Woofer

Woofer is the term commonly used for a 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). The most common design for a woofer is the electrodynamic driver, which typically uses a stiff paper cone, driven by a voice coil which is surrounded by a magnetic field. The voice coil is attached by adhesives to the back of the speaker cone. 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 resulting motion of the cone creates sound waves as it moves in and out.

The frame, or basket, is the structure holding the cone, voice coil and magnet in the proper alignment. There are two main metal frame types, stamped and cast. Cast baskets are more expensive, but are usually more rigid in all directions. Stamped steel baskets are the lower-cost approach, but cast plastic is lower cost still and has come into common use.An important woofer specification is its power rating, the amount of power the woofer can handle without damage. The power rating is not easily characterized (see below) and many manufacturers cite peak ratings safely attainable only for very brief moments.

At ordinary sound pressure levels (SPL), most humans can hear down to about 20 Hz. Woofers are generally used to cover the lowest octaves of the system's frequency range. In two-way loudspeaker systems, the drivers handling the lower frequencies are also obliged to cover a substantial part of the midrange, often as high as 1000 or 2000Hz; such drivers are commonly termed mid woofers. Since the 1990s, a sub-type of woofer (termed subwoofer), which is designed for very low frequencies, has come to be commonly used in home theater systems and PA systems to augment the bass response; they are usually handle the very lowest two or three octaves (i.e., from as low as 20 to perhaps 80 or 120 Hz).

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 (usually required to reproduce loud sounds) with low distortion, and energy storage in one or more of the moving parts (called ringing when the cone is underdamped). There are also 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 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.

An early version of the now widely used bass reflex cabinet design was patented by Albert L. Thuras of Bell Laboratories in 1932. Earlier speakers simply mounted the driver on a baffle, and low frequency performance was lost to interference. A. Neville Thiele in Australia, and later Richard H. Small of Australia and later the United States, first comprehensively adapted electronic filter theory to the design of loudspeaker 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 by competent system designers when applicable; the T/S approach does not fully apply to some enclosure types.

Speaker designers, including DIY builders, can use any of several widely available computer programs that perform the sometimes involved T/S calculations. Some are open source programs, others commercial. 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. 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. This awkward fact obviously complicates manufacturing.

Resonance frequency is one of these, 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 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 itself. As a rule of thumb, the lower the system's resonance frequency, the lower the frequency reproducible by the speaker system at some given level of distortion. The resonance frequency of the driver itself is listed in its specification sheet T/S parameters as Fs.

All woofers have electrical and mechanical properties that very strongly influence 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 (sometimes summarized for box enclosures as Hoffman's Iron Law, after the H in KLH); in essence, Hoffman observed that for a given driver, box size, system resonance, and efficiency are inter-related such that only two may be optimized in any design. These woofer characteristics 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 due to its T/S measurements. For instance, a woofer with a small maximum excursion (often those with critically hung voice coils) will not be suited to acoustic suspension designs (which 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 (i.e., controls excursion via impedance matching to the atmosphere) below that resonance frequency; cone excursions increase greatly, and for some drivers beyond safe limits. 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, different, design analyses as do transmission lines; the last has only recently had a practical and usable mathematical model for design use. Thiele/Small analyses don't really apply to these enclosure types.

Active loudspeakers
In 1965, Sennheiser Electronics introduced the Philharmonic sound system, which used electronics to overcome some of the problems ordinary woofer subsystems confront. They added a motion 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 extension of 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 sensor-equipped woofers in inexpensive 'music systems', boom boxes, or even car audio systems. This is usually done in an attempt to get better performance from inexpensive or undersized 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 the benefits from 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 that somewhat compensates for the deficiencies of a speaker system.

Equalization techniques are used in most public address and sound reinforcement applications. Here, the problem is not primarily hi-fi reproduction, but managing the acoustic environment. In this case, the equalization must be individually adjusted to match the particular characteristics of the loudspeaker systems used and the room in which they are used.

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 fiberglass 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 rubbing 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 specification is its power rating, the amount of power the woofer can handle without damage. The electrical power rating is not easily characterized and many manufacturers cite peak ratings attainable only for very brief moments without damage. Woofer power ratings become important when the speaker is pushed to extremes: applications requiring high output, amplifier overload conditions, unusual signals (i.e., non-musical ones), 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 will heat up, increase its resistance, causing "power compression", a condition where output sound power level decreases after extended high power activity. Further heating can physically distort the voice coil, causing scuffing, shorting due to wire insulation deterioration, or other electrical or mechanical damage. Sudden impulse energy can melt a section of voice coil wire, causing an open circuit and a dead woofer; the necessary level will vary with driver characteristics. In normal listening level music applications, the electrical power rating of woofers is generally unimportant; it remains important for higher frequency drivers.

