measuring impedance to optimize horn chamber

[Milkduds]

I'm trying to measure impedance of my 65 hz FLH to determine if my rear chamber volume needs to be adjusted. I used the procedure in the Loud Speaker Design handbook using a signal generator, DMV, and 1k resistor. I did a sweep from 40 hz to 110 hz and I can't see a noticeable change in impedance. except for small change at around 66-68 hz. See below. Is this enough of a change to mean anything? Am I doing something wrong? The impedance drops smoothly from 19.9 ohms at 40 hz to 14.5 ohms at 110 hz.

Hz Imp
60 16.40
61 16.23
62 16.10
63 15.97
64 15.78
65 15.47
66 15.16
67 15.25
68 15.37
69 15.51
70 15.49

Thanks,
Jim

 

[weltersys]

Jim,

Seems like you might be measuring something other than the speaker.
Check your wiring set up, easy to get things wrong.
Verify the voice coil resistance, at some point the impedance should drop close to that figure.

Good luck !

 

[Milkduds]

Forgot to mention that there is a 2.5 mH inductor in series with the woofer. I'll remove and try without. Horn is used from 65 hz to 500 hz btw.

Jim

 

[Milkduds]

Although the woofers are JBL 2220A, they measure 14 ohms DCR. They were evidently reconed with 16 ohm coils.
Jim

 

[godfrey]

The DVM, or some other part of the test setup, may be playing tricks on you. I'd repeat the test with the speaker replaced by a resistor to check that.

 

[Milkduds]

I figured this out. Redid test with no amplifier. Connected signal generator directly to mid-bass, used better Fluke meter. This time I got a very nice, discernable change in impedance at 66 hz, right at the design of the mid-bass cab. So rear chamber is fine!

Jim

 

[Scott L]

Show us your horn !

I figured this out. Redid test with no amplifier. Connected signal generator directly to mid-bass, used better Fluke meter. This time I got a very nice, discernable change in impedance at 66 hz, right at the design of the mid-bass cab. So rear chamber is fine!

Jim

Hi Jim,
I'd like to see a nice horn that covers from 65-500 Hz !

(maybe others would, too?)

 

[Milkduds]

Ok! Here you go.

 

[Scott L]

Very nice !

lookin Good, Jimmy !

They kinda look familiar. Similar or perhaps the same design I saw pictures of at one of the previous VSAC shows. JJ maybe ? From what I remember, info on that design was unobtainable.

None-the-less, those look real nice. thanks for showing.