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This article explores some of the myths and facts about crossoverdesign. It also discusses some of the mistakes often made by loudspeakermanufacturers done either as cost savings or design incompetence. It is ourhope that the reader will gain a better understanding of the mechanics ofloudspeaker crossovers so they can make a more informed purchasingdecision.
Speaker Crossover Design Software Mac Pro
The loudspeaker crossover can be considered thebrain of the loudspeaker. It directs thebandwidth of frequencies each driver is optimized to reproduce while it alsolevel matches each driver and can help to stabilize the load impedance theamplifier will see. Without theloudspeaker crossover, a loudspeaker driver such as a tweeter can be overdrivenwhich can lead to distortion and eventual failure. A loudspeaker system without a properlydesigned crossover (or none at all) can cause too much frequency overlapbetween drivers which can increase distortion and degrade overall soundquality.
Properly designing a loudspeaker crossover requiresengineering talent and sufficient budgeting to fit it into the total systemcost. There are often times manufacturerswill downplay the crossover’s importance, either because they lack theknowledge to truly understand its role, or they are simply attempting yetanother cost cutting tactic, assuming the buyer will never know since it’s outof sight, thus out of mind.
Let us briefly define some basic crossoverterminology which will help in the discussions later in this article. A High Pass Filter (HPF) bandwidth limitsfrequencies below where the driver is inefficient at (ormechanically incapable of) producing those frequencies. A Low Pass Filter (LPF) bandwidth limitsfrequencies above where the driver is inefficient at (ormechanically incapable of) producing those frequencies. Traditional filter theory deals with -3dB points where power is cut in half. However, since we are dealing with actual sound pressure (SPL) and not sound power, loudspeaker engineers typically deal with -6dB points when working with crossover networks.
Think of a tweeter trying to produce bassfrequencies. It can’t do so efficiently(or mechanically!) so we employ a HPF to block those frequencies. Conversely think of a woofer trying toproduce very high frequencies. We employa LPF to effectively filter the frequencies above which the woofer is capable ofproducing. The diagram pictured hereshows basic schematics of each filter type along with a generic theoreticalslope response. The squiggly lookingdevice is an inductor while the dual parallel adjacent lines represent acapacitor. This diagram is courtesy ofWilliamson-labs.com.
For more information on this topic, we recommendreading our article: Filter & Crossover Types for Loudspeakers
Myth #1: The SimplerCrossover is ALWAYS Better
We'veseen numerous loudspeaker companies defend their 2- or 3-element crossover (ie.Resistor/capacitor network only) as being preferred to a more complex crossovernetwork that their competitors employ on their designs. They argue that they custom designed theirdrivers to better integrate with each other, thus not needing a crossover withsteep slopes or an elaborate design to improve overall system impedance.
Tabulatedbelow is a list of filter slopes based on order for the readers reference:
- First-order = 6 dB/Oct
- Second-order = 12 dB/Oct
- Third-order = 18 dB/Oct
- Fourth-order = 24 dB/Oct
Thereality here is sometimes the exact opposite. A two- or three-element crossover on a two-way bookshelf speakertypically has no element at all on the midbass driver, allowing it to run fullrange. Unless the driver has beenmeticulously custom-designed and custom-manufactured to have a natural,precise, controlled rolloff at the upper end of its operating range (which istheoretically doable, but is often a very expensive proposition), the endresult would be break up distortion at higher frequencies which becomes audibleas the speaker is driven harder and why at least a second-order crossovernetwork is required to filter such distortion below audibility. This is especially true with stiff conedrivers that have a more prominent audible break up mode. Higher-order networks are typically neededin such cases. Having no network at allis NOT a viable solution if high fidelity reproduction is the primary goal ofthe speaker system.
First-orderfilters aren’t good at preventing distortion, particularly at or below thetweeter’s resonance frequency. They alsodon’t provide enough isolation of the bandwidths in multi driver systems. Theresult is too much of an opportunity for destructive interference, andtherefore a loss of uniformity across the crossover region with the wooferdoing most of the damage at high frequencies. It’s important to note that a tweeter still remains pistonic above andbelow Fc while a woofer does not. This is why second-order or higher filternetworks are typically employed but they are most costly to employ and complexto produce. The bigger parts are moreexpensive and more likely to cause insertion loss or burn out understress. This requires a tighter set oftolerances than a lower order network for the same amount of networkvariability.
