Speaker Crossover Filter Types
Crossover filters determine which frequencies are allowed to reach each loudspeaker driver. The most common filters used in loudspeaker systems are low-pass, high-pass and band-pass filters. Understanding these filter types is essential when designing a two-way, three-way or multi-way speaker system.
What Is a Filter?
An audio filter is an electrical, electronic or digital circuit that changes the level of different frequencies in a signal.
A filter can allow a particular frequency range to pass while attenuating frequencies outside that range.
In a loudspeaker crossover, filters are used to divide the audio spectrum between the different drivers.
Audio spectrum
──────────────────────────────────────────► Frequency
Bass Midrange Treble
│──────────────────│──────────────────────│
Filters divide these regions
between the loudspeaker drivers.
The Main Crossover Filter Types
The four basic filter types are:
- Low-pass filter
- High-pass filter
- Band-pass filter
- Band-stop filter
The first three are particularly important in loudspeaker crossover design.
Low-Pass Filter
A low-pass filter allows lower frequencies to pass while attenuating higher frequencies.
Output
│
│████████████████
│ \
│ \
│ \
│ \________
│
└────────────────────────────► Frequency
fc
In a two-way speaker, the low-pass filter is normally connected to the woofer.
Low-Pass Filter in a Speaker
Amplifier
│
▼
Low-pass
filter
│
▼
Woofer
The filter reduces the amount of high-frequency energy reaching the woofer.
This helps prevent the woofer from reproducing frequencies above its intended operating range.
High-Pass Filter
A high-pass filter allows higher frequencies to pass while attenuating lower frequencies.
Output
│
│ █████████████
│ /
│ /
│ /
│___________/
│
└────────────────────────────► Frequency
fc
In a two-way speaker, the high-pass filter is normally used for the tweeter.
High-Pass Filter in a Speaker
Amplifier
│
▼
High-pass
filter
│
▼
Tweeter
The high-pass filter prevents excessive low-frequency energy from reaching the tweeter.
This is especially important because tweeters generally cannot handle large low-frequency excursions.
Band-Pass Filter
A band-pass filter allows a middle range of frequencies to pass while attenuating both lower and higher frequencies.
Output
│
│ ███████████
│ / \
│ / \
│________/ \________
│
└────────────────────────────────────► Frequency
f1 f2
Band-pass filtering is commonly used for midrange drivers in three-way speaker systems.
Band-Pass Filter in a Three-Way Speaker
Input
│
┌──────┼──────┐
│ │ │
▼ ▼ ▼
Low-pass Band-pass High-pass
│ │ │
▼ ▼ ▼
Woofer Midrange Tweeter
The midrange driver receives only the frequency region between its lower and upper crossover frequencies.
Band-Stop Filter
A band-stop filter attenuates a selected frequency range while allowing frequencies below and above that range to pass.
Output
│
│██████████ ██████████
│ \ /
│ \___/
│
└────────────────────────────► Frequency
fc
Band-stop filters are not normally used as the basic filter for a woofer, midrange or tweeter, but they can be useful for removing unwanted resonances or peaks.
Notch Filter
A notch filter is a narrow form of band-stop filter.
It can strongly attenuate a relatively narrow frequency range.
In loudspeaker design, a notch filter can be used to reduce a strong driver resonance or response peak.
Filter Cutoff Frequency
The cutoff frequency is the point where the filter response begins to transition between its passband and stopband.
For a simple first-order filter, the output at the cutoff frequency is approximately 3 dB below the passband level.
The cutoff frequency is often represented as fc.
Passband
The passband is the frequency region that the filter allows to pass with relatively little attenuation.
For example, a low-pass filter designed for a woofer may allow low frequencies to pass while gradually reducing higher frequencies.
Stopband
The stopband is the frequency region where the filter significantly attenuates the signal.
A high-pass tweeter filter, for example, has its stopband below the crossover region.
Transition Band
The transition band is the region between the passband and stopband.
The signal is neither fully passed nor completely blocked in this region.
The width of this transition depends largely on the filter order and design.
Filter Slope
Filter slope describes how rapidly the signal is attenuated outside the passband.
Common crossover slopes are:
- 6 dB/octave
- 12 dB/octave
- 18 dB/octave
- 24 dB/octave
- 48 dB/octave
First-Order Filter
A first-order filter has a slope of approximately 6 dB per octave.
It is the simplest crossover filter.
A basic passive first-order crossover uses one reactive component per driver.
First-Order High-Pass Filter
A simple passive first-order high-pass filter can be made using a series capacitor.
Amplifier │ │ ──||── Capacitor │ ▼ Tweeter
As frequency increases, the capacitor's impedance decreases and more of the signal reaches the tweeter.
First-Order Low-Pass Filter
A simple passive first-order low-pass filter can be made using a series inductor.
Amplifier │ Coil │ ▼ Woofer
The inductor presents increasing impedance as frequency increases, thereby reducing the high-frequency signal reaching the woofer.
