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Old 02-18-2013   #1
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Default The RTA Walkthrough and Usage thread. With Video.

This is a spin off of my discussion in my Build log here:
http://www.diymobileaudio.com/forum/...-tune-log.html



This is intended to lay out the basics of measuring your Car audio system using Room EQ Wizard, which can be found here:
http://www.hometheatershack.com/roomeq/


Here goes...




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:
  1. 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.
  2. 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.



Cliffs:
  • 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.

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Last edited by ErinH; 02-20-2013 at 05:32 PM..
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Old 02-18-2013   #2
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Default Re: 8th Gen Honda Civic Sedan v.∞: My full disclosure build and tune log

I'll discuss how to use either of the two methods listed above. But first off, let's discuss the equipment you need and why you need it.

  • If doing RTA only measurements all you really need is:
    1. A CD with pink noise
    2. A mic to capture the sound.
  • If doing impulse measurements you need two things:
    1. A way to use the software's signal generator and send that signal to your audio system. This can be done by using a soundcard output and run it in to your audio system auxiliary input.
    2. A mic is used to capture the sound.

If you are looking to achieve a target curve of any sort you MUST USE A CALIBRATION FILE with the mic.



-------------------------------------


For impulse measurements, this is the gear I'm using. It's pretty simple:
  1. M-Audio Transit. This sends the signal from REW software via a 3.5mm male/female cable to my P99's auxiliary input. I use this because I don't have on-board audio.
  2. Dayton Omnimic USB microphone. It comes with a cal file.


You don't have to use what I have. In fact, you can save a good deal of money by simply using:
  1. Your laptop's built-in headphone output
  2. This Dayton mic (click link here). There are numerous other mic alternatives. This is just an easy one with one USB cable. Plug and chug. And it comes with cal files.


------------------------------------------


You know what you have to have. Why do you have to have it?
  1. The soundcard output is used to send the signal to your auxiliary source. Every headunit should now have one of these. This is how I do my testing most of the time. If it doesn't, you may be limited to using the pure RTA method only.
  2. The mic records the system output. Simple as that.



-----------------------------------------


Here's some pictures of my gear:


M-Audio Transit, using the output via a 3.5mm headphone extension cable.








Dayton Omnimic USB mic (it has electrical tape because I broke the tip; these things are pretty fragile and I made a goof):







Mic in the headrest:









3.5mm aux cable plugged in to my headunit's aux input on the face panel:


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Last edited by ErinH; 02-21-2013 at 07:20 PM..
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Old 02-19-2013   #3
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Default

Woke up early before the little one did to make this video so forgive me if I sound half dead and my video has the shakes.

This covers only the RTA aspect of using REW. I'll post another video soon showing how to use the impulse measurement method since I feel it's a bit more complete depending on what you want to do and given I'll be posting results from that kind of measurement.

Let me know if you have questions.



+ YouTube Video
ERROR: If you can see this, then YouTube is down or you don't have Flash installed.




- Erin

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Old 02-20-2013   #4
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Default Re: The RTA Walkthrough and Usage thread. With Video.

I'll pick this back up with more discussion of using REW impulse measurements for further capability (such as decay plots for targeting modal issues with Parametric EQ, Time Alignment, etc).

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Old 02-20-2013   #5
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Default Re: The RTA Walkthrough and Usage thread. With Video.

holder for impulse measurement tutorial

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Old 02-20-2013   #6
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Default Re: The RTA Walkthrough and Usage thread. With Video.

Subbed!! Thanks!

Looking forward to read the stuff that's coming..

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Old 02-20-2013   #7
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Default Re: The RTA Walkthrough and Usage thread. With Video.

Woah, so subbed here. I have used REW before but have settled on Tru-RTA lately. After your awesome video, I am thinking I will go back and try REW once again. It's so much easier to take readings and average things than Tru-RTA is. I am really looking forward to learning more about how to use it for the other features which I pretty much have no clue about really. I am especially interested in learning more on how to deal with modal issues as I think I have a nasty one that runs Car width right at the front passenger head position at 60-70 Hz. At least I think that is the cause of the dive there from my sub. I am really hoping my new 9 inch midbasses will play that frequency better than my current 6. 8 more days and I should know on that front.
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Old 02-20-2013   #8
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Default Re: The RTA Walkthrough and Usage thread. With Video.

On a more personal note, does anyone know if there is an auxillary in on the Pioneer Z110BT nav system? Or, Erin, you have any idea on how to get it connected to my Mobile PreUSB preamp? I am using this and a Behinger ECM 8000 with xlr cables. I assume this will suffice to do the impulse measurements?
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Old 02-20-2013   #9
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Default Re: The RTA Walkthrough and Usage thread. With Video.

there's an A/V input on the rear harness, iirc.

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Old 02-20-2013   #10
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Default Re: The RTA Walkthrough and Usage thread. With Video.

Quote:
Originally Posted by bikinpunk View Post
there's an A/V input on the rear harness, iirc.
Hmm, thanks. I will have to look into that one. Have to pull the damned deck again to get at it tho. Sounds like a nice weekend project at some point
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Old 02-20-2013   #11
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Default

Every time I install a deck with rear AV input, I go ahead and run a cable out to the glovebox. That way I have easy access to it in the future.

