Data AcQuisition And Real-Time Analysis
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The following is from the Daqarta Help system:



Spectrum Analyzer

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Histogram (PSTH)

THD Meter

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Precision Phase Meter

Pulse Meter

Macro System

Multi-Trace Arrays

Trigger Controls


Spectral Peak Track

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Frequency response

Distortion measurement

Speech and music

Microphone calibration

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Sound Card Calibration (.CAL and .FRD) Files

Daqarta supports its own .CAL format, as well as the emerging industry-standard .FRD format.

Calibration (.CAL) Files:

A Daqarta .CAL file is a plain ASCII text file which can be created with Windows Notepad. It contains entries for the unit name and reference sensitivity, followed by a list of frequencies and dB difference from reference at each frequency. Entries must be in ascending frequency order, and should include an entry for 0 Hz as well as the maximum frequency you expect to use.

Daqarta uses linear interpolation to convert file entries to the dB correction for each spectral line at the active sample rate. If you don't include zero and maximum frequency entries, Daqarta will use simple extrapolation. That probably won't be as good as your estimate.

Comments may appear in the .CAL file on separate lines that start with a semicolon (;), or following a semicolon at the end of a data line. Blank lines are ignored.

After any initial comment lines, the first active line should start with 'Unit:' followed by the unit name, as in:


The name may be up to 4 characters long. Leading spaces are considered part of the name, so 'SPL' and ' SPL' are two different names. Trailing spaces are ignored. Case is preserved. In general, this name will appear in the Units Name control on the User Line dialog, and will be shown at the top of the Y axis and as a column head in .TXT output files.

However, two names are given special treatment: 'SPL' and 'Pa'. 'Pa' is the abbreviation for Pascal, which is a unit of pressure that is equivalent to 93.9794 dB SPL (Sound Pressure Level). If you use either of these names, Daqarta will convert between them as required. In waveform or linear Spectrum mode, the 'Pa' unit will be used, whereas 'SPL' will be used for the Y-log power spectrum as long as the Spectrum RMS button is active. (RMS is mandatory for SPL measurements.)

The next active line in the .CAL file is the 'Sens:' line, which gives the full-scale reference (0 dB) level for the response data to follow. Data sheet response curves for microphones typically give output versus frequency, relative to some baseline value which is labeled as 0 dB on the curve. The actual absolute value corresponding to 0 dB is given separately, and is typically called the "sensitivity".

There are numerous ways sensitivity may be given on data sheets, but the .CAL file expects this to be relative to 1 volt RMS. For a microphone whose 'Unit:' entry was 'SPL', this would be the SPL required to produce a 1 VRMS output from the mic. For an output device, it would be the SPL produced by the device for a 1 VRMS drive level.

In the more general case, the Sens value would be in dB relative to 1 V/Unit, where Unit could be Pa or any unit of your choice.

As an example, the manufacturer's specifications for a 1/2-inch condenser microphone may state that it has a sensitivity of 12.5 mV/Pa, equivalent to -38 dB relative to 1 Volt/Pa. (20 * log10(0.0125) = -38.06 dB)

The .CAL file could thus use:


That means that to produce 1 VRMS from the mic, we need an RMS pressure that is 38.06 dB above 1 Pa, which is 10^(38.06 / 20) or 79.98 Pa.

Caution: Note that the manufacturer stated sensitivity as the voltage output for a 1 Pa sound pressure. Since that is only 12.5 mV, the equivalent dB value is negative. But the CAL file Sens line is the number of Pa to produce 1 Volt, which is the same numerical value only positive.

Alternatively, the .CAL file could use:


Here the Sens value is found by noting that since it takes a sound that is 38.06 dB louder than 1 Pa to produce 1 Volt, and that 1 Pa is equivalent to 93.9794 dB SPL, then the dB SPL to produce 1 Volt is the sum of these or 132.04 dB SPL.

A typical .CAL file:

;B&K 4134 microphone calibration.

;Freq       dB
0          -60
5          -4.1
15         0
8000       0
15000      1.0
20000      0
50000      -12.5

Loudspeaker data sheets typically give the sensitivity as dB SPL at 1 meter on-axis for a 1 watt driving signal (often abbreviated as 'dB/W/m', which is a bit misleading). To convert to 1 VRMS you could use the relationship that

watts = volts^2 / ohms

and rearrange it to

volts = sqrt(watts * ohms).

But since watts = 1 here,

volts = sqrt(ohms).

Now you need to know the speaker impedance, which is typically 4 or 8 ohms "nominal". For an 8 ohm speaker, the square root would give 2.83 volts RMS, which is 9.04 dB above 1 VRMS. (See the dB section for computation details.) You would thus have to reduce all the manufacturer's dB values by this amount to convert to a 1 VRMS reference.

However, it is strongly recommended that you not rely on speaker manufacturer's curves as anything other than comparative reference data for purchase decisions. If you need to know the actual response from a particular speaker in your particular setup, you really need to calibrate it yourself.

Note that .CAL files are not restricted to microphones and speakers, nor even to conventional transducers. It is perfectly possible to have a .CAL file for an electrical filter network, in which case the Unit Name might be Volt. Of course, you could have .CAL files for current-to-voltage transducers or anything else you have in mind. All that is needed is a conversion between your units and volts, which is the native signal for the sound card.

Also, note that Curve files (with a .CRV extension) use the same format as .CAL files, except the 'Unit:' and 'Sens:' lines are ignored. Curve files may be more appropriate for some applications.

Frequency Response Data (.FRD) Files:

The .FRD file format is an emerging industry standard for calibrated microphones. It is similar to .CAL except that instead of just frequency and dB columns, there is a third column for phase. Comment lines are allowed only in the header before the actual data, and must start with an asterisk (*) instead of a semicolon.

The typical .FRD file supplied with most calibrated microphones also lacks the Unit and Sens lines required by Daqarta for absolute (SPL) display or use of the Sound Level Meter option. However, such an .FRD file can be used with Spectrum Curves just as it is if you only need a relative calibration, where you just want to see the shape of the frequency response without measuring absolute levels.

You can manually add the Unit and Sens lines with Windows Notepad or any text editor, and re-save the result. To keep the file compatible with other applications that use the .FRD format, use asterisks to make the Unit and Sens lines look like comments:


Daqarta will look past the asterisks to read the Unit and Sens values, while other applications will treat these lines as comments and ignore them.

Daqarta will ignore the phase data column.

If you perform your own microphone or other calibrations, you can save them as .FRD files with the Save Y-log Trace as .CAL, .FRD, .CRV, or .LIM File option in the File Menu. These .FRD files will have the Unit and Sens values included with asterisks, as above. The phase data will be all zeros.

See also Load Mic Cal File, Formulas For Working With Sound


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