Subwoofer Low Pass Filter (LPF ) Explained

LPF (Low Pass Filter) is a crucial component in subwoofers as it controls the frequencies that the subwoofer produces, allowing only the low bass tones to pass through while filtering out higher frequencies.

In this blog post, we’ll understand the LPF in subwoofers, its impact on audio quality, and the LPF fundamentals.

Key Takeaways

  • LPF or low pass filters help you to customize the way you want to listen to a piece of music on your sound system.
  • You can set a low or specified frequency unit (measured in Hz) to block higher frequencies above the set level to pass on to the drivers.
  • Setting LPF to a lower frequency will help in boosting the bass effect.
  • The LPF setting is usually controlled by the amplifiers in most cases.

How Low Pass Frequency (LPF) Work?

The Low Pass Filter (LPF) acts like a gatekeeper, letting only the deep bass frequencies pass through.

Its main job is to make sure the subwoofer accurately reproduces powerful low-frequency sounds.

In other words, its primary purpose is to ensure that the subwoofer reproduces low-frequency sounds with utmost precision and power through a specialized audio filter that prevents higher frequencies from passing through the drivers (which other normal speakers may not produce).

These are a type of crossover filter network designed to pass low or set frequencies to the drivers.

For example; if you have set the frequency to 80Hz then the Low Pass Filter will only allow frequencies below 80Hz and pass it on to the drivers.

This means any frequency above 80Hz will be blocked by the filter from being sent or released toward the drivers.

On the other hand, HPF or high pass filters will prevent lower frequencies from being sent to the drivers. HPF settings can be used when you want to play instruments that require low or no bass effects.

Also Read: How to build a subwoofer box for deep bass? (Guide)

Subwoofer Low Pass Filter Circuits

Low pass filter circuit in subwoofer

The example that has been presented above includes most low-pass filter circuits available for subwoofers.

And the circuit presented here is based on the ST Micro Electronics opamp TL062.

The TL062 is a dual high input impedance J-FET opamp with a higher slew rate and consumes very low power.

This particular opamp has excellent digital characteristics and is extremely compatible with this circuit.

If you refer to the diagram above, you will find one of the two opamps (operation-amplifier) inside TLC062 is connected (wired) in the form of a mixer with a pre-amplifier stage.

In other words, it is wired as the mixer cum pre-amplifier stage.

For mixing, the left and right channels are connected to the inverting input of IC1a. POT R3 can be used to alter the gain of the first stage. 

The output of the first stage is linked or connected to the input of the second stage. This is done through the filter network or filter circuit which is composed of components R5, R6, R7, R8, C4, and C5.

The second opamp (operation amplifier) serves or functions as a buffer, moreover, the filtered output can is accessible at pin 7 of the TLC062.

Regulated Dual Power Supply

If you look at the below diagram it demonstrates the regulated dual power supply to the circuit.

regulated dual power supply circuit

Things to keep in mind while assembling the circuit.

  • Make sure to assemble the circuit on a  PCB of optimum quality.
  • You can power the circuit can from a +12/-12 V DC dual power supply.
  • Finally, mount the IC1 on a holder.

Single Operational Amplifier Circuit diagram

single opamp circuit diagram

As you can see from the circuit diagram above, it is a simple design where a single opamp is operating as the main active element.

The resistors and capacitors in the circuit are independently calibrated for a 50 Hz cut OFF, allowing no frequency more than 50 Hz to pass through the circuit into the output.

Subwoofer Low Pass Filter using Transistors

Subwoofer low pass filter using transistor

The above circuit diagram illustrates an active low-pass filter structure.

This specific layout can be easily assigned to have any desired cut-off point across a wide range of frequencies.

The above filter consists of a pair of NPN/PNP BJTs and an RC network.

The transistors specification displayed in the above diagram can be abruptly substituted by different types of transistors.

Moreover, there’s no need to alter the functionality of the circuit in the process.

Finally, the supplied voltage should range between 6V to 12V.

The cut-off frequency is established by the set values for capacitors C1 to C4.

