Lazy speaker...

[Ian Rogers]

Well!! The speaker is out..... I can't see any discernible problems...


Frame was straight... The connections are DEAD clean... There is no obvious signs of perishing..

When you press carefully on the speaker cone, it appears slightly out of align ( catches internally ) but that would be a problem when VERY loud..

I have ordered a new speaker... It will be here around 3:00pm

We'll see how it goes.

 

[misterT]

It would be nice to find out what was the problem. I hope you have time to investigate the problem even when you receive the replacement speaker. I have similar problems with my home stereos, but I'm pretty sure the problem is the old 80's harman/kardon PM625.. not the speakers. I just need to "twerk the volume like Miley Cyrus" and the other speaker is alive again.

 

[dr pepper]

There are different ways to spec a speaker impedance, I've allways taken it as impedance at crossover freq, bigger drivers have some wild changes around resonance in impedance.
If you measure with a meter (at dc) you get around 6 ohms and 3 ohms for 8 and 4 ohm drivers.

 

[ericgibbs]

When you press carefully on the speaker cone, it appears slightly out of align ( catches internally ) but that would be a problem when VERY loud..

I have ordered a new speaker... It will be here around 3:00pm

We'll see how it goes.

hi Ian,
Sounds as though, pardon the pun, that the speech coil has some foreign object trapped between it and the speaker magnet.

We'll see how it goes

Do you mean 'hear' how it goes.?

E

 

[MrAl]

Hi misterT,

Looks interesting but unfortunately i can not translate that language. All i see is someone applying what looks like 120vac to a speaker coil without the cone (ha ha). Cant tell what it really is though because the language is not Latin based or French or English
Maybe you can explain a little. Looks interesting anyway.

To get a 7 ohm speaker to measure 8 ohms in free air at 30Hz would take about 21mH of series inductance. That would make the impedance around 132 ohms at 1kHz which would make a very very poor speaker. Thus, the DC resistance has to be close to the AC resistance.

Oh yeah i almost forgot to mention that the coil form could come loose from the cone.
Also, the coil wire could actually be detached from the coil form itself so that when the current flows the coil wire moves but the coil form does not move much thus the cone does not move much. This would cause decreased volume but probably also distortion. This can happen when the speaker is overdriven as the wire gets hot, melts the glue that fastens it to the coil form, and breaks it loose. There could be a number of things that happen after that.

 

[audioguru]

We played with speakers enough about 10 years ago. When I worked at an amateur tv-station.. **broken link removed**
Skip to half-way of the video to see the silly thing go.. Well.. it is pretty stupid thing to do anyway and we did not have a high-speed camera.

You blew up the little speaker by feeding it 230VAC!? It was a nice explosion.

I worked for a pro-audio manufacturer and I played their sub-woofers at 1800W in the demo room.
I have one of the sub-woofer speakers. It is 18" in diameter and is very heavy. If it plays 1800W at 50Hz then it will break all the windows in my neighbourhood.
But it is brand new and has never played.

 

[misterT]

But it is brand new and has never played.

Plug it in.. play it.. I dare you, I double dare you.

 

[audioguru]

If I blast 1800W into my 18" speaker then maybe you will hear it and feel its vibrations across the Atlantic ocean in Finland.

 

[Nigel Goodwin]

To get a 7 ohm speaker to measure 8 ohms in free air at 30Hz would take about 21mH of series inductance. That would make the impedance around 132 ohms at 1kHz which would make a very very poor speaker. Thus, the DC resistance has to be close to the AC resistance.

As has already been mentioned, an 8 ohm speaker reads about 6 ohms, and a 4 ohm speaker about 3 ohms.

To measure impedance you test at 1000Hz - but a speaker isn't a plain inductor, and doesn't act like one, it's a type of electric motor.

 

[MrAl]

As has already been mentioned, an 8 ohm speaker reads about 6 ohms, and a 4 ohm speaker about 3 ohms.

To measure impedance you test at 1000Hz - but a speaker isn't a plain inductor, and doesn't act like one, it's a type of electric motor.

Hello,

I agree that there is more to it than that, but that is the main idea. The point was to illustrate the idea that the DC resistance is close to the AC 'resistance' which i think everyone here agrees about. So if we drive it with 12vdc and it can handle the power to begin with, especially if it is only for a few tens of milliseconds, it should survive without a problem.
We can always go into it in more detail if you'd like with a decent electrical model of a speaker.

I used to have an 18 inch speaker too but dont remember what the wattage was. Wow 1800 watts, i guess that is for when the astronauts get to Mars we can play them some music from right here on good ol' Earth

 

[misterT]

I agree that there is more to it than that, but that is the main idea. The point was to illustrate the idea that the DC resistance is close to the AC 'resistance' which i think everyone here agrees about.

