# RTA the right tool for acoustic measurement



## Niick (Jun 3, 2015)

What an RTA does. And what it doesn't do.

An RTA, or real time analyzer, takes the incoming audio signal and breaks it up into bands, usually 1/3 octave wide, and shows the amplitude of the audio information that falls within each band. It cannot distinguish between direct arrival energy from the speaker to the mic, and reflected energy that arrives at the mic some measure of time later. Our human hearing mechanism does, believe it or not, distinguish between direct arrival and later reflections to a varying degree that changes with both frequency and how late in time those reflections might occur, the study of which is referred to a Psychoacoustics. 

Therefore, if we are trying to use an RTA to tune a system, we are sadly mistaken if we think that the RTA's read out in some way correlates with how we hear. It doesn't. In addition to that, without the ability to tell time, it cannot measure phase. Phase is the phenomenon that directly controls how two speaker drive units coexist. It also directly affects how reflected acoustical energy combines with direct energy from the speaker at the listening position to affect what we hear. EVERYTHING affects phase. Both passive and active crossovers, eq, delay, physical placement, etc. Understanding phase and how to measure it is the key to successful system optimization. RTA's cannot, never could, and never will, be able to make these kinds of critical measurements. 

As an installer, if my intention is to be able to perform no compromise audio system installation and tuning, this then cannot be realized without the proper tools, and knowledge of how to use those tools. Once you hear a system that has been properly phase aligned, there's no going back. The rest of the audio industry has abandoned the RTA as their main tool of acoustic measurements literally DECADES ago. Yet, for some strange reason, the car audio industry continues to embrace the old dinosaurs as if they're still high technology. So, if an RTA is not the right tool for the job, then what is? 

Glad you asked...........

Dual channel FFT. 

With dual channel FFT analysis we are able to COMPARE any two signals. That is, we can compare the sound picked up by the microphone to the signal sent to the speakers, and see how the system under test has changed the signal passing thru it. Or we can measure the input vs. the output of a signal processor or passive crossover to get an EXACT measurement of eq. curves, crossover points and slopes, and phase. With dual channel FFT we can precisely set delays in order to phase align driver to driver interactions, like sub to mid bass, mid bass to midrange, midrange to tweeter. When all of the individual drivers in a system are working together, in phase, thru crossover, now you CAN shape its freq. response with EQ. 

It's often been said that when applying EQ, don't try to raise the dips, just try to lower the peaks. Well now, with a properly phase aligned system, those dips will largely disappear, because they were induced as a result of driver to driver cancellation. See, in car audio we rarely have the luxury of precisely choosing the mounting position of our drive units, relative to each other. Tweeters go in the factory locations, same with mid bass, subs go way in the back somewhere, several feet behind, not to mention the different directions all the drivers are facing and all the barriers they are playing thru and big pieces of glass they're reflecting off of to reach your ears. Once the sound from all the individual drive units reach your ears, the overlapping regions of the frequency range where two or more drive units are playing the same signal, the acoustical crossover, will almost always have big dips in the frequency response that just cannot be EQ'd away. 

Those dips are the result of phase cancellation. 

The individual drive units are not in phase thru crossover. 

With dual channel FFT we can measure the phase of the individual drive units, zero in on the critical crossover region, determine which driver is leading, and adjust that driver's delay either in the processor or the head unit until the two drivers in question are working together rather than against each other. Or, in instances where we are doing really customized work, like building custom kick pod locations or door pods, now we can easily determine what would be the best positioning for our tweeter to midrange relationship. Now those crossover dips, gone! 

See, you CAN raise dips, just not with EQ. Of course you still might have a reflection induced cancellation or two, but now you'll be able to tell exactly that, it's reflection induced, not driver to driver induced. Reflection to direct arrival interaction is an advanced topic for another day, but it's a topic that CAN be addressed, now that we have dual channel FFT analysis at our fingertips. 

Make no mistake, these types of software require considerable effort to learn how to use, factory training is almost mandatory to get the most out of them. But it's worth it, if you want to be able to say that you analyze vehicle acoustics, and you properly tune systems, you're just lying to your customers and to yourself if you attempt to do these things with a 1/3 octave RTA.