You can acquire the values of the magnitudes with the help of the two formulas listed below.

  • C1 = C2 = C3 = 7.56/fC
  • C4 = 4.46/fC

In the above calculation, the required cut-off frequency (in Hertz) is provided by fC.

The values for C1 to C4 are calculated in microfarads whereas the amplitude response is down 3dB in the above formula.

Note: If we use kHz as the unit in the calculation, the response result will be displayed in nanofarad values. Similarly, using MHz will be displayed into picofarad units.

To put it across as an example the calculated effect is indicated for a filter made with C1 = C2 = C3 = 5n6 and C4 = 3n3.

If you look at the diagram below, the ‘-3dB point’ in the displayed scenario emerges approximately at 1350 Hz.

One octave higher at 2700 Hz, the attenuation has already reached 19 dB.

diagram explaining db increase or decrease based on frequency measured in hertz

To get a technical understanding of the circuit you may refer to the information here.

Also Read: Pairing Soundbar and subwoofer: Add a Subwoofer for bass

Importance of Setting the Low pass filter (LPF)

To get the most out of a subwoofer, you have to put on the hat of an audio engineer and set the low-pass filter with skill.

Choosing the right low-pass filter frequency is important because it controls how the subwoofer responds to low frequencies and works with the rest of the audio system.

If you make the right choice, the subwoofer will fit in with the other speakers and create a smooth, pleasing soundscape.

What are the different Low Pass Filter setting modes?

There are three main types of low pass filter settings, here’s a quick table that explains the key difference in all three modes.

Filter ModeSlopeAdjustments
BypassN/ALow-pass filter bypassed
Third OrderSlope of 18dB per octaveAdjusts the upper frequencies of the subwoofer when they approach the low-pass filter frequency setting.
Fourth OrderSlope of 24dB per octaveAdjusts subwoofer’s upper frequencies as they approach the low-pass filter frequency setting

What is a Crossover slope?

A crossover slope, which is also called “roll-off,” is the rate at which the signal level changes in a filter after a crossover point. In technical terms, it indicates the amount of attenuation that occurs every octave.

An octave represents a doubling or halving of frequency, so one octave below 70 Hz is 35 Hz, and one octave above 70 Hz is 140 Hz.

The crossover slope affects how quickly the signal decreases or increases as you move away from the 70 Hz crossover point.

What is a Subsonic filter and do you need it?

The subsonic filters help to remove extremely low-frequency disturbances that are below the range of human hearing before the audio stream reaches the speaker or subwoofer.

These low-frequency signals can damage the speaker system, drain a lot of amplifier power, and cause distortion.

If you need a subsonic filter or not will depend on a number of factors, including the kind of audio system you have, the power of your speakers or subwoofers, and the specific content you are playing.

However, if your amplifier or subwoofer has a subsonic filter, you could use it, especially if you’re listening to music or sound that produces incredibly low frequencies.

Remember that if you are using a high-quality audio system with carefully constructed speakers or subwoofers that can withstand low frequencies without distortion or harm, a subsonic filter may not be necessary.

Alright! now that you know what the LPF, crossover slope, and subsonic filter mean, let’s take a quick look at factors to consider when adjusting LPF followed by integrating subwoofers with speakers.

Also Read: 2 ohm vs 4 ohm subwoofers: Ohm difference explained

Factors to consider when adjusting LPF

Subwoofer capabilities

Every subwoofer has its own set of capabilities, which are described by things like subwoofer output, frequency response, and power handling.

Understanding these characteristics and qualities helps music lovers to figure out their way through the confusing world of LPF settings and get the most out of their subwoofers.

Reading the user manual can be a good starting point to understand your subwoofer’s lowest frequency response, crossover slope, and other settings if any to achieve an optimum setup.

Speaker configuration

A comprehensive understanding of the entire audio setup is vital when configuring the LPF.

By matching the crossover points with the LPF settings, you can make sure that the sound from each speaker flows into the next one without any breaks.

Whether it’s a two-channel stereo or a surround sound system with many channels, the LPF in subs and speakers work together in sync and the transition between the frequencies should happen smoothly.

