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This app note explains how to use the acoustic timing reference feature of Room EQ Wizard (REW) with a miniDSP UMIK-1 or UMIK-2 to time-align the speakers in your home theater.

Introduction[Top]

REW can use an acoustic timing reference when doing measurements. This can be used to measure time delay between speakers so you can time-align them. A typical home theater setup is shown in this diagram:

System connections with AVR
for REW timing with UMIK-1 or UMIK-2

As indicated in the diagram, the front left speaker is used as the acoustic timing reference. Each speaker will be measured for its time delay relative to this timing reference.

Set up for measurement[Top]

Use your AVR controls to:

  1. Disable bass management (typically by setting all speakers to "large").

  2. Turn off any up-mixing or down-mixing. This ensures that one measurement channel from REW goes to just one speaker.

  3. Set all speaker distances to the same value. This makes it easier to calculate the delays between the speakers.

Set up REW for measurement through the AVR. For information on how to do this, refer to our app notes on HDMI measurement for Windows or Mac.

Measure the speakers[Top]

Click on the "Measure" button (top left of the main window). Check the frequency sweep range and level. Set Timing (towards the top right of the window that pops up) to "Use acoustic timing reference."

Select acoustic timing reference in REW

Set both Output and Ref. Output to "L":

Select left speaker to measure

Run a levels check and then the measurement sweep. You will hear a short "peep" sound from the left speaker, shortly followed by a full frequency range sweep. Rename the measurement to "Front Left".

Click on the "Measure" button again. This time, set Output to "R" but leave Ref Output at "L":

Select right and all other speakers to measure

Run the measurement sweep and label it "Front Right."

Repeat this procedure for the remaining speakers, including the subwoofer, by setting Output to each speaker in turn. Label each measurement as you go.

HDMI supports 8 channels of audio over linear PCM, so a 7.1 system can be directly measured by setting Output. (REW is not able to output audio using bitstream formats.) If you have more channels, you can physically connect each remaining speaker to the front right channel of your AVR and set Output to R. If you do this, be very very careful not to short any wires to any other channels or to each other.

When done, save all your measurements to a file.

Tabulate your results[Top]

On the REW main window, click on the Front Left speaker measurement and then on the Info button:

REW Info button

Locate the parameter "System Delay" on the window that pops up. This shows the time delay of the speaker being measured relative to the timing reference. It also shows the equivalent distance in meters and feet. For the front left speaker, these values should be at or close to zero:

System delay measurement for left speaker

Other speakers will have a different delay. For example, this is our subwoofer:

System delay measurement for subwoofer

If the relative delay or distance is positive, it means that the speaker being measured is further away than the timing reference. If the value is negative, it means that the speaker being measured is closer than the timing reference.

Click on each of the measurements and write down the relative distance shown next to System Delay. For example, here are the relative distances in meters reported by REW for our test system:

  Front left:      0.00
  Front right:     0.007
  Subwoofer:       0.922
  Center:         -0.093
  Surround left:  -0.799
  Surround right: -1.043

Set distances in the AVR[Top]

Measure the distance to the front left speaker, as this is the reference. The distance to the other speakers is that distance plus the relative distance recorded above. For example, with 2.6 meters distance to the front left speaker, our subwoofer distance is 2.6 + 0.922 = 3.52 meters. Our surround left distance is 2.6 – 0.799 = 1.8 meters.

Use your AVR controls to set the distance of each speaker and the subwoofer. If there are small differences between left and right as with our front speakers, this simply indicates that the microphone was not exactly in the middle of the listening area and can be ignored.

If you want to double-check your work, run all the measurements again – this time, all of the delay values shown by REW should be close to zero.

Wrapping up[Top]

Turn bass management back on and run additional measurements with REW and your UMIK-1/UMIK-2 to check integration of the speakers with the subwoofer. Have fun, and please let us know how you go in our forum!

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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 [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.

Microphone positioning for measuring time delay

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:

  1. If two drivers are playing at the same time, the measured signal will combine the impulse response of both drivers, and

  2. 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.

Tweeter impulse and delayed midrange impulse

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:

  1. Bypass all crossover filters. Leave the parametric EQ that flattens the driver responses as-is.

  2. 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.

    Mixer tab to measure time delay from tweeter to midrange

  3. Set the delay on the midrange driver to 1.0 ms.

  4. Zero the delays on both tweeter output channels.

  5. Double-check that there are no delays on the Dirac tab in the plugin.

  6. 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.

    Measurement settings to measure time delay from tweeter to midrange

  7. Click Start.

  8. 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.

