DAQmx Output graphs.

[StavA]

Hello there,

First off, i wasn't sure where to post this question so if a moderator wants to move my post to the proper location, feel free to do so.

My question is quite simple. What are the units on the graphs below? I have searched online and offline thoroughly, and haven't found any useful sources or links to provide an absolutely certain answer.

Context and background: My colleague and I are trying to obtain the sound pressure level (SPL) to test speakers. We are using the PCI_4472. In the graphs below, we are using a 1kHz tone (hence the spike in graph 2 at 1000 Hz) to test. However, back to my original question: what are the units on the Y-axis of both graphs? I would have thought it to be Pascals, but the values don't really make sense.

The sine wave represents the measured signal from the microphone, and the second graph is the FFT of the signal.

Settings in MAX:

Max Level (dB): 100

Scaled Units: Pascals

Sensitivity: 48.3

Iex Source: Internal

Iex Value: 4m

Sensitivity Units: mV/Pa

dB Reference: 20u

If you require more information, I will happily oblige. Thanks for reading, and any insight that you might be able to provide.

[Bob_Schor]

Well, seeing the code would certainly help, as it would tell us what units you told the Analog Input card to use.  Note that the default unit, particularly if reading voltages, is "Volts", certainly not Pascals ...

Bob Schor

[StavA]

Here's a snippet of the code. I had to obtain permission before releasing anything.

[Case_in_Point]

Hey Stav,

You mentioned in your first post about MAX settings, so I'm assuming you have a task configured in MAX. In the snippet you posted, you don't have a task feeding into the Create Virtual Channel VI, so any settings you have in MAX aren't going to be applied to this task. You also have a couple of Global Channel constants thrown down that aren't wired to anything, was that intentional?

Cason

Applications Engr., NI

[StavA]

I was getting the same values as the task in MAX as i am with the currently wired setup. I just wanted to make sure i could replicate whatever i did in MAX, in Labview. So yes, to answer your question it is intentional. It is functionally equivalent to having a task wired in. I have included a cleaned up diagram.

Should I use the open circuit sensitivity from the manufacturer which is 48.3 mV/Pa or should I use 10, which is the Labview default?

We are exposing the microphone to a 94 dB 1kHz signal and we are measuring 200 mV (Using 48.3 mV/Pa sensitivity). That 200 mV doesn't corroborate with what SPL we should be getting when we input the numbers into the formula.

[Case_in_Point]

You definitely want to use your microphone's sensisitivty, the LabVIEW default is just that, a default. I confess I'm not at all experienced with sound measurements, but some quick Googling revealed that a 1kHz 94 dB-SPl tone corresponds to 1 Pascal, correct? That's what your initial graph was measuring, right? You said what you were reading didn't make sense if it were in Pascals, what were you expecting to read?