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:

 
Unit:SPL
 

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.

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, a microphone .CAL file may have:

 
Unit:SPL
Sens:134.5
 

That means that 134.5 dB SPL produces 1 VRMS output from the mic. But we know that 93.9794 dB SPL = 1 Pa, so that would be equivalent to:

 
Unit:Pa
Sens:40.5206
 

That means that to produce 1 VRMS from the mic, we need an RMS pressure that is 40.5206 dB above 1 Pa, which is 10^(40.5206 / 20) or 106.1769 Pa.

A typical .CAL file:

 
;B&K 4134 microphone calibration.
Unit:SPL
Sens:134.5

;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 (but must nevertheless be present). Curve files may be more appropriate for some applications.

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

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