measuring impedance to optimize horn chamber

Jimmy D. · 4th January 2012, 12:47 AM

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 · 4th January 2012, 01:03 AM

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 !

Jimmy D. · 4th January 2012, 01:50 AM

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

Jimmy D. · 4th January 2012, 04:04 AM

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

godfrey · 4th January 2012, 07:57 AM

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.

Jimmy D. · 9th January 2012, 03:37 PM

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 · 9th January 2012, 07:31 PM

Quote:

Originally Posted by Jimmy D.

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?)

Jimmy D. · 9th January 2012, 11:26 PM

Ok! Here you go.65hzhorn.JPG

Scott L · 12th January 2012, 08:03 PM

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.

4th January 2012, 12:47 AM	#1
Jimmy D. diyAudio Member Join Date: Oct 2004 Location: Southeast US	measuring impedance to optimize horn chamber 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

12th January 2012, 08:03 PM	#9
Scott L diyAudio Member Join Date: Dec 2008 Location: Knoxville, TN	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.

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4th January 2012, 01:03 AM	#2
weltersys diyAudio Member Join Date: Jan 2008	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 !

4th January 2012, 01:50 AM	#3
Jimmy D. diyAudio Member Join Date: Oct 2004 Location: Southeast US	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

4th January 2012, 04:04 AM	#4
Jimmy D. diyAudio Member Join Date: Oct 2004 Location: Southeast US	Although the woofers are JBL 2220A, they measure 14 ohms DCR. They were evidently reconed with 16 ohm coils. Jim

4th January 2012, 07:57 AM	#5
godfrey diyAudio Member Join Date: Nov 2009 Location: Cape Town	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.

9th January 2012, 03:37 PM	#6
Jimmy D. diyAudio Member Join Date: Oct 2004 Location: Southeast US	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

9th January 2012, 11:26 PM	#8
Jimmy D. diyAudio Member Join Date: Oct 2004 Location: Southeast US	Ok! Here you go.65hzhorn.JPG

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