RTA Frequency Measurement Technique (Car Audio)

Leo Eskelinen

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Hi All!
I am running the latest REW with a UMIK-1 utilizing custom 90 degree mic calibration file.
Having spent countless of hours measuring, trying to find a measurement technique in addition to settings in REW that produces constant, correct and comparable results.

Below are my current settings/method.

SPL level is around 75-80dB.

The RTA "Averages" setting is chosen based on the microphone technique utilized.
I use Moving Microphone Measurement Technique, sweeping slowly from ear to ear in a slight figure eight. Starting with the ear corresponding to the relevant channel (if applicable).
Every measurement is based on 7 sweeps so the time frame is fairly constant, about 20-25 sec.

What are your method??

Please feel free to comment!


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Wayne A. Pflughaupt

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Did the “Moving Microphone Measurement Technique” get you “constant, correct, and comparable” results,” or are still looking for that?

Regards,
Wayne
 

Leo Eskelinen

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Did the “Moving Microphone Measurement Technique” get you “constant, correct, and comparable” results,” or are still looking for that?

Regards,
Wayne
Hi Wayne

I guess i’m beeing slightly unclear.

Yes. This method together with the settings mentioned gives me constant results, which is good.
The only drawback is that it is difficult to make real-time EQ adjustments.
 

Matthew J Poes

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I haven't been involved in car audio in 18 years so I can't really share much. My measurement approach however doesn't use the RTA to measure the speaker response typically. I use the sweep method which is a bit more accurate. I would only use the RTA for level setting, quick checks, or distortion measurements.

To get a response that is representative of the sound we hear, I take multiple measurements using the sweep method at each location around where my head is. I typically average the measurements to get a sense what they look like on average, but I also look at all individual measurements as that gives me clues.

The only real advantage to the impulse response measurement is that it let's you see the some additional information, only some of which is likely useful for a car. As I used to be an IASCA competitor I've often wondered about using these measurements to make a highly competitive system since todays measurements and digital signal processing far exceeds what was possible 18 years ago. I have even played around a bit with FIR filters for car correction, but I no longer use the kind of equipment I used to use.

In any case you may want to try my results and see what you see. It allows you to also look at phase, the actual impulse response, and distortion. All of this can be helpful.

Just as an example, I am measuring a pair of Pioneer speakers designed by Andrew Jones right now. These are well regarded speakers and their axial response is very flat. Looking at the response alone makes them look great. I took a quick look at the distortion graphs and noticed that up around the crossover point, there is a rise in harmonic distortion. It looks suspiciously like tweeter distortion and my guess would be that the tweeter is slightly overloading. At 75db's (at a 12 foot distance) the tweeter had about 3-4% 3rd harmonic distortion and 1.5% 4th and 5th harmonic. If I saw this in a car setup I would probably try a steeper slope and/or higher crossover point for the tweeters. If I wanted to use the lower crossover point, It would tell me I need to replace the tweeters with something that handles a lower crossover point.

REW also has a number of tests that you can use to look for rattles, which at high volume and with a lot of bass is very common in a car. Of course you may start finding resonances and rattles you don't typically hear and it could start a whole new kind of nervosa.
 

Matthew J Poes

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Hi Wayne

I guess i’m beeing slightly unclear.

Yes. This method together with the settings mentioned gives me constant results, which is good.
The only drawback is that it is difficult to make real-time EQ adjustments.
John is the best to comment since he wrote the software, but going back to my earlier post, if you use the impulse response measurement you can let REW set EQ for you which can give you better results than what you can hand calculate. The way it derives its EQ is based on the impulse response and not a simple inverse of the frequency plot. Eyeballing EQ based on the RTA measurement is only going to focus on frequency. It should give better results.
 

