Oscilloscope Trigger threshold units?
edited December 2021 in Faber Acoustical Products for Mac
Does anyone know what units the trigger threshold for the Oscilloscope trigger are? There's none shown on the menu. Is it the same as the y-axis on the OScope screen? I'm struggling to get the 'Single' trigger level right for room acoustics measurements (using an impulsive source), without the i/p signal clipping (I'm using a MiniDSP UMIK-2 mic).
The units for the trigger threshold match the units of the input channel(s). So, if you are using a UMIK-2, and you have your channel units set to Pa (pascals), then the trigger threshold will be in pascals (and, yes, it will correspond to the Y axis scale in the Oscope).
For reference, a threshold of 1 pascal would correspond to a nominal instantaneous sound level of 94 dB, 2 pascals would be 100 dB and 0.5 pascals would be 88 dB.
OK, thanks @benfaber . I've also noticed that the input gain seems to adjust automatically. I know there is an auto setting for the display range but does this also control the input gain? It make it difficult to set a consistent trigger level if the input gain is changing.
My fundamental problem is that when I do RT measurements using a pistol as a source, the impulse response appears to be clipped even when the input gain is -40dB. It is not shown as clipping when in SLM mode and the Lp is only measuring 30 - 40 dBLAmax. When I do the same measurement in the same space with a balloon as a source, which has a lower Lp, the IR seems to be fine. I'm concerned that the clipped IR will affect the starting point of the regression line and therefore the calculated RT.
SignalScope does not automatically adjust the gain (except to synchronize the two input channels of the UMIK-2 when you manually adjust either of them). It does listen for changes in the gain made in other apps (such as Apple's Audio MIDI Setup), so you might want to be sure that no other app is changing it.
The gains on the UMIK-2 units I have tested are purely digital. This means that you cannot get a higher measurement range by lowering the gain, so I would recommend leaving all the gains at 0 dB.
One of the units I tested has an acoustic clipping point of about 112.7 dB, which isn't very high.
The UMIK_2 claims a max SPL of 125 dB. Was your measurement of 112.7 dB definitely flat, or was it A-weighted?
I'm only using pistol shots in large rooms so I don't expect the SPL is that high, but I will try and set up an A-B comparison test with another mic and see if I'm overloading the mic. Thanks @benfaber
The measurement was unweighted. The mic was placed in a Larson Davis CAL200 calibrator with the calibration level set to 114 dB. The clipping was so clean it appears to be digital clipping. So, the microphone capsule may accommodate sound levels as high as 125, but the UMIK-2 appears to clip the signal digitally before it reaches the level of the calibrator at 114 dB.
Lowering the input gain of the UMIK-2 did not affect the clipping point, as measured in pascals, which suggests that the signal was clipped prior to the gain stage. It's possible that this unit's microphone element is overly sensitive relative to the input range of its ADC (analog-to-digital converter).
OK, thanks. I'll also try calibrating against a 114dB signal instead of the 94 dB I usually use and see what happens.