The question is a bit misleading because it assumes there is a blanket answer for everyone and every application, but it’s never that simple. If it was, we would just include a preset and wouldn’t bother adding the ability to manually set the crossover on your KEF subwoofer at all. It’s tough to give a quick answer for a few reasons, the first of which is that we need to understand which crossover point you are setting—your low pass filter for your subwoofer, your high pass filter for your speakers, or in many cases, both.
There’s also a bit of trickiness to the concept of the crossover point. Crossover filters are not brick walls – they don’t simply stop all frequencies below or above the exact setpoint you choose. Instead, they attenuate all frequencies below or above that set point over time. For an 80Hz low pass filter for instance, you don’t stop the subwoofer from playing anything above 80Hz. Instead, there is a steep attenuation and as the frequency gets higher you will hear those frequencies less and less. The slope that results from this crossover point is key to properly setting your HPF and LPF filters.
Let’s start with a closer look at the filters and get an idea of how they work.
The High Pass Filter (HPF) is a crossover filter network that allows frequencies above the set-point to pass onto the driver. If you set your HPF to 80 Hz, all the frequencies above 80 Hz are sent on to the driver while all the frequencies below 80 Hz are steeply attenuated, taking the crossover slope into consideration.
What is the Low Pass Filter?
The Low Pass Filter (LPF) is a crossover filter network that allows frequencies below the set-point to pass onto the driver. If you set your LPF to 80 Hz, all the frequencies below 80 Hz are sent on to the driver while all the frequencies above 80 Hz are steeply attenuated, taking the crossover slope into consideration.
What is attenuation?
Attenuation is the decrease in an electrical signal, either by natural causes such as insertion loss over a long transmission or by simply turning the volume down on an amplify. Equalizers attenuate a specific band of frequencies. With analog signals, boosting a signal (think turning up the bass on your basic radio) is not optimal because the boost also adds a phase shift that ultimately changes – or distorts – the signal. The best way to implement EQ changes is to attenuate a certain band of frequencies, to bring that band into line with the strength of the other audio frequencies. By attenuating a certain band of frequencies, you lessen that band’s ultimate volume, but you also bring surrounding frequencies into line by giving them more relative prominence.
Crossovers work on a slope that begins at the crossover setting and slowly slopes downward or upward (downward for HPF and upward for LPF). For example, in a Low Pass Filter, frequencies above the set-point will be present in the output, but exactly how present is determined by the slope. When you select a crossover point you are setting a filter which blocks more of the signal the farther you go away from the crossover point.
Regardless of the circuit they are in, electrical components cause a time-delay. Think of a motor vehicle going from standing still to top speed – it takes time to get there because of inertia, friction and other things like gravity. Electrical components also require some measure of time to get up to speed. This delay is part of why a crossover has a slope, or a period of time to reach full operation.
The slope tells us how long before the filter attenuates all frequencies to the point where we can’t hear them. Crossover slope (sometimes referred to as ‘roll-off’) defines the rate the signal increases or decreases. This is done by defining how much attenuation takes place per octave. (An octave is a doubling or halving of a frequency: One octave below 80 Hz is 40 Hz; one octave above 80 Hz is 160 Hz).
A slope of -24 dB is fairly common in hi-fi systems, so we’ll use that as our example. A -24 dB slope means that for every octave you move away from the set frequency, the signal is attenuated -24 dB. If you set the crossover HPF at 80 Hz, at 40 Hz (one octave below 80 Hz) the signal is -24 dB lower in volume than at the set point. This also means that one half octave below the set point (60 Hz in this example) the attenuation is -12 dB. At 70 Hz the attenuation is only -6dB; that’s where the need for setting the HPF and LPF at complementary points becomes critical.
The HPF and LPF set point frequencies should complement each other—remember, you’re not completely filtering out the signal above and below your selected crossover points. Between the two filters there is critical recorded information caught in the middle. You can potentially get too much or too little of the frequencies in between your setpoints which can make sub/speaker integration tricky to dial in.
The goal is striking a balance between your HPF and LPF settings to complement each other in a way that gives you a smooth frequency response throughout the entire audible range of frequencies.
To use an example, we’ll consider the LS50 Wireless II and a single KF92 subwoofer, and examine the recommended settings in the KEF Connect app. Our engineers have determined that the best starting point when pairing LS50 Wireless II to KF92 is to set the HPF to 70 Hz, and the LPF to 50 Hz. This setting allows the slopes to complement each other so that the frequencies in between the two setpoints are properly passed to the appropriate drivers.
With the HPF filter set to 70 Hz, all frequencies above 70 Hz are sent to the LS50 Wireless II at 0 dB attenuation (full-strength). At 35 Hz (one octave below), the attenuation is -24 dB, which for all intents and purposes is fully attenuated. At one half-octave below the setpoint (around 56 Hz), the attenuation is -12 dB, which is pretty low but still audible.
Setting the LPF for 50 Hz is the best complementary setting. This setting means all of the frequencies below 50 Hz are sent to the subwoofer, and frequencies at 100 Hz (one octave above) are fully attenuated. At one half octave above (75 Hz) the attenuation is approximately -12 dB.
When taken together, each driver produces frequencies in between the set-points, but at an attenuated level that allows the drivers to work together to produce the proper volume at the frequencies in-between the setpoints.
It’s important to note that these settings are merely recommended starting points. Our labs and testing rooms are going to react differently to bass than your listening room – it’s impossible to suggest a perfect setting for any piece of audio equipment! Your room, ears, tastes and equipment ae unique. You’ve got to experiment with what sounds best until you get yourself dialed in. The settings we recommend are just that: recommended places to start when setting up your gear.
Don’t be afraid to experiment – you can always back out to the factory settings if you get stuck in the weeds. You’re not going to damage anything by experimenting with the HPF/LPF settings, on the contrary, by investing a little time you’re likely to come across that optimal setting you were hoping to find!