A question for R.G. or anyone with speaker knowledge?

[boogietube]

So, I'm selling a pair of PA speakers. This guy calls me up and says he'll bring a multimeter over to test the resistance of the speaker to look for the  " shelf life " of the speaker. He claims that a speaker will measure a lower resistance as it ages- and he can tell how long he will get out of a used speaker just with a resistance measurement. I personally think he's cracked, but I thought I'd ask in case there was some merit to this "shelf life" test.  I thought that impedance (not resistance)  was reactive and would vary with frequency. ie the lower the note, the lower the impedance as seen by the source etc... Could somebody illuminate me?
Thanks,
Sean

[Paul Marossy]

So, I'm selling a pair of PA speakers. This guy calls me up and says he'll bring a multimeter over to test the resistance of the speaker to look for the  " shelf life " of the speaker. He claims that a speaker will measure a lower resistance as it ages- and he can tell how long he will get out of a used speaker just with a resistance measurement. I personally think he's cracked, but I thought I'd ask in case there was some merit to this "shelf life" test.  I thought that impedance (not resistance)  was reactive and would vary with frequency. ie the lower the note, the lower the impedance as seen by the source etc... Could somebody illuminate me?
Thanks,
Sean

I'm not an expert on the subject, but I am leaning towards agreeing with you. The impedance is reactive, and it does change with frequency.

[theehman]

The only time this will work is if it reads infinity.  That means they're probably at the end of their life (or there's a bad connection somewhere).

The man's a doof.  I bet he's hoping they're 4 ohm cabs and he'll be able to get you to come down on the price because they're "well used and near the end of their life".

[GREEN FUZ]

Sounds like a load of B****cks, but what would I know.

[Nasse]

You should solder a resistors in series inside the cabinet and perhaps you can put a figure zero in the end or 1 in front what you ask for the cabs

Sure he is bullshitting you or somethin else

[Mark Hammer]

Even if it were true, from a measurement perspective he has no freaking idea of what they measured when they were new.  So how could he know how much "aging" there had been?

Sounds to me like the gentleman is another sad victim of IRIOTIS (I read it on the internet somewhere).

[GREEN FUZ]

Tell him, his multimeter would be better served testing the "shelf life" of a certain part of his anatomy.

[anchovie]

The only time this will work is if it reads infinity.

Based on the guy's logic, this should mean the speaker will last forever!

[boogietube]

I'll go home and solder an 8 ohm resistor in (and disconnect the crossover and drivers) and then charge him full retail. lol!!

[Mike Burgundy]

Warning: the following bit is my mind unfiltered straight to keyboard. Scroll down to see me agreeing with everybody else

I assume he will disassemble the speaker first and measure the voicecoil DC resistance - if you measure between the terminals of the box, the (passive, RLC) of unknown construction and value will render any DC measurement totally useless.
That said - DC resistance only tells you something about the voice coil of that speaker chassis. Mainly if it works. Infinity means it's open somewhere, which van be caused by severe overheating (such as when feeding a woofer high-power high-freq block signals), mechanical abrasion (the voicecoil has been rubbing against the channel in the magnet structure causing it to wear through isolation and copper - can be caused by age (the speaker actually sags over time due to gravity, or severe overloading, damaging the cone and spider structure), loose voicecoil wiring due to bad construction (potting), or even the connecting wires *to* the voicecoil failing. These *can* fail due to age/degradation/material fatigue, although I've only ever seen it in tweeters where the wires are *really* thin.
If it's shorted, it's probably overheated to the point of melting/burning  or something conductive has aggressively wedged itself in the coilgap, puncturing the isolation of the coil. Extremely rare.

The voicecoil is just a big roll of copper wire - I completely fail to see how it's DC resistance (just the copper's resistance, but for a *very* long length of (thin) wire) can change during it's healthy life. I'd say the permanent magnet will change more, and that really has an effect on performance. Also every mechanical bit of the speaker (the cone, the cone rim/suspension, the spider and the suspension therein) will have a very real influence on performance over the speaker's lifetime, and this will NOT reflect in any way in a DC measurement of the coil.

Now, if he had extensive knowledge of that specific speaker (not just Thiele-Small, but also how much variation in parameters is normal for that production line, and how those values are important to actual performance - there can be "significant" differences between two speakers which, with all differences combined, amount to identical performance), and brought a signal generator/frequency counter, lab voltage source, two very accurate meters and something to measure the speakers force-factor, then he *might* be on to something.

That was a very long-winded way (sorry if you read it all, but I warned you ;P ) to say this is hogwash

Oh, and impedance roughly does get lower with frequency (DC measurement will always be lower than the stated impedance, the coil/magnet isn't fighting it) but it *will* spike quite high on the speakers resonant frequency. Looks like this for a speaker with Fs=40Hz:
http://www.quadesl.com/speaker/diyesl/woofer_24oz_z.gif

[petemoore]

  Here's my take...
  Either he's got IRIOTIS
  Such as what I wrote about my old vox bulldogs [circa like 40+ year old speakers had a LOT of BASS real loud run through them].
  Her'es about what I wrote:
  The one with the lowest ohmage blew first, another two followed suit not too long after...we're talke 40+ year old beat on speakers though.
  Of course it was also the loudest one of the 4 x 12''ers.
  ...And I had 3 similar speakers to read off of, and the one was a big gap difference the others resistances were pretty closely clustered.
  deal is, you don't know what the original new resistance {DC or impedance] was to begin with, therefore there's no 'optimum number' by which to judge the R or I of the speaker coil.
  Basically if it says 8 ohm, and the DMM set for DCR reads above 6 or 7 ohms, less than 8 ohms...ballpark.
  I wouldn't put too much salt into DCR unless like noted above, it is 0, or something really offbase.
   

