Audio crossovers are a class of electronic filters designed specifically for use in hi-fi audio applications. A commonly used dynamic loudspeaker driver is incapable of covering the entire audio spectrum all by itself. Thus, crossovers serve the purpose of splitting the audio signal into narrow frequency bands which can be handled by individual loudspeaker drivers optimised for those bands. A combination of multiple drivers each catering to a different frequency band constitutes most hi-fi speaker systems. Electronic filters are electronic circuits which perform signal processing functions. ... Closeup of a loudspeaker driver A loudspeaker is a device which converts an electrical signal into sound. ... Closeup of a loudspeaker driver A loudspeaker is a device which converts an electrical signal into sound. ... High Fidelity is also the title of a book by Nick Hornby and a film directed by Stephen Frears, based upon Hornbys book. ...

Overview

Audio crossovers (filters) are characterised by a flat phase response (typical of Bessel filters) as opposed to the flat frequency response (typical of Butterworth filters) of most industrial and data acquisition filters within their pass-band. A crossover is constructed as a combination of low-pass, band-pass and high-pass filters (LPF, BPF and HPF respectively). The number of pass-bands determines the "way" of the crossover. Thus, a 2-way crossover means it has one LPF section and one HPF section. A 3-way crossover has one LPF section, one BPF section and one HPF section (the BPF section is in turn a combination of HPF and LPF sections). The filters are so designed that each section has a -6 dB amplitude at the crossover point(s). Thus, Phase response is the relationship between the phase of a periodic input and an ouput signal passing through any device which accepts an input and produces an output signal such as an amplifier or a filter. ... Wikipedia does not have an article with this exact name. ... Frequency response is the measure of any systems response to frequency, but is usually used in connection with electronic amplifiers and similar systems, particularly in relation to audio signals. ... The Butterworth filter is one of the most basic electronic filter designs. ... In telecommunications, optics, and acoustics, passband is the portion of spectrum, between limiting frequencies (or, in the optical regime, limiting wavelengths), that is transmitted with minimum relative loss or maximum relative gain by a filtering device. ... A low-pass filter passes low frequencies fairly well, but attenuates, or blocks, high frequencies. ... The frequency axis of this symbolic diagram would be logarithmically scaled. ... In J. R. R. Tolkiens fiction of Middle-earth, the High Pass is a pass over the Misty Mountains. ... In telecommunications, optics, and acoustics, passband is the portion of spectrum, between limiting frequencies (or, in the optical regime, limiting wavelengths), that is transmitted with minimum relative loss or maximum relative gain by a filtering device. ... DB or db or dB may stand for: DB (car), a French automobile maker Darkbyte, A character in a Tales of. ...

• In a 2-way Bessel crossover, the -6 dB point of the LPF = -6 dB point of the HPF
• In a 3-way Bessel crossover, the -6 dB point of the LPF = lower -6 dB point of the BPF and the upper -6 dB point of the BPF = -6 dB point of the HPF.

Although the good old Butterworth filter is used to construct an audio crossover, it generally is not considered the ideal choice. Butterworth filters when used in a crossover, are 3 dB down at the crossover frequency(ies), which means that the summed frequency response of these filters exhibits large peak(s) at the crossover point(s) - this is not an ideal (i.e., flat) frequency response. As far as audio is concerned, a Bessel or Linkwitz-Riley (L-R) filter characteristic is preferred. The reason is that the Bessel and L-R filters, when used in a crossover are 6 dB down at the crossover frequency(ies), which means that the summed frequency response of these filters is flat. The Butterworth filter is one of the most basic electronic filter designs. ... Wikipedia does not have an article with this exact name. ...

Note: The crossover points (frequencies) must not be confused with the -3 dB cutoff frequency of filters. While the -3 dB frequency means the same thing (as in bandwidth, for example) for all filters irrespective of their order or type, the crossover point is the point (frequency) where one filter stops (or fades out) and the other filter takes over. Specifically, the Butterworth filter must have the crossover point to be the same as its -3 dB frequency in order to work, otherwise the summed response will exhibit a large or wide dip in the frequency response - this means we cannot simply design two Butterworth filters and some how cross them over at -6 dB. Bessel filters will have a -3 dB frequency away from their -6 dB points and crossing them over at their -6 dB points gives a flat summing - a result of the Bessel characteristic. In telecommunications, the term cutoff frequency fc has the following meanings: 1. ... // Analog For analog signals, bandwidth is the width, usually measured in hertz, of a frequency band f2 − f1. ...

Classification based on filter order

Just as filters have different orders, so do crossovers - depending on the filters they use.

