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Audio crossovers are a type of
electronic filter
Electronic filters are a type of signal processing filter in the form of electrical circuits. This article covers those filters consisting of lumped electronic components, as opposed to distributed-element filters. That is, using components ...
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. The crossover filters can be either
active or passive.
[ They are often described as ''two-way'' or ''three-way'', which indicate, respectively, that the crossover splits a given signal into two frequency ranges or three frequency ranges.][ Crossovers are used in ]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 " ...
cabinets
A cabinet is a body of high-ranking state officials, typically consisting of the executive branch's top leaders. Members of a cabinet are usually called cabinet ministers or secretaries. The function of a cabinet varies: in some countrie ...
, power amplifier
An audio power amplifier (or power amp) is an electronic amplifier that amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup, to a level that is high enough for driving loudspea ...
s in consumer electronics
Consumer electronics or home electronics are electronic (analog or digital) equipment intended for everyday use, typically in private homes. Consumer electronics include devices used for entertainment, communications and recreation. Usually r ...
( hi-fi, home cinema
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 audio equipment that is set up in a roo ...
sound and car audio
Vehicle audio is equipment installed in a car or other vehicle to provide in-car entertainment and information for the vehicle occupants. Until the 1950s it consisted of a simple AM radio. Additions since then have included FM radio (1952), 8 ...
) and pro audio
Professional audio, abbreviated as pro audio, refers to both an activity and a category of high quality, studio-grade audio equipment. Typically it encompasses sound recording, sound reinforcement system setup and audio mixing, and music produc ...
and musical instrument amplifier products. For the latter two markets, crossovers are used in bass amplifier
A bass amplifier (also abbreviated to bass amp) is a musical instrument electronic device that uses electrical power to make lower-pitched instruments such as the bass guitar or double bass loud enough to be heard by the performers and audien ...
s, keyboard amplifier
A keyboard amplifier is a powered electronic amplifier and loudspeaker in a wooden speaker cabinet used for amplification of electronic keyboard instruments. Keyboard amplifiers are distinct from other types of amplification systems such as g ...
s, bass and keyboard speaker enclosures and sound reinforcement system
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 ...
equipment (PA speakers, monitor speakers, 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 ...
systems, etc.).
Crossovers are used because most individual loudspeaker driver
An electrodynamic speaker driver, often called simply a speaker driver when the type is implicit, is an individual transducer that converts an electrical audio signal to sound waves. While the term is sometimes used interchangeably with the ...
s are incapable of covering the entire audio spectrum
A harmonic series (also overtone series) is the sequence of harmonics, musical tones, or pure tones whose frequency is an integer multiple of a ''fundamental frequency''.
Pitched musical instruments are often based on an acoustic resonator suc ...
from low frequencies to high frequencies with acceptable relative volume and absence of 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 ...
. Most hi-fi speaker systems and sound reinforcement system speaker cabinets use a combination of multiple loudspeaker drivers, each catering to a different frequency band
A frequency band is an interval in the frequency domain, delimited by a lower frequency and an upper frequency. The term may refer to a radio band or an interval of some other spectrum.
The frequency range of a system is the range over which i ...
. A standard simple example is in hi-fi and PA system cabinets that contain a woofer
A woofer or bass speaker is a technical term for a loudspeaker driver designed to produce low frequency sounds, typically from 50 Hz up to 1000 Hz. The name is from the onomatopoeic English word for a dog's bark, " woof" (in contrast to th ...
for low and mid frequencies and a 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 ...
for high frequencies. Since a sound signal source, be it recorded music from a CD player
A CD player is an electronic device that plays audio compact discs, which are a digital optical disc data storage format. CD players were first sold to consumers in 1982. CDs typically contain recordings of audio material such as music or aud ...
or a live band's mix from an audio console
A mixing console or mixing desk is an electronic device for mixing audio signals, used in sound recording and reproduction and sound reinforcement systems. Inputs to the console include microphones, signals from electric or electronic inst ...
, has all of the low, mid and high frequencies combined, a crossover circuit is used to split the audio signal into separate frequency bands that can be separately routed to loudspeakers, tweeters or horns optimized for those frequency bands.
