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# Acoustic FRF units with Q-Source?

Experimenter

I am using the Q-MED volume velocity acoustic source and MIMO FRF Testing to generate acoustic FRF’s. However, the sensitivity provided for the Q-source has units of mV/m^3 and therefore my measurements have units of Pa/m^3. How can I get the more common unit for acoustic FRF’s of (Pa/m^3/s^2) ?

3 REPLIES 3

# Re: Acoustic FRF units with Q-Source?

Siemens Phenom

Hello, this is correct.  The Q-MED is a volume displacement sensor.

Also, if the sensitivity is negative make sure that you reflect that.  Either enter the sensitivity as negative or add the Polarity field to Channel Setup and set that to negative.

I guess your next question will be, what to do with the FRF?   The FRF wil be Pa/m^3.  I believe if you integrate that you will get Pa/(m^3/s) and if you double integrate you will get Pa/(m^3/s^2).  I have not used that exciter myself, but that seems to make sense to me if you want Pressure over volume velocity or volume acceleration..  If not, I am sure someone will correct me.

# Re: Acoustic FRF units with Q-Source?

Experimenter

Thanks for the clarification Kevin and the important tip about the sensitivity and polarity !!

# Re: Acoustic FRF units with Q-Source?

Siemens Valued Contributor

AntMoon I agree with Kevin….

The double integration of data with units of (Pa/m^3) will indeed lead to FRF data with units of (Pa/(m^3/s^2)).

Additionally, I wanted mention a couple of easy ways perform the double intergation. One method simply changes the units in the display and another actually creates a new curve with your desired units. I have listed both methods below so can choose whichever one meets your needs.

• Start Desktop module ( Windows 7 : Start -> all programs-> Sim Center Testlab 18-> Desktop Standard & Advance)
• Go to Navigator worksheet and browse to your FRF measurements

• Create a FrontBack display by clicking on the icon next to “Create a Picture…” button near the top middle of the page. Next single click on FRF data and drag into the FrontBack display.

• From here there are two methods
• Method 1- Changing Units in Display

• Method 2 -  Creating a new data set with the new units
• Select the FRF measurement by either clicking on the curve or the selection state in the curve legend.

• Next click on integration button ( ∫ ) in the top row the tool bar

• Using the “Integration Settings” box set the number of times equal to 2 then press the “OK” button.

• An FRF with units of Pa/(m^3/s^2) will be the result displayed in the FrontBack display window and the results will be save in a folder labeled Conditioning.