Ever smelled a loudspeaker smoldering? It’s probably not the rocking love ballad you’re listening to, it’s the voice coil inside getting too hot. The voice coil pulses in and out, driving the speaker and creating soundwaves, thanks to the alternating current of electricity feeding into it. The problem is speakers only turn a tiny amount (about 3% to 5%) of the electricity from the amplifier into sound, the rest of the energy turns into heat. Designing a speaker right and managing the power feeding into are the keys to handling the heat.
If a woofer is playing music really loudly for just a couple of minutes, the voice coil inside can easily hit 200°C. The problem with that heat is it doubles the voice coil’s resistance (impedance), which reduces the speaker’s sensitivity and performance. You don’t want that.
To overcome this heated situation, you have to consider thermal management when designing a speaker and selecting the material to use for it.
Down to the Wire
One option is whether to use copper or aluminum wire for the voice coil. Copper is the default, in part because it’s slightly better at dealing with the heat. Aluminum is lighter but will expand more under hot conditions and have more resistance. Copper has a temperature coefficient of resistance of .386% per degree Celsius. Aluminum is very close behind that at .429% per degree Celsius.
What the wire is wound around is the next consideration. That bobbin can be made from paper, aluminum, fiberglass, or Kapton. Kapton is a thin film material created by DuPont in the 1960s. Its specialty is handling temperature extremes, and so it gets used in things that get as cold as -268°C and as hot as 260°C. That may seem extreme, until you consider fiberglass is rated to withstand 537°C and not melt until it gets to 1,482°C.
The size of the voice coil has a big impact on how well it will handle the heat dissipation. The larger a voice coil the better it’ll keep its cool.
As your voice coil heats up, the speaker’s sensitivity is not the only thing affected. The electrical damping decreases. This can change the sound quality of low frequencies and make your speaker sound more boomy. The crossover alignment in passive speakers can also get out of whack a bit. Crossover alignment is the point where low frequencies come out of the woofer while the higher frequencies continue to come out of the midrange speaker. Excessive heat can blur this line.
An overheated voice coil can even partially demagnetize the magnet in the speaker’s driver. This is pretty rare and the magnet should go back to its normal full strength when it cools. However, it could be a permanent problem if the speaker was designed poorly with a neodymium magnet. A neodymium magnet will begin to permanently demagnetize starting at 80°C (176°F).
Overall Design Matters
The materials used in a speaker are important for managing heat, but so is the design. A thick top plate on a speaker will help with heat but affect the acoustic characteristics. How the voice coil and magnet are aligned for the air to flow around them is also a consideration.
Of course, the speaker enclosure impacts thermal management. The materials used, the stuffing inside the box, whether there are ports, and the location of the ports will affect air flow and heat dissipation. All of these factors need to be considered when designing a speaker to not overheat.
However, even a properly designed speaker needs to be tested well to ensure it can take the heat. If you want to dive deeper, download our Loudspeaker 101 Guide.