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The setup is quite simple, the following diagram is taken from the LIMP manual:
https://artalabs.hr/download/LIMP-user-manual.pdf
Measurement Jig
This instruction is following what is in the LIMP manual linked above, which I would also suggest reading through for more detailed information.
- Before you start measuring with LIMP, there is a basic calibration process that should be completed. First, go to setup -> audio devices and make sure LIMP is configured to use the your audio interface, and preferably with the ASIO driver. For example, on my USB interface it says "Yamaha Steinberg USB ASIO". Make sure that "loopback" remains unchecked at all times in the driver settings. If your USB interface includes a “mix” knob, it should always be fully clockwise to the "DAW" or “playback” to ensure that none of the input signal is mixed with the audio output from the computer. I usually start with each input gain knob at the 12 o'clock position and adjust accordingly from there, if needed.
- Go to setup -> measurement. Here you enter the reference channel, the resistor value, sample rate, FFT size, averaging, etc. Set the resistor value to the specific resistance as measured with your multimeter, sample rate of 96kHz, and the other options as desired.
Measurement Settings - Press the "CAL" button in LIMP. This calibration process will adjust for the difference in gain between left and right input channels. Anytime the gain knobs are touched, or if the FFT size or sampling rate is adjusted in LIMP, the calibration process must be completed. Simply connect the probes as shown in the above diagram, but leave the speaker disconnected. In the calibration window, there is a "generate" button that will create a tone so you can set the level of the test signal, then simply press the calibrate button. It should provide you with a success notification as well as the channel difference.
Calibration - It is important to compensate for the test leads from the jig to the speaker. This is detailed in section 4.7.2 of the LIMP manual for reference. To compensate for the test leads from the impedance measurement probe to the driver, simply disconnect the driver and short the leads together. Your amp will be safe with this operation as long as the resistor is connected as it should be. Now simply run the measurement, select "PN PINK" under the generator drop down, Fstart of 10Hz, Fstop of 40000Hz, then press the red "play" button. This will measure roughly 0 ohms, but there will be some resistance and inductance of the test leads. Click on the “fit” button to zoom in on the measured result, then click on the graph at 1kHz to set the cursor location, then press the "RLC" button, which will provide you with the lead resistance and inductance parameters. Write these numbers down.
Lead Compensation - Now, press the "cable compensation" button, which is just to the right of the "CAL" button. Enter the resistance and inductance measured, and check the box to enable the compensation. For the example above, I would enter 0.1029 ohms, and 2470 nH. Now, if you rerun the shorted lead test, the measurement should be incredibly close to 0 ohms across the entire spectrum.
For driver free-air T/S measurement, it is important that motor vents are unobstructed, so placing face up on a desk may not be the best solution without some block spacers to keep the vent open. For complete driver T/S, we must complete two measurements with added cone mass to determine the VAS of the driver. For this process, simply take the first measurement and export it as the driver free air impedance, then also select overlay -> set as overlay curve. Now we must add some mass to the driver cone so get out your kitchen scale. If you have a spec sheet for the driver, the added mass should be about equal to the driver Mms or "moving mass", so we are doubling the moving mass for this process. If you don't have the datasheet for the driver, I would use the Mms of a driver of similar size and cone material. The idea is to lower the Fs of the driver by 25% so use that as a guide on the mass requirement as well. Run the measurement again, you will see the added mass measurement now as well as the free air overlay. export this measurement as well.
The T/S parameters can be calculated within LIMP, or the exported measurements can be used directly in VituixCAD to complete the calculations there as well.
In LIMP, simply select analyze - loudspeaker parameters - added mass method. The voice coil resistance should be measured with your multimeter and entered, as well we must enter the driver diameter and added mass used. The diameter here is determining the "Sd" parameter for radiating surface area, and it is generally measured as the cone diameter + half of the surround. In the "nonlinear LSE optimization" section of this window, check the top and bottom boxes, but not “estimate voice coil resistance Re”, and in the dropdown for the lossy inductor model, select "Le + L2||R2||K" which is more commonly known as the "semi-inductance" model, and in VituixCAD it is referred to as the "Extended Z model".
T/S Calculation in LIMP
One convenient feature here is that you don't have to manually enter these values into VituixCAD. If you press the "copy parameters" button here, then in VituixCAD enclosure section, press the + button to create a new driver, you will find a "paste" button in the lower left corner to simplify the process.
If you were to use the measured data and determine T/S parameters directly in VituixCAD, you will require all the same information. 2 measurements of free-air and added mass, and you must enter the specific added mass and driver diameter. When you create a new driver in VituixCAD, simply press the "calculate T/S" button, and enter in all the necessary information. I won't go into great detail here as I feel it is fairly straight forward.
T/S Calculation in VituixCAD