# Time alignment with impulse response



## geopango (Jun 24, 2012)

Hi There
I'm very new to this program and just trying to explore how to do a time alignment reading for my three way active speakers. The crossover is the Behringer DCX2496 but I am not using the built in time alignment function in this device due to it being disabled as a result of a mod.
I have posted a graph of three impulse response passes for bass (operating full range), midrange (filtered at 80 and 3000 hz) and tweeter (high passed at 2000 hz). Obviously these are not the filter settings I normally use.
For each pass the mic remains in exactly the same position approximately 3 meters directly in front of the speaker.
I'm measuring only the left channel and muting and unmuting drivers for each pass. 
The attached image displays three perfectly aligned impulses from 3 meters.
Am I correct or is it that I dont know what Im looking at?
Are my three ways already perfectly aligned at the measured position?
I will try to experiment by exaggerating the delay for one of the drivers in the Behringer and see if the change is reflected in the graph.


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## EarlK (Jan 1, 2010)

You need to rethink your approach .

For instance, ( it appears ) you haven't setup a T=0 ( reference channel ) to refer any of the 3 impulses back to .

That's why the program automatically references all three to T=0 ( in the absence of a reference signal/chnl ).

You need a two chnl soundcard ( with the second chnl feeding itself to create the reference time ) to do what you want to do . 

FYI, measuring flight of time can not be accomplished ( on REW ) with a USB connected mic ( such as the UMIK-1 ) .

:sn:


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## geopango (Jun 24, 2012)

Thanks.
I'll have to go back and make these changes. Are there any other programs which are better suited to this test? I see that SMAART seems to be the standard, but is way out of my price range.


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## natehansen66 (Feb 20, 2011)

REW is just fine if you do it as EarlK says, as long as you are using a mic with a separate preamp. If you are using a USB mic them you will need something like Holm Impulse which can remove the latency from the system and allow you to see the impulses as they are in relation to each device. Either approach is valid.


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## geopango (Jun 24, 2012)

All good. 
I used the loop back set up as the time reference. All impulses peaks no longer lock to "0".
I'm using a Mackie preamp with a Rode NT1. I know this is not a proper measurement mic but for impulse response testing its fine. Required Tweeter delay is 190ms, Mid delay is 138ms
As I mentioned the mic positioning was from three meters away. 
Very pleased with subjective results.
The only issues I had were getting a good impulse read at 300hz which is where the woofers crossover.
To get a clear IR peak I ran the woofers full range for the IR test. 
How critical is it to measure delay at the crossover point?


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## jtalden (Mar 12, 2009)

You may already have process in mind, but I will offer this in case it is helpful.

There are several general methods for setting driver delays. There are numerous options within the methods. The following is an overview of the general method I would recommend when utilizing IR timing for the initial setting. The detail of the steps is not provided, but sometimes a general overview is helpful. This method is intended to provide a "conventional" XO alignment, i.e., one that provides the best phase tracking and SPL support throughout the XO range. This results in a stable frontal lobe pointed at the LP and will thus provides the best image stability. It also will maximize the stability when changing vertical listening height (sit down/standup). 

*Driver delay adjustment process overview* [for IIR XO filters as a DCX or similar device provides]:
(Measure and adjust each speaker XO separately)
> Apply the desired XO filters
> If old EQ is active that is okay, but EQ filters within the XO range are best disabled for this process.
> Use REW Loopback Timing for this process.
> Speakers in normal room position*
> Mic at LP*
> Measure each driver full range [75-80 dB SPL for testing is fine.]
> Adjust the SPL levels of the 2 drivers as needed to best follow the House Curve
> Identify the XO range of interest from the SPL overlay chart
> Adjust delay until the "initial rise" of the 2 IRs are aligned**
> Measure both drivers together
> Change the polarity on the lower freq driver and measure both drivers together.
> Select the "correct" polarity for the lower freq driver by the one which provides the most SPL support in the XO range. [Use 1/6 or 1/3 octave filtering for this decision.]
> Use one of several methods to fine tune the delay if needed

Notes:
* There are options for mic/speaker positioning depending on the situation. In all cases the mic should be placed on the listening axis (line of sight to a point on the baffle between the 2 drives).
** The initial peaks of the IRs are not aligned if this is done properly. [The lower freq driver's peak will occur after the higher freq driver's as it is rising at a slower rate.]

> The delay is good when the SPL is best supported throughout the XO range.
> "Delay fine tuning" should be kept within ±90° of XO freq wavelength (XO freq period / 4). 
So for a 2000Hz XO the adjustment from the initial delay setting should not exceed 1/2000 / 4 = ±0.125ms [Delay fine tuning would be expected to be significantly less than this.]


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