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This is a speaker unit for reproducing the low-frequency portions of the audio signal. Generally, woofers with a large diameter are more effective for reproducing low-frequency sounds.
In many 3-way speaker systems, the woofer is positioned in the lower part of the enclosure because the speakers (tweeter, mid-range, and woofer) are mounted according to their frequency range. Details... Woofer is the term commonly used for a loudspeaker driver designed to produce low frequency sounds, typically from around 40 hertz up to about a kilohertz or higher. The name is from the onomatopoeic English word for a dog's bark, "woof" (in contrast to the name used for speakers designed to reproduce high-frequency sounds, tweeter). Description
Cross-section of a standard loudspeaker, not to scale. The most common design is the electrodynamic driver. These use a cone, driven by a voice coil surrounded by a magnetic field. The voice coil and magnet form a linear electric motor. When current flows through the voice coil, the coil moves in relation to the frame according to Fleming's left hand rule, causing the coil to push or pull on the driver cone in a piston-like way. The voice coil is attached by adhesives to the back of the speaker cone. The resulting motion of the cone creates sound waves as it moves in and out.
Woofer design 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. Examples of such problems include damping the cone cleanly without audible distortion at each end of the in/out cycle, managing high excursions (reproduction of loud sounds) without distortion, and energy storage in one or more of the moving parts (called ringing when the cone is underdamped). There are challenges in controlling electrical impedance so as to make possible the use of economic electronic amplifiers. Good woofer design requires effectively converting a low frequency amplifier signal to mechanical air movement with high fidelity and maximal efficiency, and is 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. The widely used bass reflex design was granted U.S. Patent 1,869,178 by Albert L. Thuras of Bell Laboratories in 1932. Earlier speakers simply mounted the driver on a baffle, and low frequencies were lost to the rear. A N Thiele in Australia, and later Richard Small in the United States, first adapted electronic filter theory to the design of loudpeaker enclosures, particularly at the low frequencies where woofers work. This was a very considerable advance in the practice of woofer subsystem design, and is now almost universally practiced (except for horn and transmission line enclosures) by competent system designers. Speaker designers, including DIY builders, can use any of several computer programs which perform the sometimes involved calculations. Some are open source programs, others are expensive commercial offerings. To use what are 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. Resonant frequency is one of these, and is determined by a combination of the compliance (i.e., flexibility) of the cone suspension and the mass of the moving parts of the speaker (the cone, voice coil, dust cap and suspension). When combined with the magnetic field strength, the electrical characteristics of the driver, and the air resistance provided by the enclosure, there will be a related, but different resonance characteristic, that of the loudspeaker system itself. In general, the lower the system's resonant frequency, the lower the frequency reproducible by the speaker system at some given level of distortion. The T/S resonant frequency of the driver is listed in its specification sheet parameters as Fs. All woofers have electrical and mechanical properties that dictate the correct box size of a given type (e.g., bass reflex, sealed enclosure, "infinite baffle", etc.) for a given desired performance and efficiency. Not all desired speaker system qualities can be maximized simultaneously. They also strongly affect the crossover components needed for a given performance in a particular loudspeaker system. A given woofer may work well in one enclosure type, but not in another. For instance, a woofer with a small maximum excursion (often those with underhung voice coils) will not be suited to acoustic suspension designs (which typically require generous excursions), nor for use in bass reflex designs without an electrical filter preventing signals much below the system resonance from reaching the woofer. In this last case, the enclosure no longer seriously loads the woofer below that resonance frequency, and cone excursions increase greatly. It is at minimum critical to know and understand the Thiele/Small parameters of a driver in order to design a satisfactory loudspeaker system using it. Horn designs have their own design procedures as do transmission lines, though these last two have only recently had a usable design analysis.