1^st order crossovers

1^st order filters have a 20 dB/decade (or 6 dB/octave) slope. 1^st order filters have a natural Bessel characteristic (unless designed with some other characteristic in mind) and are considered by many audiophiles to be ideal for crossovers. This is partially true because this crossover uses minimum parts, has relatively less losses (if passive) and is more predictable than higher order passive crossovers. The predictability factor stems from the fact that a typical loudspeaker is not a resistive load but reactive. This means that the impedance that the loudspeaker presents to the passive crossover varies with frequency - affecting the crossover point of the crossover itself. A 1^st order crossover is simplest in this respect since the factors that impair the crossover's performance are minimum. However, 1^st order crossovers are generally not found in hi-fi professional systems. They are more common in off-the-shelf component systems or micro-systems and that too in 2-way speakers. A 1^st order crossover may contain nothing more than a simple capacitor connected in series with the tweeter, the woofer being directly connected to the amplifier. The crossover point is determined simply by the woofer's natural response and the combination of the tweeter's impedance and the series capacitor. A 1^st order crossover tends to allow relatively more unwanted frequencies to get through in the LPF and HPF sections. While woofers can easily take this, the tweeters may be damaged since they are not designed to handle large powers and lower frequencies. A decade on a graphical logarithmic scale represents multiplication by 10 from the previous value. ... In music, an octave (sometimes abbreviated 8ve or 8va) is the interval between one musical note and another with half or double the frequency. ... Friedrich Wilhelm Bessel (July 22, 1784 – March 17, 1846) was a German mathematician, astronomer, and systematizer of the Bessel functions (which, despite their name, were discovered by Daniel Bernoulli). ... An audiophile (literally, one who loves sound) is one who is concerned with achieving high-quality results in the recording and playback of music. ... Closeup of a loudspeaker driver A loudspeaker is a device which converts an electrical signal into sound. ... A Sony tweeter. ... A Sony 9 inch woofer. ...

2^nd order crossovers

2^nd order filters have a 40 dB/decade (or 12 dB/octave) slope. 2^nd order filters can have a Bessel or Butterworth characteristic depending on design and choice of components used. This order is most commonly used in passive crossovers as it offers a good balance between complexity, response and tweeter protection. Where a 2-way 2^nd order crossover appears, the tweeter is wired in opposite phase to the woofer to correct for crossover phase. In active form this crossover is very simple to design and build and usually has the HPF output inverted thereby not requiring the tweeter to be wired out of phase. Friedrich Wilhelm Bessel (July 22, 1784 – March 17, 1846) was a German mathematician, astronomer, and systematizer of the Bessel functions (which, despite their name, were discovered by Daniel Bernoulli). ... Butterworth can refer to: Butterworth filter, a fundamental electronic filter S. Butterworth, the inventor of the above filter A town in Province Wellesley, Penang, Malaysia George Butterworth, a South Africa Aston Butterworth, a British automobile manufacturer This is a disambiguation page — a navigational aid which lists other pages that might...

3^rd order crossovers

3^rd order filters have a 60 dB/decade (or 18 dB/octave) slope. These crossovers usually do not have a Bessel or Butterworth characteristic and phase response is neither flat nor abrupt. These are not commonly used in passive form, although some active crossovers employ them.

4^th order crossovers

4^th order filters have a 80 dB/decade (or 24 dB/octave) slope. These filters offer the best performance but are more complex to design in passive form. They are more commonly found in active crossover form and are considered superior in all respects for audio. A 4^th order crossover with -6 dB crossover point and flat summing is also known as a Linkwitz-Riley crossover. It can be constructed by cascading the individual filter sections of a 2^nd order Bessel crossover.

Classification based on components

Crossovers can also be classified based on the components they use.

Passive crossovers

A passive crossover is made entirely of passive filters. The passive filters employ passive components like resistors, capacitors and inductors for their operation. These are relatively cheap when the cost of the entire equipment is considered. On the negative side, A resistor is a two-terminal electrical component that creates an electrical potential difference across its terminals that is proportional to the current passing through it. ... Various types of capacitors A capacitor (occasionally referred to using the older term condenser) is a device that stores energy in the electric field created between a pair of conductors on which equal but opposite electric charges have been placed. ... An inductor is a passive electrical device that stores energy in a magnetic field, typically by combining the effects of many loops of electric current. ...

• These tend to be bulky and always cause some power loss.
• They also suffer from dependence on the individual speaker drivers and the crossover point (and hence overall frequency response) tends to shift from the ideal with frequency and temperature.
• A passive crossover also does not isolate the various drivers from each other - the tweeter will "see" some of the woofer's back-emf and if the amplifier clips, this will be evident as gross distortion possibly damaging the tweeter.
• The amplifier does not see the drivers (due to the crossover in between) and hence the speakers are not as well-damped as they should be. The result is poorer transient response of the amplifier-speaker combination - bass is not as precise.

Active crossovers

An active crossover contains active components in its filters. The most commonly used active device is an op-amp and active crossovers are operated at line-level powers meant to drive amplifiers in contrast to passive ones which operate at the high-level powers meant for speakers. Active crossovers always require the use of power amplifiers for each band. Thus a 2-way active crossover needs two amplifiers - one each for the woofer and tweeter. This means that an active crossover based system might end up costing more than a passive crossover based system. It also requires the use of a tweeter protection capacitor (≥ 22µF) since the tweeter is now directly connected to the amplifier and may be damaged due to DC or the short thump an amplifier produces as the amplifier is powered on. The cost disadvantage is offset by the following gains: An operational amplifier or op-amp is an electronic circuit module (normally built as an integrated circuit, but occasionally with discrete transistors or vacuum tubes) which has a non-inverting input (+), an inverting input (-) and one output. ...

• Very accurate and predictable frequency response independent of the speaker drivers,
• Ability to easily fine tune each frequency band for the particular speaker drivers being used,
• Complete isolation of the drivers - the tweeter will never know if the woofer is being over-driven,
• The power amplifiers are directly connected to the speaker drivers, thereby maximising the damping effect and improving the transient response of the amplifier-speaker combination - bass is more precise.

References

• Article about passive crossovers
• Article about active crossovers
• Comparison of active and passive crossovers
• ESP web-site.

See also

• Electronic filter
• Loudspeaker