Electronics
How to design a 2-way speaker crossover
A speaker crossover splits the music between two drivers. In the simplest one, a first-order 2-way crossover, an inductor in series with the woofer passes the bass, and a capacitor in series with the tweeter passes the treble. Each part and its speaker form a filter whose cutoff is the crossover frequency:
The woofer's coil
A coil's reactance, XL = 2π f L, grows with frequency. In series with the woofer it lets the bass through and chokes the treble: an LR low-pass with its cutoff where XL equals the speaker's resistance. For an 8.2 Ω woofer and a 2 kHz crossover:
The nearest standard value is 680 µH, which crosses over at 8.2 ÷ (2π × 0.00068) ≈ 1.92 kHz.
The tweeter's capacitor
A capacitor's reactance falls as frequency rises, so in series with the tweeter it blocks the bass and passes the treble:
Use 10 µF, for a cutoff of about 1.94 kHz. The two cutoffs sit almost together, which is what you want.
Live experiment
Who plays the bass?
A party speaker with a woofer and a tweeter, both 8.2 Ω, fed through 680 µH and 10 µF. Slide the note from a kick drum to a cymbal and watch the cones hand over near 2 kHz. Then take the crossover out and see what the bass does to the tweeter.
“Project: Bass & Treble Filter” is the project that ends Oscillation Coast, Electronics World 4. Free with an account. Lessons unlock in order, so you’ll do the 27 before it first.
Check the handover
At the crossover frequency each speaker gets 1 ÷ √2 ≈ 70.7% of the voltage, which is half the power. The two halves add back up to the full sound, so the level stays even through the handover.
Away from it, each filter falls at 20 dB per decade, 6 dB per octave. A 100 Hz bass note is about 20 times below 1.94 kHz: 20 dB for the factor of ten and 6 dB for the doubling, so the tweeter hears it about 26 dB down, about a twentieth of the voltage.
Choosing parts
Use a non-polarised capacitor, film for preference, since the music swings both ways. Use an air-core coil, which can't saturate on loud bass, and keep its wire resistance low: every ohm in series with the woofer turns some of its power into heat.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Project: Bass & Treble Filter” is the project that ends Oscillation Coast, Electronics World 4. Free with an account. Lessons unlock in order, so you’ll do the 27 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: a low-pass filter. The coil's reactance grows with frequency, so it chokes the treble and lets the bass through. With the output across the speaker, that's an LR low-pass.
- Why not “a band-pass filter”? A band-pass needs a low cutoff and a high one. One coil in series gives only one cutoff, at the top.
- Why not “a high-pass filter”? A high-pass passes the treble and blocks the bass. A coil does the opposite: its reactance grows with frequency, so it chokes the treble.
Show the answer
Answer: 680 µH. 680 µH is closest. It gives fc = 8.2 over 2π × 0.00068 ≈ 1.92 kHz, close to 2 kHz. 1 mH would cross over at 1.31 kHz.
- Why not “1 mH”? 1 mH is further off and would cross over at about 1.3 kHz, well below 2 kHz.
- Why not “470 µH”? 470 µH is further from 653 µH and would cross over at about 2.8 kHz, well above 2 kHz.
Show the answer
Answer: −26 dB. 1.94 kHz is about 20 times 100 Hz, and ×20 = ×10 × ×2, so 20 + 6 = 26 dB down. −3 dB is only at the cutoff itself.
- Why not “−10 dB”? −10 dB is only about a third of the voltage. A first-order filter falls 20 dB per decade, so 20 times below the cutoff is about 26 dB down.
- Why not “−3 dB”? −3 dB is the level at the cutoff itself, about 1.94 kHz. 100 Hz is far below it, so it's much further down.
“Project: Bass & Treble Filter” is the project that ends Oscillation Coast, Electronics World 4. Free with an account. Lessons unlock in order, so you’ll do the 27 before it first.
Questions people ask
What does a speaker crossover do?
It splits the music by pitch. Bass goes to the big woofer, which moves enough air for low notes, and treble goes to the small tweeter, which can move fast enough for high ones. It also protects the tweeter, which overheats if it's fed bass.
What capacitor type should a crossover use?
A non-polarised one, because the music swings both ways. Film capacitors are the usual choice; bipolar electrolytics are a cheaper option. An ordinary polarised electrolytic would be reverse-biased on every other half-cycle.
What is the difference between first-order and second-order crossovers?
A first-order crossover uses one part per speaker and rolls off at 6 dB per octave. A second-order one adds a second part, a capacitor across the woofer or a coil across the tweeter, for 12 dB per octave, which keeps more bass out of the tweeter.
Related topics
- Low-pass filter explainedA resistor and a capacitor that pass the bass and fade the treble, and where the cutoff sits.
- High-pass filter explainedA capacitor in series that blocks DC and the bass, passes the treble, and couples audio between stages.
- Decibels explaineddB as a ratio on a log scale: 10 log for power, 20 log for voltage, and why gains just add.
- Capacitive and inductive reactance explainedWhy a capacitor passes treble and a coil passes bass: X_C, X_L and Ohm's law for AC.