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SETTING UP AND USING ROOM EQ WIZARD This accompanying article for the Speaker Test Jig explains how to set up and use the freely-available Room EQ Wizard (REW) or Speaker Workshop software to help you design and tweak loudspeakers. F irst off, you don’t need the Loudspeaker Test Jig described in this issue to use Room EQ Wizard or Speaker Workshop to design and test loudspeakers and drivers, but it makes it a lot easier. This article will describe setting up and using REW (and later, Speaker Workshop) assuming you have built the Loudspeaker Test Jig. If you haven’t, you can still follow these procedures; you just need to rig up a microphone preamp, power amplifier, test resistor and some other bits and pieces to perform similar functions. Essentially, what you need (and the Jig provides) for measuring driver impedance is to have your computer’s sound card feeding a power amplifier that drives the device under test (DUT) via a well-characterised 10W or similar power resistor. Both ends of that resistor then connect to the two sound card inputs. For driver and speaker frequency response plots, you instead need a calibrated microphone and microphone preamp combination that gives a flat response feeding into one of your sound card’s inputs while the output(s) drive the DUT via a small power amplifier. The Jig also does that if you have a calibrated microphone (we’ll describe an inexpensive one in an upcoming issue). By Phil Prosser Final testing and setting up REW We assume you have your computer set up and your sound card properly installed. Importantly, make sure you have the sample rate set and no effects turned on. Also check that you do not have ‘monitor recordings’ set. The critical steps to getting the Loudspeaker Test Jig operational with the REW software are provided here. There are many resources on the internet for this program, and its full details are well beyond the scope of this article. Still, let’s get it up and running. After installing and launching REW, to set it up, open the preferences pulldown and then the preferences tab – see Screen 1. Select your input and output here; usually, you would use the default sound input and output devices. Calibrating your sound card 1. Set the Loudspeaker Test Jig to ‘component test’ and make sure there is nothing connected to the Speaker and DUT connectors. 2. Make sure the Loudspeaker Test Jig attenuator is switched out. 3. In REW, open the preferences pulldown and open the preferences tab. 4.  Click on ‘Calibrate soundcard’. Note that by using the ‘Component’ test mode, the 10W reference resistor acts as the loopback mentioned in the text box that will pop up. 5.  Click Next, and a text box will appear providing instructions. Follow them. Screen 1: the REW Preferences dialog. Check that the input and output devices and sampling rate settings are set correctly. 40 Practical Electronics | June | 2024 6. Click Next and check that you have levels that are about right. You should find that with about 200mV RMS output, you see a measured signal in the region of -10dB on the loopback test. Sound cards vary in sensitivity, so your voltages may vary somewhat from ours. 7. Then click Next until the measurement sweep is made. You will get a graph similar to that in Screen 2. 8.  On the tab for the measurement you just made, add any notes you need. Then click the disk symbol on the measurement and save this file somewhere sensible. 9. Now press Alt+Tab to switch back to the preferences screen and click on ‘Make Cal File’, which is below the ‘Calibrate Sound Card’ button. 10.  Navigate to where you saved the previous measurement. Select ‘all files’ from the pulldown ‘files of type’ and then select your calibration measurement. Click ‘Save’. 11. Your sound card is now calibrated. Calibrating the Test Jig 1. Set your Loudspeaker Test Jig to ‘component’ test and ensure there is nothing connected to the Speaker and DUT connectors and that the Attenuator is out. 2.  You only need to do this on the first measurement you make. Click ‘Measure’ in the top left corner of the main REW screen (Screen 3). If you have not calibrated the SPL, you will get a message box; you can ignore it for now. 3. Now enter your sense resistor value in the Rsense box at the right of this window. 4.  Click on ‘Open Circuit Cal’ and follow the instructions. Save the file along with your others. Do the same for ‘Short Circuit Cal’ and use a known resistor value for ‘Reference Cal’. 5. You can now measure an impedance. A window similar to that shown in Screen 4 will pop up. Screen 5 shows the measured impedance of a subwoofer. Using it To measure an impedance 1. Set your Loudspeaker Test Jig to ‘component’ test and make sure there is nothing connected to the Speaker and DUT connectors and that the Attenuator is out. 2. Click ‘Measure’ in the top left corner of the main REW screen. 3. Click ‘Impedance’ in the top left of the screen, as shown in Screen 3. 4.  Click Start once you have connected your unknown impedance across the DUT terminals. Practical Electronics | June | 2024 Screen 2: this shows the frequency response REW has calculated for the measurement system, including the computer sound card. Screen 3: you make impedance and frequency response measurements using this screen in the REW software. Screen 4: calibration with our 10W test resistor is complete, and the result almost exactly matches what our Low Ohms Meter reads. To measure speaker frequency response 1. If you are testing a tweeter, put a high-value non-polarised capacitor in series to protect it from low frequencies, and consider running the sweep from, say, 500Hz up. 2. Set your Loudspeaker Test Jig to ‘speaker’ test. 41 3.  