Building a DIY Speaker: Crossover Design
The crossover design can make or break any multi-way loudspeaker. Having the world’s best drivers with a poor crossover design will yield poor results. It is important to approach crossover design with humility because it truly is an art that requires experience to perfect. Yes, it is possible to get decent sound by sending driver data sheets off for a generic crossover design. However, without proper analysis of the drivers in the intended cabinet it is a game of roulette. This design overview neglects many design considerations a professional designer may consider such as time alignment, distortion and polar response but provides a starting point.
Before designing a crossover, the cabinet must be fully assembled including ports and drivers. I typically run a separate speaker wire from each driver out of the port and label each wire. For sealed loudspeakers, connect the woofer to the binding post as this is the first driver measured in the cabinet. Using SoundEasy, each driver is measured in the cabinet to obtain frequency response and electrical impedance. These parameters are obtained following the test methodology outlined in our Loudspeaker Measurement Standard. Please note that any measurements taken near field must factor in the effects of baffle diffraction. SoundEasy allows modeling and application of baffle diffraction estimates to any frequency response data. Getting good measurement data is one of the hardest parts of loudspeaker design. It is crucial to validate your results by taking measurements using both near field and quasi-anechoic gated techniques and compare the measurements before proceeding. The measurement data is subsequently used in SoundEasy’s crossover design tool.
DIY Crossover Installed in Cabinet
Although CAD has come a long way, designing a loudspeaker crossover benefits from some background in electronics. SoundEasy has 2-way to 5-way crossover templates covering 1st order to 4th order crossover topologies. Additionally, there are templates for a wide array of typical filters and compensation circuits. The CAD framework allows implementation of any circuit or filter topology imaginable so long as it uses inductors, resistors, capacitors, operational amplifiers, potentiometers or logic gates. If you have not ever studied filter design or do not know what the previously-mentioned elements are, you may want to consider studying up to improve your chances of pulling off a working and affordable solution.
The first step I take when designing a crossover in SoundEasy is to determine workable crossover frequencies and filter orders. This requires careful inspection of the driver data sheets, frequency response measurement and driver impedance. It is imperative to make sure that response anomalies, such as the breakup of a metal cone driver, are sufficiently attenuated by the crossover so that they are not audible. It is generally a good idea to keep the crossover frequencies low enough to prevent driver beaming but high enough to prevent driver failure. For example, crossing over most tweeters below 1.5kHz is a bad idea if it is meant to play program material at 110dB or has a resonant frequency above 750Hz. Crossing an 8 inch bass driver at 5kHz will result in poor dispersion and transient response at higher frequencies. As a rule of thumb, the max frequency for a given driver diameter can be equated by taking 13560 and dividing it by the drivers effective diameter. For an 8” driver with an effective cone diameter of 7”, this means that the maximum crossover frequency should not exceed 1937Hz.
A 4th order crossover rolls off frequencies at 24dB per octave where as a 1st order crossover rolls off frequencies at 6dB per octave. Lower order crossovers have less phase shift but tradeoff power handling and masking out of band response anomalies. The designer has to determine the best set of tradeoffs for an intended design goal. SoundEasy has built-in filter templates for delay networks if a designer desires to design a time aligned loudspeaker for higher order filters. It is also important to note that the filter order does not have to match for each driver in a system but it does simplify the design.
Once approximate crossover frequencies and filter orders are determined, a filter can be developed in CAD. For the purposes of this design analysis, a 2nd order crossover was designed which approximately matches a Butterworth filter target. Additional response shaping elements were added where needed to meet the design requirements. This circuit was determined based on using a 2nd order low-pass for the woofer, band-pass for the midrange and high-pass for the tweeter. Due to the response anomalies and differences in sensitivity, additional elements were added. The final circuit design is shown below.
