Frequency Response Way Off After Band Limited Pink Noise?

W

Wbgrigs

Noob
Hello, so I've been researching and progressing slowly throughout the last couple months, using the forums and massive amounts of Youtube to learn the techniques and process of building a SQ system, and I am stuck now on one of the closing steps of tuning. System so far:

2016 Dodge Challenger base 6-speaker, 5" uConnect HU
Highs - CDTAudio Unity 8s installed in dash firing into windshield
Mids - 6x9 Kenwood Excelon woofers that came in the KFC-XP6903 component set
Rears - Kenwood KFC-1666S being fed a differential (L-R & R-L) signal and -15db attenuated
Sub - Kicker Comp R DVC 10" in a Sound Ordnance vented Box tuned to 35hz

Helix DSP.3S fed high-level signal off the uConnect
Alpine BBX-F1200 for Highs/Rears
Pioneer GM 6702 for Mids
Kicker CXA800.1T for the Sub

Ive been using Zazzi's tuning chart to make a custom House curve that is close to Andy's, without the attentuation at the upper end due to running a tweeterless setup. Ive spent a fair amount of time doing and redoing different tunes, basically going in circles, trying to get better with the EQing and figuring out my own musical preferences. Ive finally got to a point to where I'm happy with each speakers response, and with how close each side matches the other as well as the house curve:
View attachment 17953 View attachment 17956
Love the way it sounds, now just trying to nail down that center image because it was easy to hear the stage drift from center to left during sweeps, so after EQing with band-limited pink noise I have this:
View attachment 17955
Is this correct? After getting my curves matched should I stop using FR-matching with an RTA to guage if my car is tuned correctly? At this point in the tune do these measurements mean less than my ears? I just want to know when to stop chasing my own tail. There was another post recently addressing the same question, and all the answers said that everyones FR matched just as much before band-limited tuning as after, which makes absolutely no sense to me. If i am cutting the EQ per band to get my center image true at all frequencies, then these measurements CAN'T match afterwards, unless I am fundamentally mistaken on this process? Thoughts, tips, any answers are appreciated, thanks!

- I use REW 1/48th RTA with a Dayton UMM-6 for measurements
 
Your ta could very well be a few cm off if the stage bleeds to the left, fine tuning ta after setting by a tape measure is something I always do, high freq move out of phase quicker than low with ta being off slightly, and the impression that the stage pulls to one side or the other is often simply down to ta being set off a touch, our ears don’t hear the same way a tape measure measures put simply as our auditory system is very good at

set the eq back to how it was when you had it good

And then add small amounts of delay to the ta on the left driver, and there will be a point where the high freqs are focused and in line with the low freqs

also don’t go down the line of assuming the centre should be the centre of the car… the centre should be exactly in the middle of where the drivers sound like they are, ie the stage boundary’s not where the drivers are necessarily, turn off the left wideband and listen for where the right driver presents as, is it diffused or a point source, chances are it will have some spread, normally high freqs drifting inwards, this can also be an issue and cause the high freqs to move left at the centre location

it's lots to take in but there are a number of reasons why what your saying happens, and not all easy or possible to solve
 
There are definitely just a handful of the 1/3rd-octave PN bands where I hear the drift, and almost universally it is a drift from center to left-center, some more than others. It starts around ~200hz, where it will shift left and back until ~4khz or so. After 4khz the center doesn't waiver at all, pinpoint spot below the rearview mirror. It is easiest to hear during a log sweep, where the sound moves almost visually before my eyes, but can also be noticed with videos such as this one:

https://youtu.be/dNdhDCv4mzc

Where the glide from left to right is bunched up on the left side, keeping those 4 tones in a 14 inch section of windshield in front of me, until it gets to center and plays out equally spaced as it should through the right side. T/a adjustment could very well be the answer to my problems, I resisted adjusting it at all because of a few quotes in Andy's Tuning Guide referencing that situation specifically:

