Impedance VS Resistance

[jeff]

 I got some speakers I boosted out of granma's record player she was getting rid of. It had 2 12" C12RLs in it(stereo, one per channel). I measured the resistance of the coil at 12.5 ohms. I know resistance is different than impedance so does 12.5 sound right for a 16 ohm speaker. This is curious because there is an internat/external switch for speakers and the EXT. SPKR. terminals say 8 ohm(each channel). I would have thought the int and ext would be the same load.

Is it possible for a 8 ohm speaker to measure above 8 ohms resistance or is the resistance always lower than imp? I know 4s measure about 3.2 generally.

Does 12.5 sound right for a 16 ohm speaker?

Another thing is it had tweeters. Could this be an oddball(not 8 or 16) speaker that when used with the tweeters makes an 8 ohm load? The tweeters are wired in series with a cap, in parallel with the speaker.

Thanks
  Jeff

[Raybob]

12.5 ohms sounds right for a 16 ohm speaker.  Possibly the tweeters are 16 ohms wired parallel giving 8 ohms total per side.

[stingray_65]

the cap is for high pass.

If 8 ohms is your goal, then run both 12's in a cab.

I've had mixed results using old HiFi speakers, mostly good.

in general, dont run more than say 18 watts. maybe 25 for the pair.

[Frankenamp]

12.5*1.414=17.675  close enuf for 16 ohms 12.5 is your DC resistance. the impedence with an AC signal is somewhat higher. It varies with the frequency (inductance), and the mechanical compliance (back EMF) of the speaker. There will be a peak of 40-80 ohms somewhere between 60 - 100 Hz, (resonant freq) and will rise with frequency above 2K as a function of inductance. I think that the woofers may be replacements as the amp is marked 8 ohms and the tweeters are 8 ohms.

[jeff]

 Wasn't thinking that makes sense 16 woofer + 16 tweeter = 8 load. For some reason I thought the tweeters pick up where the woofer drops off so it would be a 16 load total.

 Thanks again
     Jeff.

[HotBluePlates]

12.5*1.414=17.675  close enuf for 16 ohms

Only use SqRt of 2 (1.414) when you're converting from RMS voltage to peak voltage (or in other cases where SqRt of 2 is part of the mathematical formula).

It never happens with practical speakers, but an impedance of 16 ohms could include 0 ohms of resistance, instead being pure reactance.

Strictly, resistance is constant regardless of frequency; a frequency of 0 Hz is also d.c.

Impedance is the result of a combination of resistance (not varying with frequency) and reactance (an opposition to the flow of electricity that varies with frequency). You could have inductive reactance or capacitive reactance, or a combination. Each has current 90 degrees out of phase with voltage (for a pure reactance; which one leads depends on which kind of reactance). The reactance is a different value depending on frequency.

The resistance and reactance can't be added directly, you have to use vector algebra. Regardless, you end up with some number that is larger than either individual component, and has a phase angle between +/- 0 to 90 degrees.

We often don't care about the exact phase angle, so we leave it off. We also often pretend that a 16 ohm speaker is always 16 ohms, but that ignores the reactance varying with frequency, and the mechanical effects. Just know that the impedance will always be bigger than the measured resistance.

[Frankenamp]

12.5*1.414=17.675  close enuf for 16 ohms

Only use SqRt of 2 (1.414) when you're converting from RMS voltage to peak voltage (or in other cases where SqRt of 2 is part of the mathematical formula).
<snip>

It works well enough when figuring the "nominal" (industry- not the correct definition)...

What you said is absolutely correct. I was making a 'thumbnail' simplification that works well enough in the real world. (The science of loudspeaker design is very complex when calculating the mechanical and electrical components.)

Two 16 ohm woofers in parallel = 8 ohms. (Two 16 ohm woofers in series is 32 ohms.) I'm assuming stereo (2 channels, one woofer one tweeter each) and that would make for an impedance mismatch. If it came from the factory that way- I'd think someone made a miss-steak. I think the woofers got changed out at some point or another. However, since this is a geetar forum, we don't need no steenkin' tweeters anyways. They will make a nice 8 ohm 2 X 12 for you.

[jeff]

 Yeah I'm just gonna use the two 12" woofers for an 8 Ohm load no tweeters. I knew imp was lower but I've never had 16 ohm speakers and 12.5 sounded too low so wasn't sure.

But, if a 4 measures 3.2 makes sense a 16 would measure 12.5.
  4/3.2=1.25
 16/12.5=1.28

BTW they're marked C12RL. I think:
C=ceramic
12=12"
R=wattage
L=?

Anyone know what R is rated for and what L stands for?

[PRR]

12 ohms is reasonable for an efficient cone speaker advertized as "16 ohms".

There's DCR plus everything else IN SERIES. So the audio impedance is always higher than the ohm meter says.