Crossover Circuit in CAD
The component values for the circuit design are determined using SoundEasy’s ingenious optimization technique. It is as simple as setting a target response for a single driver and selecting the components SoundEasy should manipulate to attempt to meet the target response. In this example, a 2nd order Butterworth band-pass filter target was setup in the optimization dialog as shown in black in the figure below. SoundEasy will modify the selected crossover components and employ trial and error to arrive at the optimum component values required to meet the frequency response target. The optimization technique will attempt thousands of combinations and may be run multiple times to determine the best component values. While running optimization, it is important to pay attention to the component values set by SoundEasy because it may set values that are very high inductances or capacitances and therefore expensive. This may take some playing around to get right. If you are unable to get close to the target curve you might consider changing the filter topology. The midrange for the sample project tracks the target curve pretty well but is not perfect at the high frequency knee due to response problems with the driver itself. It is possible to shape the response further but more crossover components equate to a more expensive crossover. It is easy to spend hundreds or even thousands of dollars on crossover components. It is often better to solve extensive frequency response problems with better drivers or a more in-depth diffraction analysis.
Midrange Crossover Optimization
The optimization process is applied to the low-pass, band-pass and high-pass filters and a combined response is generated. Due to response issues, it may be necessary to attenuate a target or move the crossover frequency slightly to obtain a flatter response. Ultimately, it may take several rounds of moving crossover frequencies and even filter orders to get to an acceptable system response. The combined system response after optimization for this design is shown below.
3-Way Crossover Summed Response
During the design process, it is important to pay close attention to system impedance graphs. It is no fun perfecting frequency when your design has a minimum impedance of 1.5 Ohms. SoundEasy has an option to set the minimum acceptable impedance during optimization to help reduce the risk of developing a network that dips too low for the amplifier you are using.
As a final step, it is absolutely necessary to set the values of capacitors, resistors and inductors in the CAD editor to values you can actually purchase. This usually involves rounding up or down to values you can purchase. If the crossover parts are expensive, you can attempt to reduce component values and subsequently plot their effect on the final response. This is time consuming but can save you a ton of money. A 3-way crossover of this complexity is not cheap. Using cheap electrolytic capacitors this crossover costs close to $150 for components alone.
3-Way Crossover Bill of Materials
Unfortunately, the tweeter used in this design did not measure close to the data sheet at all and required pushing the crossover frequency up. Ultimately the dip at 4kHz is fixable but the cost required outweighs the benefit.
Personally, I prefer loudspeakers that have a frequency response that tilts slightly downward so I find the response of this speaker pleasing. It did not ultimately meet the design goal of +/-3dB frequency response deviation but it plays low bass with authority and sounds very good overall. Even though it is a ton of work, there is something very rewarding about designing a loudspeaker from the ground up and the owner of the speakers discussed in article is extremely happy with his one of a kind creation.
DIY speaker Finished Product
Ultimately, designing your own loudspeaker is something you do because you like the creative side of designing things. If you are after bang for your buck sound, it is much cheaper and less time consuming to use someone else’s well-established and well-documented DIY design with pre-fabricated cabinets.
Some of my personal favorites include:
- http://www.linkwitzlab.com - Siegfried Linkwitz is a renowned loudspeaker designer that develops DIY loudspeaker projects for fun in his retirement. The new LX521 and older Orion loudspeaker projects are considered by some to be among the best speakers available, even commercially.
- http://www.troelsgravesen.dk - A Danish designer responsible for many well regarded mid to very high end DIY designs. Most require building a cabinet but the designs represent very high value.
- http://zaphaudio.com - Although this site is not updated much anymore, John Krutke has many good designs available as full kits. He has also tested a slew of drivers if you need unbiased raw driver data.
- http://parts-express.com - Many full DIY kits and parts available directly from Parts Express
- http://madisound.com - Many full DIY kits and parts available directly from Madisound. They also offer a crossover design service using very good crossover design software.
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Recent Forum Posts:
audiofox, post: 977984
I always wanted to splurge and get a copy of this software for my speaker building hobby-this is the SW that Madisound uses for their custom crossover and speaker design services.
LEAP Software - Analysis & Design of Loudspeakers, Enclosures, and Crossovers
LEAP Software - Analysis & Design of Loudspeakers, Enclosures, and Crossovers