How To Check If a Subwoofer is Blown With Multimeter
By Robert Jones · 2020-11-02
Got a silent subwoofer or one making weird noises? Before you panic, grab a multimeter. I'll show you how to quickly check if that voice coil has truly packed it in.
Right, so you've got a subwoofer that's gone quiet, or maybe it's making a noise like a badger trying to escape a biscuit tin. It's a real pain, innit? Before you shell out for a new driver or even a whole new sub, let's see if we can figure out what's actually wrong. Often, it's not as bad as you think. **Quick Verdict:** A multimeter is your best mate for quickly diagnosing a potentially blown subwoofer; it tells you if the voice coil's open or shorted, saving you a trip to the tip. There's nothing worse than thinking your beloved sub has packed it in. I've seen countless chaps bring their gear into the workshop over the years, convinced it's curtains for their bass, when in reality, it's just a loose wire or a chewed-up connection. But sometimes, yes, the voice coil *does* go south. Knowing how to check it yourself with a simple multimeter can save you a pretty penny and a lot of faff.  *How To Check If a Subwoofer is Blown With Multimeter — tested in the SpeakersMag workshop.* ## Why Your Subwoofer Might Sound Dodgy (or Not At All) Before we get stuck into the multimeter stuff, let's have a quick natter about why a subwoofer might fail. It's not always a dramatic puff of smoke, though that does happen occasionally! More often, it's a slow demise or an intermittent problem that drives you barmy. * **Overpowering:** Pushing too much wattage into a sub is a classic. You think "more power, more bass," but the voice coil can only handle so much heat before it melts the adhesive or even the coil itself. Poof! Or, more accurately, *silence*. * **Clipping:** This is often mistaken for overpowering. It's when your amplifier is driven beyond its capabilities, sending a distorted, square-wave signal to the speaker. This square wave is pure DC for a brief moment, which is hell on a voice coil, causing it to overheat rapidly. Your amp might be rated for 200W, but if it's clipping like mad trying to hit that, it's effectively sending far more destructive energy than clean, rated power. * **Mechanical Stress:** Think about it. That cone is moving a lot. Sometimes the spider (the corrugated bit that keeps the voice coil centred) or the surround (the rubber or foam ring around the edge of the cone) can tear. Not strictly a "blown" voice coil, but it'll sound awful, or stop moving properly. * **Manufacturing Defects:** Rare with reputable brands, but it happens. A weak spot in the coil winding or a shoddy connection inside. Annoying, but that's what warranties are for. * **Age and Wear:** Like anything, components wear out. The voice coil wire itself can fatigue, or the solder joints can crack over time, especially with constant vibration and heating/cooling cycles. I've seen old drivers that have just given up the ghost through sheer mileage. ### What Exactly Is a "Blown" Subwoofer? When we say a subwoofer is "blown," we're usually talking about the voice coil. This is the heart of the speaker, a coil of wire wrapped around a former that sits in the magnetic gap. When current from your amplifier flows through it, it creates an electromagnetic field that reacts with the permanent magnet, pushing and pulling the cone. Simple physics, really. If the voice coil gets too hot, it can: 1. **Melt the adhesive:** The coil unwraps or becomes loose, rubbing against the magnet structure. You'll hear a nasty scraping or buzzing sound, or sometimes just very quiet, distorted output. 2. **Melt the wire itself:** This creates an "open circuit." No continuous path for the electricity. The sub will go completely silent. This is what we're mainly looking for with a multimeter. 3. **Short circuit:** The wire insulation burns off, and adjacent turns of the coil touch each other. This reduces the number of active turns, dropping the impedance significantly. The sub might still play, but it'll sound weak, distorted, and will draw excessive current from your amplifier, possibly damaging that too. This is also something a multimeter can pick up. ## Getting Started: What You'll Need Right, before you go poking around, gather your kit. It's not much, but having the right tools makes life easier. * **Multimeter:** Obvious, I know. A cheap digital one will do the job perfectly well. You don't need a fancy Fluke for this, though if you've got one, crack on. Make