Low Pass Filter Subwoofer – A Detailed Guide

One of the most critical questions I’ve been asked often is the low pass filter on a subwoofer and how does it work? The low-pass filter is essential to any home or car audio system.

It helps to ensure that the bass frequencies are reproduced accurately and clearly, allowing for a much more enhanced listening experience.

In this article, I’ll explain the basics of low-pass filters, how you can set them, and how they can improve your listening experience. I’ll also provide some extra tips for you to get maximum performance from your new settings. (Note: There is also a high pass filter)

What’s a Low Pass Filter Setting on Your Subwoofer?

A low pass filter (LPF) setting on your subwoofer is a way to control and regulate the range of frequencies that it will reproduce. As the name says, a low-pass filter transmits low-frequency signals while blocking or attenuating higher frequencies.

For example, when you set an LPF on your subwoofer, you specify that it should only play audio signals below a certain point, known as the cutoff frequency.

The value for this cutoff frequency can vary from 80 Hz up to 120 Hz depending upon the particular driver used in your subwoofer and its characteristics combined with other speakers, making your audio system more attractive.

So, the purpose of using LPFs with subwoofers is to prevent them from reproducing any frequencies which could be better handled by other components within your system like those mentioned before.

Moreover, you will enable much better integration between all audio elements, such as speakers and subwoofers, by setting your low-pass filter at an optimal level.

I’ll also add that you can prevent any distortion or muddying effects due to conflicting simultaneous bass responses from different sources. Furthermore, most powered subs come packed with a built-in LPF adjustment feature allowing you to tune according to your preferences quickly and effectively.

However, some may even include additional settings like slope or order, which control how sharply the filter attenuates frequencies beyond its cutoff point.

Some Types of Low Pass Filters:

Here are some types of LPFs that you can use according to your requirements:

• Active Low Pass Filter: An active LPF boosts the desired frequencies while reducing the input signal of unwanted ones, creating a cleaner and clearer sound. It uses an amplifier to achieve this, resulting in improved filtering performance and reduced distortion.
• Passive Low Pass Filter: A manual low pass filter separates a signal’s lower frequencies from higher frequencies. It uses only resistors, capacitors, and inductors to block the high-frequency signals while allowing the low-frequency signals to pass through.
• Butterworth Filter: A Butterworth low pass filter provides a flat response in its frequency range. It has a roll-off rate of -20dB per decade and is commonly used to create smooth transitions between frequencies in audio systems.
• Chebyshev Filter: Chebyshev filter is an LPF used when a steep frequency roll-off is needed. They provide sharper cutoff frequency fc than other low pass filters but may introduce some ripple in the passband.
• Bessel Filter: A Bessel filter has a linear phase response, meaning it does not add any distortion to the signal. You can use it in any audio system because it produces clear and accurate sound without introducing any distortions.

Low Pass Filter Calculator Subwoofer:

Cutoff Frequency (Hz):

Nominal Impedance (ohm):

Calculate

Inspired by the12volt.com

What Should You Set Your Subwoofer Low Pass Filter To?

Generally, if your subwoofers are full-range (able to provide a wider frequency response), you should set your subwoofer low pass filter around 80 Hz. On the other hand, if the speakers do not cover all frequencies, then it is best to set the low-pass filter near or below the lowest (70%, as said by most experts like me) frequency they can reproduce. For example, if your speakers only reproduce up to 50 Hz, set the subwoofer’s low-pass filter at or below 50 Hz.

You must also ensure that the subwoofer’s low pass filter is set below the cutoff frequency filter of the AV receiver (You should also ensure the high pass filter is also set correctly).

3 Factors to Consider When Setting Subwoofer Low Pass Filter:

Here are some factors that you must follow in order to gain perfect LPF settings for your subwoofer:

1. Crossover Frequency:

You must consider the crossover frequency when setting the LPF. It ensures that each speaker in your system plays to its strengths and fills in where others may lack.

The crossover frequency is the point at which two different speakers’ frequency response intersects and begins to overlap. It’s vital to select an appropriate crossover point because if it’s too low, there will be too much overlap between your subwoofers and other speakers, resulting in a muddy, unclear sound.

Conversely, setting it too high will create a gap between them, causing an uneven and unbalanced audio experience.

2. Speaker Size:

That’s another important that some people ignore while setting the subwoofer LPF (Personal experience). Larger speakers can handle lower-frequency signals better than smaller ones, so you may lower the crossover frequency when connecting larger main speakers to your subwoofer.

You’ll make the subwoofer handle more of the low-frequency signal, resulting in a fuller sound with more impact. Conversely, smaller subwoofers may be overwhelmed if you set the LPF too low, resulting in an unbalanced or overpowering bass sound.

You should try raising the crossover frequency (when getting an unbalanced sound) so that your main left and right speakers still handle mid-range and high frequencies while allowing the subwoofer to focus on producing full bass output without compromising overall sound quality.

3. Room Acoustics:

I’ll also suggest you consider the room acoustics when setting the LPF, as it can really impact the overall sound quality. The room’s size, shape, and materials can all affect how sound waves travel through it and interact with each other.

This ultimately impacts the frequency response of speakers and subwoofers within that space. If an LPF is set too low without considering room acoustics, then some frequencies may be exaggerated or over-emphasized, which could cause an unbalanced sound with too much bass.

