vibration/octave band center frequency 3

[spike]

I have received a chart which is supposed to be a vibration reading. The y axis is labeled vibration velocity (mms), the x axis is labeled as "one twelfth octave band centre frequency (Hz)".
I do not have a clue as to how to interpret the x axis. I am used to this axis being labeled as Hertz or CPM.

Can someone help me out?

 

[IRstuff]

Why don't you ask the person who generated the chart?

TTFN

 

[spike]

Thanks for your reply TTFN. The problem is that I do not know who developed the chart. This chart apparently has been passed down from about five different parties before being sent to us, the OEM. We had been asked to generate a response by March 23.

 

[obg]

Look at it as bite of frequency as opposed to a single variable. Its just a way of simplifying the number of measurements. So treat it as Hertz.

 

[kontiki99]

I vaguely recall from undergrad classes in control systems, when we did Bode Plots we could use Octave or Decade scales in the X axis.

For each pole in the feedback system equation we would plot a change of 20 db per decade or 6 db per octave.

My memory is likely to be fuzzy here. I understood the reason for the two scales was because a lot of vibration work originated with sound and music.

There are 12 keys in every octave on a piano. That is counting white keys, sharps and flats.

On of my great entertainment pastimes as I past the age of 45 was taking up the piano and trying to understand the mathematics of what sounds good to us “naturally” and why.

 

[zappedagain]

Here is my take on it; I believe kontiki is on the right path:

"one twelfth... "

Musicians like to work in 12ths; i.e. 12 notes per octave. Every 12 lines (marks, grids) will represent an octave.

"octave band..."

An octave is a doubling of frequency; that's a relative measurement, so if there isn't a fixed frequency indicated anywhere on your graph, you won't get any fixed frequency information.

"centre frequency..."

I only think of a center frequency when I'm working with a bandpass filter. If that is what you have it could define your fixed frequency.

"(Hz)"

Uh, it isn't kilohertz!?

 

[fwcatr]

Spike,

The 12th octave bands comprising the x axis are in Hertz, or cycles per second.

1/12th octaves are usually used to get better definition of the vibration spectrum (better definition than 1/3 and 1/1 octaves) without using an FFT. If detailed FFT type frequency information is not needed, 1/12th octave bands are a good way to go.

Since the information is vibration velocity, I am assuming the data was generated using an accelerometer, and if it wasn't a charge amp type accelerometer, the usable frequency range will be limited, depending on the size of the accelerometer. You should be able to find the accelerometer type on the web page of the manufacturer to find the frequency range it should be used in.