Myths & Facts about Loudspeaker Crossovers: Identifying Legitimately High Fidelity Designs
This article explores some of the myths and facts about crossover design. It also discusses some of the mistakes often made by loudspeaker manufacturers done either as cost savings or design incompetence. It is our hope that the reader will gain a better understanding of the mechanics of loudspeaker crossovers so they can make a more informed purchasing decision.
The loudspeaker crossover can be considered the brain of the loudspeaker. It directs the bandwidth of frequencies each driver is optimized to reproduce while it also level matches each driver and can help to stabilize the load impedance the amplifier will see. Without the loudspeaker crossover, a loudspeaker driver such as a tweeter can be overdriven which can lead to distortion and eventual failure. A loudspeaker system without a properly designed crossover (or none at all) can cause too much frequency overlap between drivers which can increase distortion and degrade overall sound quality.
Properly designing a loudspeaker crossover requires engineering talent and sufficient budgeting to fit it into the total system cost. There are often times manufacturers will downplay the crossover’s importance, either because they lack the knowledge to truly understand its role, or they are simply attempting yet another cost cutting tactic, assuming the buyer will never know since it’s out of sight, thus out of mind.
Let us briefly define some basic crossover terminology which will help in the discussions later in this article. A High Pass Filter (HPF) bandwidth limits frequencies below where the driver is inefficient at (or mechanically incapable of) producing those frequencies. A Low Pass Filter (LPF) bandwidth limits frequencies above where the driver is inefficient at (or mechanically 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 bass frequencies. It can’t do so efficiently (or mechanically!) so we employ a HPF to block those frequencies. Conversely think of a woofer trying to produce very high frequencies. We employ a LPF to effectively filter the frequencies above which the woofer is capable of producing. The diagram pictured here shows basic schematics of each filter type along with a generic theoretical slope response. The squiggly looking device is an inductor while the dual parallel adjacent lines represent a capacitor. This diagram is courtesy of Williamson-labs.com.
For more information on this topic, we recommend reading our article: Filter & Crossover Types for Loudspeakers
Myth #1: The Simpler Crossover is ALWAYS Better
We've seen numerous loudspeaker companies defend their 2- or 3-element crossover (ie. Resistor/capacitor network only) as being preferred to a more complex crossover network that their competitors employ on their designs. They argue that they custom designed their drivers to better integrate with each other, thus not needing a crossover with steep slopes or an elaborate design to improve overall system impedance.
Tabulated below 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
The reality here is sometimes the exact opposite. A two- or three-element crossover on a two-way bookshelf speaker typically has no element at all on the midbass driver, allowing it to run full range. Unless the driver has been meticulously custom-designed and custom-manufactured to have a natural, precise, controlled rolloff at the upper end of its operating range (which is theoretically doable, but is often a very expensive proposition), the end result would be break up distortion at higher frequencies which becomes audible as the speaker is driven harder and why at least a second-order crossover network is required to filter such distortion below audibility. This is especially true with stiff cone drivers that have a more prominent audible break up mode. Higher-order networks are typically needed in such cases. Having no network at all is NOT a viable solution if high fidelity reproduction is the primary goal of the speaker system.
First-order filters aren’t good at preventing distortion, particularly at or below the tweeter’s resonance frequency. They also don’t provide enough isolation of the bandwidths in multi driver systems. The result is too much of an opportunity for destructive interference, and therefore a loss of uniformity across the crossover region with the woofer doing most of the damage at high frequencies. It’s important to note that a tweeter still remains pistonic above and below Fc while a woofer does not. This is why second-order or higher filter networks are typically employed but they are most costly to employ and complex to produce. The bigger parts are more expensive and more likely to cause insertion loss or burn out under stress. This requires a tighter set of tolerances than a lower order network for the same amount of network variability.
Editorial Note About Directivity, Crossover Points & Driver Selection by Dr. Floyd Toole
A good sounding loudspeaker needs to have smooth and flat on-axis frequency response and similar performance as we move far off axis. We describe this in terms of directivity as a function of frequency, and although absolutely constant directivity is not necessary, smooth and gradually-changing directivity is a good objective. In deciding on the drivers to be used in a speaker system it is necessary to ensure that at crossover frequencies the drivers have closely matching directivities. This means that when the acoustic transition is made between, say, a woofer and a midrange, or a midrange and a tweeter, there is continuity in the directional sound radiation pattern. It is not sufficient just to have a good looking on-axis response. So, in addition to selecting drivers for their useable bandwidths and power handling capabilities, we need to pay attention to their directional radiation patterns. The most difficult transitions occur when the transducers involved are very different in size.
Editorial Note by Steve Feinstein on Crossover Frequency Selection
A basic, ages-old but still true, rule of thumb states that a designer is usually safe when he crosses a driver over at double its resonant frequency. If a tweeter has an Fs of 1500 Hz, use a 3000 Hz crossover, minimum. If a midrange is 300 Hz, use 600 Hz.
Another good rule of thumb says, “18 dB down at resonance.” If a tweeter’s resonance is 1500 Hz, the voltage curve of the crossover should show the tweeter section being down 18 dB from “0 dB.” That kind of conservatism all but assures no tweeter burn-out.