Passive crossovers[ are probably the most common type of audio crossover. They use a network of passive electrical components (e.g., capacitors, inductors and resistors) to split up an amplified signal coming from one ]power amplifier
An audio power amplifier (or power amp) is an electronic amplifier that amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup, to a level that is high enough for driving loudspea ...
so that it can be sent to two or more loudspeaker drivers (e.g., a woofer
A woofer or bass speaker is a technical term for a loudspeaker driver designed to produce low frequency sounds, typically from 50 Hz up to 1000 Hz. The name is from the onomatopoeic English word for a dog's bark, " woof" (in contrast to th ...
and a very low frequency 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 ...
, or a woofer and a 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 ...
, or a woofer-midrange-tweeter combination).
Active crossovers are distinguished from passive crossovers in that they split up an audio signal prior to the power amplification stage so that it can be sent to two or more power amplifiers, each of which is connected to a separate loudspeaker driver.[ ]Home cinema
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 audio equipment that is set up in a roo ...
5.1 surround sound audio systems use a crossover that separates out the very-low frequency signal, so that it can be sent to a 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 ...
, and then sending the remaining low-, mid- and high-range frequencies to five speakers which are placed around the listener. In a typical application, the signals sent to the surround speaker cabinets are further split up using a passive crossover into a low/mid-range woofer and a high-range tweeter. Active crossovers come in both digital and analog varieties.
Digital active crossovers often include additional signal processing, such as limiting, delay, and equalization. Signal crossovers allow the audio signal to be split into bands that are processed separately before they are mixed together again. Some examples are multiband compression
Dynamic range compression (DRC) or simply compression is an audio signal processing operation that reduces the volume of loud sounds or amplifies quiet sounds, thus reducing or ''compressing'' an audio signal's dynamic range. Compression is ...
, limiting
In electronics, a limiter is a circuit that allows signals below a specified input power or level to pass unaffected while attenuating (lowering) the peaks of stronger signals that exceed this threshold. Limiting is a type of dynamic range comp ...
, de-essing
De-essing (also desibilizing) is any technique intended to reduce or eliminate the excessive prominence of sibilant consonants, such as the sounds normally represented in English by " s", "z", "ch", "j" and "sh", in recordings of the human voice. ...
, multiband 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 ...
, bass enhancement, high frequency exciters, and noise reduction
Noise reduction is the process of removing noise from a signal. Noise reduction techniques exist for audio and images. Noise reduction algorithms may distort the signal to some degree. Noise rejection is the ability of a circuit to isolate an und ...
such as Dolby A noise reduction.
Overview
The definition of an ideal audio crossover changes relative to the task and audio application at hand. If the separate bands are to be mixed back together again (as in multiband processing), then the ideal audio crossover would split the incoming audio signal into separate bands that do not overlap or interact and which result in an output signal unchanged in frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
, relative levels, and phase response
In signal processing, phase response is the relationship between the phase of a sinusoidal input and the output signal passing through any device that accepts input and produces an output signal, such as an amplifier or a filter.
Amplifiers, filt ...
. This ideal performance can only be approximated. How to implement the best approximation is a matter of lively debate. On the other hand, if the audio crossover separates the audio bands in a loudspeaker, there is no requirement for mathematically ideal characteristics within the crossover itself, as the frequency and phase response of the loudspeaker drivers within their mountings will eclipse the results. Satisfactory output of the complete system comprising the audio crossover ''and'' the loudspeaker drivers in their enclosure(s) is the design goal. Such a goal is often achieved using non-ideal, asymmetric crossover filter characteristics.[
Many different crossover types are used in audio, but they generally belong to one of the following classes.
]
Classification
Classification based on the number of filter sections
Loudspeakers are often classified as "N-way", where N is the number of drivers in the system. For instance, a loudspeaker with a woofer and a tweeter is a 2-way loudspeaker system. An N-way loudspeaker usually has an N-way crossover to divide the signal among the drivers. A 2-way crossover consists of a low-pass
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter des ...
and a high-pass
A high-pass filter (HPF) is an electronic filter that passes signals with a frequency higher than a certain cutoff frequency and attenuates signals with frequencies lower than the cutoff frequency. The amount of attenuation for each frequency de ...
filter. A 3-way crossover is constructed as a combination of low-pass
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter des ...
