Sound Card Spectrum RMS Toggle

Controls: Spectrum Dialog >> RMS
Macro: RMS

You can toggle RMS (Root Mean Square) mode from the Spectrum dialog button, or directly via CTRL+ALT+R even when the dialog is not visible.

The RMS method is useful in engineering work because it allows comparisons of different waveforms based upon their equivalent energy. When the waveforms are all sinusoids (like the individual components of a line spectrum), then this is not so important. However, RMS is the standard method for reporting magnitudes, so you will often need this for compatibility.

Standard laboratory devices like AC voltmeters are calibrated to show RMS, so showing the Daqarta Spectrum in RMS mode makes comparisons simpler.

When linear (as opposed to Y-log power) Spectrum is active, RMS mode will cause the trace and readouts to be multiplied by 0.707107 to convert sine wave amplitude to RMS. The Y-axis header will change from mV to mV RMS (or whatever calibration and User Uunits are in effect.)

When Spectrum Y-log mode is active, the display and readouts are shifted down by 3.0103 dB, which is that same factor (half the square root of 2) converted to dB. 20 * log10(0.707107) = -3.0103. The Y-axis will change from dB V FS to dB V RMS. With User Units, it will change from dB:Pa to dB:Pa RMS.

If Y-log is active and the SPL button is toggled on in the User Line dialog, either manually or by loading a calibration (.CAL) file that specifies it, then the display switches to dB SPL. Since SPL requires RMS calculations by definition, the state of the RMS button here is ignored.

The RMS button state is likewise ignored when using the Sound Level Meter, or when using the Voltmeter to read SPL in dB mode.

Macro Notes:

RMS=1 sets Spectrum RMS mode, RMS=0 removes it, and RMS=x toggles between states.

See also Spectrum Control Dialog