To avoid this issue, you should adjust your LPF and crossover settings accordingly to avoid too much distortion or boomy noise when playing music at higher volumes.

My 4 Favorite Steps to Set Subwoofer Low Pass Filter:

You can follow these easy steps to set the subwoofer LPF:

1. Determine the Crossover Frequency:

As mentioned above, the crossover frequency is when low-frequency signals are routed to the subwoofer, and high frequencies (You can use a high pass filter, too) are sent to other speakers.

A typical starting point for a 2-channel home stereo system is 80Hz, but this can vary depending on room size and personal preference.

When selecting a crossover frequency, it’s essential to consider both the maximum range of your main speakers and subwoofers.

To determine if you need a higher or lower crossover frequency, start by playing music with various sound frequencies (low bass kicks, high notes, etc.).

If you can hear certain sounds better from one speaker than another, then adjust your settings accordingly – increase or decrease your crossover setting until you find an optimal balance between both drivers.

2. Set the Low Pass Filter:

Once you know what frequency you want to use, it’s time to set the low pass filter on your subwoofer. Most subs have a dial labeled “Low Pass Filter” or “Crossover” that allows you to adjust the cutoff frequency of the speaker it powers.

You will typically set this dial to whatever crossover point was determined previously, for only frequencies below that point are reproduced through your subwoofer.

Any higher-frequency audio signals will be sent directly to other speakers connected to the same system. By adjusting this dial appropriately and not overdriving your subwoofer (which can damage its components), you can ensure that your sound system produces optimal audio output from all sources within its range. 

3. Fine-Tune the Filter:

After setting the LPF, it’s time to fine-tune your subwoofer. Start by playing some music or test-tune audio. The goal is to adjust the low pass filter until you hear a smooth transition between your subwoofer and other speakers without any gaps or overlaps in the frequency response, as mentioned above.

You’ll have to gradually increase or decrease the filter frequency (using a knob or system display) until it’s optimal. It may also help to have someone else listen so they can give their opinion on what sounds better. Once you’ve achieved a good sound, save your settings and enjoy your newly optimized system!

4. Use Calibration Tools:

You can also get help from calibration tools (if you have one). These tools act as measuring devices, providing real-time feedback and helping identify potential problems with the current setup.

It also helps accurately set up the low pass filter and determine how much bass your speakers can process and reproduce. First, you will need a sound meter or room correction software available online and offline.

With this tool, you can measure the frequency response of various points in your space to determine what adjustments must be made for your subwoofer to perform correctly.

You should position the sound meter close enough to where it picks up only one speaker’s output at a time; this will help ensure accuracy when taking readings.

Once you have taken the readings from each individual point around the room, use these measurements or analysis data from either an SPL (Sound Pressure Level) or speaker response graph outcomes from dedicated computer-based software like Room EQ Wizard (REW).

Common Mistakes to Avoid When Setting LPF:

Here are some most common mistakes that I have found people making while settings LPFs on their subwoofers:

• Setting the Filter Too High or Too Low: If LPF is set too high, the subwoofer won’t be able to reproduce lower frequencies correctly; other speakers may also handle frequencies they are not designed to, leading to distortion or damage. On the other hand, if set too low, it could cause the subwoofer to work harder than necessary and potentially cause damage and an unbalanced boomy sound.
• Ignoring Room Acoustics: Your room’s acoustic properties will affect how the sound waves travel and interact with surfaces. This can lead to uneven bass response, standing waves that cause peaks and dips in frequency response, and overall deteriorating audio quality.

Low Pass Filter Setting Modes (Short-Explanation):

Here are some popular LPF setting modes that I love to use for car audio systems:

1. Bypass:

Bypass Low Pass Filter (BLPF) setting mode is used in sound systems to filter out specific frequencies selectively. Essentially, BLPF creates an audio “high-pass” or “low-pass” filter that reduces the amount of high-frequency content in the signal.

It is typically used when adjusting sound systems for optimum performance and/or protecting speakers from mid/high-frequency overloads.

2. Third-Order Mode:

Third-order low pass filter setting mode is a filter used to limit high-frequency content in input signals. It’s also known as the Butterworth filter.

It has a steep roll-off rate of 18 dB per octave, efficiently attenuating frequencies above the cutoff frequency and reducing the amount of high-frequency noise reaching subwoofers.

This filter is often used in sound systems that require precise control over low-frequency output, like home theater systems, recording studios, and live music venues.

Moreover, it helps reduce noise and distortion caused by high-frequency content in the input signal. Third-order low-pass filters can provide excellent results with minimal effort compared to higher orders such as fifth or seventh-order filters.

3. Forth-Order Mode:

A fourth-order low-pass filter has a more precise and accurate cutoff frequency than third-order filters. It has a steeper roll-off of -24dB per octave, reducing the signal level by 24dB for every octave increase above the cutoff frequency.

This results in less overlap between frequencies being passed and those being filtered out, making it highly effective at removing unwanted high-frequency noise or interference in audio applications.

What Should Be LPF Settings on Head Unit?

You should typically set the Low-Pass Filter (LPF) to the highest Hz your subwoofers are rated for. This will prevent any high-end frequencies from overworking the subwoofers and causing potential damage. The higher range of LPF settings available on most head units can be tuned to match the speaker system you use to optimize your bass level and sound quality.

General FAQs