Precision time alignment using only noise tracks and your ears

Introduction

First of all, I’m not a professional, just someone who used to be obsessive about mobile audio. So, please forgive any misuse of terms or definitions. Digital time alignment (abbreviated as TA) was the driving force behind my first active system. Based on recommendations by this forum, I put together a laptop-based measurement setup to help with setting time delays.

Long story short, I was never satisfied with the results. I put away the microphone and cables, and decided to use a different set of measuring devices: my ears. I experimented for weeks, and developed a way to set TA by listening for specific cues. The results were fantastic. The reason my method, with practice, will achieve better time alignment than any sophisticated measurement system is simple: it’s customized to every individual, and their vehicle interior.

With correct application, this method achieves immediate improvements in imaging, staging, impact, and transparency. Bass should become thinner, but in a good way - lean, with great impact. The sub-bass shouldn’t come from the back of the car, or even up front – it should be completely unlocalizable. Midbass should be solidly up front along with the rest of the center image.

TA will be calibrated:

• Using your own ears
• Taking into account your own hearing (the same sound is heard differently by everybody... and much more so by microphones)
• With your head in your everyday driving/listening position
• Taking into account all in-cabin reflections and absorptions, including your own body’s effects

The method costs nothing but time and patience. An audio file of pink and white noise will be used for tuning and can be downloaded free here: Burn-in wave files: white noise, pink noise, frequency sweep, channel mix

The method assumes that your system is an active one, with each channel individually adjustable for time delay. It is developed and written for a 2-way front stage with a mono subwoofer channel and no rear speakers, though it can adapted for any other active system, no matter how many channels. Note that there is only one sweet spot per calibration.

Here’s a preview of the method for a typical 3-way system:

While playing pink and/or white noise through system,

1. Isolate sub and passenger side midbass (mute all other channels). Align drivers.
2. Isolate sub and driver side midbass. Align drivers.
3. Isolate driver and passenger midbasses. Verify time alignment and center image.
4. Isolate passenger side midbass and tweeter. Align drivers.
5. Isolate driver side midbass and tweeter. Align drivers.
6. Isolate driver and passenger tweeters. Verify tweeter alignment and center image.
7. Verify front stage alignment.
8. Normal up the system and evaluate.

Notice that alignment takes place between 2 non-like drivers at a time (sub/mid, mid/tweeter), and verification is done using like drivers (both mids, or both tweeters).

 

The key to the process: what to listen for

The key to the entire process is listening for specific sounds which indicate when 2 given drivers are close to being in phase. Initially recognizing these sounds is difficult, but once you understand what to listen for, calibration will become easier and quicker. Here's a quick summary: 2 drivers play the same noise track and their outputs interact. Anything out-of-phase will cause harmonics. The harmonics will exhibit the Doppler Effect when time delay approaches the correct range. At the "center" of the Doppler Effect, harmonics disappear, which means out-of-phase information is at its minimum; the drivers are in phase.

Here it is in more detail. During tuning, pink and/or white noise will be playing loudly on 2 drivers at a time, and the noise will basically fill your interior.

• The 2 seperate outputs will interact, and phase incoherencies will show up as subtle harmonics within the pink noise.
• The harmonics will be at a much lower level than the 2 direct signals, and are difficult to detect. The goal is to identify and reduce these harmonics/phase problems to a minimum.
• As time delay is adjusted, the harmonics will rise and fall in pitch. The changes in pitch will sound random at first, but repeatable patterns will appear.
• As the drivers approach correct time alignment, the harmonics will exhibit the recognizable sound of the Doppler Effect.
• The Doppler Effect will present itself in one of two ways, depending on which 2 drivers are being aligned, and their relative levels:

a) The pitch of the harmonics will increase, peak, and then decrease (Like the typical Doppler Effect) --OR--
b) The pitch of the harmonics will decrease, bottom out (the trough), and then increase again (Like an “inverse” Doppler Effect)​

• In either case, when the Doppler Effect is at its extreme (the precise peak or trough), the harmonics/phase problems are at their lowest level, and the 2 drivers are in optimal phase.
• Note: As delay is increased from 0ms upward, there are multiple occurrences of the Doppler Effect. The target is the FIRST one to present itself starting from zero delay.