sure it has an "ohms" (Ω) setting, preferably with an audible continuity function. That bleep is quite satisfying when it works. * **Small Screwdriver Set:** You'll need this to remove speaker grilles, terminal cups, or to disconnect wires from the subwoofer itself. Flat-head and Phillips (crosshead) are usually enough. * **Gloves (Optional but Recommended):** Speaker cones and surrounds can sometimes be a bit dusty or even fragile. Plus, if you're rummaging around inside an enclosure, you might find some sharp bits. * **Pen and Paper:** Always a good idea to jot down readings or sketch how things were connected, especially if you've got multiple voice coils or a complicated passive crossover. * **Good Lighting:** You need to see what you're doing, obviously. A headlamp can be a godsend if you're peering into dark corners of a cabinet or under a car seat. ## Step-by-Step: Testing Your Subwoofer with a Multimeter Alright, let's get down to brass tacks. Follow these steps carefully, and you should have an answer fairly quickly. ### Step 1: Disconnect the Subwoofer from the Amplifier This is absolutely crucial. You *must* disconnect the subwoofer from any power source. Not only is it safer, but you don't want stray voltage from the amplifier interfering with your multimeter readings. Unplug the speaker wires from the amplifier terminals and, if possible, from the subwoofer enclosure itself. If it's a self-powered (active) subwoofer, unplug it from the mains *and* disconnect any signal cables. For most car subwoofers or passive home subs, you'll just be removing the positive (+) and negative (-) speaker cables. If your sub is mounted in an enclosure, you might need to take it out to get to the terminals properly. ### Step 2: Set Your Multimeter to Ohms (Ω) Turn the dial on your multimeter to the "ohms" (Ω) setting. This is usually represented by the Greek letter omega. You're looking for the lowest resistance range, often 200 Ω or auto-ranging. If your multimeter has a continuity test (often indicated by a speaker or diode symbol), that's even better as it'll usually beep when there's a complete circuit with very low resistance. That's what we're hoping for! Push the red probe into the "VΩmA" or "positive" jack and the black probe into the "COM" or "negative" jack. Make sure they're seated properly. ### Step 3: Test the Multimeter Itself Before you go blaming the sub, make sure your multimeter is working. Touch the red and black probes together. You should see the display drop to very close to 0 ohms (e.g., 0.1 to 0.5 Ω) and, if you're on continuity mode, you'll hear a beep. If it stays at "OL" (Open Line) or "1" (meaning infinite resistance), then your multimeter is either off, the probes are faulty, or the battery's dead. Sort that out first! ### Step 4: Connect the Probes to the Subwoofer Terminals Now, carefully touch the red probe to the positive (+) terminal of your subwoofer and the black probe to the negative (-) terminal. Make sure you get good contact. Sometimes the spring terminals can be a bit fiddly, or screw terminals might have a bit of corrosion. Wiggle the probes if you're not getting a stable reading. ### Step 5: Read and Interpret the Results This is the moment of truth. What your multimeter shows will tell you a lot. #### Scenario 1: Open Circuit (OL or "1" on the display) If the multimeter reads "OL" (meaning "Open Line" or "Over Limit") or just a solid "1" on the far left of the display, this indicates an open circuit. **This is a strong sign of a blown voice coil.** It means the electrical path is broken somewhere – usually the voice coil wire has melted and snapped, or a lead wire to the voice coil has come disconnected internally. Your sub will be completely silent. #### Scenario 2: Very Low Resistance (e.g., 0.5 Ω on a 4 Ω sub) If you get a reading that's significantly lower than the stated nominal impedance of your subwoofer (e.g., 0.5 ohms on a 4-ohm sub, or 1 ohm on an 8-ohm sub), this suggests a **shorted voice coil**. The insulation on the voice coil wires has melted, causing turns to touch each other. This reduces the overall resistance, drawing excessive current from your amplifier. The sub might still play, but it'll sound weak, distorted, and could damage your amp. This is also a sign of a blown voice coil, just a different kind of failure. #### Scenario 3: Reasonable Resistance (DCR Measurement) If your multimeter shows a reading that's a bit lower than the subwoofer's stated nominal impedance, this is generally good news. For example: * A 4-ohm subwoofer will typically read between 3.0 and 3.8 ohms (or thereabouts). * An 8-ohm subwoofer will typically read between 6.0 and 7.5 ohms. * A 2-ohm subwoofer will typically read between 1.5 and 1.9 ohms. This is because the multimeter measures **DC Resistance (DCR)**, which is always slightly lower than the **Nominal Impedance** (AC impedance) that manufacturers quote. If you get a reading in this range, your voice coil is likely intact and not open or shorted. The problem lies elsewhere! This is where you breathe a sigh of relief. *Robert's Tip:* While you're taking the reading, gently push the cone in and out a few times. The resistance reading should remain stable. If it fluctuates wildly, that can indicate a loose connection or a voice coil that's rubbing and potentially shorting intermittently. Not a good sign. #### Scenario 4: Dual Voice Coil (DVC) Subwoofers If you have a Dual Voice Coil (DVC) subwoofer, you'll have *two* sets of positive and negative terminals. You need to test **each voice coil independently**. So, test the first set of terminals, then the second. Both should give you a reasonable DCR reading. If one is open or shorted, then that voice coil is blown. If both are fine, then the sub is likely okay. Sometimes people wire DVC subs incorrectly, so double-check the wiring if you're getting strange results. ### Step 6: What If the Voice Coil is Fine? (Troubleshooting Beyond the Multimeter) Right, so your multimeter says the voice coil is sound. Excellent! Don't celebrate too much yet, because if it's still not working, the problem is elsewhere. Here's where I'd look next: * **Speaker Wires:** Check for frayed wires, broken strands, or poor connections at both the subwoofer and amplifier ends. Give them a good tug. Are they properly seated? Are they corroded? Sometimes a loose connection can cause intermittent sound or no sound at all. Especially those cheap spring terminals – they can get a bit slack. * **Amplifier:** Is the amplifier working? Try connecting a different, known-good speaker to the same amplifier channel. If that speaker plays, your amp is probably fine. If not, the amp might be the culprit. Check its fuses, power light, and any protection modes it might be in. * **Signal Source:** Is there a signal actually getting to the amplifier? Check your head unit, AV receiver, or pre-amp. Try a different source if possible. Sometimes it's as simple as an RCA cable coming loose or a volume control turned down. * **Fuses:** Both in the amplifier (internal or external) and potentially in the subwoofer's terminal cup or in-line with the power cable. A blown fuse means no power. Dead easy to fix if it's just a fuse! * **Passive Crossovers:** If your sub uses a passive crossover network (more common in home audio than car audio), check that. Components can fail – capacitors can dry out, inductors can short. This usually requires a bit more expertise and sometimes soldering. * **Physical Damage:** Even if the voice coil tests fine, check the cone, surround, and spider for tears, holes, or detachment. A torn surround will cause awful flapping noises and poor bass response, even if the coil is electrically sound. Sometimes a good visual inspection tells you more than any meter. ## Can You Fix a Blown Subwoofer Voice Coil? Honestly? For most people, probably not. Re-coning kits exist, and some skilled technicians (like some of my mates, proper wizards with a soldering iron) can do a voice coil replacement. But it's fiddly, precise work. You need to be able to remove the old coil, clean the magnetic gap meticulously, shim the new coil to ensure it's perfectly centred, and then re-attach the spider and surround without glueing anything wrong. For a cheap-to-mid-range subwoofer, the cost of a recone kit plus your time and potential frustration often isn't worth it. You're better off putting that money towards a new driver. For high-end, rare, or sentimental subwoofers, a professional recone can absolutely be worth it. I've had a pair of vintage JBLs reconed before; they were worth the effort, both financially and emotionally. ## Comparison: Multimeter vs. Visual Inspection | Feature | Multimeter (DCR Test) | Visual Inspection | | :---------------- | :------------------------------------------------------ | :---------------------------------------------------------- | | **Detects** | Open voice coil, shorted voice coil, stable connection | Tears in cone/surround/spider, burnt voice coil (sometimes) | | **Accuracy** | Highly accurate for