One of the most important settings for a subwoofer is the cutoff frequency for the low-pass filter. As such, people commonly ask, what frequency should I set my subwoofer low pass filter to? In this article, we will discuss subwoofer low pass filters and what frequency you should set them to.
In general, it is often safe to set your subwoofer’s low pass filter to 80 Hz or at a value approximately equal to (or below) 70% of your main speaker’s lowest frequency response. For example, if our speaker’s frequency response goes down to 115Hz, your subwoofer’s low pass filter should be set to 80.4Hz (70%).
When considering a subwoofer’s low pass filter setting, you should also ensure that the subwoofer low pass filter is set below the cutoff filter of the AV receiver.
We discuss this further below.
What Is The Low Pass Filter Setting On Your Subwoofer?
A subwoofer low pass filter prevents a subwoofer from playing frequencies above it. For example, a low pass filter of 80 Hz will only let frequencies below 80 Hz through to the subwoofer.
That means that frequencies below the cutoff frequency Fc (in this case 80Hz) are blocked from reaching the subwoofer driver.
Setting Your Subwoofer Low Pass Filter Based On The Main Speakers Frequency Response
As a general rule, the Low-Pass Filter of a subwoofer should be set at a value approximately equal to (or below) 70% of your main speaker’s lowest frequency response.
Let’s take an example.
If your speaker’s frequency response goes down to 57 Hz, then 70% of 57 Hz equals 39.9 Hz. In this case, you could set the subwoofer’s low pass filter to 40 Hz.
If you are uncertain of your main speaker’s low-frequency response, start with a setting of 35Hz, and check the manual. If that doesn’t help, then simply experiment until you find something that sounds good.
On a subwoofer, the low-pass filter setting allows you to adjust the low-pass frequency for the left and right (RCA or speaker level) inputs. However, this setting is not applied to the LFE (RCA or XLR) inputs. Instead, bass management is done by the audio/video processor.
With bass management, a subwoofer will relieve your main loudspeakers and amplifier from the production of the lowest frequencies. Otherwise, these low frequencies would demand a lot of amplifier power and restrict your dynamics.
Setting Your Subwoofer Low Pass Filter Based On The Main Speakers size
In general, the recommended setting for a subwoofer low pass filter frequency of 80 Hz is based on the assumption that the speakers have a diameter of at least 5.25 inches.
For smaller speakers, the frequency for the High-Pass Filter should be higher than 80 Hz. The best option is to start with 300 Hz and then keep turning it down as you listen to the sound quality. When you find the sweet spot, work with that.
What Frequency Should I Set My Subwoofer Low Pass Filter To?
Most subwoofers are optimized for handling 50 Hz and below. However, you are generally safe if you set your subwoofer cutoff filter to 80 Hz or so. Many are set to 80 Hz by default. In general, the choice of cutoff frequency is often selectable.
The reason your subwoofer has a low pass filter is so that higher frequencies (above the cutoff frequency) getting to the subwoofer will not be reproduced. Otherwise, this would result in distortion and an undesirable sonic effect.
What About The Low Pass Filter In My Receiver?
If your subwoofer is connected to a receiver, there is a high chance that your receiver has a built-in low pass filter as well.
This low pass filter is however for the Low frequency Effects (LFE) output to send the lows (below the LFE low pass filter cutoff ) to the subwoofer and the highs (above the low pass filter cutoff) to the main speakers.
This is very important for keeping the low frequencies out of the main speakers that don’t handle ultra lows frequencies. It is also important for keeping the high frequencies out of the sub.
However, the subwoofer low pass filter is used to add extra control to further filter the frequencies.
However, you need to be careful with the AV receiver frequency setting because an incorrect setting can cause issues if you set the subwoofer low pass too low or too high.
Subwoofer LFE vs Stereo 2.0
In general, a subwoofer will handle either stereo (2 channels left and right) or multi-channel (LFE In) audio.
The best subwoofers must offer the ability to connect both Left In/Right In and LFE In. This ensures that you achieve an optimum setup for 2-channel listening while still allowing for multi-channel (movie) mode listening.
LFE is particularly great for watching most movies. In this case, the bass from the surrounds can be fed to the subwoofer using the low pass crossover settings from the processor’s bass management system.
Low Pass Filter Setting Modes
There are various modes to which you can set your subwoofer low pass filter:
Bypass: With this setting, the low-pass filter will pass signals with a frequency lower than a selected cutoff frequency and attenuate signals with frequencies higher than the cutoff frequency. Select this option if you plan to use your receiver/processor’s bass management to set a low pass filter. Learn more about low pass bypass filters here.
Third Order: A third-order filter has a slope of 18dB per octave which is a slower roll-off than a fourth-order and requires a third-order crossover. Learn more about third order low frequency filters.
In case you are using the subwoofer L/R inputs, this option will adjust the roll-off characteristics of your subwoofer’s upper frequencies as they approach the low-pass filter frequency setting.
Fourth Order: A fourth-order filter has a slope of 24dB per octave which is a faster roll-off than a third order. In case you are using the subwoofer L/R inputs, this option adjusts the roll-off characteristics of your subwoofer’s upper frequencies as they approach the low-pass filter frequency setting.
Check here to learn more about 4th order low pass filters.
How To Turn A Subwoofer Low Pass Filter On Or Off
You can turn the low-pass filter for the subwoofer output on or off quite easily. In fact, you can even set the function for each input to which an audio input jack is assigned independently.
You should turn the function on if you connect a subwoofer without the crossover frequency function.
Here is the process:
- Select [Setup] – [Audio Settings] from the home menu.
- Select [Subwoofer Low Pass Filter].
- Select the setting you want.
On: Always activates the low-pass filter with a cutoff frequency of 120 Hz.
Off: Does not activate the low-pass filter.