Humans with normal hearing can perceive up to 10 octaves of frequencies, from 20 Hz to 20 kHz, and full-range speakers are designed to reproduce as much of that span as possible. Realistically, most speakers can encompass eight or nine octaves, from 20 kHz or even higher at the top down to somewhere between 40 and 80 Hz, though some can go lower.

That's slightly less than the entire extent of human hearing, but it's still a very wide range, and most speaker drivers can't cover all of it by themselves. So, most speakers have more than one driver mounted within a cabinet.

For example, a so-called 2-way speaker divides the full frequency range into two smaller ranges, and each range is reproduced by a different driver: A tweeter handles high frequencies and a woofer takes care of lower frequencies. 3-way speakers divide the full range into three smaller ranges using three drivers: a tweeter, a woofer, and a midrange driver that reproduces the middle frequencies between the tweeter and woofer. Some designs, especially floorstanding tower speakers, have more than one woofer since the lower frequencies require more oomph to balance the high end.

Divide Frequencies and Conquer Accurate Sound

How is the full range divided into smaller ranges and directed to the corresponding drivers? That's the job of the speaker's crossover circuit, which consists of two or more filters. This circuit accepts a full-range signal from the power amp or AV receiver and sends the high frequencies to the tweeter using a highpass filter, the lower frequencies to the woofer(s) using a lowpass filter, and—in a 3-way speaker—the middle range of frequencies to the midrange driver using a bandpass filter (see Fig. 1).

At its heart, a passive crossover circuit is simple. It consists of three basic electronic components: inductors, capacitors, and resistors. But the devil is in the details, and the specific design of the circuit—exactly which components are used and how they are laid out—is an important part of the speaker designer's art.

SVS engineers spend a lot of time on the crossover circuit, because it has a huge impact on the final sound of the speaker. If a manufacturer tries to save costs by utilizing cheap components and a simplistic design, the speaker can sound distorted and strained at high levels, the soundstage and imaging can be degraded, and the frequency response can be inaccurate and uneven. To avoid these and other issues common to inferior crossovers, SVS uses premium components, sophisticated circuit design, detailed computer-aided modeling, and extensive real-world and anechoic testing, which we will discuss a little later, to achieve uncompromised performance from its speakers.

One part of this process is deciding exactly where to divide the entire frequency range into smaller ranges; the dividing points are called the crossover frequencies. This depends on the capabilities of the drivers. Ideally, each driver should be asked to reproduce only the frequencies it's most comfortable with—in other words, its linear operating range. If a driver tries to reproduce frequencies outside of its linear operating range, it might sound weak or even distorted.

A crossover circuit sends certain frequency ranges to different drivers: A highpass filter sends high frequencies to the tweeter, a lowpass filter sends low frequencies to the woofer, and a bandpass filter sends middle frequencies to a midrange driver in a 3-way speaker.

Slippery Slope

Equally important is how the crossover transitions from one range to the next. An abrupt, sudden transition causes all sorts of sonic problems, so it must follow a more gradual slope, called the crossover slope. As the frequency rises, the output of one filter ramps down as the output from the next filter ramps up. In fact, the frequency ranges of the filters overlap, and both drivers reproduce frequencies in the transition zone.

For example, let's say the crossover frequency between the tweeter and woofer in a 2-way speaker is 2 kHz. In this case, frequencies below 2 kHz are sent to the woofer, and frequencies above 2 kHz are sent to the tweeter. However, as lower frequencies approach 2 kHz, the lowpass filter starts reducing the signal level sent to the woofer. At the same point, the highpass filter starts increasing the level sent to the tweeter until it reaches its full level somewhere above 2 kHz.

The rate at which the level decreases to the woofer and increases to the tweeter is the crossover slope (sometimes called the rolloff). In most SVS speakers, that slope is 12 dB/octave. The two slopes cross at the crossover frequency (2 kHz in this example) where they are both 6 dB below their nominal level. But because the tweeter and woofer are both reproducing this frequency, they combine to reach the same level as the higher and lower frequencies, and the overall frequency response of the speaker is flat throughout its entire range.

