Full-range

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, fail to reproduce 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.

Typical designs

Typically, a full-range drive unit consists of a single driver element, or "voice coil", used to move and control a "diaphragm". Often the cone structure 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 such that high frequency vibrations of the coil are transferred primarily to the whizzer, 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 radiating dome in place of the usual dust-cap; it is deliberately acoustically active. In most speaker drivers such dust-caps are so constructed (or made of a material) as to be as acoustically inert as possible, i.e., contribute as little as possible to the driver's frequency response^‡. Sometimes the dome or dust cap takes the form of a small conical shape to improve dispersion at higher frequencies. Yet other designs simply modify the diaphragm and dome/whizzer materials instead of compliantly coupling the diaphragm to achieve full-range operation.

The technique 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 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 sub woofer or by a special cabinet design for low frequency reinforcement. 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 than usual, which improves sensitivity and thus lowers the power requirement at 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 for reasonable low end output.

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.

One designer, the late John Wyckhoff, elected to use a large diameter cone (12 inch diameter) to provide adequate bass response, but was forced to add another driver, a small boom mounted tweeter in front of the large driver, to manage satisfactory high frequency performance. Reviews of his last design indicate that it has much more bass performance (and improved high frequency performance) compared to more typical full-range designs.

Atypical designs

A German company, Manger, produces an unusual full-range driver in the form of a differentially flexing flat diaphragm driver, about eight inches in diameter. Manger claims performance, both maximum level and extended low frequency response, which is rather better than traditional full-range drivers.

Another unusual full-range driver design is the Walsh driver, which was originally designed to perform well from below 30Hz to beyond the upper limit of human hearing. Early models were very large, and quite expensive. Loudspeakers using the Walsh driver are still in production, though they have adopted a tweeter to cover the highest frequencies and so are now two-way systems or even three way for the largest models. A variation on the Walsh driver, from a German firm, is available in two forms (a titanium cone and a carbon fiber cone) and incorporated into a broad range of production loudspeakers.

Applications

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. There is an active do-it-yourself speaker construction group on the Web focusing on full-range drivers and cabinets for them.

Critical reaction

Critics of full range drivers cite their common inability to reproduce fast and complex musical forms. Reproducing multiple frequencies with the same diaphragm causes intermodulation distortion, a non-linear effect that occurs when one surface attempts to reproduce both frequencies concurrently. This is related to the doppler effect. The result is "frequency mixing", which adds frequencies at the sum and difference of the original frequencies. A full range driver has reduced output at either end of its frequency range, occasionally resulting in a "boxy sound", or bandwidth-limited midrange-sound, without much treble presence. This type of driver may sound "limited" compared to more conventional two and three-way designs. Partisans of full-range loudspeaker designs claim superior phase coherence, while non-partisans often dismiss them as midrange speakers forced beyond their limits.

High quality loudspeaker systems using full-range drivers are typically more expensive than many multi-way speaker systems, despite the savings in crossover components and additional drivers.

Vendors

Fostex, E J Jordan, Lowther and Feastrex are prominent producers of full-range high fidelity drivers, using many of the design techniques discussed here.

ee also

*Audio crossover

References

*http://melhuish.org/audio/theory.html