# A comparison of various measurement methods



## npdang (Jul 29, 2005)

I've seen the argument regarding whether or not a "flat" response sounds good or not. The one thing I've rarely seen mentioned is that how you go about measuring frequency response in a car will have a huge impact on the results. Let's take a quick look at a few methods and see how changing a few measurement parameters can have such differing effects on the frequency response.










The yellow line represents your "typical" measurement window of 300ms (this simply means that you're recording samples of 300ms in length, and averaging all the energy within that time frame), with 1/3rd octave smoothing (basically averaging all the energy within 1/3rd octave widths, where an octave is a doubling of frequency; this provides a smoother less jagged response as narrow peaks and dips are averaged to a flat line). Using such a large time window of 300ms ensures good resolution below 100hz, but will include nearly all the reflections in the midrange and treble frequencies. 

The blue and red line both use what is called a "flex window". Simply put, it's 3 separate plots spliced together. The first plot from 2khz includes little or no reflections. Below 2kz to 50hz, the response includes gradually more and more reflections as you move down in frequency. And below 50hz all reflections are included. The reasoning behind this is that it better reflects the way our brain processes sound tonally, in that we include only the initial arrival and early reflections in what we hear tonally, while later reflections give us cues about space.

Now the difference between the red and blue line is that the red line represents a spatial average (2 different measurements taken at 2 different locations averaged together) of 2 different measurements taken to each side of the driver's headrest. The blue line is only a single measurement taken directly in the center of the driver's headrest. As you can see, there are some significant differences between 500 and 1khz, and 2 and 5khz.










Up next we have our typical RTA measurement with 1/3rd octave resolution and infinite averaging. RTA's generally operate with a 300ms time window. I like to turn averaging on because it gives better noise immunity. 










An rta with b weighting enabled. B weighting filters out a good amount of bass below 200hz and treble above 5khz. It's meant to better approximate how our ears work at moderate listening levels.










An rta with c weighting enabled. C weighting filters out less bass and treble than B weighting, and approximates how our ears work at higher volume levels.

I hope you can see that there are many ways to measure frequency response that give significantly different results. Chances are if a flat response doesn't sound good to you, then you're not measuring correctly! ...not that anyone knows what that is


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## FoxPro5 (Feb 14, 2006)

Do you use a dash mat in your car NPD? If not, would the addition of one yield better results on the RTA?


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## npdang (Jul 29, 2005)

You know I've measured a dash mat in several cars with kickpanel located drivers and it made absolutely no difference. Putting a pad over the center console did make a large difference though.

I don't have a dash mat for my car, but I'm sure with dash mounted tweeters it would have a significant effect.


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## FoxPro5 (Feb 14, 2006)

npdang said:


> You know I've measured a dash mat in several cars with kickpanel located drivers and it made absolutely no difference. Putting a pad over the center console did make a large difference though.
> 
> I don't have a dash mat for my car, but I'm sure with dash mounted tweeters it would have a significant effect.


Ahh, sorry I thought the tweeters were mounted up high. :blush: 

But thanks for the input. I don't think I can live without mine now that I've gotten used to it.


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## shinjohn (Feb 8, 2006)

Hey N-
Thanks for the awesome data!!! It is really informative, and I'm learning. I've got some experience with weighting scales, and know their importance. A couple of questions on your first plot though:

1) Assuming I'm interpreting your data correctly, during your flex window measurements, what window sizes did you actually use for the three bandwidths?
2) What input source did you use, and how was it applied? (was it continuous, sweep, burst chirp?)
3) Why measure the top end with little or no reflections, when the reality is that in a car, it's one of the things everyone seems to talk about tuning to compensate for?
4) Given all the data above, what will be your strategy for "flat tuning" your car?


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## Whiterabbit (May 26, 2006)

shinjohn said:


> 3) Why measure the top end with little or no reflections, when the reality is that in a car, it's one of the things everyone seems to talk about tuning to compensate for?


Someone is still smarting =B

I'd say a pretty good reason would be to compare to a measurement that you know includes resonances. Gives you a very good quantitative idea what frequency ranges are affected by said resonance.


