true/false: speaker ohm reading changes as speaker deteriorates from age?

bongomania · #1 01-02-2012, 02:16 AM

I'm selling some older cabs, and a guy called and asked if I had read them with a multimeter. He claimed that when a speaker starts to go bad from age, its ohm reading will rise significantly. I've never heard of such a thing. Now, obviously if a speaker totally fails and becomes a dead open then its resistance goes up a bit;

but aside from that, is there any truth to his claim?

BTW these speakers read exactly the way they should, about 6 ohms for an 8 ohm impedance.

scottfeldstein · #2 01-02-2012, 02:23 AM

Io'm not a huge tech head, but it isn't my very first rodeo, either--and I never heard of such a thing.

brendanbassist · #3 01-02-2012, 02:27 AM

If a speaker becomes a dead short, wouldn't it's resistance go down?

Other than the voice coil burning out, I can't see how the resistance would change. I've never heard of that before; sounds like bs to me.

bongomania · #4 01-02-2012, 02:29 AM

Oh yeah duh! I was thinking backwards, an open not a short.

T-Bird · #5 01-02-2012, 02:59 AM

Hi.

I'm not a tech either, but while it's a fact that even wire-resistors tend to "creep" over time, I've never heard that the speakers resistance would significantly change regardless of how it's used. Within' reason of course.

Regards
Sam

parapentep70 · #6 01-02-2012, 04:30 AM

In a multi-driver cab, any significant resistance increase could be due to one driver being open due to extreme abuse (coil fused). I´d say in this case there are easier ways to confirm the cab has been extremely abused (I think voice coil melts or severely deformates before fusing).

I would not expect DC resistance to increase even a mere 5% due to ageing. In fact, I would not expect to get any info from a DC resistance reading.

If we say AC impedance, perhaps we could see some effect of loose suspension with age (higher Z below de box tuning freq / lower driver resonant freq / increased Vas, all of them tightly coupled).

The change of these parameters after 1 or 2 hours of use is pretty clear. So it could be reasonable to expect some small changes between say 100 and 2000 hours of use. If this is usable or not... I don't know.

1958Bassman · #7 01-02-2012, 04:46 AM

Quote:

Originally Posted by bongomania

I'm selling some older cabs, and a guy called and asked if I had read them with a multimeter. He claimed that when a speaker starts to go bad from age, its ohm reading will rise significantly. I've never heard of such a thing. Now, obviously if a speaker totally fails and becomes a dead open then its resistance goes up a bit;

but aside from that, is there any truth to his claim?

BTW these speakers read exactly the way they should, about 6 ohms for an 8 ohm impedance.

He may have read something about impedance and not understood it. A conductor's resistance will increase as it's temperature increases but it will go back to normal as the temperature falls. If you have heard of 'thermal compression', this is what causes that. As a speaker is used through a gig, the voice coil temperature increases, especially when it's being treated like a farm animal. If the ambient temperature is higher, the amp is a combo and it's tilted back, the heat from the tubes will keep the voice coil(s) at a higher temperature. You would then notice that it A) doesn't sound as loud as it did when you started the gig and B) the SPL increase isn't as noticeable if you turn the volume control up as it was when the gig started.

If you really want to give this guy a headache, find someone who has a Dayton WT-3 woofer tester and have your speakers checked, individually. This is made for speaker designers, so they can be fairly sure that the speaker parameters are accurate before they make the calculations for speaker cabinets and crossovers. Print the impedance curve (this is an actual impedance curve, not DC resistance) and phase plot, then watch as he scratches his head.

BTW- if you have a 4 speaker cab and one speaker is open, your DC resistance of the whole assembly will be higher than if all four are normal, assuming the speakers are wired parallel.

billfitzmaurice · #8 01-02-2012, 06:18 AM

Quote:

Originally Posted by bongomania

I'm selling some older cabs, and a guy called and asked if I had read them with a multimeter. He claimed that when a speaker starts to go bad from age, its ohm reading will rise significantly. I've never heard of such a thing.

All you can read with a multimeter is DCR, and that never changes. Impedance will change as the driver Fs goes down with age.