Connect your speaker across the Speaker terminals. 4. Plug your microphone in and set the microphone gain as required. 5. Set the attenuator on or off depend- ing on the level you intend to test at. 6. Click Measurement again, and this time select ‘SPL’. Screen 5: a measurement of the impedance of a subwoofer taken using REW and our Test Jig. It gives a nice smooth plot that shows resonance peaks at about 31Hz and 72Hz (driver/box) plus 850Hz and 2.5kHz (cone breakup). Screen 6: a frequency response plot of a wide-range driver made using REW. This sort of information is invaluable in speaker design and tweaking. 7. The system will run a sweep and you will hear the chirp. 8. Check that the levels are reasonable. If necessary, adjust the sound card output level, the microphone gain switch and the output Attenuator for the Loudspeaker Test Jig. You will find that once you are set up for testing, these don’t change often. 9. Watch the levels; if the outputs or inputs clip, you will get odd results. If this happens, investigate the cause and correct it. 10. You will see the result pop up in a window similar to that in Screen 6, a very rough plot of a speaker done on our workbench. 11. You can change the smoothing setting, show a waterfall plot, show distortion and a range of other plots from this measurement, which is pretty impressive. Tips ● The room will play havoc with far- field measurements. If you do this in a room, you will never get a 20Hz to 20kHz plot without all sorts of peaks and dips. Just accept this. ● You will need to apply smoothing to get a plot anything like what you see in Hi-Fi magazines, as that is what they do. ● Testing outside is good; the ground is always there, though. This will generate ‘ground bounce’, which is perfectly natural, and you need to work around this unless you point your speaker up and hang your microphone from a ladder. Yes, we have done this! From here, we recommend that you explore some of the excellent Screen 8: an impedance plot of the 10W calibration resistor in Speaker Workshop. It’s a bit noisier than the equivalent REW plot, but it demonstrates that the measurement system is accurate from about 5Hz to over 20kHz. Note that this plot was made as part of the verification process of the Loudspeaker Test Jig. 42 Practical Electronics | June | 2024 resources on the Internet for these programs. REW is more active, but Speaker Workshop has a strong following. The DIY audio community has several quite active groups. ‘DIY Audio’ is a good place to start finding like-minded people. Reproduced by arrangement with SILICON CHIP magazine 2024. www.siliconchip.com.au How to get Speaker Workshop up and running If you want to try out Speaker Workshop, read relevant parts of the ‘unofficial manual’ on the download page at: www.claudionegro.com Ignore the ‘failed to update system registry’ warning on startup. You must set up a project: 1. Create a new file by clicking on ‘File’ then ‘New’. 2. This program works by adding resources to the ‘system’. Resources might be an enclosure, driver or network. 3. You need to add a driver at minimum. To do this, open the ‘Resource’ menu and select ‘New’ then ‘Driver’. You need to select this to make measurements – see Screen 7. To calibrate the system: 1. From the ‘Options’ menu, select ‘Calibrate’. 2. Make sure there are no leads connected to the Amp and DUT jumpers on the Test Jig. 3. Switch the Jig to ‘Comp’ and switch the measurement attenuator out. In this position, both sound card channels measure the amplified output. 4. Click ‘Test’ on the channel difference box. Follow the instructions to run the calibration, finishing with ‘OK’ to accept it. After calibration, look at the bottom left of the screen. This shows the digital values read in the calibration. The maximum must always be less than ±32768 and ideally in the 10,000-20,000 region. Adjust your PC’s output level and Jig attenuator setting until you get sensible readings. We generally find that an output level in the region of 40% works well. To set the Reference, open the ‘Options’ menu, then the ‘Preferences’ tab. Click on the ‘Impedance’ tab and type the exact resistance of your reference resistor in the Impedance Jig definition box. To make an impedance test: 1. Connect your DUT between the DUT and ground terminals. 2. Select the driver we created earlier. It will become highlighted in blue. 3. Open the ‘Measure’ menu and click on ‘Impedance’. 4. Once the measurement is complete, check that the values at the lower left of the screen are reasonable. You should see a window pop up with the measurement, as shown in Screen 8. Our Low Ohms Meter measured this resistor as 10.09W. 5. If the impedance plot is very fuzzy, check that you are not clipping the sound card or amplifier. To make a speaker frequency response test: 1. Switch the Jig to SPKR. 2. Switch the attenuator next to the DUT connector in. 3. Connect a driver to the AMP output, not the DUT output. 4. Plug in your test microphone and place it close to your speaker. 5. Click on the driver icon you created and then select the ‘Measure’ pulldown, select the ‘Frequency response’ tab, then ‘Nearfield’. 6. You should get a reasonably clean frequency response. It will have more noise than one from REW and may need smoothing. If the frequency response graph is very fuzzy, check that you are not clipping the sound card or amplifier. Screen 7: to use Speaker Workshop with the Test Jig, you must create a ‘driver’ instance and set some critical parameters. 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