Speakers are the last link in your audio signal chain, and can have the most dramatic effect on the overall sound of your system. They not only determine how efficient your system is overall, but also the frequency response, transient response, and how loud you can be.
A speaker consists of a very few parts.
1) The frame – simply the metal frame in which everything else is mounted. They can be stamped metal, which is the cheaper method, lighter, and potentially not as rigid; or they can be cast metal, which is usually used in the more expensive speakers.
2) The magnet – a magnet attached to the rear of the frame. The heaviest part of the speaker. The heavier the magnet is, the better the efficiency and low end response.
3) Voice coil – A coil of wire wrapped around a “bobbin” to form a cylindrical coil, which rides in a gap in the magnet. The two ends of the coil are connected to terminals on the frame of the speaker. The output of the amplifier is connected directly to the voice coil. When power is applied to the voice coil, the magnetic field in the gap causes the voice coil to move. (the stronger the magnetic field, the more movement for a given power input, all other things being equal)
4) The cone – The conical shaped thing that faces the front of the speaker. Traditionally, these are made from a type of cardboard. Sometimes you will see metal or plastic used as cone material, but not usually in pro-audio applications. The cone is attached to the voice coil, whose oscillations are transferred to the cone, which in turn, moves air.
5) The surround – This is the wavy part around the outside edge of the cone, just inside where the cone is glued to the frame. Is is most often cardboard, but in some speakers can be cloth, or foam rubber(!). The surround doesn’t really serve any audio purpose – it exists merely to connect the moving cone to the non-moving frame.
6) Dust cover – the little dome in the center of the cone. This keeps dust from getting into the voice coil gap. These can be vented, having a small (1/2″) section of screen in the center, to aid in cooling and to allow more unrestricted movement.
Barring physical damage, such as dropping the speaker, the most common parts of the speaker to fail are the voice coil, cone, and surround.
How they fail, and why
The surround can fail due to a couple of causes, neither of which is much of a mystery. The surround can just simply wear out. The cardboard of the surround is much thinner than the cone material, and, by necessity much more flexible. Over time, the constant flexing of the surround weakens it, and it can eventually just start developing rips radially. Or, in some closed-box applications, the air pressure inside the box can actually blow a hole in the surround. A hole in the surround doesn’t necessarily make a noise itself, but it can change the frequency response of a cabinet, since it is effectively a port. Also, once the rip or hole in the surround gets bad enough, the cone can become misaligned (off-centered) which will cause the voice coil to rub, which you will definitely hear.
A cone is more trouble-free, as long as you don’t stick a screwdriver through it while installing the speaker. Unlike the surround, a cone’s job is to remain rigid, so the entire surface area moves as one unit. Like the surround, a cone can, after time, wear out from the constant movement and humidity. The cone becomes flimsy and “soft.” Once this happens, the cone no longer moves as a single unit, which will affect the frequency response and efficiency.
Check your speakers every couple of months to make sure the cone and the surround are intact. If you are REALLY paranoid, rotate your speakers 90 degrees. Yes, if they hang the same direction over a long period, and are also exposed to travel bumps, the weight of the cone over a long period can possibly cause cone misalignment.
The dust covers aren’t usually a source of much trouble, although they can occasionally come loose because the glue holding them on dries out, and the combination of the dust cover’s inertia and the air pressure behind it can cause one edge to come loose. Do NOT try fixing them with tape (even duct tape isn’t going to work, believe it or not). Even if you can get it to stick, it’s not going to hold the dust cover firmly enough to keep it from rattling, so not only will the dust cover still rattle, but now the tape is going to rattle as well. If you have a rattle and find out that it is a dust cover, you can get through a show by CAREFULLY removing the dust cover entirely. Take it to your recone expert the next day.
Voice coils are the most common part to fail on a speaker, and there are two different results when they do: The speaker will rattle; or the speaker will be silent. (There is also the not entirely un-humorous “flaming speaker” which was usually caused by a Crown DC-300 dumping its DC supply into your speakers, but that’s another story)
All of the current flowing through the wire of the voice coil causes heat. This heat is mostly transferred to the metal of the magnet, which has a heat sink to dissipate the heat into the surrounding air. The movement of the speaker itself also helps keep it cool. However, if you exceed the power handling of the speaker continuously for an extended period, the voice coil will overheat. This overheating causes the adhesive used to bind the wire to the “bobbin” to come loose, the wire will partially unwind, and will either rub the voice coil gap, which you hear as a severe rattle, or will bind completely, which you hear as silence. There’s nothing you can do at the show venue. Replace it with a spare and take it to your recone expert when you can.
The voice coil can also jump out of its gap. If it remains aligned while it’s out there, it may slip back into the gap with no ill affects, other than you will hear distortion caused by the non-linear response of the speaker. If, however, it doesn’t quite remain aligned perfectly, it won’t hit the gap, and will instead slam down beside the gap, and will be essentially “locked up.” It will be silent. The cone won’t move when you push it. You MAY be able to pull the cone outward carefully (putting equal pressure on all sides of the cone) and get it to slip back into the gap, but don’t bet on it. Be aware, that since the speaker will not move, it will draw more current than normal, and since it doesn’t have the cooling action of the movement, will overheat more easily. In other words, if you notice a speaker not working, troubleshoot it as soon as possible.
As with all repair and maintenance work, if you have to open a cabinet, while you’re in there, check all electrical connections, and check the cabinet integrity. If you see any loose braces or stripped screws, FIX THEM. If they remain unfixed, they WILL be a future source of a rattle.