, band-pass
A band-pass filter or bandpass filter (BPF) is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range.
Description
In electronics and signal processing, a filter is usually a two-por ...
and high-pass
A high-pass filter (HPF) is an electronic filter that passes signals with a frequency higher than a certain cutoff frequency and attenuates signals with frequencies lower than the cutoff frequency. The amount of attenuation for each frequency de ...
filters (LPF, BPF and HPF respectively). The BPF section is in turn a combination of HPF and LPF sections. 4 (or more) way crossovers are not very common in speaker design, primarily due to the complexity involved, which is not generally justified by better acoustic performance.
An extra HPF section may be present in an "N-way" loudspeaker crossover to protect the lowest-frequency driver from frequencies lower than it can safely handle. Such a crossover would then have a bandpass filter
A band-pass filter or bandpass filter (BPF) is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range.
Description
In electronics and signal processing, a filter is usually a two-port ...
for the lowest-frequency driver. Similarly, the highest-frequency driver may have a protective LPF section to prevent high-frequency damage, though this is far less common.
Recently, a number of manufacturers have begun using what is often called "N.5-way" crossover techniques for stereo loudspeaker crossovers. This usually indicates the addition of a second woofer that plays the same bass range as the main woofer but rolls off far before the main woofer does.
''Remark: Filter sections mentioned here is not to be confused with the individual 2-pole filter sections that a higher-order filter consists of.''
Classification based on components
Crossovers can also be classified based on the type of components used.
Passive
A passive crossover splits up an audio signal after it is amplified by a single power amplifier
An audio power amplifier (or power amp) is an electronic amplifier that amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup, to a level that is high enough for driving loudspea ...
, so that the amplified signal can be sent to two or more driver types, each of which cover different frequency ranges. These crossover are made entirely of passive components and circuitry; the term "passive" means that no additional power source is needed for the circuitry. A passive crossover just needs to be connected by wiring to the power amplifier signal. Passive crossovers are usually arranged in a Cauer topology
Electronic filter topology defines electronic filter circuits without taking note of the values of the components used but only the manner in which those components are connected.
Filter design characterises filter circuits primarily by their ...
to achieve a Butterworth filter
The Butterworth filter is a type of signal processing filter designed to have a frequency response that is as flat as possible in the passband. It is also referred to as a maximally flat magnitude filter. It was first described in 1930 by the Br ...
effect. Passive filters use resistor
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...
s combined with reactive components such as capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
s and inductor
An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...
s. Very high-performance passive crossovers are likely to be more expensive than active crossovers since individual components capable of good performance at the high currents and voltages at which speaker systems are driven are hard to make.
Inexpensive consumer electronics
Consumer electronics or home electronics are electronic (analog or digital) equipment intended for everyday use, typically in private homes. Consumer electronics include devices used for entertainment, communications and recreation. Usually r ...
products, such as budget-priced Home theater in a box
A home theater in a box (HTIB) is an integrated home theater package which "bundles" together a combination DVD or Blu-ray player, a multi-channel amplifier (which includes a surround sound decoder, a radio tuner, and other features), speaker wi ...
packages and low-cost boom box
A boombox is a transistorized portable music player featuring one or two cassette tape recorder/players and AM/FM radio, generally with a carrying handle. Beginning in the mid 1980s, a CD player was often included. Sound is delivered through ...
es, use lower quality passive crossovers, often utilizing lower-order filter networks with fewer components. Expensive hi-fi speaker systems and receivers use higher quality passive crossovers, to obtain improved sound quality and lower distortion. The same price/quality approach is used with sound reinforcement system equipment and musical instrument amplifiers and speaker cabinets; a low-priced stage monitor
A stage monitor system is a set of performer-facing loudspeakers called monitor speakers, stage monitors, floor monitors, wedges, or foldbacks on stage during live music performances in which a sound reinforcement system is used to amplify a p ...