There are really four types of power handling in loudspeaker drivers, including woofers: thermal (heat), electrical (both covered above), mechanical, and acoustic. The mechanical power handling limit is reached when cone excursion extends to its maximum limit. Thermal power handling limits may be reached when fairly high power levels are fed to a woofer for too long, even if not exceeding mechanical limits at any time. Most of the energy applied to the voice coil is converted to heat, not sound; all of the heat is ultimately passed to the pole piece, the rest of the magnet structure, and the frame. From the woofer structure, the heat is eventually dissipated into the surrounding air. Some drivers include provisions for better cooling (e.g., vented magnet pole pieces, dedicated heat conduction structures) to reduce increased coil/magnet/frame temperatures during operation, especially high power level conditions. If too much power is applied to the voice coil as compared to its ability to shed heat, it will eventually exceed a maximum safe temperature. 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.

Acoustic power handling is directly related to driver and enclosure efficiency.Some combination are much more efficient so they can handle much more applied power than less efficient combinations. Energy input which is emitted as sound does not contribute to voice coil heating. So, as a rule of thumb, a voice coil in a magnet structure will be able to safely handle more applied power if the system components working together are more efficient at creating sound output.

Public address (PA) and instrument applications
Woofers designed for public address system(PA) and instrument amplifier applications are similar in makeup to home audio woofers, except that they are more heavy duty. Typically, design variances include: cabinets built for repeated shipping and handling, larger woofer cones to allow for higher sound pressure levels, more robust voice coils to withstand higher power, and higher suspension stiffness. Generally, a home woofer used in a PA/instrument application can be expected to fail more quickly than a PA/instrument woofer. On the other hand, if you use a PA/instrument woofer in a home audio application it will not likely have the same quality of performance, particularly at low volumes. A PA woofer will not produce the same audible high fidelity which is the goal of high quality home audio due those differences.

PA system woofers typically have high efficiency, and high power handling capacity. The trade off for high efficiency at reasonable cost is usually relatively low excursion capability (i.e., inability to move "in and out" as far as many home woofers can) as they are intended for horn or large reflex enclosures. They are also usually ill-suited to extended low bass response since the last octave of low frequency response increases size and expense considerably, and is increasingly uneconomic to attempt at high levels as in a PA application. A home stereo woofer, because it is used at relatively low volumes, may be able to handle very low frequencies. Because of this, most PA 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 20 Hz. To accurately reproduce the lowest tones, a woofer, or group of woofers, must move an appropriately large volume of air; a task that becomes more difficult at lower frequencies. The larger the venue, the more air the woofer movement will have to displace in order to produce the required sound power at low frequencies.

The chart below gives an approximate account of the frequency ranges of several sizes of woofers; it is suggestive only. For instance, in special cases like full-sized horns, small drivers can reproduce surprisingly low frequency material at useful levels with low distortion. The green area represents the range a single woofer of that size can commonly manage with reasonable distortion and useful output levels, while the yellow shows uncommon extended frequency capability. The purple area at the bottom represents the fundamental musical frequency range of common instruments. The lighter purple areas note instrument ranges with 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. It's important to understand that pianos, like all instruments, produce harmonic overtones along with every fundamental note which are important in properly reproducing their sound; these are marked at the top of the chart. The "airiness" frequency block is an attempt to distinguish a frequency range with less harmonic content, but clearly present in live instrument performance, as reproduction through loudspeakers with and without filters blocking these frequencies are distinguishable; no term for the effect is really satisfactory. By comparing the instrument ranges (and harmonics) versus typical driver ranges, an indication of the difficulties faced by speaker designers can be seen. No woofer can do everything well.



Note that this chart does not include bigger woofers such as 15", 18", 21" and the rare larger sizes. It also does not show the effects of two or more woofers working together; they can move a greater mass of air, resulting in lower frequency extension or perhaps merely greater output at a given distortion level. Furthermore, it does not include the narrowing of a woofer's polar pattern at the higher end of its frequency range, which is often a significant effect.