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Old 02-20-2013   #12
 
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Default Re: The RTA Walkthrough and Usage thread. With Video.

Great post!

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Old 02-20-2013   #13
 
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Default Re: The RTA Walkthrough and Usage thread. With Video.

Very nice walkthrough so far!
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Old 02-21-2013   #14
 
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Default Re: The RTA Walkthrough and Usage thread. With Video.

Hi Erin (I hope I am spelling your name correctly )

First of all THANKS for a lovely walk through with the REW soft.

I've just downloaded beta version 5 Build 13 from their forum and I am about to use it with the new MIC from MiniDSP the UMK-1

UMIK-1 | miniDSP

Up so far I was using True-RTA in conjunction with a RadioShack SPL meter which is actually a standard Phillips MIC (just use the standard calibration file)

I really wanted a better MIC so this is why I am ordering the UMK-1 and I liked the way you used the average calculation, I also tend to balance left and right and take separate measurements of each side and as well each element on its own tone by tone and then at the end run a pink noise test.

Of course - I have a bit 1 so I do not go beyond 1/3 octave but as well, for a car, I do not see a need to go beyond that, right?

Any way - I would like to know, be tipped by you as for how to use RTA measurements and understand from it if I have any phase or time alinement issues -> I would tend to believe that such issues would be easier to debug/troubleshoot using a scope no?

Thank you very much!

Eddie (please forgive my englaize - I am not a native Speaker of this language)

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Old 02-21-2013   #15
 
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Default Re: The RTA Walkthrough and Usage thread. With Video.

Thank you!

I'll give REW a shot before I potentially shell out for True RTA.


Last edited by Sonus; 02-21-2013 at 01:38 AM..
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Old 02-21-2013   #16
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Default Re: The RTA Walkthrough and Usage thread. With Video.

Thank you Erin for posting this. This should be a big help. I am looking forward to part 2.

A few questions. I noticed your mic was not at what I would think would be ear level so is it just a matter of getting close? Also does the direction matter for example yours is horizontal. Will there be a difference if the mic is angled up say at 30-45 degrees?

I am also wondering about the mic cal. I have a DBX mic but I do not have a cal file for it. But when I play tones via REW into my hu the response is perfectly matched. For example if I play a 3000hz tone it shows the response at 3000hz. I have tried this at various frequencies and it is always correct but I have not tried all frequencies. If a cal is needed are ther places that I can send it to to get a cal file made?

A hu feeding some amps pushing some speakers
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Default Re: The RTA Walkthrough and Usage thread. With Video.

Sub'd! I'm especially looking forward to you showing how to get time alignment spot on using impulse response. Cheers
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Old 02-21-2013   #18
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Lots of good questions. Give me a day or so and I'll try to answer them all. Been working 16 hour shifts lately to get stuff out the door so have only been posting from the phone.

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Old 02-21-2013   #19
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Default Re: The RTA Walkthrough and Usage thread. With Video.

Quote:
Originally Posted by 07azhhr View Post

I am also wondering about the mic cal. I have a DBX mic but I do not have a cal file for it. But when I play tones via REW into my hu the response is perfectly matched. For example if I play a 3000hz tone it shows the response at 3000hz. I have tried this at various frequencies and it is always correct but I have not tried all frequencies. If a cal is needed are ther places that I can send it to to get a cal file made?
I believe the DBX mic's are corrected in the hardware of DBX like the driverack or such.. So the mic should be calibrated for correct FR measuring

If the played freq. is visual on your screen it means is "hears" the correct freq., the calibration is to adjust the sensitivity of the mic so it shows the "actual relative" level

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Old 02-21-2013   #20
 
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Default Re: The RTA Walkthrough and Usage thread. With Video.

This is a great thread Erin. I use REW as well and have a good handle on how to use the software, but the posts you put on the Build log have helped to "connect the dots" with respect to what I'm actually doing with it. Great work, and I thank you for it.

One question, though: I noticed in the video that all your measurement snapshots are instantaneous. I don't have it in front of me, so this is going by memory, but there's also an option to do multiple samples (4, 8, 16, etc.) for each measurement. Is there any benefit (or detriment) to doing so?
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Old 02-21-2013   #21
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Default Re: The RTA Walkthrough and Usage thread. With Video.

Quote:
Originally Posted by Woosey View Post
I believe the DBX mic's are corrected in the hardware of DBX like the driverack or such.. So the mic should be calibrated for correct FR measuring

If the played freq. is visual on your screen it means is "hears" the correct freq., the calibration is to adjust the sensitivity of the mic so it shows the "actual relative" level
Thank you Woosey. I kinda figured the cal was good since it matches on the screen. I just figured I would ask since Erin made sure to emphasize the need for a cal file if going for a target curve.

I am not sure what you mean by the mic's hardware or driverack. It is just a black version of that Dayton in Erin's video (well atleast it has the same shape but in black lol) ran thru a micmate then on to the laptop.

