Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Multi-Way Conventional loudspeakers with crossovers

Speaker measurement difficulty - What's up?
Speaker measurement difficulty - What's up?
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 18th March 2017, 03:38 AM   #1
legendre is offline legendre  United States
diyAudio Member
 
legendre's Avatar
 
Join Date: Jan 2017
Location: USA
Default Speaker measurement difficulty - What's up?

Howdy folks,

Speakers aren't my forte.. in fact, I know little to nothing about them. But I've been playing around with impedance measurements recently, and came across something I can't figure out. It's pretty simple, really..

Just for drill, I set up an impedometer to measure a pair of little KLH bookshelf units. Everything went as expected, until I decided to knock (with my hand) on one of the speakers, to see what effect it had on the reading. As I expected, the reading fluctuated while it was perturbed, but the surprise came when it didn't settle back down to the original 'static' impedance.

I thought it might be a defective speaker, so I next tried the mate of the KLH speaker - and found the same issue. Next I set aside the KLH speaks and used a bare 4" driver - and found the same dang thing. If it's perturbed, the reading will not settle back to the original value.. ack.

So what is going on, here? Are the voice coils subtly shifting (and holding) position, thus changing the Z? And if that is the case, how do I measure the DUT - driver under test - without this wandering effect? For ref, here's the basic impedometer circuit I'm using:

Click the image to open in full size.

In my case, I'm using a selected 10R part (actual 10R02) rather than 8R. Also, I'm not using the scope, as it's not necessary here. Phase angle isn't of interest, and the power is so low that distortion isn't an issue either.

Last edited by legendre; 18th March 2017 at 03:45 AM.
  Reply With Quote
Old 18th March 2017, 03:42 AM   #2
eriksquires is offline eriksquires  United States
diyAudio Member
 
eriksquires's Avatar
 
Join Date: May 2013
What exactly did you hook up? Speaker impedance varies by frequency, so to say you measured the impedance means you measured the magnitude and angle from 20 Hz to 20kHz.

From Stereophile, here is a typical impedance measurement:

Click the image to open in full size.

If you were measuring resistance, and that magically changed and would not come back chances are you disturbed a connection somewhere.

Best,

E
__________________
All of my successes and failures can be explained by my attempts at getting a girlfriend. All of them.
  Reply With Quote
Old 18th March 2017, 03:50 AM   #3
legendre is offline legendre  United States
diyAudio Member
 
legendre's Avatar
 
Join Date: Jan 2017
Location: USA
Howdy Ed, thanks kindly for your reply.

Quote:
Originally Posted by eriksquires View Post
What exactly did you hook up?
The circuit is pictured. The integrity of connections isn't the issue afaik. Point is, I can measure driver D at freq. F and level L - then after moving the cone & coil (rapping on it, etc) it will not settle back to the same impedance. And the shift isn't trivial, either..

Last edited by legendre; 18th March 2017 at 03:52 AM.
  Reply With Quote
Old 18th March 2017, 04:13 AM   #4
eriksquires is offline eriksquires  United States
diyAudio Member
 
eriksquires's Avatar
 
Join Date: May 2013
I read an early version without your pic.

That would be unusual in a working speaker. This points to mechanical friction or electrical resistance.

Double check your Vmeter battery? Just thinking of things.

By connections, I include the point to poitn wiring of the crossovers and internal wiring of the speakers.
__________________
All of my successes and failures can be explained by my attempts at getting a girlfriend. All of them.

Last edited by eriksquires; 18th March 2017 at 04:16 AM.
  Reply With Quote
Old 18th March 2017, 04:16 AM   #5
Tweet is offline Tweet  Australia
diyAudio Member
 
Join Date: Jan 2015
You might try connecting a 1.5volt battery across the speaker in an intermittent way in order to produce 'clicks' from the driver and then in the reverse connection. This might free up the suspension of the driver for it to return to normal.

