Understanding Bass Management and LFE

What is Bass Management?

You may have encountered the term bass management or maybe you’ve seen bass-management options in the software or hardware you use. Let’s demystify bass management and explain how it can benefit you. In simple terms, bass management describes how a monitor system integrates a subwoofer with your main monitors to optimize a monitor system's low-frequency reproduction. In a non-bass-managed system, the main speakers reproduce the entire frequency range, from low to high.

Contrastingly, in a bass-managed system, one or more subwoofer is added to improve the sound of the system by producing low frequencies that are difficult or impossible for the main speakers to generate. A bass-managed system, therefore, comprises main speakers with one or more subwoofers. “Management” refers to routing bass frequencies to the subwoofer and the remaining frequencies to the main speakers.

Cinema sound systems may or may not incorporate bass management, but typically include subwoofers whose sole purpose is to reproduce low-frequency effects (LFE) such as rumble and explosions. Immersive formats like Atmos contain a dedicated LFE audio channel reserved exclusively for special effects, not bass information that would normally be part of the main channels.

Movie theaters have dedicated LFE subwoofers, while home theaters may utilize the same subwoofer for bass management and LFE duties. Home theater monitor controllers (AVRs) and some studio monitor controllers allow the user to configure speaker outputs for setups that include subwoofers for bass management and/or LFE. 

When is Bass Management Necessary?

Professional mixing and mastering of music and high-end listening rooms demand a speaker system that reaches below 30 Hz. However, most nearfield studio monitors and bookshelf speakers produce a flat frequency response down to around 50 Hz, with some small systems only reaching down to 70 Hz. These nearfields certainly benefit from the addition of a subwoofer to extend their low-frequency response. Bass management extends the range of usable low frequencies of these monitor systems, enabling modest main speakers to accurately reproduce low frequencies. 

Large main studio speakers also benefit from subwoofers with bass management. They gain additional headroom and clarity with the addition of a subwoofer.  The subwoofer handles the demanding task of generating low frequencies, allowing the main monitors to preserve greater power and headroom in the critical low-mid frequency range.

When subwoofers are added to any speaker system, the audio signal should be split so that the low frequencies are directed to the subwoofer and the higher frequencies to the main speakers. This frequency-based routing can be performed by a dedicated hardware crossover, the built-in electronics of certain subwoofers, or by a monitor controller equipped with bass management functionality. In the context of home theaters, bass management typically takes place inside an Audio-Video Receiver (AVR). In the mixing studio, bass management is a bit more complicated.

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A sound source, such as a mixing console or audio interface, sends audio into a crossover that bass-manages the signal, sending the low frequencies to the subwoofer and the rest of the signal to the main monitors.

How do I Apply Bass Management?

Here is how bass management should work for stereo monitors with a single subwoofer: A crossover—either hardware or software—divides the audio into low frequencies with a low-pass filter and high frequencies with a high-pass filter. The high-frequency content passes to the main left and right speakers, while the low frequencies are combined (summed) into mono and sent to the subwoofer. For stereo subs, the low frequencies remain discreet for the left and right channels. 

While mixing for immersive music formats, the LFE signal is typically generated by using sends from individual channels in a mixer or DAW. For the best translation, the LFE signal should be low-passed below 120 Hz with a filter in your DAW, irrespective of how a monitor controller may further process the signal.  Technically, the LFE signal is not full-range, so it does not require bass management during playback where it is sent to a dedicated LFE subwoofer.

As mentioned, bass management can be achieved through the settings on an active subwoofer, an external electronic crossover, or via the settings provided by a monitor controller. The key to effective bass management lies in integrating the subwoofer in such a way that low frequencies smoothly and inaudibly transition from the sub to the main speaker.

Three variables influence this transition: The crossover frequency, subwoofer volume level, and subwoofer’s polarity (phase). The crossover frequency defines which frequencies the subwoofer handles, and which are passed on to the main speakers. The volume and phase adjustments ensure that frequencies below and inside the crossover region blend properly between the sub and main speaker.

The PreSonus Temblor T10 subwoofer’s rear control panel provides input connections for a monitor controller or interface and output connections for the main stereo monitors, along with controls for the subwoofer volume, crossover frequency, and phase (polarity).

