Hello. I am working on Electrical output of a microphone. So far I can acquire my analog signal using DAq Assistant NI USB 6009 and view my waveform graph but the plot is in time vs amplitude(V) unit. I want to plot it in frequency vs decibels unit. Also the amplitude of the acquired electrical output of a microphone is in 0.000xx range for audible signal. I don't get clear understanding why the range is very low. I am grateful if anyone can help me to get a clear knowledge of these. Thank you.
The microphone will probably output a signal in the millivolt range, because it is a passive device, with probably no built-in amplification.
If your USB device is configured to read a signal in the +-10V range, then resolution will be very low. Configure your device to a lower input range, while also checking that your acquired voltage doesn't exceeds the range.
Keep in mind, that your DAQ's input impedance is relatively low (like 150kOhm), which may cause the the microphone to start draining current, which will limit the voltage that your A/D converter sees. Consult the output impedance of your mic' as well.
The DAQ Assistant Express VI can be found on the Functions palette by going to Express » Input. Place the DAQ Assistant on the block diagram by dragging and dropping it from the Functions palette. The Assistant should automatically launch when you drop the VI on the diagram.
Are you still working on this? Lysandros offered some practical advice to consider when acquiring from passive acoustic sensors. If you still need help converting acquired voltages to sound pressure level, try the Sound Level Meter (DAQmx) example VI installed with the Sound and Vibration Toolkit which has a free evaluation. You may also want to consider using the DSA Soft Front Panel to acquire, perform FFT, visualize SPL (dB) versus frequency, and log waveform data to file.