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Guenter Villnow
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Guenter Villnow
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This app note describes how to implement an active speaker and Dirac Live room correction in a single box.

Overview [Top]

miniDSP's multichannel audio processors with Dirac Live room correction can implement advanced multiway active speakers. Any number of "ways" and speakers can be implemented up to the number of output channels:

  • DDRC-88A and DDRC-88D. When ordering, be sure to include the DDRC-88BM plugin. 8 output channels.

  • C-DSP 8x12 DL. 12 output channels.

  • Harmony DSP 8x12. The Harmony DSP 8x12 includes power amplification, and can also use external amplification if more power is needed. 12 output channels.

The C-DSP 8x12 DL and Harmony DSP 8x12 are designed for automotive use and aren't recommended for home audio applications. We recommend the DDRC series instead.

Figure 1 illustrates a 3-way active speaker system using the DDRC-88A, which we will work through in this app note. In this example, there are still two unused output channels, which can be used to add subwoofers if desired. Many other combinations are also possible – for example, a single DDRC-88A could implement four two-way active speakers, two four-ways, or two three-ways and a two-way. The C-DSP 8x12 DL and Harmony DSP 8x12 have 12 output channels and so allow even more possibilities!

Active speaker system with miniDSP DDRC-88A/BM

Figure 1. The example active system

Notes:

  1. When connecting power amplifiers directly to speaker drivers, a protection capacitor in series with each tweeter is strongly recommended.

  2. The measurement examples in this app note are taken in a typical room. If you are working in a car environment, expect to see more effects from the cabin.

  3. This app note describes one method of measuring for and designing an active speaker that doesn't require additional software. There are other techniques but they are outside of the scope of this short app note.

1. Select the speaker drivers and design the enclosure [Top]

If you are starting from scratch, you will need to select the speaker drivers. There are hundreds of drivers available for DIY use at all price levels, so it's impossible to give specific recommendations here. Peruse the online forums to see what others are using and to ask for recommendations for your particular project.

If you are building your own enclosures, you will need to design them. The most important factor is the internal volume, and if it's a ported box, the size and length of the port. Fortunately, there are a number of free programs that do the complex math for this based on the Thiele-Small parameters of the woofer.

2. Set up the Mixer tab [Top]

The Mixer tab is the key to implementing an active speaker system. This is how we set it up for the example system:

Mixer tab for active speaker

The signal from Dirac Live channel 1 is sent to output channels 1, 2 and 3. These will be used for the woofer, midrange and tweeter of the left speaker, as indicated by the labels at the top. (Change the labels on the Outputs tab.) The right speaker is set up similarly on channels 4, 5 and 6.

The Routing tab doesn't have to be set up specially for this application. However, since we only use two Dirac Live channels in this application, this can be indicated like this:

Routing tab for active speaker

3. Measure and equalize the drivers [Top]

Measure the drivers one at a time and EQ them flat. Below is an example measurement of a small woofer showing the various features of the measurement, along with the corrected response in light blue. Be careful not to correct for peaks and notches caused by the room or cabin.

Woofer measurement example

For more information on measuring drivers, see the app note Loudspeaker measurement with UMIK-1 and REW. Use smoothing or gating in your measurements as necessary.

One way of flattening the response of a driver is to use the REW EQ function. This will auto-generate cut/boost filters, but you can also manually set shelving filters to make broad corrections in the response. For more details, see the app note Auto EQ with Room EQ Wizard. If possible, make the response flat for an octave past the intended crossover frequency, as this will make the crossover easy to implement. (Be careful not to strain the tweeter if extending it lower.)

When loading the EQ filters, use the linking feature on the PEQ blocks to load the same settings into corresponding left and right drivers. Then remeasure one driver to confirm the correction. For example, here is the tweeter of the example system measured at 1 meter distance, before and after applying EQ (red and green respectively):

Tweeter measurement example

Here is the midrange driver before and after applying EQ:

Midrange measurement example

In this last example, we are seeing room effects below 600 Hz. It's important to just ignore these for now. If designing a home speaker, you can try using outdoor measurements.

Room effects make it difficult to measure and correct the woofer, which we are going to cross over at 200 Hz. Therefore, we used a near-field measurement — that is, with the tip of the microphone 1 cm from the woofer cone. (The distance should be 1/20th of the cone diameter or less.) This woofer is in a relatively small sealed box for its size, so we applied EQ to extend the low end response as well as flattening the top end of its response past the 200 Hz crossover:

Woofer nearfield measurement example

4. Implement the active crossover [Top]

The crossover can now be implemented. For the woofer-midrange crossover:

  1. Adjust the gain of the midrange to match the woofer.

  2. Adjust the delay of the midrange driver. (See Time-aligning speaker drivers.)

  3. Add a 24 dB/octave Linkwitz-Riley crossover (200 Hz in our example speaker).

  4. Confirm integration by measuring. We measured at 50 cm distance with the microphone vertically midway between the drivers.

Repeat the above process with the midrange and tweeter. We used a crossover frequency of 2 kHz. Here is our measurement of all three drivers taken at 1 meter distance, with the top graph taken indoors and the lower graph taken outdoors:

Integration of all
  three drivers

While the top graph looks pretty bad below a few hundred Hz, it's just the room doing this, as can be seen by comparing with the outdoor measurement below it. While there are techniques to obtain a more accurate representation of the actual speaker response, this is not really necessary for our purposes here.

Once you're satisfied with your crossover, copy your settings to the output channels for the other speaker or confirm that the linking feature has been set up correctly (Xover and PEQ). Confirm with a measurement on the other speaker.

Finally, don't forget to save your configuration to a file.

5. Run Dirac Live calibration [Top]

For Dirac Live version 3, we need to create a zone that includes only the Dirac Live channels that we want to calibrate. Do this from the menus in your plugin. We created Zone 1 for Dirac Live channels 1 and 2:

Setting up a zone for Dirac Live

Follow the procedure in the miniDSP Dirac Live User Manual to run your Dirac Live calibration. Here is the average of nine measurements taken around the listening area together with the predicted corrected response:

Dirac Live correction of active crossover

Once you've set up your target curve, proceed to the Export tab and export your filters into the same configuration slot that you set up with the driver EQ and crossovers. Save the project before exiting the DiracLive app.

If you wish, you can measure with REW to see the effect of Dirac Live. Remember, however, that the Dirac Live calibration is made with nine, 13, or 17 separate measurements around the listening area, so measuring at just the center of the listening area will never show a completely flat in-room response. Even so, here is our measurement at the center of the listening area (red is with Dirac Live off, green is on):

In-room measurement
  with Dirac Live on and off

Wrapping up [Top]

Once that's all done, play some music and listen to the results of your hard work! You should continue to experiment with the overall response by tuning the Dirac Live target curve. If you have spare output channels, you can use them for additional subwoofers.

Have fun, and let us know about your experiences in our forum!

 


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