Short answer
Touch a 1.5 V battery across the driver’s terminals for a moment. If the cone moves out, the terminal connected to the battery’s positive end is the driver’s positive. Rod Elliott (ESP) and Troels Gravesen both describe this test, and Gravesen notes that the current standard is exactly that: a positive voltage on the + terminal moves the cone out.
The battery test, step by step
Do it before the driver is wired into a crossover. ESP notes polarity testing is easy on a loose driver and much harder once it’s wired up and buried in a box.
- Clip two test leads to the driver’s terminals.
- Hold one lead on the battery’s negative end.
- Briefly touch the other lead to the battery’s positive end and watch the cone.
- Cone moves out: that lead is on the driver’s + terminal. Cone moves in: swap the leads and try again.
- Mark the + terminal. Gravesen puts a piece of red tape on it so it’s still right when he solders the crossover wires.
Tweeters and ribbons
ESP says the 1.5 V test works with most tweeters too, because the power involved is under 300 mW into an 8 Ω tweeter. It isn’t possible with ribbon tweeters. For those, follow the maker’s marking, or check the finished speaker with a pulse-type tester like the one in ESP’s Project 185.
Don’t trust the markings blindly
Terminals can be marked wrong. While building his polarity tester, Elliott found a small driver in his own stock whose terminals were marked with the wrong polarity. Gravesen warns that vintage drivers may need an extra check despite their markings, and cites JBL, where red can mean plus or minus depending on the driver (he gives the L100 as an example). A quick battery test settles it.
When the crossover wants a driver reversed
Correct polarity at the driver isn’t always how it gets wired. With a second-order (12 dB per octave) network, ESP notes, one driver must be connected in reverse, because the network leaves the two outputs 180° apart. Why, and why the tweeter is usually the one reversed, is covered in first-order vs second-order crossovers. ESP’s own test speaker used a first-order (6 dB) series network, and both of its drivers tested correctly polarized; he adds that this won’t always be the case. So the job is to know which terminal is really + on each driver, then wire the crossover the way the design calls for.
Check the finished speaker
Gravesen says getting the drivers connected correctly is all or nothing for the crossover. On a two-way crossed around 3 kHz, a wrong connection can be really hard to hear, but measurements show a deep suck-out at the crossover point. Sweep the finished speaker (REW basics for DIY speaker builders), and use the reverse-null test from the first crossover that actually measures. Does absolute polarity matter for sound? ESP’s view is that it’s generally inaudible, but correct polarity is still considered the right thing to do. What clearly matters is the drivers’ polarity relative to each other, through the crossover.
Before you solder
Wiring more than one driver to an amp? See how to wire speakers in series and parallel.