dB in vibration PSD specification 3

[elogesh]

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

 

[GregLocock]

Yes, you have it right. 6 dB/octave roll off is a typical first order response.

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.

 

[elogesh]

Hi,

Thanks Greglocock.

I got it.

Regards,
Logesh.E

 

[tomirvine]

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

http://www.vibrationdata.com

 

[elogesh]

Hi Tom,

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