"Once you’ve set your delays to correctly compensate for distance, leave them alone. They aren’t wrong." - p. 17
"Do not adjust the delay settings to move the image if you’ve set them correctly with the tape measure. Delay corrects for distance, it’s not an image panning tool. That’s what level controls are for.Chances are, if you’ve followed these steps, you may only have to adjust the level of the channels slightly to center the image." - p.57
 
I know what Andy’s guide says, but I’ve done it way too many times to not know it’s accurate and the tape often ends up being a few cm off, even measuring impulse response it is a little bit off… Andy actually makes mention in his guide of fine tuning the ta a cm or two to focus the image…

andys guide is a simple tuning guide, adjusting the drivers side down to centre the image is not correct for me, it’s a bodge

it also counts on both left and right drivers acting like point sources and not being diffused and ‘wide’ see last note below

So if the freq response is matched via rta and sounds identical to the ear, then left and right are the only definite you know for sure

centre should be perfectly in the middle between the two stage boundary’s if ta is set correctly AND eq is also set correctly, if it’s not one or the other isn’t set correctly, the end… that’s science and factually accurate…

play music in mono and send it to both left and right, if you swap between listening to left and right does the tonality change? If so it’s eq… if not and it sounds identical then it’s ta, it’s that simple

you also need to listen to the left and right and see how point source they are, do high/low frequency’s move onwards? Do some sounds come from the side window area? If so that’s a driver position that is in a bad spot
 
Wbgrigs said:
Is this correct? After getting my curves matched should I stop using FR-matching with an RTA to guage if my car is tuned correctly? At this point in the tune do these measurements mean less than my ears? I just want to know when to stop chasing my own tail. There was another post recently addressing the same question, and all the answers said that everyones FR matched just as much before band-limited tuning as after, which makes absolutely no sense to me. If i am cutting the EQ per band to get my center image true at all frequencies, then these measurements CAN'T match afterwards, unless I am fundamentally mistaken on this process? Thoughts, tips, any answers are appreciated, thanks!
Click to expand...

Welcome to the really fun part!

If you're doing the bandwidth limited pink noise trick like I think you are, then yes, the frequency response will change because you are ... changing it. You're not crazy, this is how it works. After you get the measurements to match your targets, and you are also satisfied with the tonality, then you can put away the microphone.

The trick to the centering is to boost and cut in pairs. If you cut -1dB of some frequency on the left, then you must also boost +1dB of that frequency on the right. This way the overall tonality should be similar, but the imaging will shift slightly toward where you want it.

Also, it's okay to use time delays to shift your image a little bit. A few tenths of a millisecond is no big deal in terms of phase response and time aligning speakers "correctly", but a few tenths of a millisecond is the difference between good center image and sensational center image. This fits with my other philosophy of there is no "wrong" way to tune a system so long as you enjoy the result. There are certain methods, however, that might be faster or easier to repeat or easier to teach someone : )
 
Justin Zazzi said:
Welcome to the really fun part!

If you're doing the bandwidth limited pink noise trick like I think you are, then yes, the frequency response will change because you are ... changing it. You're not crazy, this is how it works. After you get the measurements to match your targets, and you are also satisfied with the tonality, then you can put away the microphone.

The trick to the centering is to boost and cut in pairs. If you cut -1dB of some frequency on the left, then you must also boost +1dB of that frequency on the right. This way the overall tonality should be similar, but the imaging will shift slightly toward where you want it.

Also, it's okay to use time delays to shift your image a little bit. A few tenths of a millisecond is no big deal in terms of phase response and time aligning speakers "correctly", but a few tenths of a millisecond is the difference between good center image and sensational center image. This fits with my other philosophy of there is no "wrong" way to tune a system so long as you enjoy the result. There are certain methods, however, that might be faster or easier to repeat or easier to teach someone : )
Click to expand...