Editorial Note About Directivity, Crossover Points & Driver Selection by Dr. Floyd Toole
A good sounding loudspeaker needs to have smoothand flat on-axis frequency response and similar performance as we movefar off axis. We describe this in terms of directivity as a function offrequency, and although absolutely constant directivity is notnecessary, smooth and gradually-changing directivity is a goodobjective. In deciding on the drivers to be used in a speaker system itis necessary to ensure that at crossover frequencies the drivers haveclosely matching directivities. This means that when the acoustictransition is made between, say, a woofer and a midrange, or a midrangeand a tweeter, there is continuity in the directional sound radiationpattern. It is not sufficientjust to have a good looking on-axis response. So, in addition toselecting drivers for their useable bandwidths and power handlingcapabilities, we need to pay attention to their directional radiationpatterns. The most difficult transitions occur when the transducersinvolved are very different in size.
EditorialNote by Steve Feinstein on Crossover Frequency Selection
A basic, ages-old but still true, rule ofthumb states that a designer is usually safe when he crosses a driver over atdouble its resonant frequency. If a tweeter has an Fs of 1500 Hz, use a 3000 Hzcrossover, minimum. If a midrange is 300 Hz, use 600 Hz.
Another good rule of thumb says, “18 dB downat resonance.” If a tweeter’s resonance is 1500 Hz, the voltage curve of thecrossover should show the tweeter section being down 18 dB from “0 dB.” Thatkind of conservatism all but assures no tweeter burn-out.
This was the “rule” at a major speakercompany I used to work at, and the engineers all hated it, because it was soconservative and resulted in very high tweeter crossover points. But we almostnever lost a tweeter and our warranty costs were vanishingly low. ‘Real world’vs. ‘theory.’
Aproperly designed filter network will always present a stable load impedance tothe amplifier. It will also properly bandwidth limit the loudspeaker drivers tolower their distortion and better integrate their response resulting in a moreeven on- and off-axis frequency response and power response. The power response is simply the totalradiated acoustic output of a speaker measured spherically around it. Thisis discussed in greater detail later in the article.
Compromisinga crossover design results in MORE losses and MORE distortion than a properlyexecuted network. This is especially true when the compromise is poor partsquality, not complexity of design, which by virtue of the increased part countcan also increase losses while improving other parameters.
Themost obvious visual cue is simply size. A small cheap crossover is just that. Small and cheap.
Poorlydesigned crossover (left pic); a high quality crossover (right pic)
The crossover (above left pic) is from a two-way bookshelf speaker system we have previously reviewed. The speaker system employs a stiff cone driver whichhas no crossover circuit (namely a LPF) to limit its bandwidth to reduce itsaudible break up modes at higher frequencies. The manufacturer chose to use an electrolytic capacitor as a measure of cost savings at the expense of performance as these parts have higher resistance and performance variancesthan quality and more costly poly designs. This is bottom of the barrelcrossover design unbecoming of any serious loudspeaker design, despite the claims of “science and research” behind itsproducts. At low power levels these speakers don't sound bad but once the volume is cranked up, audible driver break up was identified by our panel of listeners in blind comparisons.
Thecrossover image (right above pic) is of a much higher caliber design by anengineer making no outrageous claims. Instead, the engineer understands the importance of proper crossoverdesign and execution to get the most out of the drivers. The designer invested an appropriate portionof the budget of his product to arguably one of the most important aspects ofthe speaker design – the crossover. Notice how air core inductors are used in critical circuit paths andthey are properly oriented and spaced from adjacent magnetic inductors to avoidunwanted cross coupling.
Thetweeter circuit runs vertically up the left side of the photo. The woofercircuit is on the right. You can see how the inductors are located in thecorners, while the smallest inductor is in the middle of the board. Tweeter/woofer nearby inductors are oriented at dual-right-angles with eachother. There literally is more woofer/tweeter crosstalk fromsingle-wiring than there is through the inductor mutual coupling.
