I should put together a "Getting Started" for Demo mode, it used to be covered in an earlier version of "getting started", but was lost in some later rewrites as the document grew and grew....you are right, using Demo mode for measurements isn't terribly clear.
You'll need to make some probes, which are essentially just resistors at the end of a shielded cable, connected to a 1/8" stereo mini-phone plug. To see a diagram of these, download and install PRAXIS, then use the Help menu to access the PRAXIS Help system. Once in Help, select the Contents tab, then double-click "Praxis Operating Modes", then "Demo Mode", then "Probes"
Then, to do low frequency impedance (or Thiele/Small) measurements on loudspeakers, you'll also need a receiver or integrated amplifier (not a bridged or tube type -- a "ground referenced output" type, designed so that the [-] terminal is tied to the amplifier input ground -- most home type amplifiers are this type) to drive the speaker. And a reference power resistor -- usually a 10 ohm resistor rated at 1/2 Watt or more (a 1/4W will work if you aren't going to be measuring for long periods). More discussion on this is available in PRAXIS Help,
(double-click "Praxis Operating Modes", then "Demo Mode", then "Thiele Small Measurements in Demo Mode").
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All of this info is also available, along with detailed instructions, if you run the Thiele/Small measurement Script -- use the "Information and Demo Scripts" button on the Main Form, then in the drop-down box select "demo Thiele Small Measurements", then "Launch".
The first time you run the script you will be taken instead to a "Probe Cal" script that will again show the diagram for the probes you need to make, and will take you through a calibration for the probes and your sound card. The cal is needed to account for varying amounts of gain and possibly different low frequency responses of the two channels on your sound card. The script will show you how to connect the probes and will automatically do the cal process --
after having you deal with the Windows mixers of your soundcard. The instructions for that are pretty wordy and maybe scary looking, since they try to cover lots of different ways soundcards and their mixers are arranged -- but essentially, you just need the recording function to use the
line inputs of your soundcard, and the play function to play only the
WAVE signal.
After the cal, you will also be asked about a "Ground Resistance" calibration. That is recommended for best accuracy (otherwise the wiring of your amplifier and cabling will get included in the measurement, which you don't want). But UNFORTUNATELY, I just ran it and now noticed that the Ground Resistance cal script has a
bug! -- for now (until I can fix it in the next update to PRAXIS), the easiest way around it is to, BEFORE YOU START THE SCRIPTS, use the Main From Menus: "Config"->"Preferences", and then arrange so that "Strict Script Running" is NOT checked for now. Then try the Ground Resistance script again. That will make the script try to continue running even if an error occurs, and will get you past this bug.
After you have finished the cal script(s), the Theiele Small script should run directly. It will guide you through connections, etc. One note: before using the "Extract" button, you should first click :"Free Air Model" (or "Mass Add Model" for a measurement made with clay weight on the cone). If all goes well, you should then get your results. The process is kind of fussy at first, but once you have it set up, it goes pretty smoothly.
The same gear should work for the LC meter script.
To make in-room RTA frequency response measurements in Demo mode, there is another script. Though, actually the analyzer in DiffMaker works better for that -- in DiffMaker, the pink noise isn't really noise but is instead a very dense set of precisely phased multitones that are keyed to the analyzer so that no averaging is required while measureing - results are there and stable within a few seconds. (Using real noise and the PRAXIS RTA requires you to average many acquisitions to get a repeatable result, since noise is only statistically predictable and you have to average for a while to find the average trends). I use the DiffMaker analyzer for setting up equalizers on subwoofers or for placing subwoofers in room, it is fast and actually better suited for that than is PRAXIS' RTA. ... I probably should add the DiffMaker-style "Analyzer" to PRAXIS.
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The only other measurements supported in PRAXIS' demo mode, impulse response and "Room Sound Convolution", are obtainable in a roundabout way (no script is provided for it, either). The measurement recording isn't actually done in PRAXIS, but with any wav recording software -- you could use the Windows "Sound Recorder", or DiffMaker's Recording tool, or various other recording programs for this. You make a CD from a file you can download from
http://libinst.com/praxis_downloads.htm#Convolution to use as the stimulus. Also get the pdf instructions from there.
The instructions are for a way to record the "Sound of a room" (including the loudspeakers in it) so that you can listen to that sound later, as if any music you later choose were being played in that room. The method is sometimes referred to as "auralization", (though I'm told that term for it is not strictly correct), used in acoustic design and analysis of rooms. But even if you aren't interested in "capturing the sound of a room", the process described includs a way to obtain a very clean and low-noise (about as good as they come) impulse response for a loudspeaker and room. From that impulse response, you can use PRAXIS' postprocessing to find the room characteristics (such as RTA, STI and Shroeder curve etc, using the "Shroeder Curve" postprocess) or if you use the "FFT" postprocess you can get the room frequency response, and if you window out the echoes, you can get the loudspeakers upper-range anechoic response (though there is other free software on the web now that can get you to loudspeaker frequency response more directly).
Hope some of that helps. Sorry about the bug(s) in the Demo Scripts, I hadn't tried them in a while, and it looks like a few later changes in the system had some side-effects in the Scripts that I need to clean up. (Or if you want to edit the script yourself to fix the "Ground Resistance" script, I can tell you what needs to change in the code.
Bill
