Nearfield/Farfield curve splicing - diyAudio
Go Back   Home > Forums > Loudspeakers > Multi-Way

Multi-Way Conventional loudspeakers with crossovers

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 15th January 2014, 01:24 PM   #1
diyAudio Member
 
speaker dave's Avatar
 
Join Date: Nov 2009
Location: The Mountain, Framingham
Default Nearfield/Farfield curve splicing

Some of us have used a technique of splicing a nearfield or dustcap measurement to farfield pseudo anechoic measurements to try and create a realistic wide band look at woofer/box response. In fact every system measurement in Stereophile magazine for several decade has used this approach.

Jeff Bagby has written a white paper on an improved method of doing this which should be more accurate than the current approach. It showed up on a discussion forum on parts express:

New Paper available on How to Get Accurate Measurements Indoors Down to 10 Hz

The actual paper, worth a read, is in PDF form here:

https://app.box.com/s/fefis558wna1d6pd07r3

The thinking has been that a dustcap measurement will give a representative response for low frequencies that is free from the standing waves of your room (or even of an imperfect anechoic chamber). At upper frequencies we can use a gating approach to measure the "anechoic" response. All we have to do is splice the two curves together at somewhere around 200 Hz and we have a wideband and accurate reflection free response curve.

The problem is that the dustcap measurement doesn't properly represent what the woofer looks like in free space because it doesn't include the effects of baffle size and diffraction. Bagby uses a diffraction modeling program to create a 2pi to 4pi correction curve and adds that to the mix. When he adds the "baffle step" correction to the dustcap measurement, it and the farfield measurement show much more agreement over a broader frequency range. Splicing becomes less arbitrary and the proper level of bass vs. midrange is revealed.

Recommended!

Note that every nearfield approach suffers this same issue, as well as microphone in the box measurements, Thiel/Small approaches, P-Spice models, etc. They all assume the driver is non-directional and don't take finite baffle size into account. In that regard they look more like the 2pi response, as infinite baffle mounting will force constant directivity from very low frequencies up to beyond piston band operation.

If you want to play with diffraction modeling try here:

Home of the Edge

Tolvan Data

(Click on “The Edge”)

Don Keele’s classic paper on nearfield measurements is here:

http://xlrtechs.com/dbkeele.com/PDF/...ld%20Paper.pdf

He gives draw away curves for a woofer both mounted in a cabinet and in a large baffle. Draw away curves for the infinite baffle case retain the dustcap response shape while the 4 pi case varies as you move away from the driver.

Regards,
David
  Reply With Quote
Old 15th January 2014, 01:40 PM   #2
diyAudio Member
 
Join Date: Mar 2003
Gets rid of the pseudo low-midrange/bass hump on spliced data that gives a too low estimate for baffle-step 'correction'. No more speaker 'voicing' required. Excellent.
  Reply With Quote
Old 15th January 2014, 02:02 PM   #3
diyAudio Member
 
speaker dave's Avatar
 
Join Date: Nov 2009
Location: The Mountain, Framingham
Quote:
Originally Posted by daniel View Post
Gets rid of the pseudo low-midrange/bass hump on spliced data that gives a too low estimate for baffle-step 'correction'. No more speaker 'voicing' required. Excellent.
Yes, I've always noticed that in the Stereophile curves. Most speakers look like they have unnaturally high bass mounds.

David
  Reply With Quote
Old 15th January 2014, 02:17 PM   #4
JMB is offline JMB  United States
diyAudio Member
 
Join Date: Apr 2004
Location: Texas
Loudspeaker Design Software is Jeff's Baffle Diffraction program (Excel based), which allows for export of the Baffle Diffraction Signature in FRD format. It can also calculate a rough room effect, as well. His other programs such as the Passive Crossover Designer and a Box Modeling Program are also there.
Jay
  Reply With Quote
Old 15th January 2014, 02:19 PM   #5
JMB is offline JMB  United States
diyAudio Member
 
Join Date: Apr 2004
Location: Texas
Just so that the same problem does not occur here as over at the PE Board, Jeff acknowledges that others have identified this approach before but he was unaware of it. Jeff and Charlie have put together software that makes it easier to do and Jeff explains it in a clear fashion that makes this approach more accessible to the masses.
  Reply With Quote
Old 15th January 2014, 02:27 PM   #6
diyAudio Member
 
Greebster's Avatar
 
Join Date: Aug 2010
Location: South of the Skyway
Quote:
Originally Posted by speaker dave View Post
Some of us have used a technique of splicing a nearfield or dustcap measurement to farfield pseudo anechoic measurements to try and create a realistic wide band look at woofer/box response. In fact every system measurement in Stereophile magazine for several decade has used this approach.

