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Bass Management in Emotiva RMC-1, RMC-1L, and XMC-2 Processor

 

Most audio coming from a surround sound disc or streaming source includes a separate channel called the Low Frequency Effects (LFE) channel. This channel contains low-frequency special effects sounds and very deep bass that is usually intended to be played through a subwoofer. Many people refer to this as the "subwoofer channel," although in systems without a subwoofer, it may be routed to your regular large speakers. In systems where some of the other speakers are small and there is a subwoofer, deep bass from other channels may also be sent to the subwoofer in addition to the LFE track.

Just to clarify the terminology, when we refer to a 5.1 or 7.1 channel system, we are referring to a system with a subwoofer, with the subwoofer being "the .1 speaker channel." However, when we refer to 5.1 or 7.1 channel audio content, "the .1 channel" refers to that LFE audio channel. This is the case even though the LFE channel of the content may not be the only audio sent to the subwoofer.

In a system with a subwoofer where all other speakers are full-range speakers, the LFE channel is usually routed directly to the subwoofer, while the other audio channels are simply sent to their respective speakers. However, in a system where some or all of the other speakers or other amplifier channels are unable to handle sufficient amounts of low bass, the low bass from those channels is often redirected to the subwoofer as well. Some systems are specifically designed this way, with a subwoofer for the low bass and smaller speakers sometimes called "satellites" for the other channels. In some systems where the front speakers are very large or have built-in subwoofers, a separate subwoofer may not be included, and the LFE channel may be routed to those large speakers instead.

The part of the processor or AVR that is responsible for routing these deep bass signals is called the bass management system. On most home theater units, there is no specific option to enable bass management, and it is simply invoked by setting one or more of your sets of speakers to "Small" instead of "Large." Setting speakers to "Small" will redirect the deep bass from those channels away from those speakers and send it to the subwoofer instead. At this point, you may also be presented with a variety of other options that may vary slightly depending on your processor make and model. In this document, we describe how this works on our Emotiva RMC-1, RMC-1L, and XMC-2 processors.

From now on, we will refer to the low frequency content that comes into the system via the LFE channel as "LFE bass," and the audio signal sent from the bass management system to the subwoofer as "summed bass content."

 

Very Basic Bass Management

In a system that includes a subwoofer and a full set of full-range speakers, the full-range audio signal from each incoming channel is sent directly to the respective speaker, and the LFE channel is sent directly to the subwoofer.

In a system that includes a subwoofer but some or all of the other speakers are NOT full range, a "crossover frequency" is configured for each of the non-full range speakers. Any audio above the crossover frequency for a given channel is routed to the respective speaker, but audio content below the crossover frequency is routed away from the speaker. This low frequency content of each channel is summed together with the content of the LFE channel, and this summed bass content is sent to the subwoofer.

In a system without a separate subwoofer, but with some large speakers or even front speakers with built-in subwoofers, both the LFE bass and the summed bass from the small speakers are routed to the large speakers.

In standard bass management terminology, any full-range speaker is referred to as "large," meaning that it sends the full frequency range of audio for its channel. Any speaker that has a crossover frequency set is referred to as "small," regardless of the physical size of the speakers involved. There is also a widely accepted industry standard crossover frequency of 80 Hz for small speakers.

It's worth mentioning that in some systems we refer to physically large speakers as "small" and configure them with a crossover frequency of less than 80 Hz to limit the low frequency content they receive, as they may do a better job of handling the rest of the audible frequency range.

A few things to keep in mind

When configuring bass management on your processor, you must have at least one subwoofer or other set of speakers configured to handle LFE content, and at least one set or subwoofer configured to handle the deep bass from your other channels. (For example, you can't configure all your speakers to Small unless you have at least one sub that handles both the LFE and summed bass information. In most cases, the processor won't allow you to choose combinations of settings that don't work.)