Isn't the coil just a piece of copper wire? Why speakers have such high dc resistance? I have measured some motors and all of them have coil resistance around 100 milliohms.. that is the "ball park". Why do speaker coils have such high "DC resistance"? I do not believe it. And sorry, I can not measure my speakers right now.

 

[Nigel Goodwin]

Why do speaker coils have such high "DC resistance"? I do not believe it. And sorry, I can not measure my speakers right now.

I'm somewhat amazed that a member with 1600+ posts hasn't ever measured the DC resistance of a speaker?, and for that matter doesn't have a few speakers lying around he could test.

As for 'not believing it' when probably every single member on here knows this, and only you apparently don't?, it's not a very 'engineering attitude' to disbelieve everyone

 

[misterT]

I'm somewhat amazed that a member with 1600+ posts hasn't ever measured the DC resistance of a speaker?, and for that matter doesn't have a few speakers lying around he could test.

As for 'not believing it' when probably every single member on here knows this, and only you apparently don't?, it's not a very 'engineering attitude' to disbelieve everyone

I measured my 8 ohm speakers with a multimeter and the figure was spot on 5 ohms. But there are all the band-pass filters included. I don't want to take the elements off just to measure the resistance..

1600 posts does not mean I am an "audioguru" I am academic that codes and does digital stuff..

 

[audioguru]

I am an Audioguru and I have measured the resistance of only one speaker (it didn't work). Its voice coil was OK but its magnet was not magnetized.

Oh yeah. I measured the resistance of one of my main stereo speakers that also didn't work and found that its flexy wires on the woofer were corroded away. The speaker was 40 years old. Its mate speaker still works perfectly and is 49 years old.

 

[tvtech]

Rambling post so deleted.

 

[MrAl]

I measured my 8 ohm speakers with a multimeter and the figure was spot on 5 ohms. But there are all the band-pass filters included. I don't want to take the elements off just to measure the resistance..

1600 posts does not mean I am an "audioguru" I am academic that codes and does digital stuff..

Hi there misterT,

I am happy to hear that you took Nigel's advice and measured at least one speaker very recently. As you found, it has DC resistance that might be higher than you originally thought. But also please be aware that if there are any other things in parallel to that speaker that they can lower the measured resistance as well. I think you already saw this. But the speaker itself should measure close to the stamped value unless of course it is stamped wrong, which is always a possibility.

For example, less than 5 minutes ago i measured the DC resistance of a speaker i use from time to time. It is stamped as "8 ohms". It measures 7.4 ohms with the multimeter that uses a DC current to make resistance measurements. It is a lone speaker with no crossover network or any other components.

Although there are similarities, speakers are not motors. Motors can have low resistance because their coils work closely with the special metal alloy cores that they are wound on. The core acts with the wire to increase the field which then acts with the permanent magnet. The speaker on the other hand has no armature core. That is necessary in order to allow high compliance to the drive signal. That means that the speaker, although working on the same basic principle, only has it's winding to act with the permanent magnet, but no core to help increase the field. That in turn means that there must be more turns of wire in order to develop a force that is great enough to move the cone. Luckily, the cone isnt too hard to move, but at higher frequencies it still has to have enough force to get it moving quickly. That requires a lot of turns, so the DC resistance necessarily goes up. If we added a core to the speaker we would have a more motor like construction, but then the mass would be so great that it would not be able to handle a higher frequency audio signal without requiring huge current levels and we'd have to take into consideration the losses in the metal at higher frequencies too.

Also keep in mind that there will be a basic tolerance for the DC resistance of a speaker. An 8 ohm speaker DC resistance spec will not be as accurate as say an 8 ohm 1 percent resistor. There is going to be some tolerance just in the spec itself without even considering any AC quantities yet. That means it may be possible to actually find 8 ohm speakers that measure anything from roughly 6 to 12 ohms, but that's just an illustration of what we might find and not a real life range that i would count on. An incorrectly marked speaker would measure just about anything from 2 ohms up to maybe 32 ohms.

Another interesting thing...you could also measure your headphones. These things have a really wide range of possible DC resistances. From maybe 8 ohms up to 64 ohms or something like that. Mine measure around 32 ohms and that's what they are supposed to be. Think of how much wire they must use, and how thin it is. The thin wire means the coil can be close to the magnet, which increases force.

Also, i happen to have a 32 ohm speaker around somewhere. It does measure pretty high DC resistance like 30 ohms.