Crossovers are components hidden within just about every loudspeaker. The only exceptions are full-range speakers and speakers that use software to separate frequencies (a few Raumfeld speakers do the latter). Crossovers are sometimes also found integrated in amplifiers or receivers, especially home cinema receivers where the subwoofer for bass playback is connected separately from the satellites. Aside from these specialized technologies, it’s generally true that every loudspeaker with more than one driver in its enclosure requires a crossover. As its name suggests, a crossover is where the unfiltered audio signal is divided according to a predefined upper or lower threshold. The crossover supplies each driver with the signal range it was designed to best reproduce. For example, crossovers ensure that tweeters only receive the highest frequencies and not the lower ones. This makes a crossover something akin to a loudspeaker’s central nervous system.
Crossovers prevent loudspeakers from reaching their resonance frequencies
In addition, crossovers prevent loudspeakers from reaching their so-called resonance frequency. This refers to the frequency at which the enclosure surrounding one or more drivers resonates when the driver emits sound. In other words, if the enclosure itself were allowed to vibrate, what sound would it make. Resonance frequencies exist because the vibrations from the driver cause everything around it to vibrate. If a loudspeaker reaches its resonance frequency, certain frequencies will oscillate with an amplitude that is disproportionately greater than the other frequencies. This causes certain tones to be reproduced more loudly than the others and with distortion. In the worst cases, the entire loudspeaker cabinet can be brought into oscillation when its natural frequency is reached. This is to be avoided at all costs when creating a harmonious sound image.
We’re all familiar with cartoons depicting opera singers who are able to shatter glass with a sustained high note. As it turns out, this isn’t so far-fetched. The following video shows what happens to a glass when its resonance frequency is reached:
Crossovers feed low frequencies to bass drivers, treble frequencies to tweeters
If we take a typical 3-way floor-standing speaker as an example, the function of a crossover can be clearly visualized. 3-way loudspeakers contain three different drivers in their enclosures: A bass driver for the lowest frequencies, a midrange driver for those frequencies in the middle, and a tweeter for the highest range. Yet the loudspeaker as a whole receives a single, unfiltered audio signal. Since it would only strain a tweeter and create a distorted sound to send bass frequencies to it, and because a subwoofer will have no ability to reproduce treble tones, it makes sense to separate this signal as precisely as possible bearing in mind the range of each individual driver. A well-configured crossover is the key. The crossovers on high-end loudspeakers can also be individually adjusted by means of high and low pass filters.
A high pass filter is an electronic filter that allows only the high frequencies to pass, but prevents all lower frequencies beneath a certain cutoff frequency from reaching a driver. Low pass filters perform the opposite function for bass and midbass drivers.
Both high and low pass filters can filter signals at different strengths. These are referred to as “orders.” Usually, only filters between the first and fourth order are used. A first order filter can block signals up to 6 dB and a fourth order filter can block signals all the way up to 24 dB per octave. All of this, however, goes on inside of the loudspeaker and cannot be observed. To learn more about high and low pass filters, check out the following instructional video:
Using Teufel Audio’s Ultima 40 Mk2 as an example, one can clearly recognize the loudspeaker’s 3-way construction which is helpful when trying to visualize how a crossover works. A total of 4 drivers – 1 tweeter, 1 midbass driver, and 2 bass drivers – carry out the task of sound reproduction which covers a range from 45 to 20,000 Hz. This nearly covers the entire human hearing range of 20 to 20,000 Hz. In order to do this, of course, the loudspeaker requires a crossover to send the frequencies to their respective drivers.
Note that the drivers are not arranged on this speaker from the highest to lowest frequency transducers as this is not required in order to produce an optimal sound. In order to recognize which driver performs which function, it is necessary to refer to the driver’s circumference. As a rule of thumb, the smaller the circumference, the higher the frequency. With the Ultima 40 Mk2, the tweeter was placed under the midbass driver as this will produce a more natural sound dispersion.
Crossovers optimize loudspeaker frequency response
In all loudspeakers, the crossover is essential for creating a frequency response that is at accurate as possible. Frequency response is generally shown as frequency amplitude measured in decibels. Depicted graphically, frequency response is usually shown as a line. This line should ideally show as few spikes as possible as these will not lead to a harmonious sound. After all, the human ear is capable of detecting differences as small as one decibel, especially in the midrange. People who are used to listening to music on high-end loudspeakers will be especially sensitive to these differences.
In reality, it’s impossible to achieve a a frequency response without any spikes, yet every loudspeaker manufacturer tries to approach a clean, or “linear”, frequency response to the greatest extent possible. High-end hi-fi loudspeakers shouldn’t deviate from reference levels by more than three decibels. For this, a high-quality crossover is required. Many high-end loudspeaker manufacturers will configure their own crossovers to be sure that the desired effect is achieved.
How crossovers are configured
In order to configure a crossover, it’s necessary to know more than simply whether one is dealing with a tweeter or a bass driver. While it’s possible to find various instruction manuals and calculators online that enable a lay person to configure a crossover, it’s not advisable to take this into own’s hands. No one is more familiar with a loudspeaker’s characteristics than the manufacturer, and so it’s best to leave this tricky but all-important task to them. Those who want to build their own speakers will, however, not be able to avoid configuring their own crossovers. A good amount of technical expertise remains, however, a precondition.
All pictures: Property of Teufel Audio