A loudspeaker driver that produces the frequency range from approximately 300–5000 hertz is known as a mid-range. They are also called, less commonly, squawkers.
Midrange drivers are usually electro-dynamic cone types or, less commonly, electro-dynamic dome types, or compression horn drivers.
The diaphragm of a cone speaker is a truncated cone, with the voice coil attached at the narrow end with the spider portion of the cone suspension, and with the cone surround at the wide end. These midranges resemble small woofers. A dome is a usually spherical section with both suspension and voice coil located at the outer edge of the dome. These midranges resemble large dome tweeters. A very few midranges are electrostatic loudspeakers, Heil drivers, planar magnetic drivers, or ribbon drivers.
Materials
The most common material used in midrange cone construction is paper, occasionallty impregnated and/or surface-treated with polymers or resins in order to improve vibrational damping. Other midrange cone materials include plastics such as polypropylene or Bextrene, special fabrics, woven Kevlar, carbon fiber, light metal alloys based on aluminium, magnesium or titanium, or various alloys.
Installation issues
The capabilities and limitations of the drivers employed above and below the midrange driver, as well as the midrange itself, influence the choice of crossover frequencies and the slope of the filters. The dramatic majority of speaker systems use passive crossovers, constructed with capacitors, inductors, and resistors to split the signal after it has been amplified. A select few hi-fi and many professional speaker systems require electronic crossovers, with active circuitry to split the signal before it is amplified, a technique called bi-amping or tri-amping.
Placement of the midrange (and tweeter) drivers on the enclosure baffle can significantly effect the output of the driver, and the material surrounding the midrange and tweeter drivers on the baffle may promote or inhibit re-radiated energy from the baffle face, further influencing the output. Grilles, especially those with structural frames, can further modify the output of the entire speaker system.
A term often used in design circles is the baffle step effect. Baffle step is the result of different dispersion effects for different frequencies. Low frequencies radiate in omnidirectionally (or in a 4-pi spherical pattern), whereas higher frequencies radiate in a more controlled pattern, termed 2-pi, or hemispherical. At the transition frequency, the same quantity of energy is radiated into half the acoustic space, creating a perceived increase in output level for listeners in front of the speaker system. As the frequency increases further, the pattern continues to reduce, and may become identifiably directional. At the upper-end of the audible range (above 10kHz) radiation may be less than 0.1-pi.
Some manufacturers mount midrange drivers on their own small enclosures, so as to isolate them from the pressure generated by the woofer. Very small enclosures can increase the Qtc of the midrange system, degrading transient response somewhat, in exchange for increased output at the lower end of the drivers frequency range.
Role
A midrange driver is called upon to handle the most significant part of the audible sound spectrum, the region where the most fundamentals emitted by musical instruments and, most importantly, human voice, lie. This region contains most sounds which are the most familiar to the human ear, and where any discrepancies from faithful reproduction will be most easily be observed. It is therefore paramount that a midrange driver of good quality be capable of low-distortion reproduction.
See also
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