electrical faults | Good for physical damage, limited for electrical faults | | **Ease of Use** | Fairly easy, requires basic multimeter knowledge | Very easy, just requires eyeballs | | **Time Required** | A few minutes | A few minutes | | **Cost** | Cost of a multimeter (£10-£30) | Free | | **Limitations** | Doesn't show mechanical damage, doesn't test impedance at frequency | Can't confirm electrical integrity of voice coil | | **Best Used For** | Confirming electrical health of voice coil | Spotting obvious physical issues before or after electrical tests | My take? Always do both. A quick visual scan for obvious tears or scorch marks, then break out the multimeter for the definitive electrical check. They complement each other nicely. ## Who Is This Guide For? * **DIYers and hobbyists:** If you like to get your hands dirty and save a few quid. * **Anyone with a silent subwoofer:** A quick check before you panic and buy a new one. * **Budget-conscious owners:** Avoid paying a technician for a simple diagnosis you can do yourself. * **Car audio enthusiasts:** Blown car subs are a common issue; this is an essential skill. ## Who Should Skip It? * **People with zero electrical comfort:** If the thought of touching wires makes you nervous, get a professional. * **Those with active subwoofers (built-in amp):** While you can test the driver, diagnosing the amp itself is more complex. Focus on the driver first, but if it's fine, you might need a pro for the amp. * **If your subwoofer is still under warranty:** Don't void it by taking things apart! Contact the manufacturer first. ## Frequently Asked Questions (FAQ) ### Q: My multimeter reads "OL" – does that definitely mean the voice coil is blown? A: Ninety-nine percent of the time, yes. "OL" means infinite resistance, an open circuit. The wire is broken somewhere in the voice coil or its connections. It's an electrical failure. The only other very slim possibility is extremely poor contact with the terminals, so double-check your probes are making firm contact. ### Q: My subwoofer plays, but it sounds horrible. Will a multimeter help? A: It might. If the multimeter shows a significantly lower-than-normal resistance (e.g., 1 ohm on a 4-ohm sub), that points to a shorted voice coil, which would definitely cause distortion and poor sound quality. However, horrible sound can also be due to mechanical issues (torn surround, rubbing voice coil) that the DCR test won't directly show. So, check both DCR and do a thorough visual inspection. ### Q: How accurate is this multimeter test for voice coil health? A: For detecting an open or shorted voice coil, it's very accurate. The DCR measurement gives you a solid indication of the coil's electrical continuity. What it *doesn't* tell you is how the voice coil performs at different frequencies or under power – that requires more sophisticated testing. But for a quick "is it dead or not?" answer, it's spot on. ### Q: Can I use this method to test other speakers, like car door speakers or tweeters? A: Absolutely, the principle is exactly the same for any passive speaker driver. Disconnect it, set the multimeter to ohms, and measure the DCR across the terminals. Just remember that the DCR will always be a bit lower than the stated nominal impedance. Tweeters often have a DC resistance around 3-6 ohms for an 8-ohm nominal rating, for example. ### Q: My sub has a dual voice coil. How do I test it? A: You'll see two separate sets of positive and negative terminals. You need to test each coil independently. So, test the first (+) and (-) pair, note the reading. Then test the second (+) and (-) pair and note that reading. Both should show a DCR slightly below their nominal impedance (e.g., two 2-ohm coils might both read around 1.5-1.9 ohms). If one reads "OL" or is significantly lower, that specific coil is blown. ## Final Word There you have it. Checking your subwoofer with a multimeter isn't black magic; it's a straightforward bit of troubleshooting that every home cinema enthusiast or car audio nut should know. It saves time, it saves money, and honestly, there's a bit of satisfaction in diagnosing a problem yourself, even if the news isn't great. Often, a "blown" sub is just a loose wire, or your amp is throwing a tantrum. But if it *is* the voice coil, at least you'll know for sure, and you can make an informed decision about repair or replacement. Don't chuck it till you've checked it, eh? If you've found this useful, do share it with your mates. Keep those speakers rumbling properly!