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## jtaudioacc (Apr 6, 2010)

it's late and i honestly got lost in that huge run on i didn't get through it. 

but i got the gist of what you are saying. i'm sure you'll get a decent amount of replies. lol


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## sirbOOm (Jan 24, 2013)

Have we abandoned paragraphs in car audio along with RTAs? 

Dual channel FFT sounds interesting. Still waiting for a tuner that just does it its damn self with a nice result, cuz this is 2015.


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## Niick (Jun 3, 2015)

That was actually an email that I wrote a while back, that I never sent because I thought it was too long, and would have just confused the person it was intend for. Then when I saw that there was a "car audio myths" section, I thought I could make use of it here. Maybe get some counterpoints.


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## gijoe (Mar 25, 2008)

You'll get counterpoints if people read it, but until you edit it and break it into paragraphs most people won't.


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## seafish (Aug 1, 2012)

very interesting, but please define "FFT".


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## Niick (Jun 3, 2015)

Thank you for the tip, I think you're probably right. It is pretty damn long! I'm at work right now, will edit when I get a chance

Ok, I broke it up into paragraphs, hope it's a little better!


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## Jepalan (Jun 27, 2013)

I agree it is important to understand what an RTA is (and *isn't*) and how to us it. This applies to *any* tool.

Note that you can also use "dual channel" RTA analysis to compare signals. 

What advantage would FFT analysis have over RTA if the user only has a GEQ to tweak the install?

There is a really good discussion on this topic here:
http://www.diymobileaudio.com/forum/technical-advanced-car-audio-discussion/98389-frequency-response-car-fft-vs-rta.html


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## Jepalan (Jun 27, 2013)

This also seems to be a good discussion of the topic for those that wish to read further...
AV: What’s The Measurement? Understanding And Properly Using RTA & FFT - Pro Sound Web


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## Niick (Jun 3, 2015)

Wow, I would have guessed people would be all over me for saying their beloved RTA's didn't belong in modern acoustical analysis, but I'm pleasantly surprised. Maybe I'm the one who is wrong, maybe people aren't relying on 1/3 octave RTA (for car audio system optimization) as much as I thought. Hmmmm..........

In response to the GEQ question, I use a dual FFT program called SysTune at work almost daily, for all kinds of things, finding factory speaker/signal wiring polarity, determining the wiring polarity of any random input on a customers vehicle who might have had something installed somewhere else, QUICKY determining the best wiring polarity for tweeters in 2 way component installs, or mid's and tweets in 3 way installs, testing factory wiring to determine its suitability as an input signal for aftermarket amplification, just all kinds of things...basically, the days of EVER guessing, or listening and adjusting, listening and adjusting, listening and adjusting, and on and on, in order to really tune a system, those days are thankfully long gone. 

I DO NOT believe that you should have to tune anything by ear, you certainly can if you want, but the way I look at it, it's not a matter of opinion or taste, my job, is as a waveform delivery service, I am to deliver the waveform from the recording, thru the electrical domain, and into the acoustical domain in the position of your head as closely to unaltered as can be done with the equipment you purchased in the vehicle it's going into. That's it. If the customer wants to add a little bass, or a little treble, then that's up to them, that's what basic tone controls, or a subwoofer bass knob is for. But the real heart of the system, from the wiring decisions, to speaker placement, to any xovers or DSP adjustment is not to suite a customers listening tastes, it's to deliver the recorded waveform as perfectly as is possible. Those things are tools, a means to an end. And without dual FFT there is no way I could get to that end anywhere near as quicky as I do, if at all.


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## jtaudioacc (Apr 6, 2010)

i'm not sure if you thought the overall IQ of sound, tuning and installation was weak overall on diyma, but i assure you, it is not.


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## Jepalan (Jun 27, 2013)

jtaudioacc said:


> i'm not sure if you thought the overall IQ of sound, tuning and installation was weak overall on diyma, but i assure you, it is not.


