Electronic – Why are DC signals bad for loud speakers


I googled and found out through some forum that:

DC has a constant amplitude which overheats and destroys the voice coil of the speaker.

Could someone clarify if this answer is complete and accurate?

Best Answer

The voice coil on a speaker is effectively a big inductor. It happens to also generate sound, but the loops of wire in a magnetic field make it act like an inductor.

Inductors change impedance with respect to frequency. This is because any change in current through the system must build up the magnetic field in the coils. The faster you oscillate the current, the more pronounced the effect. This causes inductors to have a high impedance at higher frequencies, and a low impedance at low frequencies.

So what happens at DC? Well the impedance of an ideal inductor at DC is 0. That means no resistance at all! Of course, this isn't an ideal inductor. There is a bunch of wire, and that wire will provide some resistance. However, it is trivial to see that the resistance of the coil at DC will be far less than it will be at a higher frequency.

Now most amplifiers are voltage sources. They output a specified voltage, and are designed to provide enough current to maintain that voltage across the impedance of the speaker. Thus, if you have a very low resistance, you will have a very high current, much higher than might otherwise form. This current means your coil has to dissipate a lot of heat!