What is the "crossover" in headphones and why is it important for sound quality?

What is the "crossover" in headphones and why is it important for sound quality?

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

This is a bit less than the full human hearing range, but it is still a wide range, and most headphone drivers cannot cover it all on their own. That is why most headphones have more than one driver inside the cabinet.

For example, a 2-way speaker divides the full range of frequencies into two smaller bands, each of which is reproduced by a different driver: the tweeter handles the high frequencies and the woofer takes care of the low frequencies. A 3-way speaker divides the full range into three smaller bands using three drivers: a tweeter, a woofer, and a midrange driver that reproduces the midrange frequencies between the tweeter and the woofer. Some designs, especially floor-standing tower speakers, have more than one woofer because the low frequencies need more power to balance out the high frequencies.

Divide frequencies and achieve accurate sound

How is the full range divided into smaller bands and directed to the appropriate drivers? This is the role of the crossover circuit in the speaker, which consists of two or more filters. This circuit receives the full range signal from the amplifier or audio receiver and directs the high frequencies to the tweeter using a high-pass filter, the low frequencies to the woofer using a low-pass filter, and in a 3-way speaker, the midrange frequencies go to the midrange driver using a band-pass filter.

Basically, a simple crossover circuit consists of three basic electronic components: inductors, capacitors, and resistors. But the details are what matter, and the exact design of the circuit—which components are used and how they are arranged—is an important part of the art of speaker design.

SVS engineers spend a lot of time designing the crossover circuit because it greatly affects the final sound of the speaker. If a manufacturer tries to save costs by using cheap components and a simple design, the speaker can sound distorted and strained at high levels, the soundstage and imaging can be affected, and the frequency response can be inaccurate and uneven. To avoid these and other problems common to poor crossovers, SVS uses premium components, advanced circuit design, detailed computer modeling, and extensive real-world and laboratory testing to achieve uncompromising performance from its speakers.

Part of this process is determining exactly where to divide the full range of frequencies into smaller bands; the points at which we divide are called crossover frequencies. This depends on the capabilities of the drivers. Ideally, each driver should be required to reproduce the frequencies with which it is comfortable—in other words, its linear operating range. If a driver tries to reproduce frequencies outside of its linear operating range, it can sound weak or even distorted.

slippery slope

Equally important is how the crossover transitions from one band to the next. A sudden transition causes a lot of sonic problems, so it should follow a more gradual slope, called a crossover ramp. As the frequency rises, the output of one filter decreases while the output of the next one increases. In effect, the frequency ranges of the filters overlap, and both drivers reproduce the frequencies in the transition region.

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 go to the woofer, and frequencies above 2 kHz go to the tweeter. However, as the low frequencies approach 2 kHz, the low-pass filter starts to reduce the level of the signal sent to the woofer. At the same point, the high-pass filter starts to increase the level sent to the tweeter until it reaches its full level somewhere above 2 kHz.

The ratio of the woofer's drop in level to the tweeter's rise in level is the crossover slope (sometimes called the gradient). In most SVS speakers, this slope is 12 dB/octave. The two slopes intersect 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 both reproduce this frequency, they combine to reach the same level as the higher and lower frequencies, and the speaker's overall frequency response is flat across its entire range.

Crossover design

Crossover frequencies and ramps need to work in harmony with the characteristics of the drivers and the acoustics of the cabin. In addition, the crossover circuit affects the speaker’s ability to handle power. In the SVS crossover, for example, the inductors are oriented perpendicular to each other so that their electromagnetic fields don’t interfere, and the resistors are mounted vertically with space between them so that they can handle more heat and therefore more power. The last thing you want is a speaker that weakens or distorts when the action reaches its peak!

Designing a speaker crossover is a meticulous process. First, our engineers use computer modeling to develop a basic design for the speaker cabinet and drivers. Next, they build a prototype and measure its performance in an anechoic chamber. Finally, they take additional measurements in a calibrated listening room that mimics a real-world environment. Most importantly, they listen to the speaker and form their own opinions. Surprisingly, a crossover can measure well but sound terrible. In that case, we go back to the computer to tweak the design and try again.

This iterative process is a crucial part of “tuning” or “routing” a speaker. Ideally, it results in a crossover circuit that seamlessly integrates the sound of each driver into a cohesive whole, producing great sound in almost any room and at almost any volume level.

Every SVS passive speaker features a SoundMatch crossover designed to create a wide soundstage with precise frequency response and accurate imaging of the largest possible “sweet spot.” The SoundMatch crossover is carefully tuned so that each driver seamlessly blends into the other while delivering excellent on- and off-axis frequency response and precise spatial imaging for the most compelling audio experience possible. The crossover topology and geometry are consistent within every SVS speaker, so all Prime and Ultra Series models match their sonic character to seamlessly integrate with each other for maximum flexibility when building a system.

Active Crossover vs Passive Crossover

An active crossover does the same thing as a passive design, but at a different point in the signal chain, and is more electronically sophisticated. The most common type of active crossover is digital—it takes a full-range digital audio signal and splits it into different smaller bands using digital signal processing (DSP). Like a passive crossover, the bands overlap in the transition region, and the crossover slope is typically 12 dB/octave. The split signals are then converted to analog and sent to separate amplifiers that drive each type of driver.

Active crossovers are often used in speakers with built-in power amplifiers. One such pair is the SVS Prime Wireless Pro active speaker, which has a digital active crossover and a powerful Class D amplifier. It offers multiple digital inputs (Bluetooth, Wi-Fi, Ethernet, optical Toslink) in addition to analog inputs. The analog signals are converted to digital before being split into two frequency bands by the crossover.

SVS active subwoofers also use a digital active crossover if they receive a full-range signal. In this case, the crossover acts as a low-pass 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 can be adjusted with a simple firmware update. Of course, the SVS goes through the same iterative measurement and listening process to tune its active crossover, and engineers can use DSP to improve the speaker’s performance beyond the capabilities of a passive crossover.

Clearly, the crossover is a crucial part of all multi-directional speakers. With clever design and execution, the SVS speakers disappear, leaving only the amazing experience of immersive audio.