Where is the Best Location for a Subwoofer?

Unfortunately, there is no simple rule to determine the placement of your subwoofer, but there are recommended practices to consider. First, avoid positioning the subwoofer exactly midway between your side walls, as this can overly excite one bass mode, leading to issues at the listening position, which is also midway between the side walls.

You might wonder whether placing the subwoofer away from the midpoint of your speakers will ruin the stereo image. This is not a concern since sounds below approximately 80 Hz cannot be localized, so low frequencies won't appear to emanate from a specific location in the room. However, if you choose to crossover your subwoofer above 100 Hz, you should consider locating it somewhere between the left and right monitors.

Placing the subwoofer too far from the main speakers is not advisable, as you may hear a delay or timing difference between sounds from the main speakers and those from the subwoofer. Therefore, it’s recommended to position the subwoofer somewhere along an imaginary arc that is equidistant from your listening position and runs along the plane of the main speakers (see figure below).

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A subwoofer should be placed about the same distance from the listener as the main speakers are, so anywhere along the dotted line, except the midpoint may be appropriate. 

A commonly used trick to locate the ideal subwoofer position is called “the crawl method.” Place the subwoofer on the floor at your listening position and with the main speakers off, play a song you are familiar with or low-frequency sweep tones through the subwoofer (see note at the end of this article for more about sweep tones).

Now crawl around the area where you think the subwoofer should be placed while listening for the location where the bass feels most even. This is the spot where you should position the subwoofer. Mark that location with a piece of tape and place the subwoofer there. You should find the best location for the sub before calibrating the sub as outlined below.

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How to Calibrate a Subwoofer

Adding a subwoofer can seem daunting, so here is the basic calibration process for integrating a sub with your main monitors: 

1. Mute the subwoofer and mute or turn off your right monitor
2. Play pink noise at -18 dBFS (mono, center-panned) from a channel in your DAW, using a signal generator plugin.
3. Using an SPL meter (a phone app will suffice), adjust your monitor controller volume to achieve 82 dB SPL at your listening position. The SPL meter should be set to C-weighting and slow response.
4. Next, mute the left monitor and unmute the right monitor. It should also read 82 dB SPL at the mix position. If not, use the gain adjustment on the right monitor to set it to 82 dB SPL.
5. Mute both left and right monitors and unmute the subwoofer.
6. While still playing the pink noise, adjust the gain of the subwoofer so that your SPL meter reads 79 dB SPL at the mix position.
7. Set the crossover frequency to its highest setting, or around 130 Hz, and play pink noise with the sub and both speakers turned on. Switch the polarity of the sub between 0˚ and 180˚ to determine which setting has the most even bass response while listening from the mix position.
8. Now set the sub’s crossover frequency to match your main monitors. The manual for your main monitors should suggest a crossover frequency for use with subs. If you don’t have this information, you can audition some settings between 60 Hz and 100 Hz, but a setting of 15 Hz above the -3 dB low-frequency response of your speakers is a good starting point.

You do not need to rely on a subwoofer’s built-in crossover to dial in the appropriate bass management settings. An external crossover or monitor control app can provide the control. Below is an image of the Sphere monitor app’s speaker output settings. Here it is set for a 2.1 layout— stereo speakers with a subwoofer. This setup requires an interface with at least three outputs, one for the left speaker, one for the right, and one for the subwoofer.

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In the image above, the Sphere monitor control app provides volume, delay, high-pass, and low-pass filter controls for each of the monitors. You would perform the same calibration as outlined above to match your main monitors to your subwoofer. The delay setting can be adjusted to time-align the subwoofer to the main speakers. For bass-managed audio, the app sums the left and right low-frequency signals (or all the channels for immersive listening) before sending them to the subwoofer.

For immersive mix layouts, like 7.1.4, Sphere would display twelve speaker outputs with individual bass-management controls for each of the surround channels, along with a dedicated LFE output. The manual for Sphere clearly outlines how to apply bass management in speaker various layouts.

Should I use More Than One Subwoofer?