Prep for tuning:

Save your current settings to memory. Since calibration will be done from your normal driving position, have your remote control handy, or make sure you can reach your controls while keeping your head steady and facing straight ahead. A battery charger would come in handy. TA is ideally set with the engine off, to make subtle sounds easier to hear. Make use of the charger (or drive the car) after tuning.

In your car, disable all filters/XOs external to your main DSP (stuff like on-amp filters, phase switches, bass controls, etc.). If you haven’t already, set your crossover points and slopes, and set your amp levels. Zero out all time delays and get your pink noise track ready to go, with the player set to repeat the track.

 

Tuning


1) Align sub and passenger side midbass

Mute all drivers except for these two. The sub will remain at 0 ms, and delay will be adjusted for the midbass. Begin your noise track (preferably pink noise since we're in the low frequency range) and turn up the volume loud enough to closely examine the signal.

Start increasing delay for the midbass, and listen to for changes in the sound. In the sub/midbass crossover region there are frequencies where both drivers are playing, and this overlap is out of phase. These harmonics appear as an incoherent rumble within the pink noise. As time delay reaches the correct range, the rumble will take on the pattern of the Doppler Effect.

More specifically, as delay is increased, listen for one of two possible occurrences:

a) The rumble shows itself around 60 Hz, rises in pitch to around 250 Hz, disappears, and then falls again from 250 Hz – 60 Hz.
b) The rumble shows itself around 250 Hz, falls in pitch to around 60 Hz, disappears, and then rises again from 60 – 250 Hz.

This event may occur anywhere between 0.75 and 3.00 ms. Try holding down the time delay key to scan quickly up and down between zero to 3.50 ms, this makes it easier to hear the pattern. When you think you hear it, note the approximate delay setting. Slowly scan up and down around that area and do your best to locate the center of the peak or trough, where the harmonics/rumbles disappear. This doesn’t have to be absolutely perfect yet, just get it close (the next step will fine tune the delay). Restore your normal XO settings and move on to step 1a.

(Note) If you’re having trouble hearing the Doppler Effect, go ahead and force both drivers to overlap and play the same frequencies – set both to play up to 300 Hz. Leave a 40 Hz high pass filter on the midbass to filter out low bass though. Go back and listen for the Doppler Effect again. When you’re finished with all of step 1, restore your normal XO settings and move on to step 1a.


1a) Verify your delay setting

The following is an example of how to zero in on, and verify the precise delay setting. Just as an example, say your best guess from step 1 is 2.00 ms delay.

• Hit the delay “up” button and count how many presses it takes to hear a discernible change in phase (could be up to 30+, depending on time increments).
• Say it takes 20 button presses to notice a change in phase (the rumble sound will return). Note the number 20.
  
• Return to where you started – 2.00 ms.
• Now, hit the down button the same number of times (20) – you should hear the very same rumble/phase change appear.
• If it takes more or less than 20 presses, adjust your 2.00 ms delay up or down accordingly.
• Repeat these steps until correct.

You’ve just eliminated the most apparent phase incoherencies between the 2 drivers, and they are now correctly aligned to your ears. And that’s how the method works. All other drivers repeat the same basic procedures. Each alignment depends on the one before it, so precision and patience in the first steps of the process is paramount.

(Note) This verification procedure will be repeated for all drivers.


2 and 2a) Alignment and verification: sub and driver side midbass

Mute all other drivers and adjusting delay for the driver side midbass. Note that because this midbass is closer to your ears than the passenger side, the time delay will end up slightly greater than the passenger midbass. The Doppler Effect will seem to appear and disappear more quickly as well.

Once you think you have the correct delay, verify using the same method as in (1a); this time, it should take fewer presses of the up/down button to hear a change in phase.


3) Verify alignment: both midbasses

Save your TA settings. You’ve just individually aligned your midbasses using the sub as their reference. Now it’s time to listen to both midbasses together. Mute all other drivers and return the XOs for the midbasses to their normal frequencies.

