tuning forks for pure sine waves? - diyAudio
Go Back   Home > Forums > General Interest > Everything Else

Everything Else Anything related to audio / video / electronics etc) BUT remember- we have many new forums where your thread may now fit! .... Parts, Equipment & Tools, Construction Tips, Software Tools......

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 September 2008, 02:16 AM   #1
cuibono is offline cuibono  United States
diyAudio Member
 
cuibono's Avatar
 
Join Date: May 2006
Location: City of Angles
Default tuning forks for pure sine waves?

How could someone (easily) produce pure acoustic sine waves? With tuning forks? What about at a continuous SPL?

I'm wondering how one tests distortion levels for microphones...


  Reply With Quote
Old 15th September 2008, 03:49 AM   #2
diyAudio Member
 
Richard Ellis's Avatar
 
Join Date: Sep 2006
Location: Mar del Plata, a BIG seasonal getaway city, can see the Ocean from our residence.
A/F generator
_______________________________________Rick....... ....
  Reply With Quote
Old 15th September 2008, 08:42 PM   #3
cuibono is offline cuibono  United States
diyAudio Member
 
cuibono's Avatar
 
Join Date: May 2006
Location: City of Angles
sorry, I guess I wasn't clear. I'm excluding sound produced by regular loudspeaker drivers - their distortion levels are generally higher than microphones, making them useless for distortion testing, afaik. So I'm looking for an acoustic source of non-distorted sine waves, not generated by loudspeakers...
  Reply With Quote
Old 16th September 2008, 03:45 AM   #4
Pano is offline Pano  United States
diyAudio Moderator
 
Pano's Avatar
 
Join Date: Oct 2004
Location: Milliways
Blog Entries: 4
Helmholtz used organ pipes, didn't he?
__________________
Take the Speaker Voltage Test!
  Reply With Quote
Old 16th September 2008, 04:56 AM   #5
diyAudio Moderator Emeritus
 
Iain McNeill's Avatar
 
Join Date: Oct 2007
Location: Santa Cruz, California
I think B&K calibrates microphones using another microphone. They are reciprocal devices. There's a ton of great stuff on the B&K site and I think they cover what you're asking. Been a long time since I trolled this stuff.

http://www.bksv.com/Library/Technical%20Reviews.aspx

Technical Review 1998-1 Danish Primary Laboratory of Acoustics, Microphone Reciprocity Calibration, Calculation Program for Reciprocity, Calibration

A tuning fork has two vibrating elements and will produce very strange results depending on the accuracy of the two arms frequency and the angle of measurement - not good at all.

Of all the instruments, I think the flute is the purest so maybe Helmholtz was onto something
  Reply With Quote
Old 16th September 2008, 08:14 AM   #6
diyAudio Member
 
Join Date: May 2002
Location: Switzerland
Omnidirectional condenser microphones (like all those famous BKs) are usually calibrated via an electric field (consisting of the AC test signal plus a bias voltage) applied by an electrode in front of the diaphragm. So you do in fact have a directly coupled ESL/microphone combination. There won't be any method (at least not in the universe as I know it) that is more accurate than this one. Due to the usual directionality issues - that are cancelled out this way - there must be some calcualtions done on the measurement results in order to get the actual free-field response afterwards.

Regards

Charles
  Reply With Quote
Old 16th September 2008, 09:03 AM   #7
diyAudio Member
 
Frank Berry's Avatar
 
Join Date: Sep 2003
Location: Midland, Michigan
From Wikipedia:

A tuning fork is an acoustic resonator in the form of a two-pronged fork with the tines formed from a U-shaped bar of elastic metal (usually steel). It resonates at a specific constant pitch when set vibrating by striking it against a surface or with an object, and emits a pure musical tone after waiting a moment to allow some high overtones to die out. The pitch that a particular tuning fork generates depends on the length of the two prongs.

