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A high pass filter is a feature in many car audio systems that allows the user to keep low frequency sounds from playing through front or rear full-range speakers. This is especially nice when you want low frequencies to play through a subwoofer exclusively. If a stereo system does not include a subwoofer, this feature can be used to direct higher frequencies through the front speakers, while leaving rear full range speakers to handle bass frequencies.
A user sets the filter by choosing a cutoff frequency. Sounds above the cutoff will pass through, while those that fall below the cut off will be attenuated or filtered out. Generally, high-end frequencies are channeled towards the front of the sound environment in a car, while mid-range is shared by front and rear. The rear speakers also handle bass, unless a subwoofer is present, in which case it might handle all lower frequencies.
While a high pass filter is a high-tech feature, it can be used to make low-tech speakers sound better in some cases. An automobile’s front speakers are often smaller and weaker than the rear speakers, and might not be great at handling low frequencies. Increased volume accentuates the problem, and one might end up with speakers that vibrate and buzz with every beat. A high pass filter can block those frequencies that cause the buzzing. In this case, rear speakers must be able to handle the bass in order to maintain a full overall sound.
For robust speakers, a high pass filter can work in conjunction with other audio settings to help create sound imaging. Component speakers, which separate the high-end tweeter from the low-end cone, essentially allow placement of instruments and vocals by separating higher frequencies from lower ones.
A high pass filter is just one feature of a collection of features that work together to allow a user to customize the sound of his or her car stereo. Equalization (EQ) adjustments, subwoofer adjustments (a low-pass filter), and even compression algorithms are all available features in many car audio systems today.