Active loudspeakers In 1965, Sennheiser Electronics of Austria introduced the Philharmonic sound system which used electronics to overcome some of the problems ordinary woofer subsystems confront. They added a sensor to the woofer, and used the signal corresponding to its actual motion to feedback as a control input to a specially designed amplifier. If carefully done, this can improve performance (both in 'tightness' and to extend the low frequency performance) considerably at the expense of flexibility (the amplifier and the speaker are tied together permanently) and cost. In the US, L W Erath, an oil industry engineer, introduced a line of high end speakers along very much the same lines. As electronics costs have decreased, it has become common to have active loudspeakers (in this meaning) in inexpensive 'music systems', boom boxes, or even car audio systems. This is usually done in an attempt to get better performance from inexpensive drivers in lightweight or poorly designed enclosures. This approach presents difficulties as not all distortion can be eliminated using servo techniques, and a poorly designed enclosure can swamp any attempt at electronic correction.
Equalized loudspeakers Because the characteristics of a loudspeaker can be measured, and to a considerable extent predicted, it is possible to design special circuitry which compensates for the deficiencies of a speaker system. The most notable early example of this design approach in hi-fi equipment were the Bose 1801 and 901 speaker systems of the late 1960s and early 1970s. Both of these speaker systems used several identical small drivers (18 in the 1801 and 9 in the 901). In the 901, each driver was about 4" in diameter, and was not capable of reasonable reproduction of either low frequencies (too small) or high frequencies (too large). But, with a specially designed electronic equalizer circuit (supplied with the speaker system) prior to the power amplifier, the signal at both low frequencies and high frequencies could be changed to compensate for the otherwise inherent deficiencies of the designed loudspeaker. At the expense of substantially increased amplifier power demand for typical listening volumes, and very considerable power demand for high listening volumes. The result was a speaker system (the 901) which made a considerable commercial impression and had substantial sales. The 1801 was too cumbersome and far to expensive to sell well. Equalization techniques are also used, for a very different purpose, in most public address and sound reinforcement applications. Here, the problem is not primarily hi-fi reproduction, but managing the acoustic environment (eg, resonance, reverberation, etc). In this case, the equalization must be individually adjusted to match the particular characteristics of the loudspeaker systems used and the room they are used in.
Digital filtering crossover and equalization Computer techniques, in particular DSP techniques make possible a higher precision crossover technique. By using FIR and other digital techniques, the crossovers for a bi-amped or tri-amped system can be accomplished with a precision not possible with analog filters, whether passive or active. Furthermore, many driver peculiarities (down to and including individual variances) can be remedied at the same time, using the same techniques. One of Klein and Hummel's recent designs is implemented using these techniques. Because of the complex and advanced techniques involved, this approach is unlikely to be used in lower cost equipment for some time to come.
Cone materials All cone materials have advantages and disadvantages. The three chief properties designers look for in cones are light weight, stiffness, and lack of coloration (due to absence of ringing). Exotic materials like Kevlar and magnesium are light and stiff, but can have ringing problems, depending on their fabrication and design. Materials like paper (including coated paper cones) and various polymers will generally ring less than metal diaphragms, but can be heavier and not as stiff. There have been good and bad woofers made with every type of cone material. Almost every kind of material has been used for cones, from fibreglass and bamboo fibers to expanded aluminum honeycomb sandwich panel material and mica loaded plastic cones.
Frame design The frame, or basket, is the structure holding the cone, voice coil and magnet in the proper alignment. Since the voice coil gap is quite narrow (clearances are typically in the low thousandths of an inch), rigidity is important to prevent scuffing of the voice coil against the magnet structure in the gap and also avoid extraneous motions. There are two main metal frame types, stamped and cast. Stamped baskets (usually of steel) is a lower-cost approach. The disadvantage of this type of frame is that the basket may flex if the speaker is driven at high volumes, there being resistance to bending only in certain directions. Cast baskets are more expensive, but are usually more rigid in all directions, have better damping (reducing their own resonance), can have more intricate shapes, and are therefore usually the preferred for higher quality drivers.
