When a subwoofer fails, it is often said to have blown out. This is usually characterized by a sudden and drastic reduction in bass and unbearable distortion, and it’s usually caused by either thermal and/or mechanical failure in the sub that prevents it from working like it’s supposed to.
Thermal failure typically happens when applying too much power to the subwoofer and causes the voice coil(s) to “burn”. Mechanical failure on the other hand occurs when the speaker physically rips and/or tears.
In most cases, subwoofers blow out due to accidents or carelessness, like incorrect gain settings, improper wiring, or cranking up the volume too high for an extended period of time. However, it is possible for subwoofers to fail due to age, and subwoofers that were cheaply made to begin with are more likely to blow out during normal usage as they age.
So, how do you know when a subwoofer is about to fail you or already blown? In a nutshell, these Five strong signs of a blown subwoofer will help you determine if your sub is toast.
- Distortion in the form of cracking, popping and rattling
- Lack of vibration
- Lack of bass
- Damaged cone or voice coil(s)
- Infinite/fluctuating resistance
How to Tell if a Sub is Blown
You can find out if your subwoofer is blown by:
Generally speaking, subwoofer distortion occurs mainly because of two things and those are when the subwoofer is overpowered and when it’s receiving clipped signal.
If everything was set correctly, and you suspect a blown subwoofer, set your bass and volume at a relatively low level, and listen for distortion.
If you hear unpleasant sounds like popping, cracking or rattling especially when you turn it up, that’s a major red flag. Your subwoofer is almost certainly blown.
If your subwoofer sounds weak no matter what you do to increase the bass, or if you notice a sudden and drastic reduction in bass response, that is usually a good hint that your subwoofer has partially blown out.
If the subwoofer doesn’t make any sound at all, it has definitely bit the dust.
By touching – Lack of vibration
Subwoofers convert electrical energy into sound energy. They’ll play loud when the cone vibrates a large amount, or soft when it moves a small amount.
Vibration is a common thing that you can experience by placing your hand on the surface of a fully functional subwoofer.
That said, lack of vibration is sometimes a strong sign of a blown subwoofer, but it can also be caused by a bad/weak amp or a wiring problem.
A good functioning subwoofer generates strong vibrations that you can feel on your hands. If you don’t feel any vibration at all upon checking the front of the subwoofer, you’ll need to check and see if your amp is functioning properly and that the sub wire connections aren’t broken.
By physical inspection
Physical inspection is another great way to tell if a subwoofer is blown or not. If it’s repairable or if you’re better off buying a new subwoofer.
To inspect your subwoofer, you’ll need to thoroughly check its cone for tears, cracks, holes or any damage. If the cone seems intact, use your fingers to gently push at the edges to make sure there are no splits.
A functioning subwoofer has suspension that allows for limited movement. With two hands, carefully press on either side of the subwoofer’s cone to assess the damage.
If the cone is very stiff and won’t move at all when you push down on it, the voice coils are frozen, which means the subwoofer is definitely blown. If it’s moving, make sure it’s not off-axis, and notice if you hear any scratching noises and be sensitive to any movements that are too loose or sluggish, as they may all be signs of a ripped or torn suspension.
If everything checks out okay, do not forget to check the wires. This is especially important if the subwoofer is cutting out or if it’s not producing any sound at all. Sometimes, a detached wire can prevent current from reaching the sub’s motor structure.
By Using a multimeter
One of the most easiest ways to tell whether a subwoofer is blown or not is by checking its resistance with the help of a multimeter. This way, you’ll be able to quickly and easily determine whether the voice coil or cone are busted.
Basically, what you want to do is to set the multimeter to Ohms or resistance and touch the lead of the multimeter to the subwoofer terminals. Each subwoofer has its own specific ohms reading. So, if your subwoofer is functioning properly, the ohm reading on the multimeter should be what the sub was rated at (more or less). If the multimeter shows a reading that’s far below expectation or if the reading fluctuates wildly, the subwoofer is blown.