, PA speaker or bass amplifier speaker cabinet will typically use lower quality, lower priced passive crossovers, whereas high-priced, high-quality cabinets will use better quality crossovers. Passive crossovers may use capacitors made from polypropylene
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene.
Polypropylene
belongs to the group of polyolefins and ...
, metalized polyester
Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
foil, paper and electrolytic
An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
capacitors technology. Inductors may have air cores, powdered metal cores, ferrite core
In electronics, a ferrite core is a type of magnetic core made of ferrite on which the windings of electric transformers and other wound components such as inductors are formed. It is used for its properties of high magnetic permeability couple ...
s, or laminated silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
steel cores, and most are wound with enameled copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
wire.
Some passive networks include devices such as fuses
Fuse or FUSE may refer to:
Devices
* Fuse (electrical), a device used in electrical systems to protect against excessive current
** Fuse (automotive), a class of fuses for vehicles
* Fuse (hydraulic), a device used in hydraulic systems to protec ...
, PTC devices, bulbs or circuit breaker
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the risk ...
s to protect the loudspeaker drivers from accidental overpowering (e.g., from sudden surges or spikes). Modern passive crossovers increasingly incorporate equalization networks (e.g., Zobel network : ''For the wave filter invented by Zobel and sometimes named after him see m-derived filters.''
Zobel networks are a type of filter section based on the image-impedance design principle. They are named after Otto Zobel of Bell Labs, who publish ...
s) that compensate for the changes in impedance with frequency inherent in virtually all loudspeakers. The issue is complex, as part of the change in impedance is due to acoustic loading changes across a driver's passband.
Two disadvantages of passive networks are that they may be bulky and cause power loss. They are not only frequency specific, but also impedance specific (i.e. their response varies with the electrical load that they are connected to). This prevents their interchangeability with speaker systems of different impedances. Ideal crossover filters, including impedance compensation and equalization networks, can be very difficult to design, as the components interact in complex ways. Crossover design expert Siegfried Linkwitz
Siegfried Linkwitz (November 23, 1935 – September 11, 2018) was a German American engineer who was noted co-inventor of the Linkwitz–Riley filter along with Russ Riley. He submitted several important technical papers to the ''Journal of t ...
said of them that "the only excuse for passive crossovers is their low cost. Their behavior changes with the signal level-dependent dynamics of the drivers. They block the power amplifier from taking maximum control over the voice coil motion. They are a waste of time, if accuracy of reproduction is the goal."[ Alternatively, passive components can be utilized to construct filter circuits before the amplifier. This implementation is called a passive line-level crossover.
]
Active
An active crossover contains active components in its filters, such as transistors and operational amplifiers.[ In recent years, the most commonly used active device is an ]operational amplifier
An operational amplifier (often op amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op amp produces an output potential (relative to c ...
. In contrast to passive crossovers, which operate after the power amplifier's output at high current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
and in some cases high voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
, active crossovers are operated at levels that are suited to power amplifier inputs. On the other hand, all circuits with gain
Gain or GAIN may refer to:
Science and technology
* Gain (electronics), an electronics and signal processing term
* Antenna gain
* Gain (laser), the amplification involved in laser emission
* Gain (projection screens)
* Information gain in de ...
introduce noise
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...
, and such noise has a deleterious effect when introduced prior to the signal being amplified by the power amplifiers.
Active crossovers always require the use of power amplifiers for each output band. Thus a 2-way active crossover needs two amplifiers—one for the woofer
A woofer or bass speaker is a technical term for a loudspeaker driver designed to produce low frequency sounds, typically from 50 Hz up to 1000 Hz. The name is from the onomatopoeic English word for a dog's bark, " woof" (in contrast to th ...
and one for the 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 ...
