fft (Speaker Workshop) - 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 27th June 2004, 10:57 PM   #1
diyAudio Member
 
Join Date: Jul 2003
Location: Silicon Valley
Default fft (Speaker Workshop)

Attached is a smoothed FFT plot done by Speaker Workshop. It's the response of a stereo pair of Locust I's at about two meters, in my office. The subwoofer is muted.

It looks reasonable down to 30 Hz or so. It starts rolling off around 70 Hz, as Martin's worksheet says it should. It does not roll of as quickly as predicted, being down 6 dB around 50 Hz rather than 63 or whatever. Maybe that's because of room lift in the small office. Otherwise the rolloff curve looks plausible. But what's going on below 30 Hz? I feel pretty sure those little speakers are not actually functioning at those frequencies.

I don't know how to set the FFT parameters in Speaker Workshop. For this plot, I checked the "Use entire range" button, and used uniform windowing.
Attached Images
File Type: png no-sub.png (7.3 KB, 141 views)
__________________
Davy Jones
  Reply With Quote
Old 28th June 2004, 01:07 AM   #2
diyAudio Member
 
Join Date: May 2004
Location: San Francisco
Send a message via AIM to joe carrow
Default Ideas

Quote:
Originally posted by Dave Jones
Attached is a smoothed FFT plot done by Speaker Workshop. It's the response of a stereo pair of Locust I's at about two meters, in my office. The subwoofer is muted.

It looks reasonable down to 30 Hz or so. It starts rolling off around 70 Hz, as Martin's worksheet says it should. It does not roll of as quickly as predicted, being down 6 dB around 50 Hz rather than 63 or whatever. Maybe that's because of room lift in the small office. Otherwise the rolloff curve looks plausible. But what's going on below 30 Hz? I feel pretty sure those little speakers are not actually functioning at those frequencies.

I don't know how to set the FFT parameters in Speaker Workshop. For this plot, I checked the "Use entire range" button, and used uniform windowing.

I've got a few ideas for possible sources of error;

First, (collecting data) What microphone are you using? A lot of microphones don't work right for extreme low frequencies.

Second, (processing) What's the sampling rate/bit depth? I've noticed with some software I have used that the results of a FFT in the low frequencies changes based on sampling rate and/or bit depth. If this is a software problem, it would be best to post on the forum for Speaker Workshop. It looks like they have a good forum over there.

Third, what if the data is right? How large is your office? Perhaps the speakers are setting up some sort of resonance; or perhaps its a subsonic rumble from passing trucks. If there are low frequencies present and the software is accurately processing them, then the next thing to check would be the environment.

Those are just my guesses. Good luck!

Joe Carrow
  Reply With Quote
Old 28th June 2004, 10:51 AM   #3
diyAudio Member
 
mikee12345's Avatar
 
Join Date: Nov 2002
Location: NZ
I agree. the information below 50hz seems erroneous

Search the local forums for SW help

www.audioasylum.com has some,use its powerful search utility

Take them outside and compare( if possible )
  Reply With Quote
Old 28th June 2004, 01:58 PM   #4
Ron E is offline Ron E  United States
diyAudio Member
 
Ron E's Avatar
 
Join Date: Jun 2002
Location: USA, MN
It's spurious errors caused by the inaccuracy of your soundcard. You will notice with most soundcards below ~20Hz or so, depending on how soon they start to roll off, that there are a bunch of spikes in measurements.

Ignore them, or get a better soundcard. Ithink SBlive will frequently get down to 10Hz without big errors like this, but all soundcards I have used (onboard or cheapie PCI) have had problems below 20-30Hz.
__________________
Our species needs, and deserves, a citizenry with minds wide awake and a basic understanding of how the world works. --Carl Sagan
Armaments, universal debt, and planned obsolescence--those are the three pillars of Western prosperity. Aldous Huxley
  Reply With Quote
Old 28th June 2004, 02:05 PM   #5
johnnyx is offline johnnyx  United Kingdom
diyAudio Member
 
Join Date: Jan 2004
Location: manchester
I don't use Speaker Workshop, but I have noticed some peculiarities when measuring. The microphone output from low frequency wind noise can be much greater than the signal amplitude. Look at the mic preamp output with a 'scope and you'll see the signal waveform bounce off screen. Move your arm towards the keyboard and it bounces again. I built a mic preamp for the electret insert and included a 7Hz 4th order high pass filter, which removed the bounce.

Soundcard measuring systems have a similar lf response, but even if the fft filters the low frequencies, the amplitude may be enough to clip the inputs. When the mic output is sampled, how does the fft know if the voltage is the signal picked up from the speaker, or the draught created by your body movements?

When I've auto re-measured the response, at 1 to 2 second intervals, the higher frequency response doesn't change, but the lf response changes with each aquisition.

So maybe you should keep still and out of draughts when measuring.

Hope this helps.
  Reply With Quote
Old 28th June 2004, 06:20 PM   #6
diyAudio Member
 
Join Date: Jul 2003
Location: Silicon Valley
The sound card is an Audigy II. I replaced an older SoundBlaster with it, because people told me the Audigy II was better.

The mic is the one that came with an Audio Control Richter Scale III. I do not have a calibration file for it. I'm running it through a Rolls "Mini-Mic" preamp MP13.
__________________
Davy Jones
  Reply With Quote
Old 28th June 2004, 11:49 PM   #7
Ron E is offline Ron E  United States
diyAudio Member
 
Ron E's Avatar
 
Join Date: Jun 2002
Location: USA, MN
Do your impedance measurements show spurious peaks at low frequencies as well?

IF not, it may not be the probem I describe. My measurements often show peaks at 5Hz that are higher than the resonance peak.

Perhaps it is aliasing? Try the fastest sample rate if you haven't already, or try several sample rates. I know the SB Live worked best at 48kHz because it resampled all other sample rates to that rate internally.....
__________________
Our species needs, and deserves, a citizenry with minds wide awake and a basic understanding of how the world works. --Carl Sagan
Armaments, universal debt, and planned obsolescence--those are the three pillars of Western prosperity. Aldous Huxley
  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
Speaker workshop ? A_Reef_Scene Multi-Way 14 28th February 2007 12:39 AM
My Speaker Workshop Rig bigmike216 Multi-Way 0 28th November 2004 01:24 AM
Speaker Workshop: the best and how to use it? rick57 Multi-Way 12 13th August 2004 11:12 PM
Which mic for speaker workshop? azira Multi-Way 2 12th February 2004 05:12 PM
Speaker workshop Ilianh Multi-Way 2 10th January 2003 09:11 PM


New To Site? Need Help?

All times are GMT. The time now is 02:40 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