Response with tweeter inverted

Figure 3. Windowed response with tweeter inverted

To obtain this graph:

  1. Set a 4th-order Linkwitz-Riley crossover between the midrange and tweeter.

  2. Zero the delay on the midrange driver.

  3. 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.

  4. Run a measurement sweep.

  5. Click on the Impulse button to display the Impulse response.

  6. 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):

    Impulse response window settings

  7. 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:

    Windowed impulse response of midrange and inverted tweeter
  with crossover

    Figure 4. Windowed impulse response of midrange and inverted tweeter with crossover

  8. 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.

Midrange impulse and delayed woofer impulse

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:

  1. Bypass all crossover filters. Leave the parametric EQ that flattens the driver responses as-is.

  2. 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.

    Mixer tab to measure time delay from midrange to woofer

  3. Set the delay on the woofer to 1.0 ms. If necessary, this can be set higher to separate the impulses more.

  4. Zero the delay on the midrange driver. Check that the delay on the tweeter used for the acoustic timing reference is zero.

  5. Double-check that there are no delays on the Dirac tab in the plugin.

  6. 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.

  7. 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:

  1. Set the delay on the woofer output channels to zero.

  2. 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).

  3. 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:

Delays set in miniDSP plugin

(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:

Mixer tab for a three-way 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.

 


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How to time-align speaker drivers

In this application note, we show you how to "time align" the drivers in your active DSP loudspeaker using the delay function of the miniDSP platforms.

Why time-align?

In a multi-way loudspeaker, the sound from different drivers will take slightly different amounts of time to reach your ears. The figure below illustrates a typical scenario: since the woofer cone is "deeper" than the tweeter, the acoustic center of the woofer is further away from the listener's ear than the tweeter. The sound from the woofer will therefore arrive at the listener's ear slightly later than the sound from the tweeter. This can have a detrimental effect on the speaker's response around the crossover.

Illustrating time delay in a loudspeaker

One approach to this issue is to build a slanted or stepped baffle. In passive crossover design, the designer may use specific techniques to shift the phase of one driver or another to compensate. In a miniDSP crossover, we use the Delay parameter in the output channels to delay the signal from the tweeter by a small amount of time. The result is that the acoustic waveform from both drivers will arrive at the listener's ear at the same time.

How much time delay?

Sounds travels at 343 meters per second, or 1126 feet per second. If you can measure approximately the distance between the tweeter and the woofer cone as indicated above, you can calculate the time delay with one of these formulae:

  • Delay in milliseconds = distance in cm / 34.3
  • Delay in milliseconds = distance in inches / 13.5

Alternatively, if you have an acoustic measurement program, you can calculate the delay by measuring the arrival time of the signal from each driver, and calculating the difference. The result is the time delay needed. The arrival time measurement can be done in Room EQ Wizard (REW) with the use of a two-channel soundcard - see the REW documentation for Use Loopback as Timing Reference.

Another method is to run a measurement sweep on both drivers at the same time. If one driver is significantly delayed, you will be able to measure the time difference between the impulse response peaks.

How to set time delay

Almost all miniDSP DSP platforms will support time alignment, but for the sake of clarity, we'll use the miniDSP 2x4. In the interface to any of the miniDSP 2x4 plugins (Stereo 2-way PEQ, 4-way PEQ, and so on), click on the Delay/Gain/RMS block. Set the time delay of the closest driver as indicated in the screenshot below.

Setting delay in 2x4 plugin

In the plugins with eight output channels (4x10, 10x10, miniSharc), all delays appear on the Output tab. With these plugins, you can implement three-way or four-way speakers and use time delays to time-align all drivers. Just remember to calculate delays relative to the furthest-away driver. This screenshot shows four channels of one of these plugins:

Setting time delay in 2x8 plugins

Wrapping up

Once you have delays set correctly for all drivers, your crossovers will be easier to design. Don't forget to ask on the miniDSP forum if you have further questions!

Looking for an Audio processor doing time alignment?
Check out the miniDSP webstore.

 
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In this application note, we show you how to use Room EQ Wizard (REW) and its integration with miniDSP to equalize your subwoofer.

What you will need[Top]

  • A miniDSP processor and plugin that support the REW integration feature. Almost all current processors/plugins have this feature — check the relevant User Manual.

  • A calibrated measurement microphone, for which we recommend the UMIK-1 or UMIK-2.

  • Room EQ Wizard (REW), which is a free download for Windows, Mac OS X, and Linux. If you are using a UMIK-2, get the most recent beta version from avnirvana.com (you will need to register a free account to download). This will ensure that you get a version that supports the UMIK-2.