. This means that a loudspeaker system that is based on active crossovers will often cost more than a passive-crossover-based system. Despite the cost and complication disadvantages, active crossovers provide the following advantages over passive ones:
*a frequency response independent of the dynamic changes in a driver's electrical characteristics (e.g. from heating of the voice coil)
*typically, the possibility of an easy way to vary or fine-tune each frequency band to the specific drivers used. Examples would be crossover slope, filter type (e.g., Bessel, Butterworth, Linkwitz-Riley, etc.), relative levels, etc.
*better isolation of each driver from the signals being handled by other drivers, thus reducing intermodulation
Intermodulation (IM) or intermodulation distortion (IMD) is the amplitude modulation of signals containing two or more different frequencies, caused by nonlinearities or time variance in a system. The intermodulation between frequency compo ...
distortion and overdriving
*the power amplifiers are directly connected to the speaker drivers, thereby maximizing amplifier damping control of the speaker voice coil, reducing consequences of dynamic changes in driver electrical characteristics, all of which are likely to improve the transient response of the system
*reduction in power amplifier output requirement. With no energy being lost in passive components, amplifier requirements are reduced considerably (up to 1/2 in some cases), reducing costs, and potentially increasing quality.
Digital
Active crossovers can be implemented digitally using a digital signal processor
A digital signal processor (DSP) is a specialized microprocessor chip, with its architecture optimized for the operational needs of digital signal processing. DSPs are fabricated on MOS integrated circuit chips. They are widely used in audio si ...
or other microprocessor
A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...
.[ They either use ]digital
Digital usually refers to something using discrete digits, often binary digits.
Technology and computing Hardware
*Digital electronics, electronic circuits which operate using digital signals
**Digital camera, which captures and stores digital i ...
approximations to traditional analog
Analog or analogue may refer to:
Computing and electronics
* Analog signal, in which information is encoded in a continuous variable
** Analog device, an apparatus that operates on analog signals
*** Analog electronics, circuits which use analog ...
circuits, known as IIR filters ( Bessel, Butterworth, Linkwitz-Riley etc.), or they use Finite Impulse Response (FIR) filters.[ IIR filters have many similarities with analog filters and are relatively undemanding of CPU resources; FIR filters on the other hand usually have a higher order and therefore require more resources for similar characteristics. They can be designed and built so that they have a ]linear phase
In signal processing, linear phase is a property of a filter where the phase response of the filter is a linear function of frequency. The result is that all frequency components of the input signal are shifted in time (usually delayed) by the sam ...
response, which is thought desirable by many involved in sound reproduction. There are drawbacks though—in order to achieve linear phase response, a longer delay time is incurred than would be necessary with an IIR or minimum phase FIR filters. IIR filters, which are by nature recursive, have the drawback that, if not carefully designed, they may enter limit cycles, resulting in non-linear distortion.
Mechanical
This crossover type is mechanical and uses the properties of the materials in a driver diaphragm to achieve the necessary filtering.[ Such crossovers are commonly found in ]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 driv ...
s which are designed to cover as much of the audio band as possible. One such is constructed by coupling the cone of the speaker to the voice coil bobbin through a compliant section and directly attaching a small lightweight ''whizzer'' cone to the bobbin. This compliant section serves as a compliant filter, so the main cone is not vibrated at higher frequencies. The whizzer cone responds to all frequencies, but due to its smaller size, it only gives a useful output at higher frequencies, thereby implementing a mechanical crossover function. Careful selection of materials used for the cone, whizzer and suspension elements determines the crossover frequency and the effectiveness of the crossover. Such mechanical crossovers are complex to design, especially if high fidelity is desired. Computer-aided design has largely replaced the laborious trial and error approach that was historically used. Over several years, the compliance of the materials may change, negatively affecting the frequency response of the speaker.
A more common approach is to employ the dust cap as a high-frequency radiator. The dust cap radiates low frequencies, moving as part of the main assembly, but due to low mass and reduced damping, radiates increased energy at higher frequencies. As with whizzer cones, careful selection of material, shape and position are required to provide smooth, extended output. High frequency dispersion
Dispersion may refer to:
Economics and finance
*Dispersion (finance), a measure for the statistical distribution of portfolio returns
*Price dispersion, a variation in prices across sellers of the same item
*Wage dispersion, the amount of variatio ...
is somewhat different for this approach than for whizzer cones. A related approach is to shape the main cone with such profile, and of such materials, that the neck area remains more rigid, radiating all frequencies, while the outer areas of the cone are selectively decoupled, radiating only at lower frequencies. Cone profiles and materials can be modeled using finite element analysis
The finite element method (FEM) is a popular method for numerically solving differential equations arising in engineering and mathematical modeling. Typical problem areas of interest include the traditional fields of structural analysis, heat ...
software and the results are predicted to excellent tolerances.
