It's interesting how, just when you think you have it all figured out, something breaks and it winds up being something that you've never seen before. Just like the saying goes, "you learn something new every day." I remember when I started in this business, when something like a speaker would blow up, the reasoning would be "because it was driven too hard."
Luckily, my first paying gig was at a production company that was also the main re-cone shop in town. Learning about the fundamentals of loudspeakers while learning how to make them work on show site was probably the best combination one could hope for, especially for being the young "green" kid. What was once explained as "because it was driven too hard" has evolved over time into much more in-depth reasoning. This knowledge can significantly help when setting system protection, from the smallest PA-on-a-stick-system to an arena-sized array.
There are a couple different ways a speaker can fail. Typically, a driver will thermal-out or have a mechanical failure that ultimately destructs the inner parts of the speaker.
Thermal Failure
Speaker drivers actually include different ways to dissipate heat for both low and high frequency components. Heat sinks, gap ventilation and ferrofluid are the three main ways that this is achieved.
Heat sinks are usually built into the magnet structure on a low-frequency type of component. High-frequency components will typically have them built into the back plate of the driver. These help transfer the heat to the back of the driver, away from the voice coil. Gap ventilation is the most common design to keep the voice coil of a woofer cool by the means of air movement. The way this works is that the speaker frame has a series of holes incorporated in a way that creates air movement across the voice coil as the cone moves. So the more the cone moves, more air moves across the voice coil. The other way is ferrofluid. This method is not seen as much in low frequency components these days, but more so in high frequency components. Ferrofluid is a black fluid that has iron-oxide particles in it, making it attracted to the magnetic field of a speaker's air gap. Once inserted, this fluid stays in the gap and helps get heat away from the voice coil, creating a higher power handling capability for the coil. Even with all of these techniques, it is still very possible to destroy a speaker with too much heat.
Too much heat comes from one main cause, too much voltage. What this does to a driver physically is illustrated in Fig. 2. This speaker had substantially more power than it was rated for pushed through it. The voice coil wire is wrapped around the former (which keeps the voice coil form) and has a coating on it. When the voice coil got hot enough, it started to form bubbles in that coating and the former, which can deform the coil as well. When a speaker starts to have that "rubbing" sound, this is usually what has happened. Some speakers can even get burn marks on the coil, or the voice coil leads can burn up all together. Voice coils can get very hot (upwards of a couple hundred degrees Fahrenheit), especially with the high-powered amplifiers that are now on the market.
Mechanical Failure
A mechanical failure occurs when one or more of the moving parts in a speaker (cone, spider, coil, etc.) come apart or deform, resulting in the failure of the driver. The cause of this can be from a couple different things including improper crossover setup, an improperly matched speaker to box design/port tuning, and the infamous "operator error."
Different types of speakers are designed for different applications (subwoofer, midrange, etc.) as well as a certain range of box volume and porting frequency. There is extensive research and testing done in these areas by manufacturers to get the best performance out of their products. Replacing the specified driver for any given box can change the excursion response to a point at which the driver will not necessarily overheat from too much voltage, but instead will bottom out the voice coil. Fig. 3 is an example of what can happen in this instance. This type of mechanical failure can also be caused by crossover points that are too low. This is not just limited to subwoofers – midrange and high frequency drivers have this kind of failure as well. A high-frequency compression driver is a little different – the diaphragm will either have a shattered dome, or the voice coil can become separated from the dome.
In a previous article, I wrote about generic parts versus factory parts and the importance of a good re-coner. The same principle applies in this situation. The inner moving parts of a speaker (cone, surround, spider, voice coil) are all picked specifically for that driver. The first and most obvious reason is that the combination of parts makes the speaker achieve the sonic characteristics that the designer intended. The other reason is that the surround, spider and voice coil are specifically selected and assembled in a way to work properly with the air gap depth. For example, take a driver that has a maximum mechanical excursion limit (Xlim) of 10mm (sometimes listed as 5+/-mm). When a speaker has no voltage going through it, it is sitting at a point where it can push 5mm out or pull 5mm in. It is very important that all the parts (cone, surround, spider, voice coil) are all the correct sizes, properly measured and assembled. If not, the voice coil has a good chance of either jumping the air gap or bottoming out into the back of the magnet structure before it reaches its full potential. This will wind up once again looking like Fig. 3.
{mosimage}There are many different ways a component can blow up; this column just focuses on two of the more common cases. The trick is to know what causes speakers to blow up and to use preventative measures to keep that from happening. This will lead us to next month's topic: different methods of system limiting.
