This is intended to lay out the basics of measuring your car audio system using Room EQ Wizard, which can be found here:
As mentioned above, I use REW for my testing. It's free and pretty easy to use once you figure out how to use it.
I've used every bit of software under the sun. At this point, I've just grown to like REW more than I used to so I've gone back to it for my tuning measurements. For those wanting to see how I'm using REW, here's some info.
RTA vs Impulse Response Measurement Method:
One thing to note is there are different meanings of the term, at least how we use it:
- RTA - Real Time Analyzer:
- This is simply a real time measurement of what the mic hears. Birds chirping, subs playing... whatever. It records it.
- RTAs are typically used to record pink noise.
- Impulse Response:*
- This can be a form of RTA, depending on how you look at it. An impulse is used typically to measure something before a reflection because you can gate the response. In other words, let's say I want to measure Speaker A. I know that the walls and floors create reflections occurring after 3 milliseconds (ms) that will 'tarnish' the speaker's response as measured by the mic if I let it. To keep this from happening, I look at the impulse response, tell the software to ignore everything after 3ms. Bammo... no more reflections in the measurement. Just Speaker A.
- The impulse response is measured by sweeping a sine wave and capturing the response.
* I have severely watered down my explanations here and there are caveats; especially when you get in to different window type methods. But for the sake of this post, it's fine.
The bottom line: In a car, you don't care about impulse gating. You can't really achieve a reflection free zone so there's not much point in trying. -- If you care to disagree please see discussion and reply here (link) so I don't get this one junked up). -- Therefore, we just disregard the whole gating process. That leaves us with a very long impulse response that matches what an RTA would show you, if the signal were the same (ie: pink noise). IOW, using a very long impulse window (100's of milliseconds) in the car will yield an RTA measurement.
Why use this impulse method if it essentially nets you the same thing as an RTA measurement? Because RTA measurement only gives you RTA data; SPL vs frequency. You can't get Decay or some of the other things I am looking to get. More data. That's all. I get in to it more below.
- When measuring a car, there are a couple ways to do it.
- RTA and Impulse are not the same. They each have their own use. However, when the impulse is used without filtering or gating it, it nets you the same result as an RTA.
- The benefit of using impulse measurements are you get more data such as decay, group delay, etc.
- Like a standard RTA measurement, multiple impulse responses should be taken and averaged together if you want to tune to a car.