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Phase of the Frequency Response Function - FRF - Vibration

Phase of the Frequency Response Function - FRF - Vibration

Phase of the Frequency Response Function - FRF - Vibration

I have difficulty to interpret the phase angle from my frf. I would appreciate if anyone can help me with this issue.

Attached is my FRF plot. Is it correct? and why is it noisy?
If the phase at any peak changes from -180 to 180 (not 90 degrees), is it then related to the natural frequency?

Thanks alot,

RE: Phase of the Frequency Response Function - FRF - Vibration

It might be correct, nobody can tell from a screenshot. The phase is just telling you the amount of sin and cos in each frequency line. Usually I would not look at the phase from a single channel, it's the phase difference between 2 channels that is important.

It is 'noisy' around 20 hz because there's no signal there.


Greg Locock

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RE: Phase of the Frequency Response Function - FRF - Vibration

I assume you are doing two channel measurements with accelerometer and hammer/shaker with load cell. Your Phase plot look Okay, since you are only interested in Phase at Natural Frequencies. There is a lot of tutorial information on Web. Here is a quick reference:


RE: Phase of the Frequency Response Function - FRF - Vibration

1. the FFT is performed over a specific time interval and characterizes that data including, that of the time window which introduces its own contribution.

2. by the way there is a danger in equating "amplitude" with FFT magnitude.

RE: Phase of the Frequency Response Function - FRF - Vibration

look at higest peak, where you have enough energy to calulate correctly phase.
Seems #2 resonance 12 and 37 Hz aprox. In 12Hz it is clear the change phase shift with 90degrees at resonant frequency.

In order to well understand if your measurement is correct it is usefull to use choerence function.

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