Subwoofers are known to blow which leads to failure which can be noticed by a drastic reduction in the bass quality and worse case scenarios not work at all.
These failures can be caused by a lot of things with sustained overpowering being the most common cause of this.
But before we can look at what causes subwoofers to blow, how to know when your sub has blown, and what you can do about it, it is essential to understand what a blown subwoofer is.
The term blown subwoofer refers to mechanical and thermal failures on a sub’s components that can be caused by various issues. This does not include physical damage due to errors such as dropping the subwoofer or even poking the sub’s cone using a sharp object.
It is important to understand this when you are talking about a blown subwoofer.
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What causes a Subwoofer to blow?
1. Overdriving the subwoofer constantly for sustained periods
The most common cause of sub damage is overpowering the drivers beyond what they can handle for an extended period.
Subwoofers have power ratings and this can either be the peak power also known as PMPO or continuous power handling which is also known as RMS.
The PMPO rating can only be sustained for microseconds and is not reliable but the RMS rating is more reliable as it can be sustained for longer but is almost always not an accurate measure of how much power the subwoofer can handle without causing damage.
What happens is that most people will drive their subwoofers at their continuous power rating or even beyond it.
Most subwoofer drivers have an efficiency of about 1%. This means that only 1% of the power delivered to the drivers is converted to sound while the rest is converted to heat.
So, if you are driving your subwoofer at 80 watts, only 0.8 watts is used to move the driver while 79.2 watts are converted into heat. This heat is then dissipated through convection as the driver moves air and through radiation.
You can now guess what happens when you overdrive your subwoofer. A lot of heat is generated from the excess power which cannot be efficiently dissipated to the environment. Instead, the heat builds up heating the voice coil.
Overheating the voice coil will in turn soften the glue/adhesive holding the coil to the former which deforms it and with more heat buildup, complete damage of the coil occurs.
As you can tell a deformed voice coil will not work as it was intended to and may scrap on the sides of the voice coil gap. This is why you may hear rattling, scratching, buzzing, or other unpleasant noises as the driver moves back and forth to reproduce the low-frequency signals.
On the other hand, a damaged coil will stop working altogether and you will have not have sound coming from the subwoofer at all.
2. Driving the subwoofer with overpowered clipped signals
No, it is not the clipped/distorted signals from an amp that cause subwoofer damage but the excessive power from these clipped signals.
This is something that most people often get wrong.
Most sound systems will have clipped signals which can come from the source or the amplifier. If clipped signals were to cause speaker or subwoofer damage, most sound systems would almost always get damage but this is not always the case, is it?
The problem with clipped signals is the significant increase in the amount of power they deliver to the subwoofer’s drivers.
This happens because clipped signals do not have the punch the most people would love and in the process, they increase the level as they try to compensate for this. In turn, more power is delivered to the sub with no improvement in sound quality.
And as we have seen in the first point, more power is equal to more heat which increases the likelihood of damage.
Clipped signals do not cause mechanical or thermal damage but overpowering the drivers beyond their power handling capacities does.
3. Mechanical failures of the cone and magnet assembly
Over excursion of the sub’s cone can cause failure but this is less common.
Most speaker drivers are made to withstand too much movement either inwards or outwards. But in some cases, the cones surround (cone’s frame) and the spider (a flexible ring that supports the cone) may be stretched over the limit of what it was intended.
When this happens, the suspension may tear, or in worse case scenarios, the voice coil may move out of the voice coil gap (less common).
If the voice coil moves out of place, it can easily overheat from the incoming power as it is no longer resting in the magnetic field hence does not move as intended. Therefore, the power is converted into heat instead of moving the cone.
If only the cone structure, especially the foam, tears or comes loose, fixing the sub should be possible using flexible silicon rubber gel.
Mechanical failure of the magnet assembly can occur if the glue and bolts that hold the front and back plates come loose. The voice coil can get jammed in the voice coil gap which is made to precisely fit the coil.
Failure can also occur if any objects get into the voice coil gap. These can be soil particles, a small piece of metal, a breakaway particle from the magnet, or any other foreign objects that should not be in the gap.