Speakers which use these mechanical crossovers have some advantages in sound quality despite the difficulties of designing and manufacturing them and despite the inevitable output limitations. Full-range drivers have a single acoustic center and can have relatively modest phase change across the audio spectrum. For best performance at low frequencies, these drivers require careful enclosure design. Their small size (typically 165 to 200 mm) requires considerable cone excursion to reproduce bass effectively, but the short voice coils required for reasonable high-frequency performance can only move over a limited range. Nevertheless, within these constraints, cost and complications are reduced, as no crossovers are required.
Classification based on filter order or slope
Just as filters have different orders, so do crossovers, depending on the filter slope they implement. The final acoustic slope may be completely determined by the electrical filter or may be achieved by combining the electrical filter's slope with the natural characteristics of the driver. In the former case, the only requirement is that each driver has a flat response at least to the point where its signal is approximately −10dB down from the passband. In the latter case, the final acoustic slope is usually steeper than that of the electrical filters used. A third- or fourth-order acoustic crossover often has just a second-order electrical filter. This requires that speaker drivers be well behaved a considerable way from the nominal crossover frequency, and further that the high-frequency driver be able to survive a considerable input in a frequency range below its crossover point. This is difficult to achieve in actual practice. In the discussion below, the characteristics of the electrical filter order are discussed, followed by a discussion of crossovers having that acoustic slope and their advantages or disadvantages.
Most audio crossovers use first- to fourth-order electrical filters. Higher orders are not generally implemented in passive crossovers for loudspeakers but are sometimes found in electronic equipment under circumstances for which their considerable cost and complexity can be justified.
First order
First-order filters have a 20 dB/decade
A decade () is a period of ten years. Decades may describe any ten-year period, such as those of a person's life, or refer to specific groupings of calendar years.
Usage
Any period of ten years is a "decade". For example, the statement that "du ...
(or 6 dB/octave
In music, an octave ( la, octavus: eighth) or perfect octave (sometimes called the diapason) is the interval between one musical pitch and another with double its frequency. The octave relationship is a natural phenomenon that has been refer ...
) slope. All first-order filters have a Butterworth filter characteristic. First-order filters are considered by many audiophile
An audiophile is a person who is enthusiastic about high-fidelity sound reproduction. An audiophile seeks to reproduce the sound of a piece of recorded music or a live musical performance, typically inside closed headphones, In-ear monitors, open ...
s to be ideal for crossovers. This is because this filter type is 'transient perfect', meaning that the sum of the low-pass and high-pass outputs passes both amplitude and phase unchanged across the range of interest.[ It also uses the fewest parts and has the lowest insertion loss (if passive). A first-order crossover allows more signal content consisting of unwanted frequencies to get through in the LPF and HPF sections than do higher-order configurations. While woofers can easily handle this (aside from generating distortion at frequencies above those that they can properly reproduce), smaller high-frequency drivers (especially tweeters) are more likely to be damaged, since they are not capable of handling large power inputs at frequencies below their rated crossover point.
In practice, speaker systems with true first-order acoustic slopes are difficult to design because they require large overlapping driver bandwidth, and the shallow slopes mean that non-coincident drivers interfere over a wide frequency range and cause large response shifts off-axis.
]
Second order
Second-order filters have a 40 dB/decade (or 12 dB/octave) slope. Second-order filters can have a Bessel, Linkwitz-Riley or Butterworth characteristic depending on design choices and the components that are used. This order is commonly used in passive crossovers as it offers a reasonable balance between complexity, response, and higher-frequency driver protection. When designed with time-aligned physical placement, these crossovers have a symmetrical polar
Polar may refer to:
Geography
Polar may refer to:
* Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates
* Polar climate, the c ...
response, as do all even-order crossovers.