[Paul Marossy]

Tell him, his multimeter would be better served testing the "shelf life" of a certain part of his anatomy.

[JKowalski]

  The one with the lowest ohmage blew first, another two followed suit not too long after...we're talke 40+ year old beat on speakers though.
  Of course it was also the loudest one of the 4 x 12''ers.

That's probably because the speakers were wired in parallel. The low resistance ones will probably blow first in these cases because they are drawing more current then the ones with a higher resistances. That's also why they are louder. More power dissapation.

So - this guy is right in this regard - speakers with a low resistance wired in parallel with ones with higher resistance will blow first, probably, simply because they are always drawing more power.

Obviously, the guy does not know what the hell he is talking about. However, it is right in some cases - as discussed above - and wrong in another:

For example, if you wire a low resistance speaker in series with a high resistance one, the high resistance one will be drawing more power, because the voltage drop across it is higher, while the current through both is the same.


In short:

High resistance speakers draw more power in a series connection, low resistance speakers draw more power in a paralell connection.



Having said all that, it's kind of pointless to check this. I can see how it would cause problems if one speaker is so cheaply made that it's resistance is way too low and it is drawing way too much power, but I havent really seen anything that drastic. Speakers in the same line typically have variances in DC resistance of no more then 0.3 ohms or so (at least in all the cabinets I have checked). If you have a well designed amplifier, these variances are probably going to be compensated for - you dont want to build an amplifier so that the output rides at the maximum rated watts all the time.

[Electron Tornado]

Even if it were true, from a measurement perspective he has no freaking idea of what they measured when they were new.  So how could he know how much "aging" there had been?

Sounds to me like the gentleman is another sad victim of IRIOTIS (I read it on the internet somewhere).

Ask him to cite the source for his info. Or simply call an amp tech tell him about the guy's story.

[Paul Marossy]

Ask him to cite the source for his info.

+1

[R.G.]

I keep a collection of the more entertaining things I read that people post. This is going into it.

Speakers have a DC resistance and both an electrical and an electromechanical impedance, both of which include the DC resistance as a part of it.

Multimeters do not, in general, read impedance, although there are now some vector-impedance meters specialized for measuring impedance. Measuring the DC resistance of a speaker only tells you what the resistance of the speaker's coil  is. Speaker coils are usually copper wire, but in some high power speakers are aluminum or even special-formed wires. But wire resistance does not change appreciably with age (leaving aside damage). Electrical impedance does not change significantly with age. Electromechanical impedance does, as the cone suspension loosens up, but the DC resistance part of it does not.

Your friend is misinformed.

[wavley]

I'm no expert, but part of my job when I did my time as a repair tech was re-coning speakers.

First, if he shows up with a DMM they are almost useless for checking a speaker.  If you use an analog meter (even a cheap one, but something like a Simpson is way better) it's a lot easier to read i.e. an 8 ohm speaker will read 8 ohms.  As far as the electrical side of a speaker, the voice coil either works or it doesn't, it reads what it's rated, open, or shorted.  This is not taking into account crossovers and such, just a raw speaker. 

Readings will tell you nothing about a speaker, a 50 year old speaker may still read 8 ohms because the voice coil is still fine, but it may be rubbing in the gap causing it to rattle, the paper in the cone or surround may be worn out, the magnet may be weak.  The mechanical aspects of a speaker tell you much more about it's shelf life than any electrical measurement.  I've seen speakers where the voice coil is just fine, anything on a meter would tell you it's great, but the foam surround has dry rotted and disapeared so the speaker is unusable, clean the old foam off, put a new surround on and the speaker is great again.

Just sharing my experience, I may be wrong, I don't know everything, but all sounds like a load of crap to me!

[Paul Marossy]

Your friend is misinformed.

Totally!

Readings will tell you nothing about a speaker, a 50 year old speaker may still read 8 ohms because the voice coil is still fine, but it may be rubbing in the gap causing it to rattle, the paper in the cone or surround may be worn out, the magnet may be weak.  The mechanical aspects of a speaker tell you much more about it's shelf life than any electrical measurement.  I've seen speakers where the voice coil is just fine, anything on a meter would tell you it's great, but the foam surround has dry rotted and disapeared so the speaker is unusable, clean the old foam off, put a new surround on and the speaker is great again.

All good points. The mechanical aspects are far more important in an old speaker than what the DC resistance measures - that's pretty much going to be a constant as long as that part of the speaker is not damaged.

[theehman]

So, did the guy ever come check out the speakers?  I'd love to hear what happened.

[MikeH]

The same guy probably send his cables out to be 'broken in'