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Sound Card Sound Level Meter
The Sound Level Meter is a dialog that can be invoked from the Options Menu or toggled open or closed directly via CTRL+L.
In order to use the Sound Level Meter to measure SPL, your system must first be calibrated. Auto-Calibration allows Daqarta to keep track of relative sensitivity when you change Input levels, and Full-Scale Range calibration gives an absolute voltage sensitivity for each Input line.
In addition, you must use a calibrated microphone. (A good but inexpensive choice is the Dayton Audio iMM-6, under $20.) The sensitivity of the unit is specified in the Units/Volt setting under the User Line Dialog for the Input line you are using, and Units Name must be set to 'Pa' (Pascals).
Normally, the frequency response of the microphone is specified via a calibration (.CAL or .FRD) file which includes Units and Sens entries. When loaded via the Load Mic Cal button of the User Line dialog, it sets everything automatically to enable SPL measurements.
When you activate the Sound Level Meter dialog, Daqarta automatically sets Y-log Spectrum mode and toggles User Units on, as needed. That process does not attempt to set a specific Input line, load a calibration file, or enable that line to use SPL. However, all of these states are retained across sessions, so you only need to set things up for SPL measurements once, and they will be correct as long as you don't change them.
If you later toggle any of these settings off while the Sound Level Meter is open, its display will change to show 'No Spect', 'No Y-log', 'SPL Off', or 'Uncal' until you restore the required mode.
In general, you should avoid any built-in sound card's microphone input line (and that of many USB cards as well), since internal Mic preamps are typically noisy and have poor frequency response. You may want to test yours by using a loopback cable and running a frequency response test. Otherwise, use a separate microphone preamp to drive the card's Line In connections.
Note that although the Voltmeter dB option can also read SPL, it does not include frequency response information from the calibration file, nor from weighting curves. It also uses only a simple single time constant scheme that is not automatically adjusted for true time in msec like the Response calculation of the Sound Level Meter. You may nevertheless find the Voltmeter useful as a second SPL channel in certain cases where you can compensate for these differences.
Besides measurements on live signals, you can also apply the Sound Level Meter to .DQA files created on a calibrated Daqarta system. You can use it with .WAV or other uncalibrated files by opening the User Line dialog and loading a calibration file, or by setting Units Name to "Pa" and setting the proper dB:Units/Volt, then toggling SPL on.
The Sound Level Meter dialog can be positioned by dragging its title bar in the usual Windows manner. The position and size will be maintained independently from other Daqarta dialogs, and will be saved across Daqarta sessions.
Unlike most Daqarta dialogs, the dialog size can be changed by dragging its borders. The numerical display font will be scaled proportional to the height of the dialog, allowing creation of extra-large displays for distant viewing. (On the initial use, you may need to adjust the size for your screen.)
Note: Larger characters take longer for Windows to draw on the screen. In Spectrogram or Pitch Track (Sgram/PT) modes with fast Trace Update rates and/or slow systems, this may be noticeable as slight gaps or discontinuities in the spectrogram or pitch track that repeat at 100 msec intervals (the meter update rate).
Note: To copy the Sound Level Meter displayed value to the Windows Clipboard, left-click anywhere in the central region of the meter display. This captures a copy of the current value and pops up a message box allowing you to save it to the Clipboard. CTRL+left-click saves it directly, without the message prompt. SHIFT+left-click includes the meter title above the meter value. CTRL+SHIFT+left-click includes the title and omits the prompt.
Note: The Sound Level Meter display update rate is approximately 10 times per second. A small asterisk is shown to the left of the Response Time button on alternating meter updates. This assures you that the meter is in fact updating, even if the reading is unchanged due to a very stable signal.
SPLdlg=1 opens the Sound Level Meter dialog, SPLdlg=0 closes it, and SPLdlg=x toggles between open and closed.
SPLdlg#m=1 sets the dialog to minimized if open. SPLdlg#m=0 restores it, and SPLdlg#m=x toggles between minimized and restored.
You can read the current minimized state via X=SPLdlg?m, which will set variable X to 1 if the dialog is currently open and minimized, else 0.
SPL is a read-only variable that returns the current SPL value. It behaves as a numeric variable if assigned to a macro variable or used in a math expression, but when used in a string expression returns the same string that is shown in the Sound Level Meter display. For example, if the display shows "48.0 SPL" (with Decimal Places set to 1) then Msg=SPL will display that identical string. But A=SPL followed by Msg=A may show "47.991", or by Msg=A(0.7) may show "47.9912567".
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