Woofer, tweeter and crossover: the basics of a two-way speaker

A full-range driver does everything on its own, which is why beginners like it. When you want full bass and clear treble from the same box, even at high volume, the work gets shared out: a woofer for the bass and midrange, a tweeter for the treble, and a crossover in between that sends each one its share. That is a two-way speaker. You will find the parts in Woofers and subwoofers, Tweeters and midranges and Crossovers and audio accessories; here we look at how to choose them and make them work together, using the figures from the Visaton data sheets.

Who does what

The woofer has a large, heavy cone that moves a lot of air. The Visaton W 130 S (109 mm cut-out) handles 50 W rated and 80 W maximum, has a sensitivity of 87 dB and a resonance of 50 Hz in the 4 Ω version, 52 Hz in the 8 Ω one. Visaton offers it as a woofer and also as a low-midrange driver in three-way systems. At the top it reaches 12,000 Hz; at the bottom it depends on the box.

The tweeter is small and light, built for treble only. The SC 10 N has a 25 mm fabric dome, is magnetically shielded, and Visaton specifies it for the range above 3,000 Hz. The TW 70 is a cone tweeter, rated from 800 to 20,000 Hz. Both have a sensitivity of 90 dB.

The crossover directs the traffic. Visaton's HW 2/70 NG is a ready-built two-way passive filter: crossover point at 3,000 Hz, 12 dB per octave, 8 Ω, 100 W rated. Inside there are only coils and capacitors. In the circuit Visaton publishes, the woofer goes through a 1.0 mH coil with a 10 µF capacitor, the tweeter through a 3.3 µF capacitor with a 0.3 mH coil.

Never connect a tweeter directly

In its notes on its own speaker designs Visaton puts it plainly: tweeters do not take kindly to low frequencies. That is why a tweeter's power rating is never a single number; it depends on the filter protecting it. The SC 10 N handles 100 W rated with a 12 dB per octave high-pass filter set at 4,000 Hz or above. The TW 70 handles 40 W with the crossover at 3,000 Hz and 80 W with it at 5,000 Hz.

Wired straight to the amplifier, the tweeter would also get the bass, which it is not built to reproduce. In the crossover, the capacitor and coil on the treble side are there precisely to block it.

What 12 dB per octave means

An octave is a doubling of frequency: from 1,500 to 3,000 Hz, from 3,000 to 6,000. Visaton's glossary explains that each coil or capacitor adds 6 dB per octave of attenuation, so a second-order filter, with two components per way, reaches 12 dB per octave. In practice, below 3,000 Hz the signal reaching the tweeter drops by 12 dB for every octave down, and the same happens to the woofer above 3,000 Hz. That is the HW 2/70 NG circuit: two components for the woofer, two for the tweeter.

Impedance: the most common mistake

A passive crossover is designed for a particular impedance. You can see it in the HW 3/120 NG, Visaton's three-way network: it comes in 4 Ω and 8 Ω versions, with the same crossover points (800 and 5,000 Hz) but different coil and capacitor values. So with the HW 2/70 NG, which is an 8 Ω unit, you need an 8 Ω woofer and an 8 Ω tweeter: the 8 Ω version of the W 130 S and a tweeter such as the SC 10 N or the TW 70.

There is one case where two 8 Ω drivers make 4 Ω: when they are wired in parallel. In its ALTO LINE MK II design Visaton uses two 8 Ω W 130 S woofers and a DT 94 tweeter, and the speaker comes out at 4 Ω. That design, though, has its own purpose-built crossover, at 2,300 Hz.

If the treble is too bright

Here the tweeter is more sensitive than the woofer: 90 dB for the SC 10 N, 87 dB for the W 130 S. The HW 2/70 NG has no level control, so if the treble sounds too forward you bring the tweeter down with two resistors, as Visaton explains: one in series and one in parallel with the driver, so the crossover still sees the right impedance. For 8 Ω, Visaton's table gives:

• −2 dB: 2.2 Ω in series and 22 Ω in parallel;
• −4 dB: 3.3 Ω in series and 10 Ω in parallel;
• −6 dB: 4.7 Ω in series and 8.2 Ω in parallel.

The woofer, on the other hand, is not attenuated this way: Visaton warns that a series resistor on a driver working at its own resonance spoils its impulse response.

The woofer decides the box

The tweeter asks little of the box (the TW 70, for instance, has a sealed back and needs no separate chamber). The woofer sets the volume. For the W 130 S Visaton gives three options:

• a 7 litre sealed box;
• a 20 litre bass reflex box with the BR 6.8 port, 4 cm long, tuned to 44 Hz;
• a 30 litre bass reflex box with the BR 19.24 port, 12 cm long, tuned to 39 Hz.

The figures show the difference. The 7 litre sealed box, with the 8 Ω W 130 S, has its cut-off at 86 Hz; the two reflex boxes are tuned to 44 and 39 Hz, but the box becomes three or four times bigger and the port has to be cut to the stated length. The sealed box is still the easiest place to start: nothing to tune, it just has to be properly airtight. For the larger W 170 S, Visaton suggests bass reflex boxes of 20, 30 or 50 litres.

How to build a sealed box (panels, glue, wadding at the bottom, sealed cable hole) is shown step by step in the guide Bluetooth desktop speakers, 2 × 20 W. It uses a single full-range driver, but the woodwork is the same.

Two Visaton designs worth studying

Visaton keeps its designs online even when the complete kit is no longer sold, and two of them use drivers we stock:

• ALTO LINE MK II: two 8 Ω W 130 S woofers and a DT 94 tweeter, 30 litre bass reflex box with a BR 15.34 port 14.7 cm long, crossover at 2,300 Hz.
• CENTER 130, in the archive: one SC 10 N and two SC 13s, 11.5 litre sealed box, crossover at 3,500 Hz.

They are a good way to see how a manufacturer puts the parts together, box volumes included.

What about three-way?

If you want the voices on a driver of their own, you add a midrange and move to the HW 3/120 NG (800 and 5,000 Hz). Visaton specifies the M 10 and MR 130 for the range between 1,000 and 5,000 Hz, both with a sealed back, while the W 130 S moves down to low-midrange duty. It is a later step: build a two-way first and learn to listen to it.

Wiring and checking

• Plus to plus. Follow the markings printed on the crossover and on the drivers; Visaton marks the positive terminal with a red or white dot, a red sticker or an embossed +.
• Visaton's check. If the bass only sounds full with the balance turned all the way left or right, one of the two speakers is wired the wrong way round.
• The amplifier. A board such as the DFRobot DFR0803 from our guide will do the job: with 8 Ω speakers it gives 12 W per channel at 12 V and 38 W at 24 V, according to DFRobot's table. Another option is the class D stereo module based on the TPA3116D2 chip.

Want advice on the parts?

Tell us how much room you have, which room the speakers will live in and which amplifier you will use: we will help you match woofer, tweeter and crossover.