Jeff Bagby has written a white paper on an improved method of doing this which should be more accurate than the current approach. It showed up on a discussion forum on parts express:

New Paper available on How to Get Accurate Measurements Indoors Down to 10 Hz

The actual paper, worth a read, is in PDF form here:

https://app.box.com/s/fefis558wna1d6pd07r3

The thinking has been that a dustcap measurement will give a representative response for low frequencies that is free from the standing waves of your room (or even of an imperfect anechoic chamber). At upper frequencies we can use a gating approach to measure the "anechoic" response. All we have to do is splice the two curves together at somewhere around 200 Hz and we have a wideband and accurate reflection free response curve.

The problem is that the dustcap measurement doesn't properly represent what the woofer looks like in free space because it doesn't include the effects of baffle size and diffraction. Bagby uses a diffraction modeling program to create a 2pi to 4pi correction curve and adds that to the mix. When he adds the "baffle step" correction to the dustcap measurement, it and the farfield measurement show much more agreement over a broader frequency range. Splicing becomes less arbitrary and the proper level of bass vs. midrange is revealed.

Recommended!

Note that every nearfield approach suffers this same issue, as well as microphone in the box measurements, Thiel/Small approaches, P-Spice models, etc. They all assume the driver is non-directional and don't take finite baffle size into account. In that regard they look more like the 2pi response, as infinite baffle mounting will force constant directivity from very low frequencies up to beyond piston band operation.

If you want to play with diffraction modeling try here:

Home of the Edge

Tolvan Data

(Click on “The Edge”)

Don Keele’s classic paper on nearfield measurements is here:

http://xlrtechs.com/dbkeele.com/PDF/...ld%20Paper.pdf

He gives draw away curves for a woofer both mounted in a cabinet and in a large baffle. Draw away curves for the infinite baffle case retain the dustcap response shape while the 4 pi case varies as you move away from the driver.

Regards,
David
InKtoresting, wery inKtoresting

I have been doing it the same way since getting a mic almost year ago. Suppose the engineer in me is trying to be freed. Nagging voice in the back of my head says I shouldn't have stopped building speakers over 20 years ago, but the kid was near death sick. Wadya suppose to do? He's a very healthy 21 year old today


Pssst, getting settled into your new digs yet?

Cheers,
Mike
__________________
When all your problems look like nails, then the only solution is a different hammer - MLH
  Reply With Quote
Old 15th January 2014, 02:44 PM   #7
diyAudio Member
 
Join Date: Feb 2009
Interesting, I had this same idea a couple of years ago when I started doing FR measurements. What is great is a new SW that automates the blending process. Thanks Dave for pointing here the document.

But I bring on a problem I have: what is the correct method of a near field measure if the driver has a phase plug instead of a dust cap.

Ralf
  Reply With Quote
Old 15th January 2014, 02:46 PM   #8
diyAudio Member
 
Robh3606's Avatar
 
Join Date: Nov 2002
Location: Destiny
Hello David

Thanks for starting this thread! I didn't know about this method and I am about to do some woofer measurements so this will be helpful.

Rob
__________________
"I could be arguing in my spare time"
  Reply With Quote
Old 15th January 2014, 02:52 PM   #9
diyAudio Member
 
speaker dave's Avatar
 
Join Date: Nov 2009
Location: The Mountain, Framingham
Quote:
Originally Posted by Greebster View Post

Pssst, getting settled into your new digs yet?

Cheers,
Mike
Still living in an extended stay place until the beginning of March. Bought a house last weekend.

Really enjoying the Bose environment. Lots of smart people here. I will try and get by with a superior level of bull$hit!

David
  Reply With Quote
Old 15th January 2014, 02:58 PM   #10
diyAudio Member
 
speaker dave's Avatar
 
Join Date: Nov 2009
Location: The Mountain, Framingham
Quote:
Originally Posted by giralfino View Post

But I bring on a problem I have: what is the correct method of a near field measure if the driver has a phase plug instead of a dust cap.

Ralf
Not sure about "correct" but I would just try a number of measurements near the center of the cone and see what the variation is. I think even over the phase plug a curve would look like the nearfield average, for the most part.

Nearfield curves are only accurate in the piston band and will also show cancelation nulls below the piston band from the arrival time of various concentric rings of radiation (see the Keele paper for curves on that).

Try a number of measurements and only trust them over the range where they are consistent. (And report back your findings!)

David
  Reply With Quote

Reply


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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
The Real Farfield Distance JMB Multi-Way 55 31st July 2012 03:04 AM
Farfield Calculation Solved (I think) with calculator JMB Multi-Way 3 30th June 2012 10:08 PM
headphone splicing help toontown Headphone Systems 1 1st August 2011 08:53 PM
splicing nearfield to farfield, does this look right? wintermute Multi-Way 110 6th April 2011 09:15 PM
Nearfield vice Farfield bbksv Multi-Way 16 6th October 2004 08:57 PM


New To Site? Need Help?

All times are GMT. The time now is 12:53 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2