Second-Order Filter
A second-order filter has an electrical slope of approximately 12 dB per octave.
It requires two reactive components in a basic filter section.
Second-order filters provide stronger attenuation than first-order filters.
Third-Order Filter
A third-order filter has an electrical slope of approximately 18 dB per octave.
It provides a steeper transition between the passband and stopband.
The additional filtering also introduces additional phase shift.
Fourth-Order Filter
A fourth-order filter provides approximately 24 dB per octave of electrical attenuation.
Fourth-order filtering is common in professional loudspeaker systems because it can provide strong separation between drivers.
Higher-Order Filters
Active and digital crossover systems can implement even steeper filters.
For example:
- 36 dB/octave
- 48 dB/octave
- Higher DSP slopes
The appropriate slope depends on the drivers and system design.
Filter Order and Component Count
In a basic passive filter, increasing the filter order generally increases the number of reactive components required.
| Order | Approximate Slope |
|---|---|
| 1st | 6 dB/octave |
| 2nd | 12 dB/octave |
| 3rd | 18 dB/octave |
| 4th | 24 dB/octave |
Why Use a Steeper Filter?
A steeper filter reduces unwanted frequencies more rapidly.
This can be useful for:
- Protecting tweeters
- Reducing driver overlap
- Reducing unwanted cone breakup
- Keeping drivers within their intended operating range
However, increasing the filter order can also increase circuit complexity and phase-related considerations.
Why Use a Gentle Filter?
A gentle filter allows a wider transition region between drivers.
This can sometimes provide useful phase characteristics and a more gradual acoustic transition.
The disadvantage is that each driver operates over a wider frequency range.
Filter Phase Response
Filters affect both the amplitude and phase of an audio signal.
This becomes increasingly important when two drivers operate near the same crossover frequency.
The outputs can either reinforce or partially cancel each other.
Filter Polarity
Some crossover filter alignments work correctly with both drivers connected with the same polarity.
Other topologies may require one driver's polarity to be reversed.
The correct connection depends on the complete crossover and acoustic system.
Butterworth Filters
Butterworth filters are designed for a maximally flat magnitude response in their passband.
They are commonly used in both analog and digital audio filtering.
The acoustic result of using a Butterworth electrical filter depends on the drivers and their natural response.
Linkwitz-Riley Filters
Linkwitz-Riley filters are widely used in loudspeaker crossover systems.
A fourth-order Linkwitz-Riley crossover has a 24 dB/octave electrical slope.
Its design aims to provide useful summation between the low-pass and high-pass sections when implemented correctly.
Bessel Filters
Bessel filters are designed to provide favourable phase and transient characteristics.
Their transition is generally less steep than some other filter alignments of the same order.
They can therefore be useful when phase behaviour is an important design consideration.
Active Filter Types
Active filters use powered electronic components, commonly operational amplifiers.
Common active filter structures include:
- Sallen-Key
- Multiple-feedback
- State-variable
- Other op-amp filter topologies
Passive Filter Types
Passive filters do not require an active amplification stage.
Speaker crossovers commonly use:
- Capacitors
- Inductors
- Resistors
They are normally placed between the power amplifier and the loudspeaker drivers.
Digital Filters
Digital filters are implemented using DSP.
They can provide highly adjustable crossover characteristics.
A DSP crossover can independently control:
- Frequency
- Slope
- Gain
- Delay
- Equalization
- Limiting
Low-Pass, High-Pass and Band-Pass Together
A three-way speaker can combine all three basic crossover filters.
INPUT
│
┌───────────┼───────────┐
│ │ │
▼ ▼ ▼
LOW-PASS BAND-PASS HIGH-PASS
│ │ │
▼ ▼ ▼
WOOFER MIDRANGE TWEETER
This arrangement divides the audio spectrum into three separate regions.
Band-Pass Filter Construction
A band-pass filter can be created by combining a high-pass filter with a low-pass filter.
Input │ ▼ High-pass │ ▼ Low-pass │ ▼ Output
The lower cutoff determines where the passband begins, while the upper cutoff determines where it ends.
Filter Q
The Q factor describes the relationship between a filter's centre frequency and bandwidth in certain filter configurations.
It is particularly important in resonant and higher-order filters.
Changing Q can alter the shape of the transition around the cutoff frequency.
Resonant Filters
Some filters can produce a peak around their cutoff or centre frequency.
The amount of peaking depends on the filter topology and Q.
In loudspeaker crossovers, excessive peaking is normally undesirable unless it is deliberately part of the design.
Filter Attenuation
Attenuation describes the reduction in signal level.
For example, a 12 dB/octave low-pass filter reduces the unwanted high-frequency signal increasingly as frequency moves above the crossover region.
The exact response depends on the filter alignment.
Filter Frequency Response
A filter's frequency-response graph shows how its output level changes with frequency.
When designing a crossover, it is useful to examine the low-pass and high-pass responses together.
Level
│
│ LOW-PASS █████████\
│ \
│ \
│ \
│ \
│ /████ HIGH-PASS
│ /
│ /
└────────────────────────────────► Frequency
fc
The combined response determines how smoothly the drivers transition from one to another.
