Woofer - Woofer Design

Woofer Design

Good woofer design requires effectively converting a low frequency amplifier signal to mechanical air movement with high fidelity and acceptable efficiency, and is both assisted and complicated by the necessity of using a loudspeaker enclosure to couple the cone motion to the air. If done well, many of the other problems of woofer design (for instance, linear excursion requirements) are reduced.

In most cases the woofer and its enclosure must be designed to work together. Usually the enclosure is designed to suit the characteristics of the speaker or speakers used. The size of the enclosure is a function of the longest wavelengths (lowest frequencies) to be reproduced, and the woofer enclosure is much larger than required for midrange and high frequencies.

A crossover network, either passive or active, filters the band of frequencies to be handled by the woofer and other speakers. Normally the crossover and speaker system, including the woofer, are expected to convert the electrical signal supplied by the amplifier to an acoustic signal of identical waveform without other interaction between the amplifier and speakers, although sometimes the amplifier and speakers are designed together with the speakers supplying distortion-correcting negative feedback to the amplifier.

There are many challenges in woofer design and manufacture. Most have to do with controlling the motion of the cone so the electrical signal to the woofer's voice coil is faithfully reproduced by the sound waves produced by the cone's motion. Problems include damping the cone cleanly without audible distortion so that it does not continue to move, causing ringing, when the instantaneous input signal falls to zero each cycle, and managing high excursions (usually required to reproduce loud sounds) with low distortion. There are also challenges in presenting to the amplifier an electrical impedance which is approximately constant at all frequencies.

An early version of the now widely used bass-reflex cabinet design was patented by Albert L. Thuras of Bell Laboratories in 1932. Earlier speaker system designs paid little attention to the mounting and enclosure of the driver, and the longest wavelength (lowest frequency) handled was limited by the shortness of the distance the unwanted out-of-phase signal from the back of the driver had to travel before reaching and interfering with the wanted signal from the front. A. Neville Thiele in Australia, and later Richard H. Small (who taught and did design work in Australia, England, and the United States), first comprehensively adapted electronic filter theory to the design of loudspeaker enclosures, particularly at the low frequencies handled by woofers. This was a very considerable advance in the practice of woofer subsystem design, and is now almost universally used by system designers when applicable, although the Theile/Small approach does not fully apply to some types of enclosure.

To use what are now known as Thiele/Small (sometimes called T/S) design techniques, a woofer must first be carefully measured to characterize its electrical, magnetic, and mechanical properties; these are collectively known as the Thiele/Small parameters. They are now commonly included in the specification sheets for most higher-quality woofer drivers; not all, of course, have been carefully measured, and in any case, specific drivers may vary from the average run produced. In addition, some of these parameters can change during a driver's lifetime (especially during its first few hours or days of use) and so these parameters should really be measured after a suitable burn-in period to best match the enclosure design to the driver actually being used.

Resonant frequency is an important parameter, and is determined by a combination of the compliance (i.e., flexibility) of the cone suspension and by the mass of the moving parts of the speaker (the cone, voice coil, dust cap and some of the suspension). When the resonant frequency of the driver is combined with the magnetic motor characteristics, the electrical characteristics of the driver, and the acoustic environment provided by the enclosure, there will be a related, but different resonance characteristic, that of the loudspeaker system as a whole. As a rule of thumb the lower the system's resonant frequency, the lower the frequency reproducible by the speaker system for a given level of distortion. The resonant frequency of the driver is usually listed in its specification sheet as F_s.

All woofers have electrical and mechanical properties that very strongly influence the correct size of enclosure of a given type (e.g., bass reflex, sealed enclosure, "infinite baffle", etc.) for a given performance and efficiency. There is a tradeoff between sealed enclosure size, system resonance, and power efficiency; for a given driver specifying two of these quantities determines the third. This is known as Hoffman's Iron Law (Hoffman is the H in KLH). Similar relationships apply to other types of enclosures. As high-power solid-state amplifiers are available, a common tradeoff is to sacrifice efficiency and produce a relatively small enclosure capable of reproducing low frequencies, but requiring a lot of power; in the days of vacuum tube amplifiers a 35W amplifier was large and expensive, and large, efficient, enclosures were more often used.

There are several computer programs, both open source and proprietary, that perform the complicated T/S calculations.