How to set PC microphone level for spectrum analysis?

[eepty]

I want to do some audio spectrum analysis on an appliance so I bought a Daytonaudio measurement USB microphone. I hope to use it with the REW to do the analysis.

I plugged the USB microphone to my computer and tested the "RTA" and "measure" function in REW. I think they are working and it did show the frequency domain graph with the waveform. However I am wondering how to set my PC recording device level for the correct measurement. It is because when I change the recording level in my computer, the amplitude of the waveform also change. Then how can I tell what is the sound level of the appliance in a particular frequency?

I am a newbie in this field, hopping you can help.

Thank you very much.

 

[John Mulcahy]

Did you load the calibration file for the microphone? That should contain a sensitivity figure for REW to use. If you then also make sure you have selected the mic as the input device and input REW will be able to compensate for the input volume setting.

 

[eepty]

Did you load the calibration file for the microphone? That should contain a sensitivity figure for REW to use. If you then also make sure you have selected the mic as the input device and input REW will be able to compensate for the input volume setting.

Yes I have loaded the calibration file for the microphone. In fact when I plug the microphone into my PC, and then open the REW, the REW will pop up a message notice me that I am using the UMM-6 USB microphone and it ask me to select the calibration file.

Is that mean I should not change the recording level in my PC after I opened the REW?

 

[John Mulcahy]

It shouldn't make any difference unless you set the level very, very low. If you play a test signal, such as Pink PN, the SPL figure on REW's meter should remain consistent as input volume is changed. A volume setting of 70% should correspond to no overall gain or attenuation within the mic (for Windows 10) which is typically a good setting to use.

 

[eepty]

It shouldn't make any difference unless you set the level very, very low. If you play a test signal, such as Pink PN, the SPL figure on REW's meter should remain consistent as input volume is changed. A volume setting of 70% should correspond to no overall gain or attenuation within the mic (for Windows 10) which is typically a good setting to use.

Thank you. In my computer it seems that it does affect the level. I did a test: I switch on a noisy fan, and testing it with the microphone and the SPL meter in the REW software. When I change the mic level (in Windows 10) to 70%, the result is about 65dB(A)S, but when I change it to 100%, the result is about 75dB(A)S.

Now I try use an external SPL meter to set the computer recording level, but I do not know whether this approach is correct or not.

 

[John Mulcahy]

Does your calibration file contain a sensitivity figure?

 

[eepty]

Does your calibration file contain a sensitivity figure?

I am not sure, I attached the calibration file here.

 

[John Mulcahy]

The calibration file looks correct, and it has a sensitivity figure. Have you loaded the file in REW? If you look in the Mic/Meter preferences is the calibration file shown there?

If you right click on the Windows input volume control you can change it between percentage and decibels. What is the decibels figure when the input volume is 70%, and what is when the input volume is 100%?