This app note shows you how to use a UMIK-1 or UMIK-2 with Room EQ Wizard (REW) to time-align speaker drivers.
- Overview
- Position the microphone for measurement
- Observe impulse responses
- Check phase matching
- Time-aligning the woofer and midrange
- Set the delays in the plugin
- Wrapping up
Overview [Top]
When designing an active speaker, correct time alignment between drivers is necessary for predictable vertical lobing of the speaker's radiation pattern. With a Linkwitz-Riley crossover, the vertical lobing pattern is symmetrical as long as the drivers are time-aligned. For an in-depth examination of this topic, see "Linkwitz-Riley Crossovers: A Primer" (https://www.ranecommercial.com/legacy/note160.html).
In this app note, we will describe how to measure acoustic time delay between drivers using just a UMIK-1 or UMIK-2 and Room EQ Wizard (REW). This is then used to set up time delays in a miniDSP plugin so the drivers are time-aligned. We will assume that you are already set up for measurements and know how to use REW.
Note: the method described here requires that you have already flattened the response of each driver using parametric EQ or FIR filtering and adjusted their passband levels to be equal. (See Implementing active speakers with miniDSP and Dirac Live for an example.)
Position the microphone for measurement [Top]
It's common when measuring speakers to measure at a 1 meter distance. When measuring acoustic delay between drivers, having the microphone at tweeter level as illustrated at the top of Figure 1 will give an incorrect reading. Instead, position the microphone vertically midway between the two drivers, or alternatively, at the listening position.
Figure 1. Microphone positioning to measure time delay between two drivers
Observe impulse responses [Top]
In REW, you can observe the impulse response of a driver – that is, the response in time rather than frequency – by measuring its response and clicking on the Impulse button. It follows that:
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If two drivers are playing at the same time, the measured signal will combine the impulse response of both drivers, and
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If one of these impulses is sufficiently delayed, two distinct impulses will be visible in the time domain signal.
Figure 2 is the graph we obtained to measure the acoustic time delay between the tweeter and midrange drivers of an example speaker. To separate the impulses, we applied a 1 millisecond delay to the midrange driver using the miniDSP plugin. You can clearly see the two impulses here – tweeter first and then midrange a bit over a millisecond later.
Figure 2. Tweeter impulse and delayed midrange impulse
By positioning the cursor on the graph, we can read off the time of each impulse peak. The acoustic delay between the drivers is the difference, less the 1 ms that we added to the midrange driver in the miniDSP plugin. Using the numbers from our example, the delay is:
1.148 – 0.082 – 1.0 = 0.066 ms
Here is how we obtained this graph:
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Bypass all crossover filters. Leave the parametric EQ that flattens the driver responses as-is.
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Set up the Mixer tab as shown in the next screenshot. This sends the left channel to the tweeter only – we will use this as the acoustic timing reference. The right channel sends the main test sweep to both the tweeter and midrange.
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Set the delay on the midrange driver to 1.0 ms.
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Zero the delays on both tweeter output channels.
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Double-check that there are no delays on the Dirac tab in the plugin.
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Open the Measurement screen. Turn on the acoustic timing reference and set the start and end frequencies to a range over which both drivers are flat. In our example speaker, the midrange driver goes very high, so we used 1 kHz to 20 kHz. Set Output to R and Ref Output to L. See the circled items in the next screenshot.
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Click Start.
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When the measurement completes, click on the Impulse button and adjust the graph axis limits to obtain a graph similar to Figure 2.
(The acoustic timing reference is used so that the two impulses are in predictable locations on the graph. We don't use the acoustic timing reference to find the acoustic delay because of the very small delays that we are trying to measure.)
Check phase matching [Top]
The reason for time aligning the two drivers is so that they are in phase. (We are assuming the use of a Linkwitz Riley crossover.) To verify that the two drivers are in phase, invert one driver and run a measurement sweep. If the drivers are in phase and have the same amplitude, they will cancel at the crossover frequency and create a null in the response.
Figure 3 is the response we obtained by inverting the phase of the tweeter.
Figure 3. Windowed response with tweeter inverted
To obtain this graph:
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Set a 4th-order Linkwitz-Riley crossover between the midrange and tweeter.
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Zero the delay on the midrange driver.
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Set the delay on the tweeter to the value calculated earlier (0.066 ms in our example). You may not be able to set the exact value because the plugin will quantize to the nearest sample – just pick the closest value.
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Run a measurement sweep.
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Click on the Impulse button to display the Impulse response.
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Click on the IR Windows button and set the parameters like this (you may need to adjust the Right Window value to get the best result):
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Click on the Apply Windows button. In the main measurement window, check the Window checkbox. You should be able to see that the window function (blue graph) is attenuating the first reflection:
Figure 4. Windowed impulse response of midrange and inverted tweeter with crossover
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Click on the SPL & Phase button and adjust the graph axis limits if necessary.
Time-aligning the woofer and midrange [Top]
If you are building a three-way speaker, you will need to measure the delay between the woofer and midrange. Figure 5 shows the graph we used to calculate the acoustic delay of the woofer relative to the midrange driver.
Figure 5. Midrange impulse and delayed woofer impulse
As before, the acoustic delay is the difference between the peaks, less the 1 ms added to the woofer in the plugin:
1.259 – 0.038 – 1.0 = 0.221 ms
To obtain this graph:
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Bypass all crossover filters. Leave the parametric EQ that flattens the driver responses as-is.
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Set up the Mixer tab as shown in the next screenshot. This sends the left channel to the tweeter, again for use as the acoustic timing reference. The right channel sends the main test sweep to the woofer and midrange.
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Set the delay on the woofer to 1.0 ms. If necessary, this can be set higher to separate the impulses more.
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Zero the delay on the midrange driver. Check that the delay on the tweeter used for the acoustic timing reference is zero.
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Double-check that there are no delays on the Dirac tab in the plugin.
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Run a measurement sweep. As before, turn on the acoustic timing reference and set Output to R and Ref Output to L. Set the start and end frequencies to a range over which both drivers are flat. In our case, we used 100 Hz to 2 kHz.
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Click on the Impulse button and adjust the graph axis limits to obtain a graph similar to Figure 5.
You can run a phase cancellation check if you wish but it's trickier due to room reflections. However, it's also less critical to get the time delay exactly right as the wavelength at the crossover frequency is relatively long. If you do try it, play around with the Right Window value to see if you can obtain a null.
Set the delays in the plugin [Top]
Assuming the common case in which the tweeter has the shortest acoustic time delay, then the midrange, and finally the woofer:
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Set the delay on the woofer output channels to zero.
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Set the delay on the midrange output channels to the value measured as the acoustic delay between the woofer and midrange (0.221 ms in our example).
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Set the delay on the tweeter output channels to the above delay plus the acoustic delay between the midrange and tweeter (0.221 + 0.066 = 0.287 ms in our example).
You may not be able to set the exact values, so set them to the nearest available values. Here is the output screen for our example speaker:
(Some plugins will show three digits after the decimal point while some will show two.)
Wrapping up [Top]
Check that you have all your crossover settings in place and that all settings (PEQ, Xover, delay, level) are the same on both channels. Restore the Mixer tab to normal for your speaker:
Run full-range measurements on each speaker to confirm that everything is working as expected. If all looks good, sit back and play some tunes! Have fun, and don't forget to let us know how you go in our forum.
