Driver time alignment for two or more way speakers
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This section shows how to use a UMIK-1 or UMIK-2 with Room EQ Wizard (REW) to time align your speaker drivers.
- Overview
- Position the microphone for measurement
- Observe impulse responses
- Check phase match
- Time alignment of woofer and midrange
- Set delays in the plugin
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Overview
When designing an active speaker, proper time alignment between the 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 aligned in time.
In this section we will describe how to measure the acoustic delay between drivers using just a UMIK-1 or UMIK-2 and Room EQ Wizard (REW). This is then used to set time delays in a miniDSP plug-in so that the drivers are time aligned. We will assume that you are already ready 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 equalization or FIR filtering and adjusted their passband levels to be equal. (See Implementing Active Speakers with miniDSP and Dirac Live for an example.)
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Position the microphone for measurement
It is common when measuring speakers to measure at a distance of 1 meter. When measuring the acoustic delay between the drivers, having the microphone at the level of the tweeter as shown at the top of Figure 1 will give an incorrect reading. Instead, place the microphone vertically halfway between the two drivers or, alternatively, at the listening position.
Microphone placement for delay measurement
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Observe impulse responses
In REW you can observe a driver's impulse response, that is, its response over time rather than frequency, by measuring its response and clicking the Impulse button. It follows that:
If two drivers are playing at the same time, the measured signal will combine the impulse response of both drivers, and if one of these pulses is sufficiently delayed, two distinct pulses will be visible in the time domain signal.
Figure 2 is the graph we obtained to measure the acoustic delay between the tweeter and midrange drivers of an example speaker. To separate the pulses, we applied a 1 millisecond delay to the midrange driver using the miniDSP plug-in. You can clearly see the two pulses here: first the tweeter and then the midrange just over a millisecond later.
Delayed tweeter pulse and midrange pulse
By placing the cursor on the graph, we can read the time of each pulse peak. The acoustic delay between the drivers is the difference, minus 1 ms that we added to the midrange driver in the miniDSP plug-in. Using the numbers from our example, the delay is:
1.148 – 0.082 – 1.0 = 0.066 ms
Here's how we got this graph:
- Bypass all crossover filters. Leave the parametric equalizer that flattens driver responses as is.
- Set up the Mixer tab as shown in the next screenshot. This sends the left channel only to the tweeter - we'll use this as an acoustic timing reference. The right channel sends the main test scan to both the tweeter and midrange. Mixer tab to measure delay from tweeter to midrange.
- Set the delay on the midrange driver to 1.0 ms.
- Eliminates delays on both tweeter output channels.
- Double check that there are no delays in the Dirac tab in the plugin.
- Open the Measurement screen. Turn on the acoustic time reference and set the start and end frequencies to a range where 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.
- Metering settings to measure the delay from the tweeter to the midrange
- Click Start.
- Once the measurement is complete, click the Impulse button and adjust the graph axis limits to obtain a graph similar to Figure 2.
(The acoustic time reference is used so that the two pulses are in predictable positions on the graph. We do not use the acoustic time reference to find the acoustic delay due to the very small delays we are trying to measure.)
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Check phase match
The reason for the time alignment of the two drivers is that they are in phase. (We're assuming use of a Linkwitz Riley crossover.) To verify that the two drivers are in phase, invert one driver and perform a measurement sweep. If the drivers are in phase and have the same amplitude, they will cancel out 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. Window response with inverted tweeter
To get this graph:
- Set a 4th order Linkwitz-Riley crossover between the midrange and tweeter.
- Zero delay on the midrange driver.
- Set the delay on the tweeter to the previously calculated value (0.066 ms in our example). You may not be able to set the exact value because the plugin will quantize to the closest sample - just choose the closest value.
- Perform a measurement sweep.
- Click the Impulse button to view the Impulse response.
- Click the IR Windows button and set the parameters like this (you may need to adjust the right window value to get the best result):
Impulse response window settings
- Click the Apply Windows button. In the main measurement window, select the Window check box. You should be able to see that the window function (blue graph) is smoothing the first reflection:
Figure 4. Windowed impulse response of midrange and inverted tweeter with crossover
- Click the SPL & Phase button and adjust the graph axis limits if necessary.
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Time alignment of woofer and midrange
If you're building a three-way speaker, you'll 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.
Figure 5. Midrange pulse and delayed woofer pulse
As before, the acoustic delay is the difference between the peaks, minus 1 ms added to the woofer in the plugin:
1.259 – 0.038 – 1.0 = 0.221 ms
To get this graph:
- Bypass all crossover filters. Leave the parametric equalizer that flattens driver responses as is.
- Set up the Mixer tab as shown in the next screenshot. This sends the left channel to the tweeter, again to be used as an acoustic timing reference. The right channel sends the main test sweep to the woofer and midrange.
- Set the delay on the woofer to 1.0 ms. If necessary, this can be set higher to separate the pulses more.
- Zero delay on the midrange driver. Verify that the delay on the tweeter used for the acoustic time reference is zero.
- Double check that there are no delays in the Dirac tab in the plugin.
- Perform 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 frequency to a range where both drivers are flat. In our case, we used 100 Hz to 2 kHz.
- Click the Pulse button and adjust the axis limits of the graph to obtain a graph similar to Figure 5.
You can do a phase cancellation control if you want, but it's trickier due to room reflections. However, it is also less critical to get the delay exactly right since the wavelength at the crossover frequency is relatively long. If you try this, play with the right window value to see if you can get a null value.
Set delays in the plugin
Assuming the common case where the tweeter has the shortest acoustic delay, then the midrange, and finally the woofer:
- Set the delay on the woofer output channels to zero.
- 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).
- Set the delay on the tweeter output channels to the above delay plus the acoustic delay between the midrange and the tweeter (0.221 + 0.066 = 0.287 ms in our example).
You may not be able to set exact values, so set them to the closest available values. Here is the output screen for our example speaker:
(Some plugins will show three digits after the decimal point while others will show two.)
In the end...
Check that you have all crossover settings in place and that all settings (PEQ, Xover, delay, level) are the same on both channels. Reset the Mixer tab to normal for your speaker:
Perform full-range measurements on each speaker to confirm that everything is working as expected. If everything seems good, sit down and let your system play.