A 3-way crossover works in much the same way with the addition of a bandpass filter between the lowpass and highpass filters (see Fig. 2). In this case, there are two crossover frequencies.

This graph represents the crossover rolloffs within the SVS Prime Pinnacle, a 3-way floorstanding speaker with one tweeter, one midrange, and three woofers. The crossover frequencies are 320 Hz between the woofers and midrange and 2.3 kHz between the midrange and tweeter.

The blue curve represents the lowpass filter's rolloff, the green curve is the midrange bandpass filter's rolloff, and the black line depicts the highpass filter's rolloff. The red line is the combined frequency response of the entire speaker taking all three filters into account. Notice that the rolloffs are down 6 dB from the combined frequency response at the crossover points.

Designing Crossovers

The crossover frequencies and slopes must work in harmony with the driver characteristics and cabinet acoustics. In addition, the crossover circuit affects the speaker's power-handling capabilities. In SVS crossovers, for example, the inductors are oriented perpendicular to each other so their electromagnetic fields do not interfere, and the resistors are mounted vertically with space between them so they can tolerate more heat and thus more power. The last thing you want is a speaker that wimps out or distorts just as the action reaches its climax!

Designing a speaker crossover is a painstaking process. First, our engineers use computer modeling to come up with a basic design for a speaker cabinet and drivers. Then, they build a prototype and measure its performance in an anechoic chamber. Finally, they take more measurements in a calibrated listening room that simulates a real-world environment. Most importantly, they listen to the speaker and form their own subjective opinions. Amazingly, a crossover might measure well but sound terrible. In that case, we go back to the computer to modify the design and try again.

This iterative process is a critical part of "tuning" or "voicing" a speaker. Ideally, it results in a crossover circuit that seamlessly blends the sound of each driver into a unified whole, and the speaker sounds great in just about any room at almost any volume level.

All passive SVS speakers feature a SoundMatch crossover designed to create an expansive soundstage with accurate frequency response and precise imaging for the largest possible “sweet spot.” The SoundMatch crossover is carefully tuned so each driver blends seamlessly with each other while offering excellent on and off-axis frequency response and pinpoint spatial imaging for the most convincing audio experience possible. The crossover’s topology and architecture are consistent within every SVS speaker, so all Prime and Ultra Series models are timbre-matched to integrate seamlessly together for the ultimate flexibility when building a system.

Active vs. Passive Speaker Crossovers

Active crossovers do the same job as passive designs, but at a different point in the signal chain, and they are more sophisticated electronically. The most common type of active crossover is digital—it accepts a full-range digital-audio signal and splits it into different smaller ranges using DSP (digital signal processing). Like a passive crossover, the ranges overlap in the transition zone, and the crossover slope is generally 12 dB/octave. The split signals are then converted to analog and sent to separate amplifiers that power each type of driver.

Active crossovers are often used in powered speakers—that is, speakers with built-in power amplifiers. One such speaker is the 2-way SVS Prime Wireless Pro Powered Speaker Pair, which incorporates a digital active crossover and powerful Class-D amplifier. It offers several digital inputs (Bluetooth, WiFi, Ethernet, TosLink optical) as well as analog inputs. Analog signals are converted to digital before being split into two frequency ranges by the crossover.

SVS powered subwoofers also use a digital active crossover if they receive a full-range signal. In this case, the crossover acts as a lowpass filter that prevents frequencies above the subwoofer's operating range from reaching the driver.

The beauty of digital active crossovers is that they are more accurate with much tighter tolerances than passive circuits, and they can be tweaked with a simple firmware update. And of course, SVS goes through the same iterative measurement and listening process to fine-tune its active crossovers, and the engineers can use the DSP to refine the speaker's performance beyond the capabilities of a passive crossover.

Clearly, crossovers are a critical part of all multi-way speakers. With smart design and implementation, SVS speakers disappear and leave nothing but the glorious experience of immersive sound.

If you have questions about crossovers or setting up your home audio system, please leave it in the comments or reach our the SVS Sound Experts at [email protected], 877.525.5623 or chat.