# Time Alignment & FIR filter for linear phase



## AndrewUK1990 (May 17, 2016)

Hi guys, 

I am in the final stages of producing a 3-way pair of stereo loudspeakers which have a flat baffle. I want to try and use the new acoustic reference feature in REW to allow me to accurately measure the impulse response and phase of each driver at the microphone reference point (1m from baffle and in line with tweeter) with my UMIK-1 USB mic.

I have 2 main aims here:

1. To determine exact time offset of each driver on baffle when measured at the mic ref point. In doing so I will be able to time align each driver by applying the relevant time delay for each individual driver in my Mini-DSP DRC-DA8.

2. Once the Tweeter, mid and woofer pair are perfectly time aligned, I want to accurately measure the phase of each driver at the mic ref point. The goal here is to input the accurate measured phase of each driver into the RePhase software so that I can use an FIR filter on each driver to linearise the phase in order to allow each driver to be in phase with one another and the loudspeaker as a whole to be of linear phase from 30hz to 20khz.

Now my questions relating to the above are:

How do I determine the "flight time" or driver acoustic centre to mic distance using this new acoustic reference feature in REW via looking at the impulse responses? Also, how do I determine if the phase data displayed is correct at the measurement point (as this is directly affected by any time offsets in the impulse response). I find that I need to manually move the impulse response (t offset) or still use the "estimate IR delay" button in REW in order for it to display what looks to be realistic phase data. Am I missing something here with the acoustic reference feature as i thought it would display the correct phase data its self with this option enabled?

As an FYI I have currently used more traditional methods for time alignment and I'm pretty certain the drivers are well aligned but I would like to get to grips with using this method and also to understand exactly how I can achieve accurate phase data measurements.

Many thanks in advance,

Andy.


----------



## jtalden (Mar 12, 2009)

AndrewUK1990 said:


> How do I determine the "flight time" or driver acoustic centre to mic distance using this new acoustic reference feature in REW via looking at the impulse responses?


We need to manually adjust for the overall delay in the measurement system as you noted. See Next.



> Also, how do I determine if the phase data displayed is correct at the measurement point (as this is directly affected by any time offsets in the impulse response). I find that I need to manually move the impulse response (t offset) or still use the "estimate IR delay" button in REW in order for it to display what looks to be realistic phase data. Am I missing something here with the acoustic reference feature as i thought it would display the correct phase data its self with this option enabled?


Briefly:
> Choose and apply the XOs to be used. [These may need to be changed after seeing the initial measurements.]
> For convenience use full range sweeps (away from nearby boundaries if practical).
> Use the ms location of the initial rise (or first peak) of the higher freq driver's IR (TW for M to TW XO) to manually adjust both driver IR's back to near 0ms. So, if the TW IR is located at 32.25ms, then manually offset both IRs that same amount.
[Moving them both the same amount to place the TW near 0ms retains the current relative timing difference and allows the relative phase to be more easily read.] 
> If the phase is too noisy to be easily read then a 'frequency dependent window' (FDW) can be applied to mitigate the impact of room interactions.
> Measure TW and M. Copy or remeasure the M so there are 2 identical measurements of that driver.
> Hide the first M trace and Offset the IR of the M duplicate until you achieve very close phase tracking between the TW and duplicate M. Close tracking can sometimes be achieved with the M IR polarity inverted so that is an option - just check the IR invert box if needed. The chosen XO filters will impact whether the best tracking is achieved with the M driver at positive or negative polarity.
> The needed delay change is the time difference between the shifted M IR and the shifted duplicate M IR. [Edit: I want to more clearly indicate that both M IRs have the same initial offset as the TW has, but only the duplicate M is shifted to achieve tracking alignment.]
> Close phase tracking can be fine tuned if desired by adjusting the XO filter slopes and other tweaks if desired.

Please provide some basic description or pictures of the speakers and your setup if you need elaboration on some of these brief points as I don't want to make the wrong assumptions about what the situation is.

I can post example charts if you like, but it is better to use your own files or charts for questions. You will need to 5 posts before pictures and .mdat files are allowed. There is a post padding thread if needed.


----------



## AndrewUK1990 (May 17, 2016)

Thanks jtalden,

That's the most clear and precise description of this method I have seen so your help is greatly appreciated. The penny has finally dropped as to how the acoustic reference works now as well. My main issue previously was understanding what the time offsets in the Impulse response actually represented in terms of system delay, relative acoustic offset of drivers and absolute offsets but after reading your previous post this is now much clearer.

I will give this method a go at the weekend and post my results here. I'll post my system details as well although your assumptions so far are spot on.


----------



## AndrewUK1990 (May 17, 2016)

OK this weekend I have carried out more testing and tweaking and carefully adjusted the phase response of each individual driver of my 3-way loudspeakers using FIR filters generated in RePhase. I have uploaded the FIR filters to my Open DRC-DA8 and I've attached the measured results below. The measurements were all taken on axis, in line with the tweeter and 1m from baffle. I also carried out an additional off axis response measurement of the final system, 1m from baffle, at vertical height of tweeter and approximately 20-25 deg off axis laterally.

I am very satisfied with the outcome and I also found it easy carrying out time alignment as per your suggested method Jtalden so thank-you for your advice. By ear they sound very natural, especially with a well mastered recording and the overall response seems very balanced and unbias. One point to also note, I have gated all measurements to 3.3ms to eliminate reflections, as such the low frequency response isnt shown properly here. I have however carried out near field measurements - at the drivers and port and previously calculated the response using these measurements and it would suggest that the frequency response remains very flat to 30hz where it rolls off (the port is also tuned to 30hz, by measurement rather than calculation as I have found calcs to be inaccurate on this as explained here: http://www.troelsgravesen.dk/vent_tuning.htm ).

I have attached the gated direct measurement results below FYI if anyone has any queries! Thanks  

Tweeter Scanspeak D2905/9500:
http://i245.photobucket.com/albums/gg54/waddmyster/Tweeter.jpg

Midrange Morel EM 428:
http://i245.photobucket.com/albums/gg54/waddmyster/Midrange.jpg

Woofers 2x Scanspeak 18w/4531G00:
http://i245.photobucket.com/albums/gg54/waddmyster/Woofers.jpg

System Response On Axis:
http://i245.photobucket.com/albums/gg54/waddmyster/System.jpg

Impulse Response On Axis:
http://i245.photobucket.com/albums/gg54/waddmyster/System Impulse Response.jpg

System Response 20-25 deg Off Axis:
http://i245.photobucket.com/albums/gg54/waddmyster/System Off Axis 20-25deg.jpg


----------



## jtalden (Mar 12, 2009)

Andrew,
You didn't need much help. :smile: 
The overall IR and SPL/Phase suggests the timings and are spot on and that you achieved a very linear phase. 
Well done!


----------

