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?

• They're even bad for quiet speakers... May 10, 2016 at 21:44

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!

• Kind of like how a stalled electric motor draws far more current than one which is turning at the proper speed: no back emf is generated if it is stalled. Poof!
– user56384
May 9, 2016 at 23:23
• Presumably it could also damage your amplifier as that won't be designed to give such a high current. May 10, 2016 at 7:58
• Also, a speaker with DC applied may well not be moving, which hurts cooling of the voice coil. Low impedance (Because you just see the DC resistance), combined with poor cooling (because the coil is not moving in the air in the gap) can easily equal overheating. In addition, the DC will introduce a shift in the position of the cone possibly increasing distortion as the thing is more likely to run out of linear excursion in one direction, usually not damaging, but not good for the audio either. Oct 17, 2016 at 8:50
• However DC does not mean the current keeping unchanged, it can be a DC square wave, the amplitude is changing. Jul 26, 2018 at 8:39
• @eepty in the context the op is looking at, it is unlikely that that definition of DC would be used Jul 26, 2018 at 14:16

ALL current will heat the voice coil of a speaker. But AC current is useful to reproduce sounds (which is what a speaker is made for).

On the other hand, DC current will produce the equivalent amount of heating as an equivalent AC current, but it will produce nothing but a fixed offset (versus moving the cone in and out to produce sound). And while you can hear AC current, and you can hear when it is "too loud" and distorting the speaker, you cannot hear DC, so you don't know whether your speaker voice-coil is sitting there frying until you see the smoke.Also DC current biases the cone off center which could increase even harmonic distortion.

For these reasons it is never a good idea to allow DC current to go into a speaker voice-coil.

• That depends on how much you "push" it, a little bit will be OK but there is always a signal that is large enough to damage the speaker. May 9, 2016 at 18:56
• There is not a clear 1:1 correlation between distortion and damaging heating. There are some speakers that will distort before you reach dangerous heating, and there may be some speakers that will start overheating before you hear distortion. But is is likely a reasonable rule that hearing distortion from a speaker is probably an indication of abuse. May 9, 2016 at 19:03
• If you hear 'rubbing' or 'scrapping' sounds the cone is damaged or the voice coil has burnt from either AC or DC overload. Modern amplifiers intentionally check for any DC content in the output stage, and try to cancel it out. Else, they will cut off the amplifier to protect the speakers. Low cost amplifiers may not offer this protection.
– user105652
May 10, 2016 at 1:20
• @RichardCrowley coming from a computer user, but doesn't know much about electronics, I'm interested about distortion and its effect to the speaker. Does it mean that playing distorted audio (e.g due to clipping from audio source, not because of volume control) may damage the speaker? Or is this unrelated at all? May 10, 2016 at 4:20
• @Andrew.T No, distortion does NOT necessarily mean that the speaker is being over-driven. In most cases the distortion is likely happening "upstream" somewhere and the speaker is only faithfully reproducing it. Playing "pre-distorted" audio does not harm the speaker. Or there would be lots of burned out guitar speakers out there! :-) May 10, 2016 at 4:44

Sound consists of pressure changes in the air.

You can generate these pressure changes using a loudspeaker.

The loudspeaker generates these pressure changes (sound waves) by moving a diaphragm back and forth.

This diaphragm is moved back and forth by a voicecoil consisting of a "pipe" with some electrically conductive wire wound onto it.

This voicecoil is suspended in a magnetic field provided by a permenent magnet.

If you would use the loudspeaker properly and only apply an AC signal to it the voice coil would move some distance to the front and the same distance to the back. This is because the average of the signal you're applying is 0 (zero), the signal has a DC value of zero. On average (over a some time) the position of the voice coil is at it's center point, the "resting" position, the same position it would have if you applied no signals to the loudspeaker.

Now if you would apply a DC signal there would be a constant force working on the voice coil constantly moving it a little bit to the front or (if you reverse the polarity) a little bit to the back. If you would also apply an AC signal the loudspeaker would still work but on average it would not be in it's center "resting"position.

This DC signal induces a constant force on the voice coil but it also heats it up as there is a current flowing and since the voicecoil's electrical wire has some resistance (4 or 8 ohms usually) some power will be dissipated heating up the voice coil.

Another side effect is that good loudspeakers are designed such that the voice coil can move a certain distance to the front and a similar distance to the back. If you apply a DC voltage then you offset this as the distance the voice coil can travel will be asymmetric. If the voice coil can move 10 mm to the front and 10 mm to the back but you offset it with a DC signal by 5 mm to the front, the voice coil can only move 5mm to the front and 15 mm to the back. This will result in more distortion and worse sound quality.

• It is possible to move the voice coil entirely out of the gap and lodge it there also, or to smack it against the back of the structure hard enough to dent it, so that it binds after that.
– user56384
May 9, 2016 at 23:24

No, it's not complete and it's not accurate. A few tens of mV of DC are not a problem with most speakers.

Amplifiers that are output transformerless and lack bulky blocking capacitors will have a bit of offset voltage on the output.

If there is too much DC component then you get excessive $I^2R$ heating in the voice coil just from the DC (and the DC resistance of the coil is lower than the rated impedance- usually about 70-80%). Too much temperature rise can damage or destroy the speaker. The total amount of heating will be from the RMS sum of the sound and the DC components.

There is a significant omission in the referenced statement. It should be "... which could overheat..."
It all depends on the DC power applied versus the power handling capability of the speaker. But even if the speaker can handle the DC, it absolutely makes no sense to apply it. Speakers are designed to reproduce sound and DC only produces "noise" when it is first applied.

Compared to an AC signal with the same peak-to-peak amplitude as a DC signal's voltage, a DC signal has more power (if you're wondering, this is the significance of RMS voltages when working with AC signals - the RMS voltage of an AC signal is the voltage of a DC signal with equal power). Because DC signals have more power, more power will be dissipated in the speaker coil which may cause it to overheat.

Another way of looking at this is by considering the duty-cycle of an AC signal and the fact that the AC signal doesn't remain at the peak amplitude all the time, therefore the speaker coil has a chance to "cool down" between peaks in the signal and doesn't overheat, whereas a DC signal remains at the same voltage all the time so the coil doesn't "cool down" and so the heat accumulates until the coil overheats.

DC signals also affect the movement of the speaker cone which may result in a reduction of audio quality, although this doesn't damage the speaker.

• This is completely irrelevant.
– Mast
May 27, 2016 at 7:07
• @Mast it is not irrelevant, because it explains that DC signals have more power than AC signals which is a major cause of speaker damage from DC signals. May 27, 2016 at 8:10
• It's extraneous energy, energy being wasted by definition because it doesn't translate into motion. So yes, it adds heat, just like adding any form of useless energy to it. That has nothing to do with it being DC, it has everything to do with it not being AC.
– Mast
May 27, 2016 at 8:39
• @Mast "not AC" and "DC" are the same thing. May 27, 2016 at 12:52