Not sure how "pure" the waveform would be. Might be a good idea to record the waveform produced by a tuning fork and looking at it on a scope or on audio editing (Adobe Audition) software.
__________________
Frank
  Reply With Quote
Old 16th September 2008, 03:38 PM   #8
cuibono is offline cuibono  United States
diyAudio Member
 
cuibono's Avatar
 
Join Date: May 2006
Location: City of Angles
Quote:
Originally posted by phase_accurate
Omnidirectional condenser microphones (like all those famous BKs) are usually calibrated via an electric field (consisting of the AC test signal plus a bias voltage) applied by an electrode in front of the diaphragm.

Thanks, I just saw that. I guess it works only for pressure sensitive mics (omnis), and that cardioid mics would then have to reference these mics.

I've been looking into distortion levels for mics, and found an interesting article on Linkwitz's modified wm61a electret capsule, published by audioXpress - pdf is here.

The panasonic capsule is compared with a B&K reference mic, with pretty decent results, about .5% THD at 125dB SPL. My next question is how this compared with studio mics, and apparently its about average, with small diaphragm condensers having somewhat less distortion, and large diaphragms a little more. Not bad, sounds like the panasonic capsules can be used for reference.

The next thing I wonder is what the THD levels are at lower SPLs. I read something saying you could extrapolate, for each 6dB drop in SPL, you'd expect half the THD. For the modified panasonic capsule, at 95dB, that implies about .02 %THD, or about -74dB down. Not bad, but I wish it were better - some drivers have distortion around this level, which might make measuring them problematic. Sure show the primary limitations on fidelity are the transducers (loudspeaker drivers, headphones, microphones)....
  Reply With Quote
Old 16th September 2008, 04:15 PM   #9
diyAudio Moderator Emeritus
 
Iain McNeill's Avatar
 
Join Date: Oct 2007
Location: Santa Cruz, California
At low levels the limiting factor is noise: room ambient, FET 1/f, resistor etc. Its hard to get more than 60dB SNR in an acoustic environment.

That's what you pay for with an expensive mic - good sensitivity and low noise. The Panasonics have very good response but generate more noise than your lab mic.
  Reply With Quote
Old 16th September 2008, 04:44 PM   #10
jcx is offline jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
loudspeakers could be acoustic filtered as mentioned above, indirect measurement of microphone IMD products in the sound feilds of 2 speakers driven with different frequencies could avoid the harmonics in each speaker's output

for a single sine drive the 2nd harmonic would decrease ~ with level, n-th higher order harmonic percentage are expected to decline as n-1 power

and as usual, 2nd harmonic distortion isn't expected to be audible at even 3% of a sine wave, but 2nd order nonlinearity generates IMD difference products with complex waveforms that can be lower in frequency than the excitation and therefore lie on the steep "front" slope of the masking curve and should be much more readily audible

this points to higher harmonics but shows a masking curve:

http://www.listeninc.com/files/pdf/Rub&Buzz.PDF


of course Klippel is the expert on dynamic speaker distortion:

http://www.klippel.de/pubs/papers.asp


and Geddes (who is active here) has studied nonlinear distortion audibility and speakers for a while now and his current position seems to be that loudspeaker driver nonlinear distortion is seldom a problem and most effort shoud be put into the "linear" part of loudspeaker/room design such as frequency response/directivity/early reflection/diffraction control

http://www.gedlee.com/
  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
SMPS to Pure Sine EUVL Power Supplies 2 11th June 2009 04:24 PM
FS:Pure Sine Inverter 300w rubydac Swap Meet 0 25th April 2008 04:39 AM
sine waves on nos dac regal Digital Source 5 11th March 2007 09:38 PM
Sine and square waves percieved pitch, why? baggystevo82 Everything Else 33 11th March 2005 04:18 AM
Pure sine wave? PeteMcK Multi-Way 12 4th July 2004 06:31 AM


New To Site? Need Help?

All times are GMT. The time now is 01:45 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