A hu feeding some amps pushing some speakers

Last edited by 07azhhr; 02-21-2013 at 01:21 PM..
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Old 02-21-2013   #22
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Default Re: The RTA Walkthrough and Usage thread. With Video.

awesome stuff erin. i about about to go to tintbox's this weekend to play around and ill snag REW and play around haha.


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Old 02-21-2013   #23
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Default Re: The RTA Walkthrough and Usage thread. With Video.

Quote:
Originally Posted by eddieg View Post
Hi Erin (I hope I am spelling your name correctly )

First of all THANKS for a lovely walk through with the REW soft.

I've just downloaded beta version 5 Build 13 from their forum and I am about to use it with the new MIC from MiniDSP the UMK-1

UMIK-1 | miniDSP

Up so far I was using True-RTA in conjunction with a RadioShack SPL meter which is actually a standard Phillips MIC (just use the standard calibration file)

I really wanted a better MIC so this is why I am ordering the UMK-1 and I liked the way you used the average calculation, I also tend to balance left and right and take separate measurements of each side and as well each element on its own tone by tone and then at the end run a pink noise test.

Of course - I have a bit 1 so I do not go beyond 1/3 octave but as well, for a car, I do not see a need to go beyond that, right?

Any way - I would like to know, be tipped by you as for how to use RTA measurements and understand from it if I have any phase or time alinement issues -> I would tend to believe that such issues would be easier to debug/troubleshoot using a scope no?

Thank you very much!

Eddie (please forgive my englaize - I am not a native Speaker of this language)

Use whatever mic you want. ecm8000 works well and you can get them calibrated by Herb @ Cross Spectrum Labs. The Dayton mic I linked will work as will the MiniDSP mic you linked. There are likely numerous others I don't know of or haven't seen. As long as you have the means to use it (ie: you don't need special equipment) and a calibration file is provided, get what you can afford and call it a day.


As for phase/ta, I'm not really sure how you would use a scope to determine the acoustic alignment at your seated position, relative to each speaker.

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Old 02-21-2013   #24
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Default Re: The RTA Walkthrough and Usage thread. With Video.

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Originally Posted by charliekwin View Post
One question, though: I noticed in the video that all your measurement snapshots are instantaneous. I don't have it in front of me, so this is going by memory, but there's also an option to do multiple samples (4, 8, 16, etc.) for each measurement. Is there any benefit (or detriment) to doing so?
Yep. I actually thought of that after I posted the video (on my way to work). I'll have to check this out again. When I used TrueRTA, I use averaging to essentially slow down the response (keep it from jumping so wildly). That number was usually '50'. But, I'll need to go back and see how REW is set up for this. I really don't use it's RTA feature. I only use the impulse function because, frankly, it's easier as odd as it may sound. I don't have to worry about pausing the CD when I am not measuring. I like using the built in generator from the impulse method. (trueRTA has a built in generator for all sorts of samples)

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Old 02-21-2013   #25
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Default Re: The RTA Walkthrough and Usage thread. With Video.

Quote:
Originally Posted by 07azhhr View Post
Thank you Erin for posting this. This should be a big help. I am looking forward to part 2.

A few questions. I noticed your mic was not at what I would think would be ear level so is it just a matter of getting close? Also does the direction matter for example yours is horizontal. Will there be a difference if the mic is angled up say at 30-45 degrees?

I am also wondering about the mic cal. I have a DBX mic but I do not have a cal file for it. But when I play tones via REW into my hu the response is perfectly matched. For example if I play a 3000hz tone it shows the response at 3000hz. I have tried this at various frequencies and it is always correct but I have not tried all frequencies. If a cal is needed are ther places that I can send it to to get a cal file made?
Quote:
Originally Posted by Woosey View Post
I believe the DBX mic's are corrected in the hardware of DBX like the driverack or such.. So the mic should be calibrated for correct FR measuring

If the played freq. is visual on your screen it means is "hears" the correct freq., the calibration is to adjust the sensitivity of the mic so it shows the "actual relative" level
Quote:
Originally Posted by 07azhhr View Post
Thank you Woosey. I kinda figured the cal was good since it matches on the screen. I just figured I would ask since Erin made sure to emphasize the need for a cal file if going for a target curve.

I am not sure what you mean by the mic's hardware or driverack. It is just a black version of that Dayton in Erin's video (well atleast it has the same shape but in black lol) ran thru a micmate then on to the laptop.

Woosey is right.

Think of a calibration file as not one that calibrates the frequency but the SPL measured at a given frequency.

You need this ability if you want to target a curve of any sort. Otherwise, you won't be guaranteed any accuracy.

For example:
Let's say your mic has a natural rise in response above 5khz (most do). If you don't have a calibration file to compensate for this and flatten this out, then this rise will show up in every measurement. In a fictional world, let's say your Car measured flat. If your mic has a rise above 5khz, you'd measure an apparent rise above 5khz and you'd start cutting it to flatten it (or whatever you'd do to target X curve). You'd be cutting something that's caused by the mic; not by the car.

Make sense?

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