A woofer or bass speaker is a technical term for a
loudspeaker
A loudspeaker (commonly referred to as a speaker or speaker driver) is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referred to as a "speaker" or " ...
driver designed to produce
low frequency
Low frequency (LF) is the ITU designation for radio frequencies (RF) in the range of 30–300 kHz. Since its wavelengths range from 10–1 km, respectively, it is also known as the kilometre band or kilometre wave.
LF radio waves exh ...
sounds, typically from 50
Hz up to 1000 Hz. The name is from the
onomatopoeic
Onomatopoeia is the process of creating a word that phonetically imitates, resembles, or suggests the sound that it describes. Such a word itself is also called an onomatopoeia. Common onomatopoeias include animal noises such as ''oink'', ''m ...
English word for a dog's bark, "
woof" (in contrast to the name used for loudspeakers designed to reproduce high-frequency sounds, ''
tweeter
A tweeter or treble speaker is a special type of loudspeaker (usually dome, inverse dome or horn-type) that is designed to produce high audio frequencies, typically deliver high frequencies up to 100 kHz. The name is derived from the high ...
''). The most common design for a woofer is the electrodynamic driver, which typically uses a stiff paper cone, driven by a
voice coil
A voice coil (consisting of a former, collar, and winding) 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. The term ...
surrounded by a
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
.
The voice coil is attached by adhesives to the back of the loudspeaker cone. The voice coil and the magnet form a linear
electric motor
An electric motor is an Electric machine, electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a Electromagneti ...
. When current flows through the voice coil, the coil moves in relation to the frame according to
Fleming's left hand rule for motors
Fleming's left-hand rule for electric motors is one of a pair of visual mnemonics, the other being Fleming's right-hand rule (for generators). They were originated by John Ambrose Fleming, in the late 19th century, as a simple way of working out ...
, 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 or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophone ...
levels (SPL), most humans can hear down to about 20 Hz.
Woofers are generally used to cover the lowest octaves of a loudspeaker'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 2000 to 5000 Hz; such drivers are commonly termed ''mid woofers''. Since the 1990s, a type of woofer (termed
subwoofer
A subwoofer (or sub) is a loudspeaker designed to reproduce low-pitched audio frequencies known as bass and sub-bass, lower in frequency than those which can be (optimally) generated by a woofer. The typical frequency range for a subwoofer is ...
), which is designed for very low frequencies only, has come to be commonly used in
home theater
Home cinema, also called home theaters or theater rooms, are home entertainment audio-visual systems that seek to reproduce a movie theater experience and mood using consumer electronics-grade video and home audio, audio equipment that is set ...
systems and
PA system
A public address system (or PA system) is an electronic system comprising microphones, amplifiers, loudspeakers, and related equipment. It increases the apparent volume (loudness) of a human voice, musical instrument, or other acoustic sound sou ...
s to augment the bass response; they usually handle the very lowest two or three octaves (i.e., from as low as 20 to 80 or 120 Hz).
Woofer design
![Speaker-cross-section](https://upload.wikimedia.org/wikipedia/commons/c/cd/Speaker-cross-section.svg)
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
A loudspeaker enclosure or loudspeaker cabinet is an enclosure (often rectangular box-shaped) in which speaker drivers (e.g., loudspeakers and tweeters) and associated electronic hardware, such as crossover circuits and, in some cases, power ...
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
In statistics, the mid-range or mid-extreme is a measure of central tendency of a sample defined as the arithmetic mean of the maximum and minimum values of the data set:
:M=\frac.
The mid-range is closely related to the range, a measure of sta ...
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
In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...
-correcting
negative feedback
Negative feedback (or balancing feedback) occurs when some function (Mathematics), function of the output of a system, process, or mechanism is feedback, fed back in a manner that tends to reduce the fluctuations in the output, whether caused by ...
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 may mean:
Vibrations
* Ringing (signal), unwanted oscillation of a signal, leading to ringing artifacts
* Vibration of a harmonic oscillator
** Bell ringing
* Ringing (telephony), the sound of a telephone bell
* Ringing (medicine), a ri ...
, 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
In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit.
Quantitatively, the impedance of a two-terminal circuit element is the ratio of the comp ...
which is not too far from 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
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial research and scientific development company owned by mult ...
in 1932.
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
L, or l, is the twelfth Letter (alphabet), letter in the Latin alphabet, used in the English alphabet, modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is English alphabet#Let ...
, 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 may refer to:
Mechanisms
* Servomechanism, or servo, a device used to provide control of a desired operation through the use of feedback
** AI servo, an autofocus mode
** Electrohydraulic servo valve, an electrically operated valve that ...
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
A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also distribute those sounds ...