The "cut-one-side-&-boost-the-other" is definitely news to me! Makes sense when working towards the goal of centering the sound, might as well work from both ends rather than just adjusting the strong side down. This will still affect the frequency responses, but I understand that they've served their purpose already. And as far as sweating the small stuff goes, what I am after is the 'sensational.' I love this as a hobby, and honestly don't think there will ever be a time where I sit and listen and say "Yep, there it is, its perfect," and be done. The knowledge-ceiling on car audio is 200 stories high and I'm still cruising in the first 10 floors, which kind of makes this more fun, knowing that I'm not going to run out of skills and concepts to learn, probably ever.
I think what I really need to do is go to a local IASCA meet or just find time to get up to 5 Star Car Audio as their shop is ~20 minutes away and get the oppourtunity to hear a real, professionally tuned SQ car. Gotta hear what's possible with the best installs and tuning, give me something real to work towards. As of right now I've never heard any other car that has a DSP, much less actual tuning time.

When tuning with band-limited PN, is it done with speaker pairs, or whole sides playing? The Unity 8s are handling the lion's share of the spectrum, just curious if people do it with highs then mids then rears or side vs side
 
I have always saved band limited PN for the last part of tuning. Once everything else is dialed in and close to perfect center image. I do band limited PN with all speakers playing at the same time. Let it play as a sweep from 20 Hz to 20,000 Hz. Sit with your eyes closed and visualize where the PN is located on the windshield. If they are all over the place then you have bigger problems. If they all fall within 6-8 inches of center then it is OK to adjust EQ a little.
 
Wbgrigs said:
The "cut-one-side-&-boost-the-other" is definitely news to me! Makes sense when working towards the goal of centering the sound, might as well work from both ends rather than just adjusting the strong side down. This will still affect the frequency responses, but I understand that they've served their purpose already. And as far as sweating the small stuff goes, what I am after is the 'sensational.' I love this as a hobby, and honestly don't think there will ever be a time where I sit and listen and say "Yep, there it is, its perfect," and be done. The knowledge-ceiling on car audio is 200 stories high and I'm still cruising in the first 10 floors, which kind of makes this more fun, knowing that I'm not going to run out of skills and concepts to learn, probably ever.
I think what I really need to do is go to a local IASCA meet or just find time to get up to 5 Star Car Audio as their shop is ~20 minutes away and get the oppourtunity to hear a real, professionally tuned SQ car. Gotta hear what's possible with the best installs and tuning, give me something real to work towards. As of right now I've never heard any other car that has a DSP, much less actual tuning time.

When tuning with band-limited PN, is it done with speaker pairs, or whole sides playing? The Unity 8s are handling the lion's share of the spectrum, just curious if people do it with highs then mids then rears or side vs side
Click to expand...

Hah! I love that, I know the feeling. It took many years before I decided my car was the way I liked it, and I haven't touched it since because I know how much work it was to get it juuuuust right.

Good for you too, wanting to learn more and get involved in the community and attend a meet. This is absolutely the #1 best way to listen to other cars and meet people and learn what is possible. My car only sounds the way it does because I've listened to hundreds of other cars and carefully chose which aspects of them I liked and didn't like, then formed an ideal target in my mind. So yes, please, absolutely go meet and listen to other systems and see what's possible!

When tuning with band limited pink noise, I like to have all the speakers playing. I will sometimes have the luxury of a global equalizer where I can adjust all the left channels at the same time and all the right channels at the same time. If I don't have a global EQ, then I will make adjustments to all the left channels equally (like cut -1dB on front left woofer and midrange and tweeter). At this point I do not want to change the way the individual channels relate to eachother, I only want to change the balance between all of the left vs all of the right. I might also make small time delay changes too, but adjusting only one side is needed for that.

My favorite trick is to put something on the dashboard (like a rock) where I want center to be, or a piece of tape on the windshield, etc. This gives me a very clear way to judge if a noise is just left or just right or right on.
 
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