As you can see, setting up a subwoofer requires careful placement and calibration, so incorporating two subwoofers will introduce additional challenges. Research indicates that for stereo music, a mono subwoofer provides the listener with the same experience as stereo subwoofers. However, the use of two or more mono-summed subwoofers provides specific advantages, such as smoothing out the modal response (standing waves) in a room or helping provide a wide listening area, such as in a home theater.

The benefits of multiple subwoofers may not be as noticeable in smaller spaces, like a typical bedroom studio. You may benefit from more than one subwoofer if you have a larger space and are willing to dedicate the time and effort to a proper setup. 

Prominent acousticians, like Carl Tatz, often recommend the use of two subwoofers for stereo systems and possess the expertise to properly calibrate such a system. When the time comes for this type of monitor system, you may want to consult with a professional acoustician to achieve optimal results.

Film mixing stages and theatrical listening rooms often employ more than one sub dedicated to the LFE channel along with more subs dedicated to the bass-managed main monitors. This type of system requires a fair amount of space and careful planning but can achieve incredible results. 

Choosing a Subwoofer

For seamless and easy integration, consider exploring subwoofer options from the same manufacturer as your main monitors. However, you might discover alternative choices that offer superior performance at more favorable prices. In a bedroom-sized control room, opt for a subwoofer with a minimum power of 300 watts. For larger rooms, a recommendation of at least 700 watts and possibly multiple subs is advised.

Subwoofer designs come in sealed, ported, and passive radiator variations, each with its own set of pros and cons. Regardless of the design and driver size, you should prioritize a subwoofer capable of producing audio down to below 30 Hz, and one that also includes any controls that you deem necessary, like polarity, volume, and crossover frequency. 

When using the Sphere app or an electronic crossover, the subwoofer doesn't require built-in crossover features. However, if you prefer that the subwoofer perform the bass management, it will need to offer controls for volume, polarity, and crossover frequency. Some people prefer that subs provide a remote on/off switch, but since the sub is an integral part of your main monitor system, it's advisable not to disable the sub.

The incorporation of a subwoofer can enhance the performance of most nearfield and many midfield speakers by handling the reproduction of low frequencies that may be challenging or impossible for the main speakers. In such a system, a crossover—either in software or hardware — must be used to distribute the audio signal between the subwoofer and the main speakers. Some hardware monitor controllers and some software monitor apps provide the necessary control, while some users opt to perform bass management using the built-in controls of their subwoofer.

In essence, bass management in audio systems involves the strategic distribution of low-frequency signals between the main speakers and a subwoofer. Bass management not only improves overall audio quality but also ensures that each speaker handles frequencies within its effective range, contributing to a more balanced and immersive listening experience.

Notes About Sweep Tones 

Sweep tones help us analyze and understand the bass response of your speakers. The tones can help a listener locate the best listening position in a room as well as help integrate a subwoofer into a bass-managed setup. Sweep tones can also be used to diagnose problems in the unevenness of the frequency response of a listening environment. Here is a simple experiment that will assist you in learning how to use sweep tones:

• Web over to https://onlinetonegenerator.com/
• Select “Sweep Generator” from the menu on the left side of the page
• Set start frequency to 25 Hz, end frequency to 200 Hz, duration to 30 seconds, and sine wave. Adjust your computer’s sound output so that when you press play, the tone sweep comes from your main speakers.
• Now play the sweep tones while listening from your main listening position. 
• While watching the on-screen frequency counter, take note of any frequency ranges that are much quieter or louder than the average volume. You will undoubtedly hear a roller-coaster of volume changes.
• If you are using a subwoofer with your main speakers, this test will highlight if the frequency range produced by your subwoofer (ie: below 60 Hz) sounds louder or softer than the frequencies produced by the main speaker (ie: above 100 Hz). The volume should also remain even while passing through the crossover range (ie: 60 to 100 Hz)
• Adjust the volume, polarity, and crossover frequency of your sub and repeat the sweep tone to fine-tune the integration of the sub with the main speakers.
• If you are not using a subwoofer and you hear large dips and boosts during the sweep tone, this indicates room mode problems that can be addressed by fine-tuning your listening position, speaker position, and acoustical room treatments.

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