materials are used in the construction of Compression Driver diaphragms including titanium, aluminium, phenolic impregnated fabric, polyimide and PET film, each having its own characteristics.
The diaphragm is glued to a voice coil former, typically made from a different material than the dome, since it must cope with heat without tearing or significant dimensional change. Polyimide film, Nomex, and glassfibre are popular for this application.
The suspension may be a continuation of the diaphragm and is glued to a mounting ring, which may fit into a groove, over locating pins, or be fastened with machine screws.
The diaphragm is generally shaped like an inverted dome and loads into a series of tapered channels in a central structure called a phase plug, which equalizes the path length between various areas of the diaphragm and the horn throat, preventing acoustic cancellations between different points on the diaphragm surface.
The phase plug exits into a tapered tube which forms the start of the horn itself. This slowly expanding throat within the driver is continued in the horn flare. The horn flare controls the coverage pattern, or directivity, and as an acoustic transformer, adds gain.
A professional horn and compression driver combination has an output sensitivity of between 105 and 112dB/watt/meter. This is substantially more efficient, and less thermally dangerous to a small voice coil and former, than other tweeter construction.