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## durwood (Mar 7, 2007)

what about using a real dummy for testing?  I've always wanted to try this since it should be more realistic. The damn things have synthetic ears modeled after real human ears. They are expensive though


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## npdang (Jul 29, 2005)

The ultimate fail proof strategy... use your ears  I hate to say it, but I haven't really found any measurement strategy that's nearly as good. I generally measure to get perfect time alignment, check driver phase, and to level match and ensure that my crossovers are functioning properly. I also use it to look at broad patterns or spot *possible* areas of concern. I will never look at a plot and say hmm that spike at 1khz is definitely a problem, only a possible problem that should be investigated and if my ears don't like it I leave it alone.

I use a chirp stimulus for my measurements because it's fast and has good noise immunity. Swept sine would take forever! MLS is good too, but chirp is just better.

The statement I made earlier was a bit misleading... I measure all the early, strong reflections but NOT the low level reflections that come later in time. The reason behind this is if you average all those little reflections it will show your tweeter level down quite a bit, because you're averaging all these low level reflections into your measurement. Look at this plot of the impulse response (yellow line) for my tweeter:










I only include the heavy early reflections from 7ms to 9ms, but not those little squiggles past 9ms because 1) they come much later in time 2) they are of such low amplitude that it'd screw up your frequency response if you averaged them and 3) because your ears tonally only include the first few heavy reflections and not reflections that come later in time.

The windowing I used was 9ms, then 100ms, and then 1 second.


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## andycph (Dec 21, 2005)

3) Why measure the top end with little or no reflections, when the reality is that in a car, it's one of the things everyone seems to talk about tuning to compensate for?
[/QUOTE]

I was wondering the same thing, but it seems the the time window is critical in the FFT of the impulse reponse. From the reference manual of MLSSA:

"When assessing the spectrum of a sound source (as opposed to the spectrum of room reverberation), the ear tends to ignore late reflections in the treble region while giving them some weight in the mid-range and much more weight in the bass region."

Check this out, I pasted it from the manual which I'm sure some of you guys may be interested to know more about time windows:








[/IMG]


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## t3sn4f2 (Jan 3, 2007)

durwood said:


> what about using a real dummy for testing?  I've always wanted to try this since it should be more realistic. The damn things have synthetic ears modeled after real human ears. They are expensive though


Alpine uses one of those when they tune their show cars, saw it on that TLC show "Rides" when they did a program on the build.


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## B&K (Sep 20, 2005)

durwood said:


> what about using a real dummy for testing?  I've always wanted to try this since it should be more realistic. The damn things have synthetic ears modeled after real human ears. They are expensive though


They are only $25k, but B&K does make a headphone like apparatus that has mics instead of cans that is closer to $5k. Then it would be the transfer function with you in the car. I used to be able to use them for free 

I just noticed this thread, now but earlier today was trying to have the same conversation. It seems to me there are a few logical ways to do this:
1) MLS or some other gated FFT to get the true impulse response. Advantage is that you can ignore reflections. Disadvantage is that those reflections might make a difference.
2) Long average on white/pink noise for FFT/nth octave analysis. 
3) A periodic chirp. Basically an FFT that is flat at all frequencies converted by running an inverse FFT to get the signal into a time domain. Then this signal is played through the speakers and the response is measured with a microphone. Conveniently since the whole time signal is aligned with your FFT lines no windowing is required like when playing back noise preventing leakage.
4) Stepped sines or a steady state sine algorithm. Basically stepping a sine at quantized frequencies until you have a measurement value at each one. I used steady state as typically you would set your averages so that at each sine tone your response within your averaging window is no longer changing.

Of course there are others, but each technique is going to yield completely different numbers and responses. I always find it amusing when people just use noise and a short average 1/nth octave calling it a RTA analysis as if it is actually good for something, especially considering that most RTA's are not true 1/3 octave analyzers but FFT analyzers that are synthesizing the 1/nth octave. A change in the FFT time window or resolution will greatly affect that result.


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## mattyjman (Aug 6, 2009)

good read


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