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                    Speaker Failure Analysis            
   
When a woofer or tweeter fails in use, people most often say it has "blown" or "burnt out". But the real situation and what actually causes speakers to suddenly fail is a topic that is rarely explained in any detail. To learn what kills woofers, keep reading here
Because of the lack of solid info, strange myths and weird science abound about speaker failures, and these are sometimes exploited by those who supply and repair speakers to the disadvantage of their customers.
Here is a list of the most common types of speaker failure, in approximate order of likelihood.
·         Overheating damage of the voice coil
·         Feed and flexible wire failure
·         Glue failure of the voice coil attachments
·         Mechanical failure of the cone structure
·         Foreign objects in the magnet gap
·         Mechanical failure of the magnet structure
·         Failure due to over excursion of the cone

1 - Voice Coil Facts
Speaker voice coils are wound from either copper, aluminium or copper clad aluminium (CCA) wire. The wire may be of round section or sometimes edge wound strip wire is used. Aluminium is half the weight of copper for the same resistance so is preferred for high efficiency drivers and horn diaphragms. In all cases, the wire is coated in an insulating material, generally a synthetic enamel. Aluminium wire may simply be anodised to provide sufficient insulation.

When a current is passed through a voice coil, heat is generated. The amount of heat in watts is given by the very simple formula:
P = I² * R - where I is the RMS current and R is the actual resistance of the wire, at any temperature
With either copper or aluminium wire in use, the R value increases linearly by 0.4% for each degree C rise in temperature. This means the R value doubles if the temp of the whole coil rises by 250°C. Keep that 250°C number in mind as it connects to info to come soon.

2 - Voice Coil Construction
Voice coils are wound on short, cylindrical formers made from a variety of materials - the oldest and cheapest is treated paper. High powered speakers generally use formers made from aluminium and more recently high temperature plastics like Kapton. Fibreglass is also quite common, and can be seen in the destroyed voicecoil shown in Figure 5. Of these materials, aluminium is generally the best, because it is thermally conductive and acts as a heatsink for the voicecoil. It's not an especially good heatsink, because it's small and of thin material, but none of the other common formers have any useful heatsinking ability at all.

Figure 1 - 40mm, Dual Layer Voicecoil On Aluminium Former
To stand up to strong drive forces, the wire must be physically held in place by an adhesive applied at the time of winding. It is crucial that this adhesive be able to withstand high temperatures without softening or burning. The best available adhesives (epoxy or polyester/ polyurethane resins) can stand actual temperatures of up to 250°C - at least in the short term.
Heat generated in the voice coil is lost into the air and the magnet structure surrounding the coil. The main mechanism is conduction (aided by convection) across the tiny air gap each side of the coil into the iron pole pieces of the magnet. Magnetic gaps are made from soft iron, as it is easily the best material for the job, while the energising magnet itself is made of Alnico 5, ferrite or more recently Neodymium.
With most high powered woofers, air is also pumped back and forth through a hole in the centre of the pole piece by low frequency cone excursions and this helps to reduce voice coil temperatures. Needless to say, if the driver is used for mid to high frequencies and has little cone movement, there is no pumping effect, so the only cooling mechanism is via conduction/ convection. Some (albeit very few) makers have used small fans to force airflow. These are commonly powered from the audio signal, using a simple rectifier and filter, with the ability to limit the fan voltage to a safe value. There have also been some bizarre arrangements - for example having the magnet assembly (with heatsink) mounted in front of the cone so it gets outside (cool) air rather than the potentially warm air inside the enclosure.

   Important Fact:
If ever the temperature of a voice coil, or a part of it, exceeds the softening temperature of the insulation or adhesives used, that coil will fail by coming apart and/ or smoking and burning as the insulation or adhesives simply give up.

 
Figure 2 - Loudspeaker Motor Construction
The general details of a loudspeaker motor are shown above. There are many variations, but the basics don't change. For example, not all drivers have a vented centre pole-piece, and the position and size of the spider can vary widely. Magnet structures also change, but all loudspeakers feature the centre pole, front and rear plates, voicecoil former, etc.
For a much more informative cutaway view of a real speaker, click HERE. The image is from Calco Cutaways - a company in the US that specialises in making cutaways of actual products for many different industries. My thanks to David Kasper for allowing the link.
This info has been included for anyone who has never dismantled a speaker, and doesn't know about all the hidden parts. It helps to understand where everything goes, as it is obviously important to know the internal construction so the rest of this article makes sense.
 
 

GUANGZHOU SHANGJIA ELECTRONIC CO., LTD.      Mobile: 0086+18122769630  North of Anzhi area of Tuanjie village, Pingbu road, xinhua town, Huadu district, Guangzhou city., P.R.China  Tel:020-36983568  Fax:020-36985018