Full-range
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A Full-range loudspeaker drive unit is defined as a driver which reproduces as much of the audible frequency range as possible, with high-fidelity, within the boundaries imposed by the physical limitations of the specific design. This is so difficult that there are very few manufacturers producing full range drivers for high fidelity use. Most single driver systems, such as those in small radios, or the smaller computer speaker designs, make no pretense of reproducing the entire (or almost) audio range and are tolerable only because our ears are forgiving. It is the attempt to do so which distinguishes the full-range driver.
Typically, a full-range drive unit consists of a single driver element, or voice coil, used to move and control a diaphragm. Often the diaphragm includes optimizations to enhance high-frequency performance. For example, a small low-mass horn or whizzer cone can be mounted where the voice coil and diaphragm meet, thereby increasing the output efficiency (and hence frequency range) at high frequencies. The larger (or main) diaphragm in these designs is usually coupled to the voice coil using compliant material so that high frequency vibrations of the coil are transferred only to the whizzer and not to the main diaphragm. The shape and materials used in the cone are generally highly optimized as well.
Another arrangement, which doesn't employ a whizzer, uses a dome in place of the dust-cap – looking rather like a dust cap, but deliberately acoustically active. In contrast, in other speakers a dust-cap is so constructed (or made of a material) as to be as acoustically inert as possible, i.e., it contributes as little as possible to the speaker's frequency responseǂ. Sometimes the dome takes the form of a small conical shape to improve dispersion. Yet other designs simply modify the diaphragm and dome/whizzer materials instead of compliantly coupling the diaphragm to achieve full-range operation.
This arrangement of using a compliantly coupled (or modified) diaphragm for the low frequencies and auxiliary whizzer or modified dust-cap (dome) for the high frequency response of a speaker is a kind of mechanical crossover.
Since the requirements of a full-range driver include both good low and high frequency response (which are essentially contradictory in terms of physical construction), a full-range driver is usually limited to covering the audio spectrum above perhaps 100Hz -- leaving lower frequencies to be handled / augmented by a separate subwoofer or by a special cabinet design for low frequency reproduction. These requirements usually mean that the full-range must have good sensitivity (for lower frequencies) with a light voice coil (for high frequencies) – these speakers commonly use a larger or more powerful magnet which improves sensitivity and thus lowers the power requirement for low frequencies as well as allowing a lighter voice coil. In addition, many have limited maximum excursions, requiring special enclosures which do not require large excursions at low frequencies.
There are rare exceptions that use multiple elements to drive a common diaphragm, but these should not be mistaken for units that use separate and concentrically aligned elements to achieve the desired range of reproduction. The latter are commonly referred to as coaxial drive units, and are not strictly classified as full-range drivers.
Full-range drive units may be found in applications ranging from inexpensive multimedia loudspeakers (low cost is helpful), to costly esoteric cabinets, the latter often using large transmission line or horn loaded enclosures to increase low frequency output.
E J Jordan, and related companies, are a prominent producers of full-range high fidelity drivers, using many of the design techniques discussed here. Lowther and Fostex are others.
ǂ – In practice, the dust cap will always add its something to the overall response of a driver since it is also a part of the moving system of a speaker. However its contribution is controlled by making the dust-cap small, light, rigid, and dense enough to merely stop dirt from entering the voice-coil gap. Rarely, a dust cap can be modified to also double up as a high frequency radiator or dome.