Blown Voice Coil Symptoms
Signs of blown voice coil can be subtle. Here are five of the most common signs that the voice coil is broken:
- No output at all
- Excessive distortion in the form of rattling, and popping
- Crunchy scratching noises when you press on the subwoofer’s cone
- Frozen cone
- Lack of resistance or wildly fluctuating resistance when checked with a multimeter
To test whether a voice coil is blown, use a multimeter, and set it to measure resistance.
Each subwoofer has its own specific resistance (measured in ohms). So, if the multimeter displays a reading that’s far below expectation or if the reading is fluctuating wildly, the sub’s voice coils are blown.
What Causes Subwoofers to Blow Out?
Mechanical failure occurs when a subwoofer is driven past its mechanical limits. This makes the components of the sub start banging together causing permanent damage to the cone or the magnet structure.
For instance, a subwoofer can be mechanically damaged if the former is smashed into the backplate multiple times. This occurs because of applying too much instantaneous power in a certain enclosure.
Over-excursion also happens when a driver is pushed past its mechanical limits. It’s more prevalent on subwoofers because they’re made to produce low frequencies, which by nature requires more cone movement. The most common damage caused by over-excursion is broken glue joints and ripped or torn spiders.
Mechanical failure can be the result of mounting the subwoofer is a lager enclosure, or because of improperly vented enclosure. In the former situation, the mechanical suspension of the subwoofer will be compromised, causing the spider or surround to physically tear and/or possibly separate from the speaker’s frame. In the latter situation, Improper tuning can cause the speaker’s mechanical power handling to be compromised below that of the “tuned” enclosure frequency.
Thermal failure occurs when a subwoofer is overpowered. Constant over-powering or excessive power leads to a buildup of heat, which in turn causes the voice coils to “burn”.
Thermal failure can be the result of incorrect gain settings or improperly wired subwoofers, which can cause “impedance mismatch” load on the amplifier.
Why, and How, Do Subs Blow Out?
Before we dive into why and how subs blow out, let’s take closer look at the different parts a subwoofer is made of :
- The Dust Cap: Usually made of the same material a cone is made of, a dust cap is a gently curved dome mounted over the central hole of the loudspeaker diaphragm. It’s meant to the inner mechanics (such as the voice coil, the pole pieces) from small particles and debris (which can cause rubs). Dust caps can also contribute structural integrity to the voice coil assembly or the cone.
- The Cone: Along with the voice coil, is the only moving part, making this piece one of the most important. Subs cones are usually made of a variety of materials including Polypropylene, Aluminum and Magnesium, Kevlar…etc. Each of which has its own acoustical properties. Polypropylene for instance is lightweight and prevents mold, moisture, and mildew damage. Kevlar on the other hand is exceptionally durable, lightweight, and very flexible. And the best thing about it is that it can preserve its shape under a lot of pressure; thus, allowing for better sound displacement.
- The Surround: The Surround is part of the sub’s suspension system. It protects the cone and prevents it from being displaced as it flexes out from the basket. It’s usually made of durable materials such rubber surround or treated foam to withstand the strenuous job of producing bass frequencies. The surround can also be made of Urethane, Santoprene, and Butyl. Rubber surrounds are slightly stiff, but last longer, while foam surround allows for high excursion.
- The basket: The basket is basically the frame that holds all the components of a subwoofer. Its rigidity and resistance to resonance affect to a great extent the sound quality.
- The Spider:The spider is a piece of the subwoofer that fits around the voice coil and is attached to the speaker basket. Along with the cone, the spider helps maintain linear movement of the voice coil. Spiders are generally made of cotton, poly-cotton blend, or conex.
- The Voice Coil:The voice coil is the most critical piece of a subwoofer. It’s basically a set of windings wound on an aluminum, nomex, kapton or other material form that’s placed in the magnetic gap in the center of the motor.The voice coil has two main parts. The former (bobbin) which holds the voice coil. The second is the coils, which create the alternating field within the motor structure.
- The Tinsel lead: The tinsel lead (also referred to as Pigtail) connects the voice coil to the speaker Terminal.
- The Terminals: The terminals are the positive and negative wire connection of a subwoofer.