We will assume that you're already set up with the miniDSP plugin and REW. If you are new to REW, refer to our app note UMIK-1 setup with REW. The miniDSP processors are very flexible and there are a lot of ways they can be set up, so the procedure below might not apply exactly to your system. REW is also very flexible, so we will describe one set of REW parameters; we suggest that you try it the way we describe here, then experiment with different parameters.

1. Make the initial measurements[Top]

  • In the plugin interface, disable all equalization.
  • Usually, you will also want to disable all crossover filters. This is so that the equalization can be applied independently of the crossover.
  • Mute all output channels except the one that you want to measure (i.e. the sub channel).

Run a measurement sweep in REW with the microphone in the center of the listening area. You can proceed with this measurement if you wish. However, since the measured response varies depending on the microphone location, it's often recommended that you make several measurements and average them. Figure 1 shows four measurements in our test system (light colors) and their average (heavy line).

Initial measurements for input to auto EQ

Figure 1. Initial measurements for input to REW auto EQ (displayed on Overlays screen)

(To average a set of measurements, click on the All SPL button, select only the measurements you want averaged, and click the Average the Responses button in the lower left of the display.)

2. Set up parameters in REW[Top]

On the main REW screen, select the average measurement in the list of measurements at the left. Then click on the "EQ" button.

REW EQ button

Set the parameters as shown in the following screenshots.

Equalizer

REW equalization for miniDSP equalizer setting

Select the option that matches your plugin. Here is a summary table:

  miniDSP Processor    Sample Rate    Nr. of PEQs    Setting  
2x4 (original)
C-DSP 6x8
10x10 HD
PWR-ICE with 2x2-FIR plugin
48 kHz 6 miniDSP
4x10 HD
nanoDIGI
96 kHz 5 miniDSP-96k
2x4 HD
Flex with MiniDSP-2x4-HD plugin
SHD Series
HA-DSP
PWR-ICE with 2x2 plugin
96 kHz 10 miniDSP 2x4 HD
C-DSP 8x12 192 kHz 10 C-DSP 8x12
DDRC-24
Flex with DDRC-24 plugin
C-DSP 8x12 DL
Harmony DSP 8x12
DDRC-88A
DDRC-88D
48 kHz 10 C-DSP 8x12 DL
nanoAVR 96 kHz 10 nanoAVR

In these cases, REW may generate too many filters. To prevent this, use the EQ Filters window to disable some filters.

Target Settings

REW equalization for miniDSP target settings

Set Target Type to "None" and deselect "Add Room Curve" to set a flat target. (Feel free to experiment with different target settings later.) Set the Target Level so that the desired EQ can be accomplished with cut filters. You will want to experiment with the Target Level as different values can give different results.

Filter Tasks

REW filter tasks

Set the parameters for generating the correction filters. Shown in the screenshot above are the values we found to work best for our example. You should experiment with these values to get the best result.

Since we are working with an average value, there is no need to try and get the graph completely flat.

3. Set manual filters[Top]

Sometimes, it's helpful to set up some filters manually. Click on the EQ Filters button to view the correction filters (initially blank). Here you can select the Manual option for a filter and then set its parameters. Here in our example we set a shelving filter to boost the low-frequency response of our subwoofer:

REW manual filter

4. Match the target response[Top]

In the Filter Tasks section, click on "Match response to target." REW will generate filters to optimize the response. Here is our example with the measured response in indigo, the corrected response in purple, the target in blue and the correction filters in green:

REW EQ screen showing measured and
predicted response with correction filters

You can see the filters calculated by REW by clicking on the "EQ Filters" button:

REW
calculated correction filters

5. Export and import correction filters[Top]

Once you have a correction that looks satisfactory, click on "Save filter coefficients to file" in the Filter Tasks section and save to a file. (Note: "Export filter settings as text" doesn't save the filters to the format needed to import into a miniDSP plugin. Make sure to choose the correct option!)

rew autoeq filter tasks annotated

Then return to the miniDSP plugin and open the PEQ block that you want to put the filters in. Set it to Advanced mode and click on IMPORT. Select the file that you just saved, click PROCESS and the plugin will display the correction filter:

miniDSP nanoAVR parametric EQ screen after loading filters

Confirm your result by running your measurement sweeps again. Note that if you equalized to an average, no single microphone location will be flat.

Wrapping up[Top]

Now you can re-enable your crossover filters and do more measurements to fine-tune your integration. Have fun! Don't forget to let us know how you go in our forum.