dB in vibration PSD specification
dB in vibration PSD specification
(OP)
Hi,
Recently I received PSD data input.
In that PSD input 5 to 20 Hz, some constant PSD input was specified. Then between 20 to 200 Hz, the input was mentioned as negative slope of 6dB per octave.
To my understanding 6 dB per octave indicates the vibration level will reduce half per doubling of frequency (Octave).
Say at 20 Hz, the PSD level is 'x'. Then at 40 Hz it is 'x/2' as per 6dB per octave concept.Whether am I right?
If not pl. correct me.
Is there any specific reason to charactize the input with slope of dB per octave.
Thanks for going through this thread.
Regards,
Logesh.E
Recently I received PSD data input.
In that PSD input 5 to 20 Hz, some constant PSD input was specified. Then between 20 to 200 Hz, the input was mentioned as negative slope of 6dB per octave.
To my understanding 6 dB per octave indicates the vibration level will reduce half per doubling of frequency (Octave).
Say at 20 Hz, the PSD level is 'x'. Then at 40 Hz it is 'x/2' as per 6dB per octave concept.Whether am I right?
If not pl. correct me.
Is there any specific reason to charactize the input with slope of dB per octave.
Thanks for going through this thread.
Regards,
Logesh.E





RE: dB in vibration PSD specification
I think you'll find it gives constant energy per octave as well.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: dB in vibration PSD specification
Thanks Greglocock.
I got it.
Regards,
Logesh.E
RE: dB in vibration PSD specification
First you need to determine whether you are dealing with units of G or G^2/Hz.
A 6 dB increase doubles the G value, but quadruples the G^2/Hz value.
I have posted a conversion utility at:
http://www.vibrationdata.com/dboct.exe
Tom Irvine
www.vibrationdata.com
RE: dB in vibration PSD specification
Thanks for posting the conversion utility.
Based on the conversion utility,the PSD value reduces quadrupally.
Earlier, I calculated the conversion based on the following formula,
dB = 20 log (X/Xref)
With Xref = PSD value in G^2/Hz. Therfore I thought X = 2*Xref (For positive slope of 6dB per octave).
whether the above formula is right!
If not, Can you specify the formula!
With Thanks and Regards,
Loganathan.E