Exactly.

OP, you might want to do some more exploring here - you may be surprised with the level of sophistication of installs, tunes and tools at DIYMA. Many of us use calibrated mics & a SW tool called RoomEQ Wizard (REW), which is much more than a simple RTA. It can perform 3 dimensional analysis, FFT based analysis, RTA-like analysis, and much more.

Read on...

http://www.diymobileaudio.com/forum/how-articles-provided-our-members/163234-first-timers-guide-measuring-your-system.html


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## 14642 (May 19, 2008)

Of course there are things that cannot be measured DIRECTLY with an RTA. There are also things that are measured incorrectly (in terms of correlation with what we hear). That doesn't mean that it's a worthless tool and should be abandoned. 


Should I throw away my adjustable wrench when I buy a set of wrenches or sockets? 

The statement in the original post that I really object to is the idea that we should be adjusting delay to alter the phase at the crossover. Delay should be used to correct for the effects of differing distance between the listener and the speakers. An all pass filter is the one that should be used to change phase at the crossover, but that shouldn't be necessary. Set delays according to distance, choose LR4 for both high and low pass and EQ the various speakers to match that target closely. Then, they'll sum correctly--or pretty close, so long as you don't have some massive reflection problem.


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## Niick (Jun 3, 2015)

So, I guess I should have been a little more precise, what I was referring to was, for instance, using the phase trace as a more accurate means of insuring that you have good summation thru crossover, because not always is one able to use the impulse response to line up arrival times, especially when low freq. are in question. in an ideal word, all car audio systems would be fully active and have multi-channel processors with all pass filtering as an option, but, far more often people might just have a deck with the ability to add delay to the pre outs. When this is the case, using that delay to correct for the distance differences that manifest themselves as phase shift thru crossover is ABSOLUTELY better than not doing so. You could TRY to measure the physical distance from your subwoofer (which is facing a different direction and firing thru a setback/rear deck-lid) to your head, and "math out" the difference to your midbass driver in the door, which doing so DOESNT take into account the phase shift brought on by whatever filters your sub and midbass are seeing, or you can be smart about it and use the phase trace to determine how much delay should be added to the leading driver to get good summation. And anyway, whether you use all pass filters or delay or passive inductors and physical positioning, or WHATEVER, to get good phase coherence thru crossover, my point was that there's no way to tell EXACTLY how much, and even sometimes, to what driver, the correction needs to be applied, with a low resolution, time blind, 1/3 octave RTA.


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## Niick (Jun 3, 2015)

https://bobmccarthy.wordpress.com/2...-of-subs-why-i-dont-use-the-impulse-response/

straight from the guy who literally "wrote the book" on sound system optimization. Yes, he's referring to pro-sound subs and mains, but it all still applies.


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## Niick (Jun 3, 2015)

jtaudioacc said:


> i'm not sure if you thought the overall IQ of sound, tuning and installation was weak overall on diyma, but i assure you, it is not.


Not at all, this is the "myths" section.........is it not?


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## Niick (Jun 3, 2015)

Jepalan said:


> Exactly.
> 
> OP, you might want to do some more exploring here - you may be surprised with the level of sophistication of installs, tunes and tools at DIYMA. Many of us use calibrated mics & a SW tool called RoomEQ Wizard (REW), which is much more than a simple RTA. It can perform 3 dimensional analysis, FFT based analysis, RTA-like analysis, and much more.
> 
> ...


I in no way think that people here aren't downright brilliant at times, that's why I posted this in the "car audio myths" section..........

I certainly did not mean to imply any disrespect.