Interestinglythe philosophy can be continued that it’s better to run a midrange driver full rangein a 3-way system with no HPF element at all because it contributes to the overall bass output ofthe product. While there is some validity to this approach, care must be taken so that the actual driver can handle the stresses of being run full-range. It is noteworthy to mention asmall midrange driver isolated in its own enclosurewill limit its bass contribution to 80Hz or so. Any additional bass output the midrange may beproviding is wasted below those frequencies where the small driver isinefficient at converting the power to sound. At the same time, not crossing over the midrange driver essentiallykeeps it in parallel with the rest of the system lowering the overall systemimpedance at low frequencies forcing the amplifier to supply more current tothe loudspeaker that would ordinarily be needed if the crossover instead employed a HPF. It can be argued that this wastes amplifier power and increases theloudspeaker’sdistortion and thechance of possibly shutting the amplifier down as a result of an unstable loadimpedance. A loudspeaker designed like this when turned up in volume has the potential to experience audible break-up from the midrange driver. This is especially true as it exceeds its excursion limits because of a lack of protection at highinput levels at frequencies below the useful range of the driver. There are always exceptions to this, that hopefully a loudspeaker designer considers when choosing not to employ a HPF network on a dedicated midrange driver.
Werounded up several tower speakers a couple of years ago and found both trainedand untrained listeners were able to identify a particular speaker running itsmidrange without a crossover in a blind listening test as subjectively having its vocalsbeing slightly colored and tubby sounding, while also sounding strained at high outputlevels. In our option, a simple fix to their crossoverby inserting a HPF would have greatly improved the sound quality of thisspeaker which in itself wasn’t a bad sounding speaker to begin with. It just needed a crossover fix that wasn’ttoo costly, but whose absence was easy to hear by even casual listeners in acontrolled blind listening test. The speaker itself still scored very highly in our listening tests but we as Audioholics are always picking nits with all products we review to keep pushing manufacturers into making better next generation products that we can all salivate and eventually upgrade to.
Editorial Note about Running a Midrange Driver with No HPFby Paul Apollonio
By eliminating the series high-pass capacitor (at theVERY minimum) needed to protect the midrange driver from dangerous levels ofpeak low frequency content, this lowers the impedance of the system in a rangewhere the output of the midrange driver adds NOTHING to the output of theWoofer(s); hence lowering system sensitivity. The low frequencycontent can cause increased voice coil movement and possibly cause it to goout of the gap if driven too hard, thereby allowing the low frequencies to modulate (read distort) themidrange the speaker produces. A sinewave sweep test to measure this problem will be unrevealing inthis case. To see this problem, one mustput in two frequencies simultaneously and view the output on a spectrumanalyzer. (One can see distortion products as sum and difference frequencies)This is a simple process and one all audio engineers are familiar with.
Even if the Midrange driver is made INCREDIBLY stiff, and placed in a very verysmall sealed enclosure minimizing excursion and hence this distortion,subjecting the midrange voice coil to the heat caused by the low frequency content is generally not betterthan saving the price of the series capacitor.
There is such a thing as recommended practice and procedures, and the practiceof eliminating the high pass filter, even if only a single series capacitorfrom the midrange driver is, in my opinion, NOT a good idea by any stretch of imagination. Allowing the large peak amplitudes of lowfrequency content to get to a midrange speakers voice coil is, in my opinion, not a very good idea.
Bottom Line: The KISS principle doesn't always work whenit comes to building a crossover network for a loudspeaker. Take pause if you open the speaker box andsee a 2 or 3 element crossover like the above left picture above, recognizingthis was, in our opinion, done for cost reducing purposes and or design incompetence. While the speaker can still offer respectable performance nonetheless, it's likely not 'state of the art' in performance like you would find in more robust and often more expensive alternatives.