These objects can short the voice coil, melt, and stick on the coil or cause abrasion to the surface insulation of the winding wire. There is no explaining how much damage this can cause to your subwoofer.
4. Failure of the wire connecting the sub’s leads to the voice coil
There is usually a flexible wire that connects the positive and negative leads on the sub to the respective terminals on the voice coil.
These wires can fail/break and stop the flow of current to the coil.
If this is the reason your subwoofer is blown, soldering another feed wire to the respective terminals should easily solve the problem but is not always a possible undertaking.
How to tell if your subwoofer is blown
It may not always be obvious if your subwoofer is blown unless it stops working completely. However, there are several ways to diagnose if it is blown and which component is damaged.
1. Listen for distorted bass, scratching, buzzing, and other noises
Playing a soundtrack on your source with your subwoofer connected to the system is one way you can diagnose a blown subwoofer.
If the bass produced is weak and distorts with an increase in volume, you may have a blown sub in your hands. The same case applies to hearing buzzing or scratching noises that can be a sign of voice coil damage.
2. Physical inspection of the cone and the voice coil
This will involve checking if the suspension has come loose or checking for tear (may be small) in the cone itself or at the attachment between the surround and the cone.
You can also use your hands to gently press on the sub’s cone. The cone should move back with ease and get back into its resting position once you release it.
If the cone wobbles/dances around, you may have a loose/spider on your hand. There may also be a lack of movement which is an indication of a jammed voice coil. The voice coil may be stuck in the magnetic gap or may have moved out of place.
If on pressing the cone you hear scratching noise, then the sub’s voice coil is distorted due to thermal damage.
You can also visually inspect the voice coil by opening up your subwoofer and carefully taking the driver out of its enclosure. The coil may be discolored partially or entirely, it may look burned and the coiled wire may be hanging loose. All these are telling signs of a blown coil.
3. Using a multimeter to test for wire resistance
To do this you will need to disconnect the subwoofer from the power outlet and audio source if it is an active sub and from the audio source if it is passive.
Connect the positive probe on the sub’s positive terminal and negative to the negative terminal and check the reading on your multimeter.
If you get a reading of 1 ohm, the voice coil is still functional. A reading of 0 ohms is bad news as it shows that the coil is completely blown.
You can also take a step further to ensure that it is not the flexible wire connecting the terminals on the coil to the leads that is the root of the problem.
You will need to make sure that the multimeter probes touch the respective terminals on the voice coil. If you get a reading or 1 ohm but had gotten a 0-ohm reading before then the flexible wire is the problem. It may need to be replaced if possible.
Can you fix a blown subwoofer?
Well, this will depend on the extent of damage and the damaged component.
If the damage is only mechanical such as a tear on the cone or the surround, you can fix it before it gets worse by using flexible glue.
Voice coil damage or spider failure will mean a full reconning process. You will need to buy a whole cone kit which includes the coil, cone, surround, flexible lead wires, the gasket, the dust cap, and adhesives. This kit can cost as much as the subwoofer itself.
It also goes without saying that the processing of installing a new cone is delicate and should be done by an expert who will also charge you for the service. This is because foreign objects may get into the magnetic gap making the process futile.
It is also easy to damage the cone kit while installing it.
If the damage has only occurred on the magnet assembly to a small extent and it is still intact, repair may be possible but it will involve taking the magnetic components apart, demagnetization and re-magnetization. This is a complex process that not many repair experts will try to do.
The thing with blown subwoofers is that you are better off replacing the whole thing with one that is more power-efficient, has a better quality build, and is more efficient at dissipating heat.
When replacing a blown sub, you should get one with better power handling and it should also have a higher sensitivity rating than what you currently own. This will reduce the risk of future power damage.
You will also need to use your amplifier reasonably making sure that you do not overpower the sub. This will apply to both passive and active subs.
Set a level limit according to your sub’s RMS rating. The sub should not be driven close to or at the power handling limit. More so, you should never exceed the power handling limit.
Blowing a subwoofer is not easy but this does not that it is impossible and, therefore, it is important to take great care of it.