What is a speaker crossover?

The crossover is a component that is built into almost every loudspeaker, yet very few people know about it. All the more reason to give the crossover its moment in the spotlight.

The crossover and its tasks

Crossovers are used almost everywhere when it comes to loudspeakers . They are only absent in full-range drivers and in speakers where software handles the frequency separation. This is the case, for example, with some Teufel Wi-Fi speakers. A crossover can also be used outside the speaker, for instance in the amplifier or receiver.

This is easiest to understand with home cinema amplifiers, as you connect a separate subwoofer here that is solely responsible for bass reproduction. Apart from these technologies, every speaker with more than one driver needs a crossover if it receives an unfiltered signal from the amplifier. The crossover then determines where a specific part of the signal above or below a defined frequency is routed. The task of the crossover is to supply each driver with exactly the signals it can process best. For example, it will only pass low frequencies on to the woofer.

Speakers from Teufel

Avoid resonance frequencies at all costs

The crossover also prevents the so-called resonance frequency of a speaker from being hit. In practice, this looks like this: If the resonance frequency of a speaker is reached, the driver may amplify the membrane’s amplitude beyond the desired excursion. If that happens, individual tones are reproduced particularly loudly and with distortion. In the worst case, the entire speaker cabinet can be set into vibration. This must be avoided at all costs, as this effect destroys the sound image.

The following video shows why the resonance frequency should be avoided, demonstrating how the natural frequency of a glass is hit.

The crossover assigns: bass for the woofer, highs for the tweeter

The function of the crossover can be explained clearly using the example of a 3-way floorstanding speaker. A 3-way speaker has three different drivers in the cabinet. Their names already give an indication of the frequency range that the crossover feeds to them. The low frequencies go almost exclusively to the woofer, mid frequencies to the midrange driver, and the highs, of course, to the tweeter.

To ensure that everything sounds exactly as it should at the end of the signal chain, it is necessary to set the crossover correctly. It is present in every home cinema speaker that contains at least two different drivers and receives an unfiltered signal from the amplifier. Some particularly high-end speakers can also be individually configured using high-pass and low-pass filters.

High-pass and low-pass filters

High-pass and low-pass filters either allow only high or only low frequencies to pass through and block or reduce the rest. Caution, there is a slight risk of confusion here! The high-pass filter can also be referred to as a low-cut filter. This means it only lets high frequencies “pass”. It’s even easier to remember if you look at the English terms: the high-pass filter is called “high-pass filter” or “low-cut filter”. The low-pass filter naturally does the opposite and only allows low frequencies through, so it would be used for a woofer.

Different filter orders

Low-pass and high-pass filters can filter with different strengths. This is referred to as a certain order. As a rule, only first- to fourth-order filters are used, with the filter attenuating between 6 dB (1st order) and 24 dB (4th order) per octave. You don’t notice any of this yourself. This process takes place inside the speaker on the crossover:

How the crossover works in a 3-way speaker

Using the Ultima 40 from Teufel as an example, the way a 3-way speaker works and the function of the crossover can be clearly illustrated.

The distribution of the drivers

As can be seen in the illustration, a total of four drivers are responsible for signal output in the Ultima 40 cabinet. Contrary to a common assumption, the driver for high frequencies does not always have to be at the very top. This is not technically mandatory either. However, as a rule of thumb, you can assume that the driver diameter gets smaller the higher its intended frequency range is. In this speaker, the tweeter is installed below the midrange driver. This brings the sound image closer to a natural sound source.

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At the bottom of the speaker cabinet are two woofers (not common in every floorstanding speaker), above them the tweeter mentioned earlier, topped by the midrange driver.

The crossover and the frequency response

The crossover must route the frequencies correctly and accurately. It has to work in such a way that the frequency response of the speaker achieves the most accurate value possible. This is only possible if the resonance frequency of the speaker is not hit and no distortion occurs, or the speaker cabinet is even set into vibration. The frequency response usually refers to the amplitude frequency response, i.e. the specification of the sound pressure level, measured in decibels. Graphically, the frequency response is usually shown as a line that should ideally have no peaks or dips.

In reality, this is hardly achievable, but the aim is to get as close as possible to the ideal frequency response. After all, the human ear can already perceive deviations of one decibel, especially in the midrange. Good hi-fi speakers should therefore allow deviations of no more than 3 dB from the reference level.

To achieve this, however, the speaker must be equipped with a very well-adjusted crossover. Ideally, the manufacturer takes care of the setup, because calculating a crossover requires solid expert knowledge.

Can I calculate the crossover myself?

To calculate a crossover, far more knowledge about the speaker is required than just whether it is a tweeter or a woofer. While there are some guides and calculatorsOpens in new tab available online that can help with the calculation, beginners should keep their hands off them.

No one knows the actual characteristics of a speaker better than the manufacturer itself, which is why such work should be left to them. However, anyone who wants to build a speaker themselves will inevitably have to calculate the crossover. In that case, the speaker builder should first acquire solid theoretical knowledge.

Ultima 40: our powerful all-rounder