On our processors, both the summed bass and the bass from the LFE channel are treated as mono. Each subwoofer output can be individually adjusted with a manual EQ and is individually calibrated by Dirac Live, but the same monaural bass signal is routed to all of them.
Connecting Your Subwoofers

Our processors support up to three subwoofers. If you have a single subwoofer, we generally recommend connecting it to the center sub output for the most flexibility, but you can also connect it to the left sub output if you prefer. (If you only connect one subwoofer to the left/right sub outputs, you MUST use the left sub output.) If you have two or three subwoofers, you can connect them to the left/right sub outputs, or you can connect one to the center sub output and one to the left sub output. (This allows you to "split" the subs as described below.)

 

Configuring Bass Management on your Emotiva RMC-1, RMC-1L, or XMC-2 Processor

Our processors allow you to configure up to three subs defined as a center sub and a pair of left/right subs. Additionally, any set of regular speakers you have can be configured as Large or Small. (Most speakers, such as front or surround speakers, are configured in pairs or sets; the only exception is the center channel, which is a standalone set.)

For each set of speakers configured as Small, you have the option to set a crossover point for bass management. (The default crossover frequency for main and surround speakers is 80 Hz; the default crossover frequency for height speakers is 250 Hz.)

IF YOU DON'T HAVE SUBWOOFERS - You MUST have at least one set of speakers configured as Large. In this situation, the LFE channel and the Bass Managed Bass from all of your small speakers are combined into a single summed bass channel that is then sent to all of your large speakers.

IF YOU HAVE ONE OR MORE SUBWOOFERS - If you have one or more subwoofers and they are all equally capable of handling both sound effects and musical content, you will probably want to configure all of your subwoofers as mono. In this configuration, all of the speakers configured as Large are treated as full range, the LFE channel and the Bass Managed Bass from all of your Small speakers are combined into a single summed bass channel, and that summed bass channel is then sent to your sub or split between your multiple subs.

ASSOCIATE THE CENTER SUB AS LFE - Some systems may include one or two subwoofers that sound great with music and another that is great for providing thunderous special effects but really doesn't sound good when music is playing. Others may include large front speakers with built-in subwoofers, or multiple full-range speakers set as large, but also a subwoofer that's good for special effects. In this situation, you can configure the center sub output as LFE instead of mono. (You only have this option if you have at least one left/right sub or at least one set of speakers set to "large.")

With this configuration, the LFE channel is routed to the center sub, while the summed bass from the other channels is not.

- If you also have left/right subs and some speakers are set to "small," the speakers you have set to "large" will be treated as full range, and the summed bass from your small speakers will be routed to the left/right subs.

- If you do NOT have one or two left/right subs, and you have some speakers set to Small but also some Large speakers, the speakers you have set to Large will be treated as full range, and the summed bass from your small speakers will be routed to your large speakers.

- If you do NOT have left/right subs, you MUST configure at least some of your speakers to Large (otherwise the bass management system's summed bass will have no place to go since all your speakers are set to Small).

 

Generally, we don't recommend this option unless you have subwoofers with very different capabilities or sound qualities. However, it can offer benefits in some situations, so feel free to experiment. One benefit is that by assigning LFE and summed bass from the bass management system to separate output channels and physical speakers, you have the ability to adjust the levels of each one separately (either by using channel adjustment on the processor or by physically changing the level on the subwoofers themselves.) You may also find that even similar subwoofers perform differently in different locations. For example, a particular subwoofer may sound quite musical when placed in the middle of a wall, but deliver louder but less well-defined bass when placed in a corner. This option allows you to use the subwoofer in the middle for music and the one in the corner for sound effects.
Enhanced Bass

There is another bass management feature called Enhanced Bass that is ONLY available in the Size menu if your fronts are set to Large. Enhanced Bass is specifically designed to allow the subwoofer channels to "play along" with your front large speakers when listening to stereo or 2.0 format content. (If your fronts are set to Large, nothing will normally be routed to your subs, as 2.0 channel content does not have an LFE channel. Enhanced Bass configures the subwoofer to play along with the front speakers in this situation, even if no bass content that would normally be routed to the subwoofer is present. NOTE that this setting essentially overrides the normal operation of Bass Management and is therefore less musically accurate.)

