HOW TO SET UP A LOUDSPEAKER MEASUREMENT SYSTEM THE EASY WAY AND GET RESULTS LIKE THE PROS!
HARDWARE
I realize that building speakers without a measurement system is like shooting in the dark. You basically rely on your ears and the sofware you use to tell you whether the approximation is good enough. Everyone listened to my RS MTMs and complimented me on how good it sounded, even with the 5dB dip at the crossover frequency. But for the non-engineer, non-technical speaker DIYer like myself, understanding this process ain't easy. It took me a long time to understand such concepts as phase, gated response, and what a resistor does, so accomplishing this feat isn't small. What I can do here is demonstrate how you can set this up easily, from the point of view of an idiot who had to read hundreds of pages from dozens of different web sites to just scratch the surface of this process. So, here we go:
You'll need the following:
 
1. A microphone and mic preamp. I got mine second hand from npdang, the administrator of diymobileaudio.com. You can float a want-to-buy request on one of these sites and get a good deal. You can get a new, calibrated mic and preamp from www.ibf-akustik.de, a German firm for less than $200.
 
2. A soundcard. I use the USB Transit from M-Audio.
3. A small receiver or amp/preamp between 10 and 50 watts per channel. Yes, I'm a hardcore DIYer and built mine from a Velleman kit.
 
4. A simplified jig to do impedance and T/S measurements. We're talking really simplified, coz I couldn't figure out the Wallin jig. I am a dummy... I used a RadioShack plastic box (4x6x2), 4 panel mount RCA jacks, and a pair of binding posts. I plegiarized the idea from www.purebits.com, which published the schematic posted below. You can use a 10 ohm resistor, or any value, but I happen to have a 300 ohm lying around, so I used it.
 
5. 2 sets of mini jack/RCA patch cables as shown on the pic.
SOFTWARE
Go to the ARTA web site and download the ARTA software. Arta consists of 3 applications, Arta, Limp, and Steps. Arta and Steps are both for FR measurements, and Limp is used for impedance and T/S. This is shareware, and costs around $100. You can however try out the free demo, which is only limited by the fact that you can't save and load your projects. You can still measure, copy your graphs to another application (I use Photoshop), save them as jpg images, and print them - all you need, in other words. Installation is straight forward on Wintel PCs.
MEASURING FREQUENCY RESPONSE USING A SINGLE CHANNEL SYSTEM
1. Connect the USB Transit soundcard to your laptop or desktop using the included USB cable, assuming that you've installed the driver from the provided CD-Rom, or use your existing soundcard. ARTA provides a list of soundcards that have proven to work well with their software. I used the M-Audio Transit because 1) it is recommended by Arta, 2) I can hook it up to my laptop and be portable. You won't need the jig for this process, so put it aside for now.
 
2. Connect both mini plug/RCA patch cables to the Transit.
 
2. Connect the mic to the mic preamp LINE IN using a standard RCA cable. Connect the mic preamp LINE OUT to the LEFT IN channel of the soundcard, using a female RCA connector between the 2 RCA cables.
 
3. Connect both channels of the LINE OUT of the soundcard to the receiver, then connect the speaker under test to the receiver. See pic below for connections.
4. Follow the manufacturer's procedures to calibrate the mic preamp. If your microphone comes with a calibration file, you will need it before measuring with Arta. NOTE: if you buy the mic from Europe, the calibration file denotes all decimals with a comma. Just use a text editor to replace all commas (,) with periods (.), then save the file with a .mic extension. 
 
5. Fire up the ARTA application, and follow the instructions to set up and calibrate your hardware in the system. If you have a mic calibration file, load the file by clicking on the "FR Comp" icon.
 
6. Position your mic and select the "FR1" icon (Fig. A). You have to play around with the distance from the mic to your driver here, since no one that I know has access to an anechoic chamber. In my garage, I got good results by using 1" for raw drivers, and from 12" to 39" (1 meter) for loudspeaker systems.
7. Click on the red "Start Rec" icon to start recording. Use the volume on your receiver or the gain knob on your mic preamp to adjust the magnitude of the frequency response (Fig. A).
 
8. Click on the red "Stop Rec" icon to stop recording (Fig. A).
 
9. Click on the "IMP" icon to convert the data to Impulse Response (Fig. A).
 
10. Click on the "Smooth FR" icon to display the frequency response graph (Fig. B). You can select the degree of smoothing, or to overlay the graphs in this window for on and off axis measurements (Fig. C). It's that easy.
Figure A
Figure B
Figure C
MEASURING IMPEDANCE - Go to Next Page