How to Set High-Pass and Low-Pass Frequency Filters on a Car Stereo Receiver

by Blaze Johnson

Many car audio stereo receivers on the market feature an integrated and user-adjustable crossover system, designed to route certain sound frequency ranges to specific speakers connected directly to the head unit, or through low-level outputs designated for external audio amplifiers. "High-pass" filters within a “two-way” crossover system work by restricting bass frequencies from mid- to high-range speakers, while "low-pass" only allow low-frequency bass signals to reach larger subwoofers. Properly adjusting the high-pass and low-pass filter settings will promote speaker longevity and maximum sound quality.

1

Examine the documentation supplied with your speakers and subwoofers, if applicable. Note the “frequency response” range listed. The frequency response range for car audio speakers and subwoofers is measured in Hz and kHz. If the vehicle does not have after-market speakers installed, disregard this step.

2

Note the frequency slope diagram listed in the speaker documentation. For high-range or coaxial speakers, note the specific frequency in which the diagram shows a sharp decline for the low-frequency range. For subwoofers, note the decline in the high-frequency slope.

3

Determine the appropriate high-pass and low-pass frequency cutoff points for the speaker system. To achieve even sound reproduction across all frequencies and speakers, consider choosing HPF and LPF cutoff frequency points that will provide a “flat” transition between the high- and low-range speakers connected to the system.

4

Examine the documentation for your stereo receiver and note the available frequency crossover points. Decide on the closest and most appropriate crossover points for the speakers. Although determining the appropriate crossover cutoff points from the documentation supplied with the speakers and receiver will get you in the ballpark, keep in mind the mounting position of the speakers, resonant frequencies of the vehicle and enclosure type for the subwoofer system will have a dramatic affect on the final adjustments.

5

Turn on the car stereo and navigate to the appropriate crossover menu for the “front” speaker high-pass filter. Select the appropriate frequency cutoff option. Repeat the same adjustment for the “rear” speaker high-pass filter option. Adjust the low-pass filter for subwoofer to the appropriate setting; ideally, the crossover points for the LPF and HPF settings should be the same frequency to promote a smooth transition between speaker components.

6

Turn off the subwoofer system or adjust the amplifier “Gain” setting to the lowest setting. Play music that features a wide range of high and low sound frequencies. Adjust the volume to the loudest position, right before the point of distortion.

7

Sit in the driver's seat and adjust the fader and balance controls to your preference. Adjust the crossover on the subwoofer to the “pass-through” or “Off” position. Because you will be controlling the LPF settings from the stereo receiver, you do not need to use the amplifier's crossover system.

8

Adjust the amplifier's “Gain” setting to the loudest setting, before the point of distortion. Adjust the volume of the receiver to a normal listening level.

9

Insert a frequency test CD that features a “sine sweep” test track. The sound test will produce a continual sound that “sweeps” between the low- and high-frequency ranges.

10

Note any sound level differences between the subwoofer system and speakers. Make adjustments the “dB/Oct” slope levels for each filter setting. Repeat the test and make more adjustments to the crossover settings as necessary.

11

Listen to a wide range of music and make small adjustments to the bass, treble and other sound field options if desired.

Items you will need

About the Author

In the spring of 2008, Blaze Johnson decided to share his expertise through writing. He studied business administration at a local community college and runs his own driveway mechanic service, specializing in computer-controlled vehicles and custom car audio installs. Johnson also serves as the de facto computer repair person for his family, friends and coworkers.