Check Your Tuning With an Impedance Sweep
Every sub pushes back on the amp more at some notes than others. That push back is its impedance, measured in ohms. In a ported box it rises into two humps, and the low point between them is the box’s tuning. Read that curve and you know your tuning for certain, before you ever take the car to a show.
New to how ports tune? Start with How a Port Works.
What You Need
The easy way: an impedance tester or sweep tool. You clip it to the sub, it plays a quiet sweep and draws the curve for you.
The old way: a signal generator or test tones played through an amp at low volume, a resistor of about 1,000 ohms (5 watt) wired in series with the sub, and a multimeter on AC volts across the sub’s terminals. The big resistor keeps the current nearly the same at every note, so the voltage across the sub rises and falls with its impedance. You don’t need exact ohms, just where the readings peak and dip.
Step by Step
- 1
Take the amp out of it
Unplug the speaker wires from the amp. The tester has to be the only thing driving the sub, and an amp left connected will spoil the reading.
- 2
Measure the box on its own first
Sit the box on the floor outside the car, port clear of walls. That gives you the box’s own tuning, with nothing else mixed in.
- 3
Hook up and sweep
Connect the tester to the sub’s terminals the way the sub is wired for the amp. Sweep from about 10 Hz to 100 Hz. With the old resistor method, step a few Hz at a time near the bottom and write each reading down.
- 4
Find the two humps and the dip
A ported box shows two humps. The lowest point between them is where the box tunes. A sealed box shows just one hump, at the box’s resonance.
- 5
Check for leaks
Tall, sharp humps mean the box is tight. If they are short and soft and the dip sits high, air is usually getting out somewhere. Heavy stuffing can soften them too. Fix any leak before anything else.
- 6
Adjust and sweep again
A longer port tunes lower and a shorter one tunes higher. Use the port tuning calculator to work out how much to change, make the change, then sweep again.
- 7
Measure again in the car
Once the box is where you want it, put it in the vehicle and sweep again. In a closed car the curve usually looks much the same as outside. If it shifts or grows an extra bump, the vehicle and its openings are acting like another chamber. The note the meter cares about is where the vehicle is loudest, which can sit a few Hz from the box’s tuning, so finish on the meter.
Watch the Dip Move
Here is one box with three different port lengths. The humps stay roughly the same shape, and the dip between them slides to wherever the port tunes it.
When the Numbers Don’t Match the Plan
A box landing a few Hz off is normal. A port running along a wall acts a little longer than it measures, bracing and the sub itself take up air, and every build is slightly different. Measure, adjust, and measure again. A port that slides or a few spare port pieces make this a lot easier.
If the curve looks strange, with extra small bumps, something inside may be rattling or a panel may be ringing. See Building a Box That Doesn’t Flex.