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
digital signal processing
Digital signal processing (DSP) is the use of digital processing, such as by computers or more specialized digital signal processors, to perform a wide variety of signal processing operations. The digital signals processed in this manner are ...
(DSP), make possible a higher precision crossover. By using
finite impulse response
In signal processing, a finite impulse response (FIR) filter is a filter whose impulse response (or response to any finite length input) is of ''finite'' duration, because it settles to zero in finite time. This is in contrast to infinite impulse r ...
(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 such as in
Klein and Hummel
Klein may refer to:
People
*Klein (surname)
*Klein (musician)
Places
*Klein (crater), a lunar feature
* Klein, Montana, United States
*Klein, Texas, United States
*Klein (Ohm), a river of Hesse, Germany, tributary of the Ohm
*Klein River, a rive ...
's recent designs. This approach is complex and thus not likely to be used in lower cost equipment.
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
Ringing may mean:
Vibrations
* Ringing (signal), unwanted oscillation of a signal, leading to ringing artifacts
* Vibration of a harmonic oscillator
** Bell ringing
* Ringing (telephony), the sound of a telephone bell
* Ringing (medicine), a ri ...
). 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
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
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 glass fiber, bamboo fiber, to expanded aluminum honeycomb sandwiches, and
mica
Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
-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) are 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 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
A voice coil (consisting of a former, collar, and winding) 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. The term ...
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 three types of power handling in loudspeaker drivers, including woofers: thermal (heat), electrical (both covered above), and mechanical. 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
An electromagnetic coil is an electrical conductor such as a wire in the shape of a coil (spiral or helix). Electromagnetic coils are used in electrical engineering, in applications where electric currents interact with magnetic fields, in de ...
can fail. Each of these events will damage the woofer, perhaps beyond usability.
Public address (PA) and instrument applications
Woofers designed for
public address system
A public address system (or PA system) is an electronic system comprising microphones, amplifiers, loudspeakers, and related equipment. It increases the apparent volume (loudness) of a human voice, musical instrument, or other acoustic sound sou ...
(PA) and
instrument amplifier
An instrument amplifier is an electronic device that converts the often barely audible or purely electronic signal of a musical instrument into a larger electronic signal to feed to a loudspeaker. An instrument amplifier is used with musical ins ...
applications are similar in makeup to home audio woofers, except that they are usually designed more ruggedly. 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, a PA/instrument woofer in a home audio application will not 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 to 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,
[ 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 room, the more air the woofer's movement will have to displace in order to produce the required sound power at low frequencies.
]
See also
* Loudspeaker enclosure
A loudspeaker enclosure or loudspeaker cabinet is an enclosure (often rectangular box-shaped) in which speaker drivers (e.g., loudspeakers and tweeters) and associated electronic hardware, such as crossover circuits and, in some cases, power ...
* Audio crossover
Audio crossovers are a type of electronic filter circuitry that splits an audio signal into two or more frequency ranges, so that the signals can be sent to loudspeaker drivers that are designed to operate within different frequency ranges. Th ...
* Full-range speaker
A full-range loudspeaker drive unit is defined as a driver which reproduces as much of the audible frequency range as possible, within the limitations imposed by the physical constraints of a specific design. The frequency range of these drive ...
* Tweeter
A tweeter or treble speaker is a special type of loudspeaker (usually dome, inverse dome or horn-type) that is designed to produce high audio frequencies, typically deliver high frequencies up to 100 kHz. The name is derived from the high ...
* Super tweeter
A super tweeter is a speaker driver intended to produce ultra high frequencies in a multi-driver loudspeaker system. Its purpose is to recreate a more realistic sound field, often characterized as "airy-ness". Super tweeters are sometimes found in ...
* Midrange speaker
A mid-range speaker is a loudspeaker driver that reproduces sound in the frequency range from 250 to 2000 hertz, Hz. It is also known as a squawker.
Mid-range drivers are usually cone types or, less commonly, dome types, or Compression_driver, co ...
* Subwoofer
A subwoofer (or sub) is a loudspeaker designed to reproduce low-pitched audio frequencies known as bass and sub-bass, lower in frequency than those which can be (optimally) generated by a woofer. The typical frequency range for a subwoofer is ...
* Thiele/Small Thiele/Small parameters (commonly abbreviated T/S parameters, or TSP) are a set of electromechanical parameters that define the specified low frequency performance of a loudspeaker driver. These parameters are published in specification sheets by dr ...
* Motional Feedback
Motional Feedback (MFB) is an active high fidelity loudspeaker system which was developed by the Dutch Philips brand in the early 1970s. The loudspeakers have built-in amplifiers and feature a feedback sensor on the woofer. The sensor measures th ...
References
{{Reflist
Loudspeakers