Room acoustics

The LPF setting is heavily affected by the shape of the room where we listen.

How the sound waves move through a room depends on its size, shape, and materials (used in subwoofers), which can cause resonances, echoes, and cancellations.

By changing the LPF on an amp to properly account for these things, we can get rid of the strange sounds and make a musical environment that surrounds and pleases.

Also Read: Active VS Passive Subwoofers

Integrating subwoofers with other speakers

When connecting subwoofers together with other speakers, they must work well together.

When the LPF and crossover settings are in sync, the subwoofer moves in sync with the other speakers.

This integration makes sure that the bass notes come from the depths and blend easily with the midrange and treble notes, wrapping the listener in a symphonic hug.

Here’s a step-by-step instruction to integrate subwoofers with other speakers:

Step 1: Check the Specifications

Look at both the subwoofers and the other speakers’ specs and pay close attention to the frequency response range, the amount of power it can handle, and the choices for input and output.

Step 2: Put the subwoofers together

Follow these steps to connect the subwoofers to the rest of the speakers:

  • Find the audio port on your amplifier or receiver that is made for subwoofers, often these slots or ports are marked as “Sub Out” or “LFE Out.”
  • Connect the subwoofer output of your amplifier or receiver to the input of your LPF subwoofers using a high-quality RCA wire.
  • If your subwoofers have more than one outlet, choose the right one for the source of the sound. Line level (RCA) inputs or speaker level inputs are two of the most common choices.
  • Set the sound on your subwoofers to a level that isn’t too loud or too quiet.

Step 3: Determine and set the Crossover Point

The frequency at which the subwoofers take over from the other speakers is the crossover point.

It’s important to find the right crossing point to make a smooth shift between the two.

As a general rule, set the crossover point just above the lowest frequency response of your main speakers. So, the subwoofers can handle the low sounds without overlapping with the main speakers.

What is the ideal LPF setting for subwoofers (in Hz)?

subwoofer audio frequency range diagram

The recommended setting to set the Low pass filters should be equal to or below 70% of your speaker’s lowest frequency value.

For example, if your main speaker frequency value goes down to 100 Hz, you should set your subwoofer low pass filter to 70 Hz. This value is 70% of the lowest frequency of your main speaker.

However, in most cases, you can set the Low pass filter of your subwoofer to 80 Hz to be on the safer side. Setting the LPF frequencies below 80 hz will ensure that the driver covers a wide range of frequencies that are under 80 hz.

That being said, give priority to set the subwoofer’s low pass filter value below the AV receivers cutoff frequency settings.

Here’s a quick table with an ideal crossover frequency range.

Main Speaker Lowest Frequency (Hz)Recommended Subwoofer Low Pass Frequency (Hz)
40 Hz28 Hz
60 Hz42 Hz
80 Hz56 Hz
100 Hz70 Hz
120 Hz84 Hz
150 Hz105 Hz
200 Hz140 Hz

Note: These are approximate values calculated based on setting the subwoofer low pass frequency to 70% of the lowest frequency of the main speakers. It’s recommended to read the user manual before you set the lpf on your amp. 

Also Read: Woofer VS Subwoofer: Difference between woofer and subwoofer explained

Wrap Up

Connecting speakers and subwoofers can seem complex when we start, but when you read the user manual and when you understand how the speaker’s frequency response goes you can achieve the best possible sound.

Just remember that, while connecting the subs with wires such as rca or xlr cable you may need to adjust the low-pass frequency and see how it sounds.

And continue to tweak the lfe settings until you get a balanced bass response.

Lastly, connecting the subs correctly does not mean you will get mind-boggling sound as other factors such as room acoustics and subwoofer placement play a key role in order to make your subwoofer sound best.

Armed with a Diploma in electrical engineering and a remarkable 12 years of expertise, he excels in rejuvenating music systems, washing machines, dryers, and laundry-related appliances. Manish's profound insights and practical know-how establish him as a credible authority in appliance and music system repairs. Contact: manish.singh (at) subwooferblog.com. He's always ready to lend a helping hand!