Power handling An important woofer characteristic is power handling, that is the average amount of power the woofer can handle without damage. This driver rating cannot be well characterized and many manufacturers cite rather optimistic numbers. In most cases, the only time amplifier wattage ratings become important are in very high-volume (loudness) situations, low-power amplifier situations (amplifier overload-clipping-can produce dangerous signals), unusual signals (i.e., non-music), very low frequencies at which the enclosure provides little or no acoustic loading (and so there will be maximum cone excursion), or amplifier failure. In high-volume situations, a woofer's voice coil can overheat and distort, causing scuffing, shorting due to wire insulation deterioration, or other mechanical damage. They can even melt. In normal listening level music applications, this rating is generally unimportant, at least for woofers. There are two types of power handling: thermal (heat) and mechanical. Mechanical power handling limits are reached when cone excursions exceed maximum limits. Consider a ported enclosure (also known as bass reflex, or vented enclosure), for which there is little loading of the diaphragm below a limiting frequency, after which cone motion is essentially uncontrolled. In this frequency region, the woofer can physically travel too far and be physically damaged. Thermal power handling may be reached when too much power is fed to the woofer for too long, even if not exceeding mechanical limits at any time. Energy not transferred to the air is converted to heat, almost all in the voice coil, and if not dissipated somehow, will eventually exceed the maximum temperature the voice coil can safely endure. Adhesives can melt, the voice coil former can melt or distort, or the insulation separating the voice coil windings can fail. Each of these events will damage the woofer, perhaps beyond usability.
Public address (PA) and instrument applications Woofers designed for public address (PA) and instrument applications are similar in makeup to home audio woofers. Typically, design variances include: cabinets built for repeated shipping and handling, larger woofer cones to allow for higher sound levels, more robust voice coils to withstand higher power, higher suspension stiffness, etc. Generally, a home woofer used in a PA/instrument application can be expected to fail more or less quickly. A PA/instrument woofer used in a home application is not likely to have the same quality of performance, particularly at low volumes. Pro audio woofers usually have high efficiency, and high power handling capacity. The trade off for high efficiency at a reasonable cost is a relatively low excursion capability (they cannot move in and out as far as many home woofers do) as they are intended for horn or large reflex cabinet mounting. They are also usually ill suited to extended low bass response since the last octave of low response increases size and expense considerably. Because of this, most pro audio woofers are not well suited to use in high quality high fidelity home applications and vice versa.
Frequency ranges At ordinary sound pressure levels (SPL), most humans can hear down to about 16 Hz.[citation needed] A loudspeaker which can produce adequate bass down to 48 Hz will sound full-range to most people when reproducing most musical material.[citation needed] The only real exception, except some electronic music, is recordings from very large organs, some of which have very low frequency pipes. Many modern small loudspeakers are designed to produce bass down to around 80-100 Hz,[citation needed] where it is assumed the end user will be using a subwoofer to cover the bottom two octaves. To accurately reproduce those bottom octaves, a woofer must be large enough to move an appropriate volume of air, and this becomes more difficult at lower frequencies. The larger the room, the larger the woofer will have to be in most cases to produce the required SPL in the room at low frequencies. Both requirements are incompatible with small drivers in speaker systems. The chart below gives the approximate frequency ranges of different sized woofers. In special cases, for instance full-length horns, small drivers can reproduce unusually low frequency material at useful levels with low distortion. The green area represents the range a woofer can commonly manage, while the yellow is the extended frequency where performance may be compromised. The purple area at the bottom represents the fundamental musical frequency range of common intruments. The lighter purple areas extend the instrument range to rarely played notes, for instance, the first and last 10 keys on a standard piano. (The frequency range of the notes on a standard 88-key piano is 27 to 4,096 Hz, but note that pianos, like all instruments, produce harmonic overtones which are important in properly reproducing their sound.) By comparing the instrument ranges versus the nominal driver ranges, some of the problems confronted by speaker designers can be seen. No woofer can do everything well. 
Image copyright Peter J Smith 2005 Note that this chart does not show bigger woofers such as 15", 18", 21" and rare larger sizes. It also does not show the effects of two or more woofers working together to move a greater mass of air, ideally resulting in lower frequency extension.
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