This was the “rule” at a major speaker company I used to work at, and the engineers all hated it, because it was so conservative and resulted in very high tweeter crossover points. But we almost never lost a tweeter and our warranty costs were vanishingly low. ‘Real world’ vs. ‘theory.’
A properly designed filter network will always present a stable load impedance to the amplifier. It will also properly bandwidth limit the loudspeaker drivers to lower their distortion and better integrate their response resulting in a more even on- and off-axis frequency response and power response. The power response is simply the total radiated acoustic output of a speaker measured spherically around it. This is discussed in greater detail later in the article.
Compromising a crossover design results in MORE losses and MORE distortion than a properly executed network. This is especially true when the compromise is poor parts quality, not complexity of design, which by virtue of the increased part count can also increase losses while improving other parameters.
The most obvious visual cue is simply size. A small cheap crossover is just that. Small and cheap.
Poorly designed 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 which has no crossover circuit (namely a LPF) to limit its bandwidth to reduce its audible 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 variances than quality and more costly poly designs. This is bottom of the barrel crossover design unbecoming of any serious loudspeaker design, despite the claims of “science and research” behind its products. 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.
The crossover image (right above pic) is of a much higher caliber design by an engineer making no outrageous claims. Instead, the engineer understands the importance of proper crossover design and execution to get the most out of the drivers. The designer invested an appropriate portion of the budget of his product to arguably one of the most important aspects of the speaker design – the crossover. Notice how air core inductors are used in critical circuit paths and they are properly oriented and spaced from adjacent magnetic inductors to avoid unwanted cross coupling.
The tweeter circuit runs vertically up the left side of the photo. The woofer circuit is on the right. You can see how the inductors are located in the corners, while the smallest inductor is in the middle of the board. Tweeter/woofer nearby inductors are oriented at dual-right-angles with each other. There literally is more woofer/tweeter crosstalk from single-wiring than there is through the inductor mutual coupling.
Interestingly the philosophy can be continued that it’s better to run a midrange driver full range in a 3-way system with no HPF element at all because it contributes to the overall bass output of the 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 a small midrange driver isolated in its own enclosure will limit its bass contribution to 80Hz or so. Any additional bass output the midrange may be providing is wasted below those frequencies where the small driver is inefficient at converting the power to sound. At the same time, not crossing over the midrange driver essentially keeps it in parallel with the rest of the system lowering the overall system impedance at low frequencies forcing the amplifier to supply more current to the loudspeaker that would ordinarily be needed if the crossover instead employed a HPF. It can be argued that this wastes amplifier power and increases the loudspeaker’s distortion and the chance of possibly shutting the amplifier down as a result of an unstable load impedance. 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 high input 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.
We rounded up several tower speakers a couple of years ago and found both trained and untrained listeners were able to identify a particular speaker running its midrange without a crossover in a blind listening test as subjectively having its vocals being slightly colored and tubby sounding, while also sounding strained at high output levels. In our option, a simple fix to their crossover by inserting a HPF would have greatly improved the sound quality of this speaker which in itself wasn’t a bad sounding speaker to begin with. It just needed a crossover fix that wasn’t too costly, but whose absence was easy to hear by even casual listeners in a controlled 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 HPF by Paul Apollonio
By eliminating the series high-pass capacitor (at the VERY minimum) needed to protect the midrange driver from dangerous levels of peak low frequency content, this lowers the impedance of the system in a range where the output of the midrange driver adds NOTHING to the output of the Woofer(s); hence lowering system sensitivity. The low frequency content can cause increased voice coil movement and possibly cause it to go out of the gap if driven too hard, thereby allowing the low frequencies to modulate (read distort) the midrange the speaker produces. A sinewave sweep test to measure this problem will be unrevealing in this case. To see this problem, one must put in two frequencies simultaneously and view the output on a spectrum analyzer. (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 very small 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 better than saving the price of the series capacitor.
There is such a thing as recommended practice and procedures, and the practice of eliminating the high pass filter, even if only a single series capacitor from the midrange driver is, in my opinion, NOT a good idea by any stretch of imagination. Allowing the large peak amplitudes of low frequency 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 when it comes to building a crossover network for a loudspeaker. Take pause if you open the speaker box and see a 2 or 3 element crossover like the above left picture above, recognizing this 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.
Editorial Note by Phil Bamberg
Low-slope designs also allow higher tweeter excursion, leading to distortion or outright failure. For this reason (and those previously stated), most low-order designs sound strained when turned up loud. Designers that are not qualified to develop crossovers properly often tout the simple filter networks. They don’t have the knowledge and experience to handle more complicated circuits, or time delay, or phase, for example. For anything more complex than a second order crossover, the designer really needs a good modeling software program with a built-in optimizer. This is why I believe that some companies which are great at building quality cabinets still don’t have properly designed crossovers inside them. Things are improving in this regard, as more audiophiles are not accepting of poor sound from inferior crossovers installed inside beautiful cabinets.
- Identifying Legitimately High Fidelity Loudspeakers: The Economics of Cost Cutting
- Myths & Facts about Loudspeaker Cabinets: Identifying Legitimately High Fidelity Designs
- Loudspeaker Drivers: Identifying Legitimately High Fidelity Parts
- Comb Filtering, Acoustical Interference, & Power Response in Loudspeakers