Progettare un diffusore a 3 vie con DSP e crossover attivo

Design a 3-way loudspeaker with DSP and active crossover

    Design an active 3-way loudspeaker with miniDSP Flex Eight

      In this section, we will show you how to design an active 3-way loudspeaker with the Flex Eight miniDSP processor.

      • What will you need
      • Overview
      • Select speaker drivers and design the enclosure
      • Connect
      • Set the path
      • Measure and equalize drivers
      • Align the drivers
      • Add the crossover
      • Configure the other speaker
      • Phase correction (optional)
      • What will you need

      A miniDSP Flex Eight. Make sure you have the software up and running before starting this section.

      Ability to perform acoustic measurements. You will need a measurement program such as the free Room EQ Wizard (REW) software and measurement hardware for which we recommend the UMIK-1 or UMIK-2.

      Six or more amplification channels, which can be 3 stereo power amplifiers, or 6 mono amplifiers. We recommend the use of Class D amplifiers as the quality/price ratio is unbeatable. See the Class D Amplifiers section.

      • Overview

      Figure 1 shows the signal flow through the Flex Eight.
      miniDSP Flex Eight signal flow for active crossover.

      3-way Active Crossover Flows
      Figure 1. Signal flow through the miniDSP Flex Eight.

      On the output channels:

      • The PEQ (parametric EQ) block corrects the response of individual drivers;
      • The Crossover block implements crossover filters;
      • The gain/delay block aligns the drivers in terms of level and tempo.

      On input channels:

      • The PEQ is used to shape overall response and to tame room issues.
      • FIR blocks are optionally used to correct 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 design than a two-way speaker. If you haven't had experience designing your own active speaker yet, you might want to start with a double entendre:

      Design of a 2-way active loudspeaker with miniDSP

      • Select speaker drivers and design the enclosure

      If you're starting from scratch, you'll need to select 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 advice in this section.

      If you're building your own speaker, you're going to have to design it. The most important factor is the internal volume and, if it is a speaker with reflex ports, the size and length of the port. Fortunately, there are a number of free programs that do the complex calculations for this based on the woofer's Thiele-Small parameters. For example, a popular Excel-based program is Unibox.

      • Connect

      Figure 2 shows the connections from the miniDSP to the "active" speakers:

      three-way active speaker connections

      miniDSP connection Flex Eight 3-way speaker

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

      We recommend placing a large capacitor in series with each tweeter as shown. This will help protect the tweeter from being switched on and off by the amplifier or accidentally being sent low frequency test signals. Generally with super tweeters capacitors from 1.0 to 1.5mF are used.

      ATTENTION: When carrying out important imports or changes, it is always advisable to switch off the final amplifiers to avoid damaging the drivers.

      • Set the input/output paths

      Flex Eight allows you to route or mix any input to any output. This is a key element of its flexibility. To implement a three-way crossover, rename the input and output channels and set the routing as shown in this screenshot:

      Path Input-Output 3 ways
      MiniDSP three-way active speaker routing

      • Measure and equalize drivers

      Once you've built the speaker and mounted the drivers, you'll need to measure the drivers one at a time. (Do this for one loudspeaker only.) For more information on how to measure a loudspeaker driver, see "Speaker Measurement with UMIK-1 and REW".

      A three-way speaker is more difficult to measure than a two-way speaker. The relatively low crossover point between the woofer and midrange is often in the region where ambient effects are evident in the measurement. However, it is also too low in frequency to measure with controlled measurement. If possible, take measurements outdoors, with the speaker raised as high as it is safe to do.

      Use PEQ blocks on each output channel to shape each driver's response to be flat across the operating range and at least one octave above the intended crossover frequency. You can do this with the help of the Room EQ Wizard's AutoEQ feature.

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

      REW Midrange and Tweeter graph
      Correction of midrange and tweeter response

      The woofer is more difficult to measure, even outdoors there are reflections off the ground and possibly buildings etc. which affect the response. However, we can measure near-field response, i.e. with the microphone right inside the woofer cone. This demonstrates that no response correction is needed, except at low frequencies where we extended the response a bit lower (this can be done with a Linkwitz transform, or just a simple shelving filter):

      REW measurement of woofer response
      Woofer response correction

      • Align the drivers

      Adjust the levels of the drivers to match each other. To do this, it measures each driver over its operating range and compares the levels to the octave attenuation. Use the gain in each output channel to adjust the level of each driver to match the intended crossover frequency.

      Here are the settings for our test speaker:

      REW align drivers, align response times

      Time alignment and level alignment driver in a three-way loudspeaker

      Once the levels are matched for each driver, calculate 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 section "Time alignment of speaker drivers with UMIK-1 and UMIK-2".

      • Add the crossover

      If you followed the time alignment procedure above, you already have a Linkwitz-Riley 24dB/octave crossover installed. For the three-way system, make sure all crossovers are enabled and all reversed drivers are not reversed again. The crossover screen should look like this:

      3-way speaker miniDSP crossover
      Three-way active speaker crossover

      The result of the figure above 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.

      Now you can measure the response of the entire 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 from the drivers. Here is our test speaker measured at 2 meters:

      REW 3-way speaker measurement
      MiniDSP three-way active speaker measured at 2m

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

      You may find that the bass response sags a bit as you move the mic further away from the speaker. This is related to "loss of deflector pitch". At closer range the deflector pitch loss was not fully compensated for. You can adjust this if needed with a shelving filter in the input PEQ blocks. For example:

      Shelving filter miniDSP 3-way speaker
      Shelving filter for baffle pitch compensation

      • Configure the other speaker

      CAUTION: Before proceeding, export the current configuration to a file.

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

      • Connect the PEQ blocks to the corresponding drivers (left woofer to right woofer, etc.).
      • Connect the Crossover blocks to the corresponding drivers.
      • Copy the gain and delay settings to the corresponding driver.

      After connecting both speakers, measure the second speaker to verify that it is the same as the first.

      Export the configuration again to a file.

      • Phase correction (optional)

      If you're running the standard Flex Eight, you can do phase correction using FIR blocks on the input channels. This is described in this section:

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

      In the end...

      Place the speakers in designated locations in your listening room and take a measurement in the room. If running the standard Flex Eight, use the PEQ blocks on the input channels to correct room modes and run other room-related EQ as desired. If you're running the Flex Eight with Dirac Live, run a Dirac Live Calibration to perform room correction and customize your sound with target curves.

      Happy listening!

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