FFT and Windowing - 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 30th August 2013, 01:04 PM   #1
JMB is offline JMB  United States
diyAudio Member
 
Join Date: Apr 2004
Location: Texas
Default FFT and Windowing

I was hoping to open up a discussion amongst you EE types to expound upon the advantages and disadvantages in different windows in different situations. Would anyone be kind enough to explain this in light of frequency response, THD and IMD measurements, burst measurements, and CSD? John K had advised to just stick with Blackmann Harris for SE while the ARTA manual suggests different windows for different circumstances. thnx.
  Reply With Quote
Old 30th August 2013, 01:29 PM   #2
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Surprisingly, the Wikipedia article is pretty comprehensive (though it leaves out my beloved H-G window):
Window function - Wikipedia, the free encyclopedia

National Instruments has a very clear tutorial:
Windowing: Optimizing FFTs Using Window Functions - National Instruments

Note the summary in Figure 8, which I think is exactly what you're looking for.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 30th August 2013, 01:56 PM   #3
Elias is offline Elias  Finland
diyAudio Member
 
Elias's Avatar
 
Join Date: May 2003
Location: Where you live
Very surprisingly, they also have window function and accompanied Fourier transform for Taj Mahal:

Click the image to open in full size.


Click the image to open in full size.
__________________
Liberate yourself from the illusion of two speaker stereo triangle
Dipole Bass vs Monopole Bass Stereophonic Sound from a Single Loudspeaker 3 Speaker Linear Stereo Matrix Wavelets
  Reply With Quote
Old 30th August 2013, 05:37 PM   #4
JMB is offline JMB  United States
diyAudio Member
 
Join Date: Apr 2004
Location: Texas
Thanks, SY. I had seen those links and several others. I am not able to make the leap from leakage, side lobes, etc. to which works best for what function in loudspeaker measurement. An impulse windowing in loudspeaker measurement for dummies would be great! thnx
  Reply With Quote
Old 30th August 2013, 05:56 PM   #5
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
What are you trying to measure and how? Frequency response is one thing, distortion is another. Impulse vs. MLS will also influence your choice.

Blackman-Harris is a good default choice if you're not sure.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 30th August 2013, 06:09 PM   #6
JMB is offline JMB  United States
diyAudio Member
 
Join Date: Apr 2004
Location: Texas
First an impulse response is obtained. I then process it with either Soundeasy or ARTA to derive frequency response, CSD, Tone Burst, etc. IMD and THD both use their own signals to obtain their results. IMD uses two separate frequencies to measure while THD uses Stepped Sine, I believe, in SE and I believe it uses swept sine in ARTA.
  Reply With Quote
Old 30th August 2013, 06:41 PM   #7
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
For the frequency response measurements, B-H is likely your best bet.

For IMD and THD, use a flat-top, or if that's not an option, a Hanning. You'll get some side lobes which you should ignore- you're interested in the ratios of the peak values of the fundamental and the harmonics.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 30th August 2013, 09:29 PM   #8
JMB is offline JMB  United States
diyAudio Member
 
Join Date: Apr 2004
Location: Texas
Thanks, SY. Any chance that you might explain the why of this? I can see the differences in side lobes and in the rate of decent from the peak of the signal under each window but I am trying to understand where there is benefit to each window and why.

I guess to be more specific, why is B-H better than Hanning or flat top, etc for FR? Why Flat top or Hanning and not Uniform for IMD or THD? What about each window profile makes it preferable to the others? Thanks in advance. Jay
  Reply With Quote
Old 30th August 2013, 10:55 PM   #9
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
The "why" is mathematical. The practicalities are that, in choosing a window or apodization, you're trading freedom from skirts or spreading in the frequency domain for accuracy of peak height. Peak height accuracy is what you want in distortion measurement since distortion percentage is defined by the ratios of the harmonics and intermod products to the fundamental(s); the spreading isn't very critical.

There's lots of different measurement situations, so there's lots of tools! For example, if I wanted to resolve sidebands from power supply intermodulation or two closely spaced tones, I'd want to choose an apodization that had the least spreading so they wouldn't get buried in the mathematical noise around the test frequency.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 30th August 2013, 11:10 PM   #10
JMB is offline JMB  United States
diyAudio Member
 
Join Date: Apr 2004
Location: Texas
Thanks for your patience and for your explanation. For FR you want the most accurate frequency data; would this be reflected by the lowest side lobes, greatest side lobe roll off (I guess reflected by the -3dB bandwidth), or a combination of the two (or am I way off base here)?
  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
FFT, now even faster benb Everything Else 9 19th January 2012 05:21 AM
FFT Analyzer Radioman62 Parts 0 1st April 2009 08:13 PM
Waterfall windowing problems elvischan917 Multi-Way 3 7th October 2008 09:54 AM
what is an FFT graph? keantoken Everything Else 23 10th September 2006 12:13 AM
FFT Analysis RobPhill33 Everything Else 22 19th March 2003 07:01 PM


New To Site? Need Help?

All times are GMT. The time now is 12:10 AM.


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