Time Alignment

Drivers Alignment

Drivers alignment refers to the alignment of the tweeter and woofers of one speaker so that the impulse response overlaps. This to ensure coherence of sound so the sound from the tweeter and the woofer reach you at the exact same time. In general, you will noticed that the tweeter needs to be delayed as they are usually closer to you than the woofer. You can see that the graph above has a few peaks in the first few milliseconds. They belong to the mis-alignment of the drivers. However, being a linear phase crossover as Bessels is, the overall step response is not bad at all!

In contrast, this is a typical step response curve you see in Stereophile megazine and this one belongs to Magico V3.

Measurement

Instead of using rulers, we can do it acoustically with Acourate. Measuring acoustically has an advantage of knowing exactly the measurement end points. There has been some debates about where from the driver should the physical measurement be done, the coil or the diaphragm? The center of the speaker or the front panel? With this technique, there is no issue as it is the sound ware that we measure.

What is the resolution of this method? Let’s do some maths. The resolution is 1 sample. 1/44100 = 0.0226 ms or 0.68 cm. In other words, this technique can measure a minimal of 0.68 cm drivers misalignment. The higher the sampling frequency, the higher the resolution.

First, I need to define a 4 ways speaker crossover – to avoid overlapping of the woofer and sub that I am having now. you will find measurement a lot easier if you select linear phase filters for this part because we do need to identify the peaks of each drivers later on and it is very easy to confuse with the first reflections. Here is my setup:

Next thing we need to do is to artificially space out each drivers in timing so the peaks can be easily detecting at a later stage.

The BruteFIR configuration files allows the definition of delays specified as samples. Now, for my 4th way system the artificial delay with the subwoofer is 0 sample, the woofer is 2000 samples later, the mid is 3000 samples and the tweeter is 4000 samples later. By delaying 2000 samples, we are delaying 2000/44100 sec = 45.45 ms or ~13 meter measured by sound.

Now we proceed to logsweep recording again. To make things easier, recording are done in the following sequence

All drivers connected – full
All drivers except subwoofer – nosub
All drivers except woofer – nowoof
All drivers except mid – nomid
All drivers except tweeter – notweet

Such arrangement is such that it is easier for you to identify the relative peaks. The following picture showed the pulse on nomid

This picture shows the green part from the recording – nowoofer

You can clear see now the green part refers to the mid impulse response

After you have identified the mid, you can select it and identify the highest point in the right side

In this example, the highest point is at 5003 sample

By entering the info into the delay calculator, this is what you will get in the end

The delay is confirmed with physical measurement just to be sure. That’s it for drivers alignment.

Alternative approach

For those who find the identification of pulse peak too difficult, here is a different way of doing it. Do a logsweep recording with the delays.

Load the crossover filter as shown – In the following example, the delay between tweeter and mid is being tested.

Apply convolution for pulseL and crossover to get the following pictures

The 2 pulses are now clearly seen. Make sure you are looking at the pulse, not step response!

Then, you can calculate the delay manually.

Now, let me explain how to calculate the delay manually. Let’s use the above reading as examples.

L: 2131,4009,5003,6000
R:2121,4005,5003,6000

Now, we need to check the relative delay reference to the sub

L:0,1878,2872,3869
R:0,1884,2882,3879

Now, we need to compensate the 1000tap delay we set at the crossover

L:0,-122,-128,-131
R:0,-116,-118,-121

Negative means delay. The above number means the tweeter arrive to the mic first.

The delay is

L:0,122,128,131
R:0,116,118,121

Now, align with tweeter

L:0,122,128,131
$:10,126,128,131

This is exactly what we got!

This technique is best used to check the final alignment. As you can see in the picture below, after convolution of the measured pulse, all pulse are aligned

LEFT / RIGHT SPEAKER ALIGNMENT

This step align the L/R speakers so they are the same distance from you. This is option as this will also depends on your listening position! But for completness, you can use Audacity to do some recording. First create a dirsc pulse with Acourte

This is what you get at the time domain window

create another pulse and delay 1000 samples

Save stereo WAV will give you a stereo file to playback and record in Audacity

You may even want to change the gain setting for one of the pulses so that the Left/Right can be easily identified. The actual difference between the recorded pulse gives you the time delay!

Check out my latest time aligned impulse response!