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Tutorials
Learn how to do a bunch of stuff from AoN's online tutorials.
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Tuning Your Car Using A PC Based Measurement System
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This article is a very basic, step by step look at how to tune your car using a pc based measurement setup.
First, position the mic in the listening area. Here, I placed the mic just in the middle of where my head would normally be.
Now we will measure the time arrival of all the speakers.
Start by disabling/muting all the speakers. Starting with the left side tweeter, enable or un-mute it. Begin your mls measurement, looking at the time response (impulse response). I use WinMLS software for this application, a calibrated pre-amp/mic combo from Germany, and the M-Audio transit usb card. Be sure to read the other article on assembling a pc based measurement system for other options. Repeat again, testing each speaker by itself with the others muted.
It is critical when doing this that you make certain your soundcard is "time synched". Most sound cards have a delay between the time that you begin playing a signal, and you begin recording. To make it even more frustrating this delay can vary between measurements. You would just want to make sure that your software+hardware is setup to compensate for this delay, so you can get accurate time measurements.
You should see something like this, which shows you the time arrival of the left tweeter in RED, and the right tweeter in BLUE.
Now, notice the first big spike with the RED line tweeter is going negative. That mean's your tweeter is wired in reverse polarity. A quick trip to your tweeter or amp to rewire and the problem is fixed. Also look at the BLUE line tweeter. The first initial spike, which corresponds to when sound from that tweeter first reaches the microphone, is exactly .6 milliseconds behind the RED tweeter. Now you know that you need to delay your RED tweeter by .6 seconds.
Now look at both tweeters. The initial spike is going positive, which means the tweeters are in the correct polarity. Also, the spikes are "almost" overlapping. The time difference between the 2 tweeters is now about .1 milliseconds which is good enough.
Basically, you would want to continue doing this until all the speakers are in the correct polarity, and the initial spikes are all overlapping.
Next up, we want to set the equalizer. To do this we would measure the frequency response of each driver separately and then take a look. You want to make sure that you include strong early reflections in your measurement, but not later ones. Generally, early reflections are responsible for tonal aspects, while later low amplitude reflections account for a sense of space.
RED = left tweet
BLUE = right tweet
BLACK = left mid
GREEN = right mid
If you're not using an accurate mic+preamp with a known, generally flat response make sure to take your frequency response measurements with a grain of salt.
(NOTE: in the bottom time window, the green/black lines are the mids time response. See how the initial spikes lineup? That shows you that the drivers are correctly time aligned. Look at the second large spike. That is possibly the cone breakup or a strong reflection from the Seas Excel w18 mid/bass. Also, note that although this driver shows a +12db spike when measured in an anechoic chamber (in the spec sheet), in MY car door there is no audible cone breakup at the listening position, and thus no need for a notch filter.)
Let's do an example starting with the left side. I like to begin with the tweeter. Looking at the RED line, we can see that below 3khz there's a bit of a dip, and above 10khz there's a -5db rolloff. So you would probably want to cut out a bit between 3 and 10khz, and boost a little bit around 2khz in order to flatten out the response. I level matched the mid and tweet pretty good by ear so I'll just leave that alone (BLACK and RED lines) Here are the settings I used, inputted into my Behringer dcx2496 processor.
Now let's do the left side mid (BLACK line). It's pretty obvious there's a big hole centered around 750hz, and some bumps around 100hz, 200hz, 400hz, and 1800hz...probably from door cover diffraction and resonance.
Here are the settings I used for the mid. Notice how the boost and cuts corellate to the peaks and dips in the (BLACK) line frequency response:
Now to setup the crossover points. Look at the black line and the red line around 1-2khz. We know that for the Seas w18 Excel used in this example, distortion rises above 2khz. So keeping this driver below 2khz would be a good start. Also, the ribbon tweeter used in this example begins to show higher distortion around 2khz as well. So it looks like 2khz is a good starting point. I also like to use steep crossover slopes in order to minimize distortion, and make sure that the driver really isn't stressed too far beyond the crossover frequency.
