18dB Crossover

Calculation of a 3rd order loudspeaker crossover with 18dB attenuation per octave

Crossover Calculator

18dB Crossover (3rd Order)

Professional crossover with three components per way for maximum separation. Attenuation: 18dB per octave (Butterworth characteristic).

Ω
Speaker impedance
Hz
Crossover frequency
Component Values
Low-pass
Inductor L₁:
Inductor L₂:
Capacitor C₁:
High-pass
Capacitor C₂:
Capacitor C₃:
Inductor L₃:

Circuit Diagram

18dB crossover circuit diagram
 
 
 
 
 
 

Circuit diagram of an 18dB crossover (3rd order)

The calculated values are automatically inserted into the circuit diagram. 3rd order requires 3 components per way.

Low-pass Formulas

Inductor L₁
\[L_1 = \frac{3 \cdot Z}{4 \cdot \pi \cdot f_C}\]
Inductor L₂
\[L_2 = \frac{Z}{4 \cdot \pi \cdot f_C}\]
Capacitor C₁
\[C_1 = \frac{2}{3 \cdot \pi \cdot f_C \cdot Z}\]

High-pass Formulas

Capacitor C₂
\[C_2 = \frac{1}{3 \cdot \pi \cdot f_C \cdot Z}\]
Capacitor C₃
\[C_3 = \frac{1}{\pi \cdot f_C \cdot Z}\]
Inductor L₃
\[L_3 = \frac{3 \cdot Z}{8 \cdot \pi \cdot f_C}\]

Variable Legend

\(L_1, L_2, L_3\) Inductors (Henry)
\(C_1, C_2, C_3\) Capacitors (Farad)
\(Z\) Impedance (Ohm)
\(f_C\) Crossover frequency (Hz)
\(\pi\) Pi ≈ 3.14159
Important Note

With 18dB crossovers, polarity reversal of one speaker is required, as the phase rotation is 360°.

Filter Characteristics

Butterworth Filter

Best compromise between amplitude and phase. Maximally flat response in passband.

Bessel Filter

Optimal phase behavior and transient response, but worse amplitude behavior.

Chebyshev Filter

Best amplitude behavior, but worse transient and phase behavior.

Characteristics of 18dB Crossover (3rd Order)

Operation

A 3rd order crossover requires 3 components in each branch and provides a slope steepness of 18dB per octave. The attenuation at the crossover frequency is 3dB. This crossover is based on the Butterworth characteristic and represents the optimal compromise between amplitude and phase behavior.

Advantages
  • Very steep separation (18dB/octave)
  • Minimal frequency overlap
  • Professional applications
  • Precise frequency separation
Disadvantages
  • Many components required
  • High costs
  • Complex circuit
  • Polarity reversal required

Technical Details

Phase Behavior

The phase rotates from 0° to 360° depending on frequency. At the crossover frequency the phase rotation is 270°, which requires polarity reversal of one speaker.

→ Polarity reversal of one speaker needed!
Butterworth Characteristic

The 18dB crossover uses the Butterworth characteristic as optimal compromise between different filter types. 3dB attenuation at crossover frequency.

Attenuation at f_c: -3dB
Typical Application

18dB crossovers are used in professional applications where maximum frequency separation is more important than simple installation.

Professional Audio & Studio
Low-pass Calculation Example

Given: 8Ω speaker, crossover frequency 2400Hz

L₁:
\[L_1 = \frac{3 \times 8Ω}{4π \times 2400Hz} ≈ 0.80\text{ mH}\]
L₂:
\[L_2 = \frac{8Ω}{4π \times 2400Hz} ≈ 0.27\text{ mH}\]
C₁:
\[C_1 = \frac{2}{3π \times 2400Hz \times 8Ω} ≈ 11.1\text{ µF}\]

Comparison of Crossover Orders

Order Attenuation Components per way Phase behavior Application
1st order 6dB/octave 1 (L or C) Polarity reversal needed Simple systems
2nd order 12dB/octave 2 (L and C) No polarity reversal HiFi standard
3rd order 18dB/octave 3 (L-C-L or C-L-C) Polarity reversal needed Professional

Filter Characteristics in Detail

Butterworth

Advantages: Flat frequency response, good compromise
Disadvantages: Medium phase behavior

Used in this calculator
Bessel

Advantages: Best phase behavior
Disadvantages: Less steep slopes

Ideal for impulses
Chebyshev

Advantages: Steepest slopes
Disadvantages: Ripple in passband

Maximum steepness

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Other loudspeaker functions

Loudspeaker crossover 6 dB  •  Loudspeaker crossover 12 dB  •  Loudspeaker crossover 18 dB  •