All one can do is experiment a little and observe the result.
There may have been a DC component flowing in the test circuit at the time of testing and then having tapped the cone it has reset itself to what is normal, thus the impedance might now be a different value. Loudspeaker drivers have a limited Xmax (limited linear movement) particularly small drivers and such may only be 2 millimetres thereabout.
Take care not to be too heavy handed when doing such things.

C.M
  Reply With Quote
Old 18th March 2017, 04:22 AM   #6
eriksquires is offline eriksquires  United States
diyAudio Member
 
eriksquires's Avatar
 
Join Date: May 2013
To trouble shoot, I would take the speaker apart and start with the drivers. Ensure the pair measure identically.
__________________
All of my successes and failures can be explained by my attempts at getting a girlfriend. All of them.
  Reply With Quote
Old 18th March 2017, 04:59 AM   #7
legendre is offline legendre  United States
diyAudio Member
 
legendre's Avatar
 
Join Date: Jan 2017
Location: USA
So am I correct, in that the impedance of a driver will vary, depending on the position of the voice coil in the gap? That is, if the resting point of the cone (and hence, the voice coil) are a little further 'in' or 'out' of the gap, after being disturbed, will change the 'static' Z of the driver?

I am seeing changes of well over 10% simply by doing a little knock-around with either the bare driver or the speaker. And yes - I give the instruments time to settle. The meter in use is a cal'd Fluke 8050A.

FYI, the 'speakers' here are little KLH 2-way bookshelf models.. model is KLH 9900 Satellite. 8R. They are cast-offs from some HT system or whatnot. Just keep them around for utility purposes.

Last edited by legendre; 18th March 2017 at 05:01 AM.
  Reply With Quote
Old 18th March 2017, 05:05 AM   #8
eriksquires is offline eriksquires  United States
diyAudio Member
 
eriksquires's Avatar
 
Join Date: May 2013
Well check both of them.

In theory, any resistor in place will work, assuming your measurement tools have enough resolution, and/or your input voltage is high enough maybe try smaller? Try something around 10 Ohms so you have enough signal to move the driver. With a 4 Ohm speaker, the voltage at the meter could be very small. I've not tried making my own rig, but worth trying.

Best,

E
__________________
All of my successes and failures can be explained by my attempts at getting a girlfriend. All of them.
  Reply With Quote
Old 18th March 2017, 05:21 AM   #9
wintermute is online now wintermute  Australia
just another
diyAudio Moderator
 
wintermute's Avatar
 
Join Date: Aug 2003
Location: Sydney
Speaker measurement difficulty - What's up?
Have a read of sb-accoustics document on measuring T/S params http://www.sbacoustics.com/index.php...le/-/view/191/

Tony.
__________________
Any intelligence I may appear to have is purely artificial!
Some of my photos
  Reply With Quote
Old 18th March 2017, 09:28 PM   #10
Jack Hidley is offline Jack Hidley  United States
diyAudio Member
 
Jack Hidley's Avatar
 
Join Date: May 2007
Location: Danville, CA
Speaker measurement difficulty - What's up?
If you are measuring the impedance of a woofer at higher frequencies, the impedance will vary a fair amount depending on the voice coils rest position. Generally the father towards the back of the motor, the greater the inductance and the higher the impedance.

In addition at low frequencies, the impedance will change for a time period after disturbing the driver due to mechanisms related to creep. This behavior is quite complex.
__________________
Jack Hidley
ACS
  Reply With Quote

Reply


Speaker measurement difficulty - What's up?Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
speaker measurement geometry astrojet Multi-Way 4 3rd April 2016 06:29 PM
speaker measurement questions Josephjcole Full Range 35 3rd June 2005 03:19 AM
Speaker measurement help in Atlanta david yost Multi-Way 2 27th April 2005 10:01 PM
speaker measurement software ucla88 Multi-Way 22 23rd February 2005 04:54 PM
speaker's load measurement GAK Multi-Way 1 8th February 2004 11:30 PM


New To Site? Need Help?

All times are GMT. The time now is 08:18 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.79%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright ©1999-2017 diyAudio
Wiki