- The Magnet:The Magnet is the driving force of the subwoofer. There are different magnet materials and the size of the magnet doesn’t matter that much. Motor strength (also known as “BL”) which is the counterforce between the top Plate and the Bottom Plate to the voice coil is way more important.
- The Top Plate: The top plate is mounted underneath the basket. It sits above the magnet and directs flux into the voice coil gap. This works directly with the t-yoke to create and maintain a magnetic field.
- The Bottom Plate: The Bottom Plate is also known as the T-Yoke. It creates the opposing force in the magnetic field along with the top Plate. It also acts as a heatsink shunting heat away from the voice coil and keeping it running cool.
- The Pole Piece: A pole piece is a structure composed of material of high magnetic permeability that serves to direct the magnetic field produced by a magnet.
Overpowering your subwoofer
In a perfect world, the signal that an amplifier sends to a subwoofer looks like the sine wave representation below
The vertical axis represents voltage, while the horizontal axis represents time. In AC (alternating current), current flow changes between positive (upward) and negative (downward) direction.
In the representation above, the Point A and point B represent the points when the amplifier is telling the subwoofer’s voice coil and cone to move toward the front and toward the rear, respectively. The +V1 and -V1 represent by how much the cone should move forward and backward.
So, by traveling back and forth rapidly, the cone pushes air and makes sound.
With that in mind, let’s take a look at what happens when you give your subwoofer too much power or when you turn it up for an extended period of time.
Let’s say that points A and B (and +V1 and –V1) are the manufacturer recommended ratings for our subwoofer. If you exceeds those ratings, you risk damaging the subwoofer.
Now, if you crank it up too high, the higher voltage (+V2) now tries to move the cone even further toward the front (C). At this point, even if the signal is still clean and undistorted, it’s still not safe for the subwoofer as it’s trying to make its cone and voice coil move much further than they were designed to go. This could end up tearing the cone, spider, and surround apart.
On the other hand, when the signal (-V2) tells the voice coil to move too much further toward the rear (D), the voice coil gets overworked and ends up crashing violently into the back plate of the magnet assembly. This results in cracking the voice coil and its former, and probably jamming it in the voice coil gap.
Underpowering your subwoofer
Underpowering a sub isn’t necessarily bad for a thing. It just means that the subwoofer will sound weak and the music will lack a lot of detail.
The worst thing for a subwoofer is when that amount of power it’s receiving is coming from an amplifier that’s being overworked and thereby sending out a clipped signal.
A clipped signal doesn’t only make your subwoofer sound bad, but it can also end up damaging your subwoofer. This is because a clipped signal by its nature pushes the subwoofer beyond its physical limitations, and tries to make it do things it’s not designed to do, which leads to it overheating and burning out.
Basically, a clipped signal is a form of distortion that occurs when an audio signal is amplified past the maximum allowed limit. This happens usually when an amplifier attempts to deliver an output voltage or current beyond its maximum capability.
If, for example, you try to increase the volume but your amplifier is overdriven and can’t play a signal more powerful than what voltage V1 can produce, you’d get a clipped signal because the amp couldn’t produce more voltage.
As you can notice in the “clipped signal” representation above, the sides of the clipped signal are vertical. What this means is that the signal tries to move the cone too quickly from all the way forward (point E) to all the way to the rear (F) in no time. when this happens, the subwoofer will either tear itself apart trying, or the flapping cone wobbles hard and end up jamming the coil into the magnet’s voice coil gap, killing the sub.
Furthermore, because the rounded peaks and troughs (the highest and lowest points of the sound wave) of a clipped signal were cut — or clipped — off, the clipped signal has now horizontal lines which represent the times the signal is telling the cone to stay all the way forward or all the way back. This makes the voice coil burn through one or more of its windings, or heats up enough to deform its shape so that it jams in the magnet’s voice coil gap.
The worst thing that could ever happen to a subwoofer is when its voice coils are subjected to both overpowering and clipped signals. In other words, when a clipped signal carries twice the RMS power of clipped signal of the same amplitude (height). In such cases, not only is the signal telling the voice coil to pop into a position and sizzle, it’s doing it with almost twice the power of the sub’s maximum capacity. This will simply wreak havoc in a sub’s motor structure.