It is commonly thought that there will always be a phase
Phase or phases may refer to:
Science
*State of matter, or phase, one of the distinct forms in which matter can exist
*Phase (matter), a region of space throughout which all physical properties are essentially uniform
* Phase space, a mathematic ...
difference of 180° between the outputs of a (second-order) low-pass filter and a high-pass filter having the same crossover frequency. And so, in a 2-way system, the high-pass section's output is usually connected to the high-frequency driver 'inverted', to correct for this phase problem. For passive systems, the tweeter is wired with opposite polarity to the woofer; for active crossovers the high-pass filter's output is inverted. In 3-way systems the mid-range driver or filter is inverted. However, this is generally only true when the speakers have a wide response overlap and the acoustic centers are physically aligned.
Third order
Third-order filters have a 60 dB/decade (or 18 dB/octave) slope. These crossovers usually have Butterworth filter characteristics; phase response
In signal processing, phase response is the relationship between the phase of a sinusoidal input and the output signal passing through any device that accepts input and produces an output signal, such as an amplifier or a filter.
Amplifiers, filt ...
is very good, the level sum being flat and in phase quadrature, similar to a first-order crossover. The polar response is asymmetric. In the original D'Appolito MTM arrangement, a symmetrical arrangement of drivers is used to create a symmetrical off-axis response when using third-order crossovers. Third-order acoustic crossovers are often built from first- or second-order filter circuits.
Fourth order
Fourth-order filters have an 80 dB/decade (or 24 dB/octave) slope. These filters are relatively complex to design in passive form, because the components interact with each other, but modern computer-aided crossover optimisation design software can produce accurate designs.[ Steep-slope passive networks are less tolerant of parts value deviations or tolerances, and more sensitive to mis-termination with reactive driver loads (although this is also a problem with lower-order crossovers). A 4th-order crossover with −6 dB crossover point and flat summing is also known as a Linkwitz-Riley crossover (named after its inventors][), and can be constructed in active form by cascading two 2nd-order Butterworth filter sections. The low-frequency and high-frequency output signals of the Linkwitz–Riley crossover type are in phase, thus avoiding partial phase inversion if the crossover band-passes are electrically summed, as they would be within the output stage of a multiband compressor. Crossovers used in loudspeaker design do not require the filter sections to be in phase; smooth output characteristics are often achieved using non-ideal, asymmetric crossover filter characteristics.][ Bessel, Butterworth, and Chebyshev are among the possible crossover topologies.
Such steep-slope filters have greater problems with overshoot and ringing][ but there are several key advantages, even in their passive form, such as the potential for a lower crossover point and increased ]power handling
Audio power is the electrical power transferred from an audio amplifier to a loudspeaker, measured in watts. The electrical power delivered to the loudspeaker, together with its efficiency, determines the sound power generated (with the rest ...
for tweeters, together with less overlap between drivers, dramatically reducing the shifting of the main lobe of a multi-way loudspeaker system's radiation pattern with frequency,[ or other unwelcome off-axis effects. With less frequency overlap between adjacent drivers, their geometric location relative to each other becomes less critical and allows more latitude in speaker system cosmetics or (in-car audio) practical installation constraints.
]
Higher order
Passive crossovers giving acoustic slopes higher than fourth-order are not common because of cost and complexity. Filters with slopes of up to 96 dB per octave are available in active crossovers and loudspeaker management systems.
Mixed order
Crossovers can also be constructed with mixed-order filters. For example, a second-order low-pass filter can be combined with a third-order high-pass filter. These are generally passive and are used for several reasons, often when the component values are found by computer program optimization. A higher-order tweeter crossover can sometimes help to compensate for the time offset between the woofer and tweeter, caused by non-aligned acoustic centers.
