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Guenter Villnow
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Guenter Villnow
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In this application note, we will show you how to design an active 3-way speaker with the miniDSP Flex Eight processor.

Contents

1. What you will need[Top]

  • A miniDSP Flex Eight (Dirac Live upgrade optional). Make sure you have all the software installed and working before starting this app note! (See the User Manual for the standard version or the Dirac Live version).
  • Ability to run acoustic measurements. You will need a measurement program such as the freeware Room EQ Wizard (REW), and measurement hardware for which we recommend the UMIK-1 or UMIK-2.

  • Six or more channels of amplification.

2. Overview[Top]

Figure 1 shows the signal flow through the Flex Eight.

miniDSP Flex Eight signal flow for active crossover

Figure 1. Signal flow through the miniDSP Flex Eight

On the output channels:

  • The PEQ (parametric EQ) block corrects for the response of the individual drivers

  • The Crossover block implements the crossover filters.

  • The Gain/delay block aligns the drivers in level and time.

On the input channels:

  • The PEQ is used for overall response shaping and to tame room issues.

  • The FIR blocks are optionally used to correct the phase shift in the crossover.

If you have Dirac Live running on your Flex Eight, the approach is the same except that the input channel PEQ and FIR blocks are replaced by Dirac Live for response shaping and room correction.

Note that a three-way speaker is a significantly more difficult project than a two-way speaker. If you haven't yet had some experience designing your own active speaker, you may want to start with a two-way:

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

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

If you are building your own box, you will need to design it. 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. For example, a popular Excel-based program is Unibox.

4. Getting connected [Top]

Figure 2 shows the connections:

connections for a three-way active speaker

Figure 2. Connections for a three-way speaker with the miniDSP Flex Eight

It's recommended to put a large capacitor in series with each tweeter as shown. This will help to protect the tweeter from any turn-on and turn-off surges from the amplifier, or if you accidentally send low frequency test signals to it.

5. Configure routing [Top]

The Flex Eight allows any input to be routed or mixed to any output. This a key element of its flexibility. To implement a three-way crossover, rename the input and output channels and set up the routing as shown in this screenshot:

miniDSP three-way active speaker routing

6. Measure and equalize the drivers [Top]

Once you have built the box and mounted the drivers, you will need to measure the drivers one at a time. (You only need to do this for one speaker.) For more information on how to measure a speaker driver, see our app note Loudspeaker measurement with UMIK-1 and REW.

A three-way speaker is more challenging to measure than a two-way speaker. The relatively low crossover point between the woofer and midrange is often in the region where room effects are evident in the measurement. However, it is also too low in frequency to be measured with a gated measurement. If possible, do your measurements outdoors, with the speaker elevated as high as you can safely.

Use the PEQ blocks on each output channel to shape the response of each driver so that it is flat over its operating range and at least an octave past the anticipated crossover frequency. You can do this with the aid of the AutoEQ feature of Room EQ Wizard.

This graph shows the tweeter and midrange measured at 1 meter distance, before and after correcting their responses:

Correction of
response of midrange and tweeter

The woofer is harder to measure - even outdoors there are reflections from the ground and possibly buildings and so on that affect the response. However, we can measure the near-field response – that is, with the microphone right inside the woofer cone. This demonstrates that no response correction is needed, except at the low frequencies where we extended the response a little lower (this can be done with a Linkwitz Transform, or just with a simple shelving filter):

Correction of
response of woofer

7. Align the drivers [Top]

Adjust the levels of the drivers so that they match the other. To do this, measure each driver over its operating range and compare the levels with octave smoothing. Use the gain in each output channel to adjust the level of each driver so that they match at the anticipated crossover frequency Here are the settings for our test speaker:

Time-aligning and
level-aligning drivers in three-way speaker

Once the levels are matched, work out the time delay between the drivers and set the output channel delays to time align the drivers. The screenshot above shows the delays for our test speaker. The procedure is described in the app note Time-aligning speaker drivers with UMIK-1 and UMIK-2.

8. Add the crossover [Top]

If you followed the time alignment procedure above, you already have a Linkwitz-Riley 24dB/octave crossover in place. For the three-way, ensure that all crossovers are enabled and any inverted drivers are uninverted again. The crossover screen should look like this:

Crossover for three-way active speaker

That is:

  • A low pass filter on the woofer.

  • A low pass filter and a high pass filter on the midrange.

  • A high pass filter on the tweeter.

You can now measure the response of the complete speaker. To do this, you will need to measure at a sufficient distance from the speaker to avoid discrepancies caused by level and time differences due to different distances to the drivers. Here is our test speaker measured at 2 meters:

miniDSP three-way active speaker measured at 2m

As you can see, there are effects from reflections even with this outdoor measurement. Don't try and correct these (for example, the dip around 170 Hz).

You may find that bass response droops a little when you move the microphone further from the speaker. This is related to "baffle step loss". At the closer distance the baffle step loss was not fully compensated. You can adjust for this if necessary with a shelving filter in the input PEQ blocks. For example:

Shelving filter for
  baffle step compensation

9. Set up the other speaker[Top]

Before proceeding, export the current configuration to a file.

Set up the channels for the other speaker if you haven't already:

  1. Link the PEQ blocks to corresponding drivers (Left Woofer to Right Woofer and so on).

  2. Link the Crossover blocks to corresponding drivers.

  3. Copy the gain and delay settings to the corresponding driver.

When you have both speakers connected, measure the second speaker to confirm that it is the same as the first.

Export the configuration to a file again.

10. Phase correction (optional)[Top]

If you are running the standard Flex Eight, you can optionally perform phase correction using the FIR blocks on the input channels. This is described in this app note:

  • Active speaker phase correction with the miniDSP Flex Eight (in preparation)

Wrapping up[Top]

Put the speakers in their designated locations in your listening room and perform an in-room measurement. If running the standard Flex Eight, use the PEQ blocks on the input channels to correct for room modes and perform other room-related EQ as desired. If running the Flex Eight with Dirac Live, perform a Dirac Live calibration to do room correction and tailor the sound with target curves.

That's it for this app note! Have fun, and let us know about your active speaker experience in our forum.

Appendix A. Subwoofer integration (optional)[Top]

Output channels 7 and 8 are still available for subwoofer integration. For a single subwoofer, set the routing like this:

miniDSP three-way active speaker routing with subwoofer

To integrate the sub, the procedure is essentially as described in this app note:

Note that you will need to treat three output channels as "one speaker" when doing your sub integration. The high pass filter on "the speaker" (in the app note) goes on the woofer output channels.

For more advanced users, use two subwoofers and the Multisub Optimizer program, as described in this app note (and again, treating the woofer/mid/tweeter channels as a single speaker):

 


  

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