Electrical Filters and Real Loudspeakers
A theoretical filter normally assumes a known load.
A real loudspeaker driver has an impedance that changes with frequency.
Therefore, the electrical filter response can differ from the simple calculated response.
This is especially important for passive crossover design.
Filter Types for Tweeters
The tweeter normally requires a high-pass filter.
Possible filter orders include:
- First-order
- Second-order
- Third-order
- Fourth-order
- Higher-order active or DSP filters
The correct choice depends on the tweeter and crossover design.
Filter Types for Woofers
The woofer normally uses a low-pass filter.
The filter prevents excessive high-frequency energy from reaching the woofer.
The slope can be selected according to the woofer's natural response and the desired crossover behaviour.
Filter Types for Midrange Drivers
A midrange driver normally uses a band-pass filter.
The lower high-pass section removes bass frequencies while the upper low-pass section removes high frequencies.
The resulting passband is the intended operating range of the midrange.
Choosing a Filter Type
The filter type should be selected according to the role of the driver.
| Driver | Typical Filter |
|---|---|
| Subwoofer | Low-pass |
| Woofer | Low-pass |
| Midrange | Band-pass |
| Tweeter | High-pass |
| Compression driver | High-pass |
Filter Type and Crossover Frequency
The filter type tells us what frequencies are passed or attenuated. The crossover frequency determines where the transition occurs.
These are two different design parameters.
For example:
Filter type: High-pass Crossover: 2 kHz Slope: 24 dB/octave Driver: Tweeter
Filter Type and Slope Are Different
A high-pass filter can have different slopes.
For example:
- 6 dB/octave high-pass
- 12 dB/octave high-pass
- 18 dB/octave high-pass
- 24 dB/octave high-pass
The filter type remains high-pass while the slope changes.
Why Filter Choice Matters for Tweeters
Tweeters are vulnerable to excessive low-frequency energy.
A suitable high-pass filter reduces the amount of energy below the tweeter's operating range.
A steeper filter can provide stronger protection, although the final choice depends on the driver's characteristics and acoustic design.
Why Filter Choice Matters for Woofers
Woofers can reproduce some higher frequencies, but their response may become increasingly directional or irregular as frequency increases.
A low-pass filter helps limit operation to the desired range.
Filter Choice for Compression Drivers
Compression drivers normally require a high-pass filter.
The required crossover frequency and slope depend on:
- Driver design
- Diaphragm
- Voice coil
- Phase plug
- Horn
- Power level
Always consider the driver's specified operating limits.
Filter Choice in Active Systems
Active crossovers make it easier to change filter characteristics.
The designer can adjust:
- Crossover frequency
- Filter order
- Filter alignment
- Output level
- Delay
DSP systems provide even greater flexibility.
Filter Choice in Passive Systems
Passive filters require physical components.
Changing the filter characteristics generally requires changing capacitors, inductors or resistors.
This makes passive crossover design less flexible than DSP but also allows a simple speaker system to operate without additional electronics.
Common Filter Design Mistakes
- Confusing cutoff frequency with crossover frequency.
- Assuming a driver has constant impedance.
- Ignoring driver resonance.
- Choosing the filter order without considering the driver.
- Ignoring phase.
- Ignoring driver polarity.
- Using a high-pass filter that is too low for a tweeter.
- Assuming a calculated filter is automatically acoustically correct.
- Ignoring the natural response of the drivers.
- Failing to measure the finished loudspeaker.
Practical Filter Selection Workflow
- Identify the driver.
- Determine its usable frequency range.
- Check its impedance.
- Check its resonance.
- Determine the desired crossover frequency.
- Select low-pass, high-pass or band-pass operation.
- Select an appropriate filter slope.
- Calculate or configure the filter.
- Build or program the crossover.
- Measure the response.
- Check phase and driver integration.
- Optimize the final design.
Key Takeaways
- Low-pass filters pass low frequencies and attenuate high frequencies.
- High-pass filters pass high frequencies and attenuate low frequencies.
- Band-pass filters pass a selected middle frequency range.
- Band-stop filters attenuate a selected frequency range.
- A notch filter is a narrow band-stop filter.
- Woofers normally use low-pass filters.
- Midrange drivers normally use band-pass filters.
- Tweeters and compression drivers normally use high-pass filters.
- Filter order determines how steeply frequencies are attenuated.
- First-order filters are approximately 6 dB/octave.
- Second-order filters are approximately 12 dB/octave.
- Third-order filters are approximately 18 dB/octave.
- Fourth-order filters are approximately 24 dB/octave.
- Butterworth, Linkwitz-Riley and Bessel are common filter alignments.
- Filters affect both amplitude and phase.
- Real loudspeakers do not have constant impedance.
- The electrical filter response is not necessarily the same as the final acoustic response.
- Filter selection should always consider the actual driver, crossover frequency, slope, phase and acoustic behaviour.