Classification based on circuit topology
Parallel
Parallel crossovers are by far the most common. Electrically the filters are in parallel and thus the various filter sections do not interact. This makes two-way crossovers easier to design because, in terms of electrical impedance, the sections can be considered separate and because component tolerance variations will be isolated but like all crossovers, the final design relies on the output of the drivers to be complementary acoustically and this, in turn, requires careful matching in amplitude and phase of the underlying crossover. Parallel crossovers also have the advantage of allowing the speaker drivers to be bi-wired, a feature whose benefits are hotly disputed.
Series
In this topology, the individual filters are connected in series, and a driver or driver combination is connected in parallel with each filter. To understand the signal path in this type of crossover, refer to the "Series Crossover" figure, and consider a high-frequency signal that, during a certain moment, has a positive voltage on the upper Input terminal compared to the lower Input terminal. The low-pass filter presents a high impedance to the signal, and the tweeter presents a low impedance; so the signal passes through the tweeter. The signal continues to the connection point between the woofer and the high-pass filter. There, the HPF presents a low impedance to the signal, so the signal passes through the HPF, and appears at the lower Input terminal. A low-frequency signal with a similar instantaneous voltage characteristic first passes through the LPF, then the woofer, and appears at the lower Input terminal.
Derived
Derived crossovers include active crossovers in which one of the crossover responses is derived from the other through the use of a differential amplifier.[ For example, the difference between the input signal and the output of the high-pass section is a low-pass response. Thus, when a differential amplifier is used to extract this difference, its output constitutes the low-pass filter section. The main advantage of derived filters is that they produce no phase difference between the high-pass and low-pass sections at any frequency.][ The disadvantages are either:
# that the high-pass and low-pass sections often have different levels of attenuation in their stopbands, i.e., their slopes are asymmetrical,][ or
# that the response of one or both sections peaks near the crossover frequency,][ or both.
In the case of (1), above, the usual situation is that the derived low-pass response attenuates at a much slower rate than the fixed response. This requires the speaker to which it is directed to continue to respond to signals deep into the stopband where its physical characteristics may not be ideal. In the case of (2), above, both speakers are required to operate at higher volume levels as the signal nears the crossover points. This uses more amplifier power and may drive the speaker cones into nonlinearity.
]
Models and simulation
Professionals and hobbyists have access to a range of computer tools that were not available before. These computer-based measurement and simulation tools allow for the modeling and virtual design of various parts of a speaker system which greatly accelerate the design process and improve the quality of a speaker. These tools range from commercial to free offerings. Their scope also varies. Some may focus on woofer/cabinet design and issues related to cabinet volume and ports (if any), while others may focus on the crossover and frequency response. Some tools, for instance, only simulate the baffle step response.
In the period before computer modeling made it affordable and quick to simulate the combined effects of drivers, crossovers and cabinets, a number of issues could go unnoticed by the speaker designer. For instance, simplistic three-way crossovers were designed as a pair of two-way crossovers: the tweeter/mid-range and the other the mid-range/woofer sections. This could create excess gain and a 'haystack' response in the mid-range output, together with a lower than anticipated input impedance. Other issues such as improper phase matching or incomplete modeling of the driver impedance curves could also go unnoticed. These problems were not impossible to solve but required more iterations, time and effort than they do today.
See also
* Bass management
The fundamental principle of bass management (also called LFE crossover) in surround sound replay systems is that bass content in the incoming signal, irrespective of channel, should be directed only to loudspeakers capable of reproducing it, whet ...
* Electrical characteristics of a dynamic loudspeaker
* Powered speakers
Powered speakers, also known as self-powered speakers and active speakers, are loudspeakers that have built-in amplifiers. Powered speakers are used in a range of settings, including in sound reinforcement systems (used at live music concerts), bo ...
* 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 ...
* 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 driv ...
* 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
* 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 ...
* Woofer
A woofer or bass speaker is a technical term for a loudspeaker driver designed to produce low frequency sounds, typically from 50 Hz up to 1000 Hz. The name is from the onomatopoeic English word for a dog's bark, " woof" (in contrast to th ...
* 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 ...
References
{{DEFAULTSORT:Audio Crossover
Analog circuits
Audio electronics
Loudspeaker technology
Tone, EQ and filter