Now that I've decided on a 4th order Linkwitz Riley crossover at 2khz, let's look again at the frequency response of the RED and BLACK lines at 2khz. The BLACK line or mid/bass already naturally rolls off at 24/db octave roughly at 2khz. So we don't need to set anything here. The RED line or tweeter "almost" seems to rolloff at 2khz 24db, but actually starts a little later at 1.5khz. So, I decided to try a 1st order (-6db) rolloff at 2khz to see what would happen.
Here is the final response of the left side mid and tweet. Notice the frequency response is much smoother overall, the mid/bass plays down flat to almost 40hz (no highpass set), and the crossover point actually ended up being around 2khz, 4th order (-24db). You also want to make sure that the tweet and mid are playing at the same level.
And here is the final summed frequency response for the left side. Tweet and mid/bass measured together. Notice around 2khz, the transition is smooth and seamless.
Now you basically want to repeat all that for the right side. Make sure that the right side is as good a copy of the left side as possible. Here is the frequency response of both the right and left side independently:
And here is the summed frequency response for the entire system, compare it to the unequalized, un-tuned frequency response at the very start (below). The optimized response is about +/- 3db ... so not bad. The system with only "textbook crossover" of 2khz 18db, and level matching by ear and no equalization has a response of about +/- 10db.
Finally, a good place to highpass the mids would be around 80hz -12db. It keeps the mid from distorting at high output levels, and it's generally a good place to bring the subwoofer in.
Here is a frequency response of the sub (located in the trunk rear firing) with no eq or crossover overlayed with the rest of the system:
Notice the dip around 60hz, and the natural 24db rolloff at 80hz. Doesn't it seem redundant to use a 24db slope on your sub if it's already got a natural rolloff at that point? I'll leave the issue of sub integration as an excercise, but my first thought would be to lower the output a little, boost the response at 60hz, and highpass the mid/bass at 80hz -24db.
Now that was pretty easy wasn't it?
Some points and conclusions I'd like to sum up that I think are fairly obvious.
First off, "one size fits all" passive crossovers and/or texbook crossovers just don't work. That's because we can see that the in car frequency response is completely different from the frequency response on the spec sheets, which were taken in an anechoic chamber. Also, we have to take into consideration the actual natural rolloff of the driver before implementing a crossover.
Also, equalization is nearly a MUST have for typical car setup. Here I have the mids in the doors, and the tweeters in the kickpanels. Everything is heavily dampened. But still notice the somewhat extreme, and strange eq settings I had to use just to get the frequency response flat. It could take weeks or even months of discriminating listening to find all those flaws.
Also, notice the shape of my equalization curves above. Those would be almost impossible to achieve unless you are using a parametric equalizer. Notice that I am equalizing the tweeter and mids separately from each other, as well as from each side. In order to get the best possible/flattest response and good imaging, an equalizer with this kind of flexibility and power is absolutely needed... and chances are you won't find it in an analog unit.
And make sure that even though you have a flat frequency response, you want to cross the speakers over where they will not distort or strain. Always bear that in mind when choosing crossover points, or boosting on the eq.
Lastly, notice how the summed system response looks a bit different from the left/right side response. If that happens, you can go back in and eq both sides to adjust the response flatter. However, although your tonality may improve from having a flatter summed response, the further apart your left and right side frequency response is from each other the worse your imaging will be. Try and fool around with small adjustments and balancing the need for tonal improvement with imaging together, and of course ALWAYS let your ears be the final judge!
In this case, I was very pleased with the sound right out of the box. Definitely one of the best cars I've heard so far. Imaging and staging is nearly perfect (for a car), and tonally it's very good. With the ribbon tweeter response easily goes out flat to 20khz unlike most tweeters. However, I still feel that it will take weeks of hard listening and fine tuning by ear to get that last 5%.... although you could certainly stop at this point since it sounds quite good.
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