In this application note, we will show you how to design an active 2-way loudspeaker. This application note will use the 2x4 series. If you're looking for higher DSP power, please check out the 2x4HD application note instead.
What you will need
- A miniDSP. For a two-way speaker, a miniDSP kit, miniDSP 2x4, or a miniDSP Balanced 2x4 are suitable.
- A miniDSP plugin. For a 2-way loudspeaker, the 2-way Advanced plugin is most suitable.
- Ability to run acoustic measurements. You will need a measurement program such as Room EQ Wizard (REW), and measurement hardware, such as the UMIK-1.
- Four channels of amplification matching your speaker needs
This is the block diagram of the two-way plugin. A good approach is to use the output channel Parametric EQ to correct for the response of the individual drivers, and the input channel Parametric EQ for overall response shaping and taming room issues.
1. Select the speaker drivers
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. For a small two-way loudspeaker, a 5" or 6.5" woofer and a 1" dome tweeter are common choices. Peruse the online forums to see what others are using and to ask for recommendations for your particular project.
If you are modifying an existing speaker from passive to active, then you have the drivers. In this case, you will most likely need to remove the internal crossover and add a second pair of binding posts.
2. Design the enclosure
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.
3. Measure and equalize the drivers
Once you have built the box and mounted the drivers, you will need to measure the drivers one at a time. When performing acoustic measurements of a loudspeaker, it's important to try and minimize reflections. Position the speaker so the tweeter is half-way between floor and ceiling, as far away from walls as possible and angled at 45 degrees to the walls. Position the microphone level with the tweeter, and 1 meter (3' 3") away. For a typical 2-way speaker (tweeter and woofer close to each other), you can leave the microphone at the same position for woofer and tweeter.
You will need to use the output parametric EQ blocks to shape the response of each driver so that it is flat over its operating range. Use "Peak" type filters to flatten peaks (with negative gain so they create a notch) and "High-Shelf" and "Low-Shelf" type filters to straighten out the overall response.
Let's start with the woofer. Ideally, this measurement should be done outdoors, but you can still get good results with indoor measurement if you are careful. Note that at low frequencies there will be peaks and notches caused by the room. You should not correct for those at this point.
The figure below shows the before-and-after responses of a typical small woofer in a ported box, annotated with the key features that you can expect to see in such a measurement. In this example, the baffle step loss and the cone breakup peak are corrected for, but peaks due to the room are left untouched.
Then measure and equalize the tweeter, When performing a tweeter measurement, start the sweep at a frequency so as not to strain the tweeter (for example, start at 1 kHz, not 20 Hz). Here is an example tweeter measurement, before and after equalization:
4. Add the crossover and fine-tune
In the Crossover block, set a low pass filter on the woofer and a high pass filter on the tweeter. As a starting point, try using Butterworth (BW) 18 dB/octave or Linkwitz-Riley (LR) 24 dB/octave filters. You can then experiment with lower or higher slopes, from 6 dB/octave up to 48 dB/octave.
Then you can measure the response of the complete speaker! Use the output level controls to match the signal levels from the woofer and tweeter. If you have a dip at the crossover frequency, you may need to invert the phase of one driver. You will most likely need to fine-tune the crossover settings to get the smoothest response around the crossover frequency:
- Time align the drivers
- Move the filter corner frequency for one driver up or down a little
- Use an asymmetrical crossover, for example BW 18 dB/octave lowpass on the woofer and LR 24 dB/octave high pass on the tweeter
- Adjust the equalization of one driver or the other near the crossover point
This REW plot shows the response of the woofer and tweeter in our example speaker with crossover filters in place, and the combined response, all after crossover fine-tuning:
5. Set up for listening
Copy the settings for the channel you have just done to the other channel. Then save the configuration.
You can now set the speakers up properly in their intended location in the room, and move the microphone to the listening position. Measurements made from now on are "in room" measurements. You will notice that the peaks and dips up to a few hundred Hz will have moved - that is why they are not corrected in the measurement made at 1 meter.
The parametric EQ block in front of the crossover can now be used to adjust the final response as you desire. You can use this to tame resonances in the bass region, for example, or to provide a small amount of shelving boost in the bass "to taste."
Be warned that you may not get it quite right the first time. It's a learning curve, but once you get going with active speakers, you'll never look back! Don't forget to ask on the miniDSP forum if you have questions.