Woofer - AbsoluteAstronomy.com

Woofer is the term commonly used for a loudspeaker

Loudspeaker

A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...

driver

Speaker driver

A speaker driver is an individual transducer that converts electrical energy to sound waves, typically as part of a loudspeaker, television, or other electronics device. Sometimes the transducer is itself referred to as a speaker, particularly when a single one is mounted in an enclosure or as...

designed to produce low frequency

Low frequency

Low frequency or low freq or LF refers to radio frequencies in the range of 30 kHz–300 kHz. In Europe, and parts of Northern Africa and of Asia, part of the LF spectrum is used for AM broadcasting as the longwave band. In the western hemisphere, its main use is for aircraft beacon,...

sounds, typically from around 40 hertz

Hertz

The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

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
Tweeter
A tweeter is a loudspeaker designed to produce high audio frequencies, typically from around 2,000 Hz to 20,000 Hz . Some tweeters can manage response up to 65 kHz...

). The most common design for a woofer is the electrodynamic driver, which typically uses a stiff paper cone, driven by a voice coil

Voice coil

A voice coil is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it...

which is surrounded by a magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

. The voice coil is attached by adhesives to the back of the speaker cone. The voice coil and magnet form a linear electric motor

Electric motor

An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

. When current flows through the voice coil, the coil moves in relation to the frame according to Fleming's left hand rule

Fleming's left hand rule

Fleming's left-hand rule , and Fleming's right-hand rule are a pair of visual mnemonics that is used for working out the direction of motion in an electric motor, or the direction of electric current in an electric generator...

, 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.

At ordinary sound pressure

Sound pressure

Sound pressure or acoustic pressure is the local pressure deviation from the ambient atmospheric pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water...

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 type of woofer (termed subwoofer

Subwoofer

A subwoofer is a woofer, or a complete loudspeaker, which is dedicated to the reproduction of low-pitched audio frequencies known as the "bass". The typical frequency range for a subwoofer is about 20–200 Hz for consumer products, below 100 Hz for professional live sound, and below...

), which is designed for very low frequencies, has come to be commonly used in home theater

Home cinema

Home cinema, also commonly called home theater, are home entertainment set-ups that seek to reproduce a movie theater experience and mood with the help of video and audio equipment in a private home....

systems and PA systems to augment the bass response; they usually handle the very lowest two or three octaves (i.e., from as low as 20 to perhaps 80 or 120 Hz).

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

Loudspeaker enclosure

A loudspeaker enclosure is a purpose-engineered cabinet in which speaker drivers and associated electronic hardware, such as crossover circuits and amplifiers, are mounted...

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 mid-range and high frequencies.

A crossover network

Audio crossover

Audio crossovers are a class of electronic filter used in audio applications. Most individual loudspeaker drivers are incapable of covering the entire audio spectrum from low frequencies to high frequencies with acceptable relative volume and lack of distortion so most hi-fi speaker systems use a...

, 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

Distortion

A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...

-correcting negative feedback

Negative feedback

Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...

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

Ringing (signal)

In electronics, signal processing, and video, ringing is unwanted oscillation of a signal, particularly in the step response...

, 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

Electrical impedance

Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...

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

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

, and later Richard H. Small

Richard H. Small

Richard H. Small is an American scientist, who worked mainly on electroacoustics. The Thiele/Small parameters are named after him and Neville Thiele.-Career:...

(who taught and did design work in Australia, of England and the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

), first comprehensively adapted electronic filter theory

Audio crossover

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 T/S approach does not fully apply to some types of enclosure.

To use what are known as Thiele/Small

Thiele/Small

"Thiele/Small" commonly refers to a set of electromechanical parameters that define the specified low frequency performance of a loudspeaker driver. These parameters are published in specification sheets by driver manufacturers so that designers have a guide in selecting off-the-shelf drivers for...

(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 complicates manufacturing.

Resonant

Resonance

In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...

frequency

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the 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

KLH

KLH is*an audio/video company, see KLH *the abbreviation for Korean Light Helicopter*the abbreviation for Keyhole limpet hemocyanin...

). Similar relationships apply to other types of enclosure. 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

Vacuum tube

In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

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.

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

Servo drive

A servo drive is a special electronic amplifier used to power electric servomechanisms.A servo drive monitors the feedback signal from the servomechanism and continually adjusts for deviation from expected behavior.-Function:...

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

Digital signal processing

Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...

techniques make possible a higher precision crossover technique. By using FIR

Finite impulse response

A finite impulse response filter is a type of a signal processing filter whose impulse response is of finite duration, because it settles to zero in finite time. This is in contrast to infinite impulse response filters, which have internal feedback and may continue to respond indefinitely...

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 polymer

Polymer

A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s 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

Mica

The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...

-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

Voice coil

A voice coil is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it...

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 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 combinations 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

Instrument amplifier

An instrument amplifier is an electronic amplifier that converts the often barely audible or purely electronic signal from musical instruments such as an electric guitar, an electric bass, or an electric keyboard into an electronic signal capable of driving a loudspeaker that can be heard by the...

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