EditorialNote by Phil Bamberg
Low-slope designs also allow higher tweeterexcursion, leading to distortion or outright failure. For this reason (and those previouslystated), most low-order designs sound strained when turned up loud. Designers that are notqualified to develop crossovers properly often tout the simple filternetworks. They don’t have the knowledgeand experience to handle more complicated circuits, or time delay, or phase,for example. For anything more complexthan a second order crossover, the designer really needs a good modelingsoftware program with a built-in optimizer. This is why I believe that some companies which are great at buildingquality cabinets still don’t have properly designed crossovers insidethem. Things are improving in thisregard, as more audiophiles are not accepting of poor sound from inferiorcrossovers installed inside beautiful cabinets.
The crossoverdesign can make or break any multi-way loudspeaker. Having the world’s best drivers with a poorcrossover design will yield poor results. It is important to approach crossoverdesign with humility because it truly is an art that requires experience toperfect. Yes, it is possible to get decentsound by sending driver data sheets off for a generic crossover design. However, without proper analysis of thedrivers in the intended cabinet it is a game of roulette. This design overview neglects many designconsiderations a professional designer may consider such as time alignment,distortion and polar response but provides a starting point.
Before designing acrossover, the cabinet must be fully assembled including ports anddrivers. I typically run a separatespeaker wire from each driver out of the port and label each wire. For sealed loudspeakers, connect the wooferto the binding post as this is the first driver measured in the cabinet. Using SoundEasy, each driver is measured inthe cabinet to obtain frequency response and electrical impedance. These parameters are obtained following thetest methodology outlined in our LoudspeakerMeasurement Standard. Pleasenote that any measurements taken near field must factor in the effects ofbaffle diffraction. SoundEasy allowsmodeling and application of baffle diffraction estimates to any frequencyresponse data. Getting good measurementdata is one of the hardest parts of loudspeaker design. It is crucial to validate your results bytaking measurements using both near field and quasi-anechoic gated techniquesand compare the measurements before proceeding. The measurement data is subsequently used in SoundEasy’s crossoverdesign tool.
DIY Crossover Installed in Cabinet
Although CAD hascome a long way, designing a loudspeaker crossover benefits from somebackground in electronics. SoundEasy has2-way to 5-way crossover templates covering 1st order to 4thorder crossover topologies. Additionally, there are templates for a wide array of typical filtersand compensation circuits. The CADframework allows implementation of any circuit or filter topology imaginable solong as it uses inductors, resistors, capacitors, operational amplifiers,potentiometers or logic gates. If youhave not ever studied filter design or do not know what the previously-mentionedelements are, you may want to consider studying up to improve your chances ofpulling off a working and affordable solution.
The first step Itake when designing a crossover in SoundEasy is to determine workable crossoverfrequencies and filter orders. Thisrequires careful inspection of the driver data sheets, frequency responsemeasurement and driver impedance. It isimperative to make sure that response anomalies, such as the breakup of a metalcone driver, are sufficiently attenuated by the crossover so that they are notaudible. It is generally a good idea tokeep the crossover frequencies low enough to prevent driver beaming but highenough to prevent driver failure. Forexample, crossing over most tweeters below 1.5kHz is a bad idea if it is meantto play program material at 110dB or has a resonant frequency above 750Hz. Crossing an 8 inch bass driver at 5kHz willresult in poor dispersion and transient response at higher frequencies. As a rule of thumb, the max frequency for agiven driver diameter can be equated by taking 13560 and dividing it by thedrivers effective diameter. For an 8”driver with an effective cone diameter of 7”, this means that the maximumcrossover frequency should not exceed 1937Hz.
A 4thorder crossover rolls off frequencies at 24dB per octave where as a 1storder crossover rolls off frequencies at 6dB per octave. Lower order crossovers have less phase shiftbut tradeoff power handling and masking out of band response anomalies. The designer has to determine the best set oftradeoffs for an intended design goal. SoundEasy has built-in filter templates for delay networks if a designerdesires to design a time aligned loudspeaker for higher order filters. It is also important to note that the filterorder does not have to match for each driver in a system but it does simplifythe design.
Once approximatecrossover frequencies and filter orders are determined, a filter can be developedin CAD. For the purposes of this designanalysis, a 2nd order crossover was designed which approximatelymatches a Butterworth filter target. Additional response shaping elements were added where needed to meet thedesign requirements. This circuit wasdetermined 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 differencesin sensitivity, additional elements were added. The final circuit design is shown below.
