How High and Low Pass Filters Relate to Speaker Crossovers


Within the audio/home theater realm, you may have come across the phrases “high and low pass filters” and “crossovers.” But what do these really mean, and how do they fit together? Recently, a friend new to this hobby asked me about them, giving me a chance to dive into the details. In this article, we will cover the basics of these concepts and how they matter in home theater systems.

What are High and Low Pass Filters?

Filters are electronic circuits that modify the frequency spectrum of an audio signal. Filters can be incorporated into the physical board, either as static designs right from the start (two-way speakers), or as components of integrated circuits (ICs). In the case of ICs, they can be programmed and adjusted after the circuit is already in use, as part of an AVR for example.

Filters can either boost or attenuate certain frequency bands, depending on the type and settings of the filter. There are many types of filters used in the audio world, but for home audio the most common types are high pass and low pass filters.

High pass and low pass filters are a filter type that allow certain frequencies of sound to pass through to your speakers, while blocking or attenuating others. They are often used in audio systems when wanting to limit the frequency range sent to a speaker. For instance, to separate the low frequencies (bass) from the high frequencies (treble), and send them to different speakers or amplifiers.

High Pass Filter: A high pass filter (Green Line) allows only high frequencies to pass through, while low frequencies are filtered out.

Low Pass Filter: A low pass filter (Red Line) allows only low frequencies to pass through, while high frequencies are filtered out.

What are Crossovers?

A crossover, short for crossover network, is a tool used to split an audio signal into two or more frequency bands, each of which can be processed separately and sent to the appropriate speaker driver. There are two main types of crossovers: passive and active.

Passive Crossover: A passive crossover employs elements like resistors, capacitors, and inductors to split the signal. These crossovers find widespread use in multiple driver speaker setups, where they divide the signal into various frequency ranges to match different speaker drivers. Passive crossovers are inserted between the amplifier and the individual speaker drivers and work by dividing the audio signal’s frequency spectrum using the inherent electrical characteristics of these components.

For instance, a typical two-way speaker configuration consists of a woofer for low frequencies and a tweeter for high frequencies. The crossover splits the signal into two segments: one for the woofer and another for the tweeter. By doing so, each driver exclusively receives the frequencies it can handle proficiently. This approach reduces distortion and improves efficiency in the sound the speaker produces.

Example of a passive crossover made by Dayton Audio

Active Crossover: Active crossovers are typically electronic circuits that require power or are done with Digital Signal Processing (DSP) and are usually placed before amplification. The signal is split into different frequency bands before amplification, directing each frequency range to a dedicated amplifier channel that drives a specific speaker driver. This allows for greater control and flexibility in adjusting the crossover points, slopes, and levels of each frequency band, which can lead to more precise tuning of the audio system and greater flexibility. An example of this type of crossover is the setting on your AVR or audio processor, where you can choose the crossover frequency.

Example of adjustable active crossover in an AVR’s settings.

How Do High and Low Pass Filters Relate to Crossovers?

High and low pass filters are essentially the building blocks of crossovers. A crossover can be seen as a combination of high and low pass filters that create different frequency bands for different drivers. For example, a two-way crossover can be made by connecting a high pass filter to the tweeter and a low pass filter to the woofer. The cutoff point of the filters determines the crossover frequency, which is the frequency where the two drivers have equal output.

The picture below depicts a crossover network that consists of a low pass filter (Red Line) and a high pass filter (Green Line), where the crossover point is 100hz.

There are different types of crossovers that affect the shape or slope of the filter. Crossover slopes determine how quickly the audio signal is attenuated as it approaches the crossover frequency. There are several types of crossover slopes, each with their own characteristics and applications; however, to go into detail on each slope is beyond the scope of this particular article.

Picture showing different crossover slopes: 12dB/octave (green), 24dB/octave (blue), 36dB/octave (red), and 48dB/octave (white).

What Crossover is Best For Your System ?

There is no definitive answer to which crossover is best for your home theater system, as it depends on various factors such as your speaker’s specifications, your room’s acoustics, your personal preferences, and the type of content you are listening to. However, a common recommendation is to start with 80Hz, which is the THX standard for home theater systems, and then fine-tune it according to your speakers and ears.

For a more detailed procedure for determining the best crossover for your system, take a look at our article “What Crossover to Choose for Your Speakers,” or feel free to contact us either by comment or clicking the card below.

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