Play the pink noise and listen for the width and center of the presented soundstage. It should be quickly apparent that the image is solidly centered. If soundstage is not cohesive (almost like drivers are wired out-of-phase), or pulling to one side, then the best thing to do is go back to steps 1 and 2, and refine/re-verify your time delay settings. Making any arbitrary time adjustments during this verification step will do more harm than good.

An incoherent or off-centered soundstage can be caused by zeroing in on the wrong Doppler Effect (the second one that appears as opposed to the first one). As you get better at recognizing the Doppler Effect during the first 2 steps, the midbasses tend to automatically end up correctly aligned, and no changes need to be made in this step.

Don’t continue until you’re confident of your midbass time delay settings.


4 and 4a) Alignment and verification: passenger side midbass and tweeter

We’ll continue the process with the passenger mid/tweeter combo. Since this step deals with the midrange frequencies, playing back white noise instead of pink may be better (but not required). Use your normal XO settings, mute all other drivers and play the noise track.

The midbass will now be the reference; its predetermined delay setting will not change. Adjust tweeter delay until the Doppler Effect is heard. It will be more pronounced and easy to hear this time because the XO region is now in the midrange, and because the mid and tweeter radiating surfaces are physically quicker. The tweeter’s time delay should end up relatively close to that of the mid. If you have problems hearing the effect, use the XO filters to cut the high-end response of the tweeter, and cut the low frequencies of the midbass.

Verify using same procedure as above. It will take fewer button presses to hear a phase change.


5 and 5a) Alignment and verification: driver side midbass and tweeter

You know what to do. This one is a bit tricky because of the angle and proximity of the 2 drivers relative to your head. The Doppler Effect will come and go quickly. During verification, it should take very few presses of the up/down buttons to hear a change in phase.


6) Verify alignment: both tweeters

Save, or write down your TA settings. Restore both tweeters’ high-pass filters to their normal frequency. Listen to noise on both tweeters together and verify your settings from the last few steps. Again, listen for the width and center of the presented soundstage. Same deal, if it’s pulling to one side or incoherent, go back to steps 4 and 5 and refine/re-verify your time delay settings.


7) Verify alignment: front stage

Normal up the XOs for your entire front stage. Leave the sub off. Still playing the noise track, double-check for coherency and general staging. This is just a final verification of your settings, and if your verifications have been good up till now, this should not require any adjustments.


8) Normal up all settings and evaluate

This is the moment of truth. Normal up all of your XO settings, and save the configuration to memory. Play some music through your system and evaluate.

If you’ve never set TA by ear, and you’ve followed the steps with confidence in your determinations, there should be a very noticeable difference. Each time you go through the process, you’ll be better at recognizing the Doppler Effect and further refine your sound.

Improvements

With correct application, this method achieves immediate improvements in imaging, staging, impact, and transparency. Bass should become thinner, but in a good way - lean, with great impact. The sub-bass shouldn’t come from the back of the car, or even up front – it should be completely unlocalizable. Midbass should be solidly up front along with the rest of the center image.

That said, TA doesn’t magically transform your system into a rolling 5-digit audiophile’s dream. It just reduces the tendency for your speakers to fight against each other. Hearing correct TA for the first time can be downright shocking - but this is simply the moment we realize how much the speakers fight against each other within the confines of an automobile.

And, of course, this method is no replacement for a carefully installed system with well-placed drivers. But, it will definitely help a modest system with drop-in speakers bridge that gap somewhat.

Thanks for reading.

 

Sorry for the incomplete thread. The forum required it to be reviewed by moderators before being posted. The rest of the method is now up.

I developed the method with an Eclipse HU with time delay steps in 0.1ms increments, and then refined the method with a DRZ9255 (0.02ms steps). Can be used on any number of channels as long as they're each individually amp'd as you mentioned.

Yeah, low frequencies are the most difficult to time align, it gets easier as you go higher up in freq. Pink noise helps to bring out the specific cues needed to verify time alignment. I haven't aligned 2 subs before, but aligning a sub with midbasses is covered in the method above.