Crossover Circuit inCAD
The componentvalues for the circuit design are determined using SoundEasy’s ingeniousoptimization technique. It is as simpleas setting a target response for a single driver and selecting the componentsSoundEasy should manipulate to attempt to meet the target response. In this example, a 2nd orderButterworth band-pass filter target was setup in the optimization dialog asshown in black in the figure below. SoundEasy will modify the selected crossover components and employ trialand error to arrive at the optimum component values required to meet thefrequency response target. Theoptimization technique will attempt thousands of combinations and may be runmultiple times to determine the best component values. While running optimization, it is important topay attention to the component values set by SoundEasy because it may setvalues that are very high inductances or capacitances and therefore expensive. This may take some playing around to getright. If you are unable to get close tothe target curve you might consider changing the filter topology. The midrange for the sample project tracksthe target curve pretty well but is not perfect at the high frequency knee dueto response problems with the driver itself. It is possible to shape the response further but more crossovercomponents equate to a more expensive crossover. It is easy to spend hundreds or eventhousands of dollars on crossover components. It is often better to solve extensive frequency response problems withbetter drivers or a more in-depth diffraction analysis.
Midrange CrossoverOptimization
The optimizationprocess is applied to the low-pass, band-pass and high-pass filters and acombined response is generated. Due toresponse issues, it may be necessary to attenuate a target or move thecrossover frequency slightly to obtain a flatter response. Ultimately, it may take several rounds ofmoving crossover frequencies and even filter orders to get to an acceptablesystem response. The combined systemresponse after optimization for this design is shown below.
3-Way CrossoverSummed Response
During the designprocess, it is important to pay close attention to system impedance graphs. It is no fun perfecting frequency when yourdesign has a minimum impedance of 1.5 Ohms. SoundEasy has an option to set the minimum acceptable impedance duringoptimization to help reduce the risk of developing a network that dips too lowfor the amplifier you are using.
As a final step,it is absolutely necessary to set the values of capacitors, resistors andinductors in the CAD editor to values you can actually purchase. This usually involves rounding up or down tovalues you can purchase. If thecrossover parts are expensive, you can attempt to reduce component values andsubsequently plot their effect on the final response. This is time consuming but can save you a tonof money. A 3-way crossover of thiscomplexity is not cheap. Using cheapelectrolytic capacitors this crossover costs close to $150 for componentsalone.
3-Way Crossover Billof Materials
Conclusion
Unfortunately, the tweeter used in this design did not measure close tothe data sheet at all and required pushing the crossover frequency up. Ultimately the dip at 4kHz is fixable but thecost required outweighs the benefit.
Personally, Iprefer loudspeakers that have a frequency response that tilts slightly downwardso 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 andsounds very good overall. Even though itis a ton of work, there is something very rewarding about designing aloudspeaker from the ground up and the owner of the speakers discussed in articleis 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 thecreative side of designing things. Ifyou are after bang for your buck sound, it is much cheaper and less time consumingto use someone else’s well-established and well-documented DIY design withpre-fabricated cabinets.
Speaker Crossover Design Software Machine
Some of mypersonal favorites include:
Free Speaker Design Software
- http://www.linkwitzlab.com - Siegfried Linkwitz is a renowned loudspeaker designer that developsDIY loudspeaker projects for fun in his retirement. The new LX521 and older Orion loudspeakerprojects are considered by some to be among the best speakers available, evencommercially.
- http://www.troelsgravesen.dk - A Danish designer responsible for many well regarded mid to very highend DIY designs. Most require building acabinet but the designs represent very high value.
- http://zaphaudio.com - Although this site is notupdated much anymore, John Krutke has many good designs available as fullkits. He has also tested a slew ofdrivers if you need unbiased raw driver data.
- http://parts-express.com - Many full DIYkits and parts available directly from Parts Express
- http://madisound.com - Many full DIY kits andparts available directly from Madisound. They also offer a crossover design service using very good crossoverdesign software.