If you pair an 8 ohm woofer with a 4 ohm tweeter, what is the speaker? Four? Six? Something under three? I get that question a lot, so I loaded one of each into a crossover program and watched the impedance chart. The number I ended up with wasn't any of the ones the parallel math predicts, and the reason comes down to which driver gets the power.
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Watch the impedance charts · 7 min
See the 8 ohm woofer and 4 ohm tweeter modeled
Jump to: The first misconception about 8 ohms · Adding a crossover
The question behind the question
The question usually comes in two parts. Can I use an 8 ohm woofer with a 4 ohm tweeter, and if I do, what's the nominal impedance of my speaker? Behind that is a third one I hear just as often: people want to end up with an 8 ohm speaker and don't know which drivers get them there.
For the example I used a Dayton Audio PC83-8, an 8 ohm woofer, and a Dayton Audio ND20FB-4, a 4 ohm tweeter. Nothing about the answer is special to those two. It works the same with any drivers.
An 8 ohm driver isn't 8 ohms
The first misconception is that an 8 ohm speaker has a flat 8 ohm response. It doesn't. When I pulled up the woofer's impedance chart in PCD, it sat around 6 ohms in one place and 39 ohms in another, and it moved with every frequency. That's still a nominal 8 ohm driver.
The tweeter was the same story. It sat somewhere around 3.5 ohms, rose to about 5 and came back down, and it was only actually at 4 ohms in about three spots. It's still a nominal 4 ohm tweeter. "Nominal" is a label for a curve, not a measurement at one point.
What the parallel math says
With no crossover at all, PCD showed the two drivers simply wired in parallel. That gave a nominal impedance of about 2.67 ohms, and the line was almost straight across. Wired in series, they'd be about 12 ohms. Most people don't want either of those, and this is the math most of the questions are based on.
What a crossover changes
Next I added a crossover at about 3,000 Hz, second order on both the woofer and the tweeter. Adding it doesn't change either driver's own nominal impedance. What changed was the system curve. It didn't look anything like the straight 2.67 ohm line. The low end looked a lot like the woofer's own curve, and the top end looked a lot like the tweeter's. Clicking between the high pass and low pass sections in PCD made it obvious how closely each half followed its driver.
The reason is that the crossover sends each range to a different driver. Above the crossover, the amplifier is powering the tweeter, not the woofer, and below it the reverse. With a properly designed crossover, the drivers don't really sit in the parallel configuration people picture.
So what is it?
With the crossover in place, the system curve started around 6 ohms and went down to about 4 in places. Is it a 4, a 6, a 39? It's a nominal 8 ohm speaker. I said in the video that I could have saved everyone a lot of time by just saying this first: the woofer section basically sets the nominal impedance. An 8 ohm woofer gives an 8 ohm speaker. Two 4 ohm woofers wired in series in an MTM also give an 8 ohm speaker.
The why is power. The lower frequencies use more wattage than the higher ones, so the amplifier is dedicating more of its power to the woofer's range. That range is what the speaker gets rated by.
Scoreboard
- Woofer alone (PC83-8): about 6 to 39 ohms across the band, nominal 8.
- Tweeter alone (ND20FB-4): about 3.5 to 5 ohms, at 4 ohms in only about three spots, nominal 4.
- Parallel, no crossover: about 2.67 ohms. Series: about 12.
- With a 3 kHz second-order crossover: follows each driver in its own range, nominal 8.
The principle that carries over
The same power split explains something else that confuses people. It's why a much lower wattage tweeter can be paired with a much higher wattage woofer. Because of the way the power divides, the woofer section gets far more of it. I said it wouldn't be surprising to see around 0.1 watt going to the tweeter at 20 kHz while the low end is getting 10 to 20 watts. The woofer sets both the impedance rating and most of the power demand.
What this doesn't cover
This was all modeled in PCD, not measured on a built speaker, and I kept the power dissipation part short on purpose. It also shows something worth keeping in mind about labels: even the "8 ohm" result dipped to about 4 ohms in places. A nominal rating tells you which driver dominates, not the lowest point on the curve. The video doesn't get into what a 4 ohm tweeter does to the crossover parts or the tweeter level next to an 8 ohm woofer, and I'm not going to guess at that here.
How it shows up in my builds
The passive DINAS plans list the finished speaker as nominally 4 ohms, so the receiver or amplifier it's paired with has to be rated for a 4 ohm load. That single number on a plans page is exactly what this video is about. It's a summary of a curve, and the curve is mostly decided by the drivers handling the low end.
Prefer to watch it on YouTube? Here's Can I use an 8ohm woofer with 4 ohm tweeter? Impedance Questions Explained on the Toid channel.
Where crossovers go from here
A passive crossover is what makes the system impedance follow each driver. When I replaced a passive crossover with DSP, each driver got its own amplifier channel instead, which changes the impedance picture completely. That post is the other side of this one.
Keep building: I replaced a passive crossover with DSP · The DINAS bookshelf speakers · All of my DIY builds
Parts Used and Mentioned
These are the drivers mentioned in this post.
Some links below are affiliate links. Toid may earn a commission at no extra cost to you.
| Item | Used or mentioned | Link |
|---|---|---|
| Dayton Audio ND20FB-4 3/4" soft dome tweeter | Mentioned: the 4 ohm tweeter I modeled in PCD. | Parts Express |
