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Sound Level Meter AC / DC Response
This button defaults to AC, which means the Sound Level Meter ignores the 0th (DC) spectrum line when computing SPL. For the default sample rate of 48000 Hz, this means that all frequencies below 46.875 Hz (SampleRate / 1024) will also be ignored.
You should normally leave this button in the AC state. The reason is that the Sound Level Meter uses a microphone calibration (.CAL or .FRD) file to correct the actual microphone response to be equivalent to a "perfect" microphone with a flat response. If the calibration file shows a dip (negative dB entry) in a certain frequency region, the Sound Level Meter adds an equal but positive dB value to the spectrum at that region, thus flattening the response.
However, most real-world microphones (which you might be using to measure SPL) do not respond to DC or very low frequencies, so their calibration files typically have corresponding large negative dB entries there.
But the microphone output signal will still have some energy at these frequencies, just due to random noise in the circuitry. In a quiet environment this electrical noise may be comparable to the actual acoustic noise you are trying to measure. Erroneous SPL readings can arise if this DC and low-frequency noise is boosted, often by many 10s of dB, in a vain attempt to obtain a flat response.
AC mode ignores this region to give greater accuracy, sometimes much greater, without having to "trick" the system by modifying the calibration curve to show a flat response down to 0 Hz.
Note, however, that this action is only applied to the Sound Level Meter readings... it does not affect the Spectrum display, so you may still see an erroneous large DC peak there. You can still use the cursor readouts to read its apparent value, and still get an erroneous reading from the Sigma cursor readout if the left-hand cursor includes this region.
Alternatively, you can apply the Z-Weight.CRV weighting curve via the Weighting Curve button (just to the right of the AC / DC button, defaulting to 'Flat') which will open the Spectrum Curves dialog. Z-Weight will affect both the Sound Level Meter readings and the spectrum display. It will additionally remove spurious high frequency noise boost caused by large negative dB entries above 20000 Hz in the calibration curve. (Or above 40000 Hz if you use Z-Weight96.CRV.)
SPL_DC=1 sets DC mode; SPL_DC=0 sets AC, and SPL_DC=x toggles the existing mode.
See also Sound Level Meter
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