Smith and Larson - Speaker Tester

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Posted by Wayne Parham [ ] on January 19, 2008 at 23:41:01:

I've been using the Smith and Larson Speaker Tester (actually WTpro) for a few weeks now and I really like it. I'm planning to do a pretty thorough write-up, at least in as much as I've used the system to verify my cabinets and optimize my crossovers. I purchased the system for the ICD because it is an extension of Spice. Since my crossovers were designed using Spice models, the Smith and Larson system is a natural progression for me, allowing me to use ZMA (measured impedance) files in place of equivalent circuits for loudspeaker loads. But for right now, I want to write a few of my initial comments.

The Smith and Larson system uses an outboard digital/analog box that connects to the computer using a USB cable. It serves both as the ADC/DAC and the connector jig box. It has a small signal output port for doing impedance measurements with built-in sense. This is where you connect a speaker to do impedance measurements and/or T/S measurements. It uses a pretty low signal level, just a few millivolts. There is also high level port, which is designed to be used with an external amplifier. You connect an amplifier output to one pair of connectors and the speaker under test to the other, and this way the box can sense current and voltage passing through. The high power port is rated at 200 watts RMS. On the other side of the box, there is right and left line outputs, right and left line inputs and a pair of microphone inputs. Measurement microphones are included.

When I first got everything setup and connected, I connected a speaker to do an impedance test thinking it would be the easiest thing to do. Right away, I noticed it thought the speaker was open. The connections were all right, the speaker was good. So something was wrong.

Something told me the problem was Vista. If you are running Vista and something doesn't work, blame it on Vista. Chances are, you'll be right. I called Keith Larson and told him I was having trouble with the low level port doing impedance measurements, and he told me to check the connections and what not, of course. I proffered the information that I was running Vista, sort of apologetically knowing that was probably throwing a wrench into the equation and Keith promptly told me that Vista has some issues and while it can be made to work, it might take new drivers. Best to back up to XP, so I did. That did it, impedance worked just fine.

After doing a few measurements, I wanted to look into the Vista problem a little more, and after some research, found that the audio drivers for USB have been completely rewritten. It's not just new code, it's a whole new standard for the way the hardware is accessed. At first, I thought maybe it was going to be a pretty big deal to get the system to work with Vista drivers, if it could even be made to work at all. But after reading a little more, I began to think that even though it was different under the hood, it probably should work. So I began to look at configuration options.

Without going through all the details, the solution is actually pretty simple. The default setting is mono, but the Smith and Larson tester needs the microphone channel to be set for stereo. Once the default value is changed, the Speaker Tester worked just fine on Vista. So I phoned Keith and told him that, and grabbed a few screen shots of the screens you need to go through to set it up. He is planning to put that in future versions of the user's manual, so check there if you are using Vista. If you need to know how to setup under Vista right now, E-Mail me and I'll send you the scoop.

One of the cool things the S&L Speaker Tester Pro does is to use the T/S measurements it makes and model a box with them. It's like having BoxPlot built-in. You can measure the T/S specs and then rather than typing them into a modeling program, the S&L system simply imports them into its own internal modeling program. You can start designing boxes right then and there, without going to an external program to do it.

Better still, once you've built the box, you can measure it and the S&L system will use the actual box measurements to make an even better model. It measures fb for sealed boxes or fl, fb and fh for vented boxes and then plots the response of the system using these values. This is a very accurate way of modeling and will give a better indication of the cabinet's bass response than acoustic measurement will, if done indoors.

I've gone through about half of my line of speakers so far and verified the cabinets with the Smith and Larson system. No surprises there, but it was a fun way to warm up to using the system. It is definitely a useful tool and gives a DIY'er excellent visibility.

The really impressive part of the system to me is the ICD. That's what I bought it for. I have LMS, and it's great for measuring response. It is perfect for knowing what a finished loudspeaker does. But it isn't really a design tool. The ICD is. And it's cooooool. With the ICD, you use a Spice model of the crossover and it becomes an active crossover. The S&L system uses DSP to make a digital filter that exactly emulates your Spice model. So you can try a crossover, measure acoustic response, and then change the crossover and measure again, just by changing a few lines of code.

This thing is sweeeet!


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