What Is a Speaker Crossover?

A speaker crossover divides the audio frequency range between different drivers. A typical loudspeaker may use a woofer for low frequencies and a tweeter for high frequencies.

                 ┌── Woofer
Amplifier ───────┤
                 └── Tweeter
                     ↑
                  Crossover

A passive crossover uses inductors, capacitors and sometimes resistors to direct different frequency ranges to the appropriate drivers.

Crossover Frequency

The crossover frequency is the approximate frequency where the signal transitions from one driver to another.

For a simple two-way speaker:

              Low frequencies
Amplifier ───────────────────► Woofer

              High frequencies
Amplifier ───────────────────► Tweeter

The crossover frequency is normally chosen according to the frequency response, power handling and acoustic characteristics of the drivers.

First-Order Crossover

A first-order passive crossover has a slope of approximately 6 dB/octave.

The basic low-pass filter uses a series inductor:

Amplifier ─── L ─── Woofer

The basic high-pass filter uses a series capacitor:

Amplifier ─── C ─── Tweeter

For a first-order low-pass filter:

L = R / (2πf)

For a first-order high-pass filter:

C = 1 / (2πfR)

where R is the nominal speaker impedance and f is the crossover frequency.

First-Order Crossover Calculator

Example — 8 Ω at 2.5 kHz

For an 8 Ω speaker with a crossover frequency of 2,500 Hz:

L = R / (2πf)

L = 8 / (2π × 2500)

L ≈ 0.509 mH

For the high-pass capacitor:

C = 1 / (2πfR)

C = 1 / (2π × 2500 × 8)

C ≈ 7.96 µF

Practical standard values close to these calculated values can then be selected.

Second-Order Crossover

A second-order crossover provides a nominal slope of approximately 12 dB/octave.

A commonly used Butterworth second-order alignment uses:

L = 0.707 × R / (2πf)

C = 0.707 / (2πfR)

These equations provide starting component values for an idealized second-order Butterworth network.

Second-Order Crossover Calculator

Second-Order Low-Pass Network

Amplifier ─── L ───┬── Woofer
                   │
                   C
                   │
                  GND

The inductor is placed in series with the woofer while the capacitor is connected in parallel with the driver.

Second-Order High-Pass Network

Amplifier ─── C ───┬── Tweeter
                   │
                   L
                   │
                  GND

The capacitor is placed in series with the tweeter while the inductor is connected in parallel with the driver.

Third-Order Crossover

A third-order passive crossover has a nominal slope of approximately 18 dB/octave.

Third-order networks use three reactive components per filter section. They can provide steeper attenuation outside the driver's operating band, but their phase characteristics and component interactions are more complex.

For practical loudspeaker design, the exact network should normally be designed from the actual driver impedance and frequency-response data rather than treating the speaker as a perfect resistor.

Speaker Impedance

The impedance printed on a speaker, such as 4 Ω, 6 Ω or 8 Ω, is only a nominal value.

A real loudspeaker's impedance changes substantially with frequency.

Impedance
   │
   │       ╭───╮
   │  ╭────╯   ╰──╮
   │──╯            ╰────
   │
   └────────────────────── Frequency

Consequently, crossover component calculations based only on nominal impedance are approximations.

Woofer Low-Pass Calculator

Tweeter High-Pass Calculator

Crossover Frequency Calculator

The first-order high-pass and low-pass formulas can also be rearranged to calculate crossover frequency.

For a capacitor:

f = 1 / (2πRC)

For an inductor:

f = R / (2πL)

Speaker Crossover Capacitor

Non-polarized capacitors are normally used in passive speaker crossovers because the audio signal is alternating.

Common crossover capacitor technologies include:

• Polypropylene film
• Polyester film
• Non-polarized electrolytic

Film capacitors are commonly used where low loss and stable characteristics are desired. Non-polarized electrolytic capacitors can be useful where relatively large capacitance values are required at lower cost.

Crossover Inductor

The series inductor in a low-pass crossover presents increasing impedance as frequency increases.

XL = 2πfL

Therefore, the inductor passes low frequencies more easily while increasingly opposing higher frequencies.

Important inductor characteristics include:

• Inductance
• DC resistance
• Current rating
• Core saturation
• Core material
• Wire size

Capacitor Reactance

The capacitive reactance is:

XC = 1 / (2πfC)

As frequency increases, capacitive reactance decreases.

This allows a series capacitor in a high-pass crossover to pass higher frequencies while increasingly attenuating lower frequencies.

Inductor Reactance Calculator

Capacitor Reactance Calculator

L-Pad Attenuator

A tweeter can sometimes be more sensitive than the woofer. An L-pad attenuator can reduce tweeter level while maintaining approximately the same nominal impedance presented to the amplifier.

For a desired attenuation in dB, the linear voltage ratio is:

K = 10^(-dB / 20)

For an L-pad using a nominal speaker impedance R:

Rseries = R(1 - K)

Rparallel = R × K / (1 - K)

L-Pad Attenuator Calculator

Common Crossover Frequencies

These are general starting ranges rather than universal design rules. The actual crossover frequency should be selected according to the specific drivers and enclosure.

Crossover Slope

A steeper slope provides greater attenuation outside the passband, but higher-order networks generally require more components and require more careful consideration of phase and driver response.

Crossover Phase

The electrical crossover network introduces phase shift. The acoustic output of the drivers also has its own phase characteristics.

Consequently, simply calculating electrical component values does not guarantee perfect acoustic summation at the crossover frequency.

Driver spacing, acoustic centers, polarity, enclosure design and frequency response can all affect the final result.

Why Nominal Impedance Is Only an Approximation

The formulas on this page assume a resistive load. Real loudspeakers are not purely resistive.

For example, the voice coil has inductance and the driver's mechanical resonance can produce a large impedance peak.

For serious crossover design, impedance and frequency-response measurements should be used with crossover-design software.

Practical Crossover Design

A complete passive crossover should consider:

• Woofer frequency response
• Tweeter frequency response
• Actual impedance curves
• Driver sensitivity
• Desired crossover frequency
• Crossover slope
• Phase response
• Acoustic offset
• Power handling
• Inductor resistance
• Capacitor ESR
• Resistor power rating

Common Mistakes

• Using the speaker's nominal impedance as its impedance at every frequency.
• Using a polarized electrolytic capacitor directly in series with an AC speaker signal.
• Ignoring inductor DC resistance.
• Ignoring tweeter sensitivity differences.
• Choosing crossover frequency without considering driver limitations.
• Assuming electrical crossover frequency equals acoustic crossover frequency.
• Ignoring driver phase and acoustic alignment.
• Using resistors with insufficient power ratings.
• Assuming a calculated textbook network will automatically produce a flat acoustic response.

Key Points

• Crossovers divide the audio spectrum between drivers.
• A first-order network has an approximate 6 dB/octave slope.
• A second-order network has an approximate 12 dB/octave slope.
• Speaker impedance strongly affects crossover component values.
• Real speaker impedance varies with frequency.
• Inductors are commonly used for low-pass filtering.
• Capacitors are commonly used for high-pass filtering.
• Higher-order networks provide steeper electrical slopes.
• Driver sensitivity can be adjusted using an L-pad.
• Accurate crossover design should ultimately use real driver measurements.