Leo Eskelinen

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Great feedback.
I really appreciate your thoughts and that you share your knowledge.
One major goal is to be able to create measurements that reflect real life listening impressions.
I’ve used the REW EQ to create automatic adjustments to match my house curve.
It is very useful and powerful tool which also give an insight of combining filters with different Q values.
In my DSP i can import these filters to each individual channel.
The restricting factor is that i cannot add a additional filter created by REW EQ to a previously created filter setting in the DSP.
All imports delete already made settings.
Kind regards, Leo
 

Matthew J Poes

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Measurements that are consistent with our subjective is important, but man is that a tough topic to tackle. There are a number of problems with the idea that make it as much an art as it is a science. I think groups like Harman have helped really make it a better science. I do a similar kind of work, measuring human perception in an objective way.

Let's take something concrete for a minute and make it a little fuzzy to show the point. Depression is a disorder, its a kind of mental disease. It isn't caused by bacteria or a virus, nor can we do a blood test to measure its existence. Yet clearly some people are depressed and some people are not. It's a thing and its not good when people have it. How do we measure it. Well, I can create a measurement tool to examine a person's behavior and develop a scoring rubric that assesses if someone is depressed. I can create a questionnaire as well, people fill it out and I create cut-off scores that reflect different levels of depression. On the face of it, these seem like objective scientific tools, but who says those scores reflect depression? How can I assert that? There are ways to do that of course, we can validate the tool against a known accurate measure. For example multiple trained mental health professionals might use an accepted diagnostic criteria to classify someone as depressed or not and we can compare that score to my tools. If the accuracy of my tool, measured by its specificity and sensitivity scores are sufficiently high, I can assert these as objective measures. Acoustic measurements are a step beyond that, they are more like a blood test, but the problem is, who says those measurements reflect what I hear? Who says that flat is better? We can only know that by developing a criteria for interpreting the measurements and validating them against a known accurate tool, what trained listeners tell us they prefer. I think the problem is we have a lot of bad objective tools. THD and Harmonic Distortion is a bad objective tool because it measures something that doesn't correlate well with what we hear. There are better objective tools but they simply aren't used. Our overall room measurement technique may also be a bad objective tool. Binaural measurement and 3D acoustic measurement are more accurate representations and bring in additional important information. I also have concern with some of the validations. My main work right now is looking at the non-average in a study. Studies by design, including those that validated what is "good sound" rely on an average. What did the average person prefer? We don't care what the unusual person preferred. The assumption is that the average really reflects what the majority of people like. That may not be a valid assumption at all, and in fact, even if it is, it may leave out a lot of non-average people who like something different for a valid reason. In my own work we are designing different trial designs that allow us to untangle why our average doesn't apply to everyone and find patterns of association between subgroups which can be directly explained by a particular mechanism. It may be that there are in fact different preferences that are not actually personal (meaning everyone is unique) but specific to measurable attributes of a person, their speaker type, or their room.

Measurements are objective, we all know this. But there is a problem. Not everyone is equally good at taking them or interpreting them. So while the measurement is objective, it may not be right or valid. Now even if we can assume that the measurement is right, it IS NOT an objective representation of what we hear. Our ears are a) not in the same place as a single microphone position, and b) does not hear the way a microphone does. We have to approximate through measurements what our ears are actually hearing and then use our own understanding of hearing to interpret what the objective data is telling us. That is really the hard part. Averaging over an area around our head is thought to do that, but I would just say its still an imperfect approximation. I used to use the exact method you use for in room measurements, which I took from Stereophile. I stopped doing it a while back and began using a more methodical approach. Partly because handling the mic alone can cause problems with the measurement. Best to avoid any kind of extra handling and potential shadow in the response. Instead what I did was start doing as I mentioned, I setup a kind of grid around my head, including dead center. Then I just took lots of measurements over and over. This let's me not only average all of them, which is the same as the spatial averaging with an RTA, but also examine the individual ones. I like having all that information saved. Of course, none of this gets at interpretation and its the interpretation that gets at what we hear. That's really tricky, I'm not even sure the research is so strong there yet. We know what kind of in room measurements people perceive as better vs not. Smooth and with a 1db's per octave rise between 20khz and 20hz, meaning the bass should be 10db's louder at 20hz than the treble at 20khz. That's just one piece of information though.