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## Jepalan (Jun 27, 2013)

Niick said:


> So, I guess I should have been a little more precise, what I was referring to was, for instance, using the phase trace as a more accurate means of insuring that you have good summation thru crossover, because not always is one able to use the impulse response to line up arrival times, especially when low freq. are in question. in an ideal word, all car audio systems would be fully active and have multi-channel processors with all pass filtering as an option, but, far more often people might just have a deck with the ability to add delay to the pre outs. When this is the case, using that delay to correct for the distance differences that manifest themselves as phase shift thru crossover is ABSOLUTELY better than not doing so. You could TRY to measure the physical distance from your subwoofer (which is facing a different direction and firing thru a setback/rear deck-lid) to your head, and "math out" the difference to your midbass driver in the door, which doing so DOESNT take into account the phase shift brought on by whatever filters your sub and midbass are seeing, or you can be smart about it and use the phase trace to determine how much delay should be added to the leading driver to get good summation. And anyway, whether you use all pass filters or delay or passive inductors and physical positioning, or WHATEVER, to get good phase coherence thru crossover, my point was that there's no way to tell EXACTLY how much, and even sometimes, to what driver, the correction needs to be applied, with a low resolution, time blind, 1/3 octave RTA.


Yep, you can (and should) try to optimize the phase relationship between sub & mid-bass with time-alignment. But you can't fix room modes - and these can be nasty in car audio. 

There are some *excellent* discussions here on these topics. Here are two...

http://www.diymobileaudio.com/forum/technical-advanced-car-audio-discussion/75959-extra-delay-your-time-alignment.html#post966750

http://www.diymobileaudio.com/forum/technical-advanced-car-audio-discussion/169915-time-correction-philosophy.html



Niick said:


> I in no way think that people here aren't downright brilliant at times, that's why I posted this in the "car audio myths" section..........
> 
> I certainly did not mean to imply any disrespect.


No worries.


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## Niick (Jun 3, 2015)

Niick said:


> Wow, I would have guessed people would be all over me for saying their beloved RTA's didn't belong in modern acoustical analysis, but I'm pleasantly surprised. Maybe I'm the one who is wrong, maybe people aren't relying on 1/3 octave RTA (for car audio system optimization) as much as I thought. Hmmmm..........
> 
> I feel I should clear up my intention with my post.
> This being the "myths" section, I thought (maybe wrongly) that it would be a good place for what I perceived to be an ongoing "myth" of sorts relating to car audio. Now, understand that I only very recently joined DIYMA, but have read different topics on here from time to time.
> ...


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## Niick (Jun 3, 2015)

Jepalan said:


> Note that you can also use "dual channel" RTA analysis to compare signals.
> 
> [/URL]


What I meant was that the comparison done by the type of FFT analysis that I was referring to takes time into consideration, therefore, you can choose to see the information displayed as "spectrum" (1/1-1/98th octave)if you want, which IS your traditional RTA type display, just WAY higher resolution, but you can also choose to display the impulse response, transfer function magnitude, or, most importantly for this discussion, PHASE. You can see in real time how changes made to any parameter, be it physical or electrical, affects the phase response of the system under test. 

The ability to directly measure phase has so many far reaching implications......basically, think of what REW does, but continuously in real time.


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## gumbeelee (Jan 3, 2011)

Just read your thread and i have to say they are some very interesting points that u made.


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## Niick (Jun 3, 2015)

Thanks, I think that was one of the very first posts I ever made on this forum. In retrospect, probably not the best way to approach what I was trying to get at, which was a discussion of how/why the 1/3 octave RTA has remained such a mainstay in CAR audio when all other fields of audio stopped using such EXTREMELY limited tools decades ago. 

I think it comes down to the fact that Audio Control (obviously a car audio company) builds one, and it was used in IASCA comps for so long, and that most installers focus on learning to fabricate above all else, that it just took hold and refuses to let go. 

IN MY OPINION, a shop that considers themselves, or is considered by others, to be "high end", a shop that installs DSP processors/OEM integration, and yet uses one of these antiquated devices with a single mic is out of touch with the current reality of audio technology. 

I also think that the main reason for the fact that few shops have technicians who truly understand acoustic analysis/system optimization is because fabrication is what everybody wants to do, it's what's valued as a skill by their peers, and by the buying public. 

So they learn to fabricate, and ironically very often never learn how WHAT THEY'RE FABRICATING effects the sound, for better or worse. 

Ya know........as long as it looks cool. 

I don't share this point of view.


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