> I thought the tweeters pick up where the woofer drops off so it would be a 16 load total.

This is correct.

With the cap, the "tweeter" is over 1K at 20Hz, over 100 ohms at 200Hz, doesn't get close to speaker impedance until 1K or up. So at least 20Hz-1KHz, it's just the woofer.

If woofer and tweeter were both -perfect- 8 ohm resistances, you'd have 8 ohms at low frequency and 4 ohms at high frequency.

If you cared, you'd put the opposite reactance, an inductance, in series with the woofer. You can hit "perfect" 8 ohms at all frequencies.

Real woofer impedance, as Frankie says, "varies with the frequency (inductance)", generally rising above 1KHz. So as the cap lets the tweeter load the amp, the woofer's inductance is un-loading the amp. The woofer and cap+tweeter together can make a pretty constant impedance.

And tube amps such as this probably was are not that fussy. 10 or 20 is all the same for a "16" tap. 8 or 32 won't hurt nothing.

In hi-fi, I like 1.2 multiplier. 6 or 7 ohms DC is surely "8 ohms" on the spec-sheet.

For high-efficiency speakers, PA and such, 1.3-1.4 may be more likely. Even up near 2 in the most extreme cases.

Either way, unless you suspect the speaker was made to an oddball impedance (or weighs 30 pounds), just round-up to the next common number. 12=16.

> an impedance of 16 ohms could include 0 ohms of resistance...

Zero DCR, in our dreams. Or nightmares. We would really like the Acoustic Resistance (actual work done on air) to be much larger than the copper-loss. That never happens. The best are a few extreme horn-loaded drivers which are 9-10 ohms of copper and show 15-20 ohms in the 500Hz-5KHz band WHEN coupled to a good horn. What we get with cones is (neglecting bass-rise and inductance) is like 7 ohms copper and 0.5 ohms actual work against the air.

[stingray_65]

Yeah I'm just gonna use the two 12" woofers for an 8 Ohm load no tweeters. I knew imp was lower but I've never had 16 ohm speakers and 12.5 sounded too low so wasn't sure.

But, if a 4 measures 3.2 makes sense a 16 would measure 12.5.
  4/3.2=1.25
 16/12.5=1.28

BTW they're marked C12RL. I think:
C=ceramic
12=12"
R=wattage
L=?

Anyone know what R is rated for and what L stands for?

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&_trksid=p4340.l2557&item=390299407176&nma=true&rt=nc&si=XBCuhDkPHzSmbrr08ZqkEHYWSy4%253D#ht_865wt_754

http://cgi.ebay.com/2-Jensen-C12R-12-Guitar-Speakers-for-Fender-NEW-09-411_W0QQitemZ220323601823QQcategoryZ43375QQcmdZViewItemQQ_trksidZp4340.m263QQ_trkparmsZalgo%3DSIC%26its%3DI%252BC%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%26otn%3D10%26pmod%3D390299407176%26ps%3D63%26clkid%3D8117969765356380709#ht_2409wt_959

Both C-12-R one is a vintage L

The L is likely a feature that Jensen was asked to design into that specific speaker for that speciific manufacturer.

I cannot seem to find any "de coder" for that designator, but that doesn't mean it's not out there.

Ray
Both claim 25 W

[HotBluePlates]

I hadn't seen anything to make me think "Jensen" in relation to these speakers. Maybe grandma's record player had Jensen's, maybe not.

I used to have a pair of RCA speaker cabinets that were alnico Jensen speakers, but with an RCA label and some codes that had no relation to anything I'd ever seen.

Deciphering codes often seems to be a manufacturer-specific thing. Just look at the enormous books they have to print to help you decipher what bulb or wiper blade goes on your car...

[jeff]

I hadn't seen anything to make me think "Jensen" in relation to these speakers. Maybe grandma's record player had Jensen's, maybe not.

  I forgot to mentioned it but it was stamped 220442. I believe 220 indicates Jensen and the last three are year and date. 4 for '54 or '64 and 42 is 42nd week. I wrote to the site that sells Jensens. They said that the R meant 14W for old speakers(R=25W for modern Jensens) but didn't know what the L was. I also wrote to a guy that recones speakers and he didn't know either. I've been looking at some pictures of a vintage jensen C12R and they look pretty much the same as my speakers. The major differences I noticed are the terminals are different. Instead of just a + and - there is a third terminal between them. The cap goes from the + to this terminal and the tweeters are connected to this point. Also the dome(dust cover?) looks like it was made with a hole in the center with felt in it, not solid paper.

 Oh well, they are what they are good or bad. Just gotta hook 'em up and listen. I'll wire them up parallel for a 8 ohm 28W cab.

Thanks for the help ID-ing them as 16 ohm
                   Jeff