I made the argument in my article on distortion that linear distortion is the biggest concern and non-linear distortion is a minor concern. That is true based on the assumption of a competently designed speaker system. What if they aren't? What if its someone's DIY affair and they didn't know what they were doing? What if its a car and the system uses all active crossovers and the person didn't know what they are doing? You could make a system that measures with a perfect frequency response and have it still sound terrible. In fact, I recently listened to a speaker that, while doing a lot of things right, sounded bad to me and another reviewer. I didn't have a chance to fully explore but suspect there is a distortion of some kind I didn't measure. It's frequency response was very flat and its harmonic distortion was unusually low. Those weren't the problem. Yet the speaker had a kind of harshness to the treble in the 3-5khz range.
 

Leo Eskelinen

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This is poetry to my ears!
Thank´s a million. I absolutely agree with you.
I attached a document about Moving Microphone Technique that i find interesting.
This method together with "Exponential 0.88" average setting in the RTA window gives me a suitable level of display inertia to get a stable reading.
 

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mojozoom

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I also would rather use sweeps for measurement in the car but find that it's just not consistent enough in the upper frequency range. The comb in a car is just vicious - if you move the mic even a fraction of an inch you get very different results.

Taking a bunch of sweeps with different mic locations and averaging them is possible, but very time consuming. If I take 8 sweeps and average them for my three way system with a sub that's 56 sweeps and some time spent averaging them together afterward, and the phase data all gets dropped anyway one they get averaged together. It's still probably an hour of work though.

The RTA method allows you to start averaging and then move the mic around your ear for a while until the graph stabilizes, maybe 10 seconds. Then on to the next driver. You can test the entire system of 7 drivers in under 5 minutes.

The results you end up with from each method don't look the same, but they're pretty close. I don't know which is really "better", as neither allows you to see a phase plot, only amplitude.
 

Matthew J Poes

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I also would rather use sweeps for measurement in the car but find that it's just not consistent enough in the upper frequency range. The comb in a car is just vicious - if you move the mic even a fraction of an inch you get very different results.

Taking a bunch of sweeps with different mic locations and averaging them is possible, but very time consuming. If I take 8 sweeps and average them for my three way system with a sub that's 56 sweeps and some time spent averaging them together afterward, and the phase data all gets dropped anyway one they get averaged together. It's still probably an hour of work though.

The RTA method allows you to start averaging and then move the mic around your ear for a while until the graph stabilizes, maybe 10 seconds. Then on to the next driver. You can test the entire system of 7 drivers in under 5 minutes.

The results you end up with from each method don't look the same, but they're pretty close. I don't know which is really "better", as neither allows you to see a phase plot, only amplitude.
I can completely understand why you would prefer to spatially average. It is a lot quicker. I was just expressing my concern with its accuracy as compared with other methods.

To obtain a phase plot from a spatially averaged measurement you would need to use vector averaging. I'm not sure how I would feel about using that for actual crossver work though. I might know someone I can ask about measurements in a car when precision is needed. The comb filtering of Fourier methods is a well known problem and it's well known that this is not reflective of what we hear. It only shows up when you have early reflections in the first Ms or so and so isn't an issue in anechoic conditions. Which is how crossovers and EQ are typically designed at such high frequencies. It is possible that cars are measured differently to address this. If I can find anything out that is adaptable to the DIY community I'll pass it along. I wonder if reflections are temporarily absorbed during initial measurements?
 

Sonnie

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Great feedback.
I really appreciate your thoughts and that you share your knowledge.
One major goal is to be able to create measurements that reflect real life listening impressions.
I’ve used the REW EQ to create automatic adjustments to match my house curve.
It is very useful and powerful tool which also give an insight of combining filters with different Q values.
In my DSP i can import these filters to each individual channel.
The restricting factor is that i cannot add a additional filter created by REW EQ to a previously created filter setting in the DSP.
All imports delete already made settings.
Kind regards, Leo
Resurrecting an old thread here... hoping Leo is still around and can chime in.

You mentioned not being able to add an additional filter to a previous created filter. Coudn't you simply add another filter in addition to the previously created filter... manually?

Also... how are you generating the mono pink noise... do you have it recorded on a USB or disc and play it back thru your headunit... or are you tapping from your laptop input to an input on the headunit?
 
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