You can change the frequency setting at which the subwoofer starts playing when Enhanced Bass is enabled by temporarily disabling Enhanced Bass, setting your fronts to Small, setting the crossover point on the fronts to the desired value, and then setting the fronts back to Large and re-enabling Enhanced Bass. (The Enhanced Bass option uses the "Small" crossover setting from the fronts to determine the crossover point used for mixing in the subwoofer.)

 

Crossover Frequency and Filter Slope Settings

The choice of whether to set a speaker as Small or Large is NOT necessarily based on the physical size of the speaker. Setting a speaker to Large simply means that it will be treated as a full-range speaker. Setting a speaker to Small means that bass management will be enabled for that speaker and a crossover frequency can be set. Anything above that crossover frequency will be routed to the speaker, while the bass management system will route frequencies below that crossover point to where it has been configured to route summed bass.

Most speakers are really not capable of fully reproducing all frequencies from 20Hz to 20kHz. Asking them to do so may result in increased distortion or require excessive amplifier power. Therefore, in some cases you may choose to set even large speakers to Small to route very low bass frequencies from those channels to the subwoofer.

Once you've set a speaker to Small in the processor's Size menu, you have an additional option to set the specific crossover frequency at which the Bass Management system begins to direct bass away from that speaker. There are default values ​​in the processor for each channel that generally work well with physically small speakers, but the user should be encouraged to adjust these based on their setup. For example, if you have physically large front speakers, you can set them to Small, but try a lower crossover frequency setting like 50 Hz.

There is no right crossover setting for any particular speaker or system, but there are some rules of thumb that can help you get started with your crossover settings. It's helpful to look at the specs of the speakers connected to each channel, note the lowest value listed in that speaker's frequency response, and set the crossover about an octave above that. For example, the Emotiva T1+'s frequency response is specified as 37Hz - 28kHz, so you might want to try a crossover frequency of 50 or 60Hz. (This setting would allow the T1+ to "work a little less hard" and continue to play cleanly at slightly higher levels, while allowing your subwoofer to handle some of the "heavy lifting.")

There is also an option in the Size menu for the crossover slope - either 12 or 24dB per octave. This setting only applies to the low-pass crossover applied to the summed bass sent to the subwoofers, as the crossover slope for the high-pass filters is set at 12dB per octave. This setting determines the sharpness of the crossover filter - with a 24dB per octave filter being sharper. While the sharper setting is arguably “better,” there is no specific right setting here, and you may find that using one or the other will allow you to achieve a smoother transition between the speaker and your subwoofer in a particular system and room.

 

Bass-Management, Crossover-Settings & Dirac Live

There are a few things we should keep in mind when considering how the processor interacts with Dirac Live in terms of bass management and crossovers. In short, the processor handles ALL bass management tasks, including crossover settings, while Dirac Live measures and calibrates each individual speaker output channel separately.

Dirac Live treats each speaker as if it were a full range. Dirac Live analyzes the response of each speaker, determines the range of frequencies the speaker can reproduce, sets its "curtains" accordingly, and then calibrates the speaker. Frequencies "outside the curtains" are not blocked or filtered out, they are simply passed through the Dirac Live filter uncorrected. When running Dirac Live, you can adjust the curtains to expand or narrow the range of frequencies Dirac Live is trying to correct.

The size and crossover setting you make in the processor does not affect how Dirac Live calibrates each speaker. However, Dirac Live will not calibrate speakers that are set to None in the speaker preset you are currently using.

This separation of duties between the processor and Dirac Live allows you to change your speaker assignments between Large and Small, or change your crossover frequencies without having to create a new Dirac filter. Dirac corrects each speaker individually across the entire frequency range when creating filters. Regardless of whether the speaker is set to Large or Small, or what frequency the crossover point is set to, the filter's Dirac adjustments applied to every part of the frequency range will be used for that channel by the processor.

Final Note

Bass management is both an art and a science. We urge every user to experiment freely and choose the settings and configurations that work best in your system and your room with your subwoofers. There are many theories about how your subwoofers should be positioned, how bass management should be configured, what you need to do to make your system sound good, and even which and how many subwoofers you need. None of them work well all the time for every system, and there is no single magic measurement or graph that will ensure you get the sound you want. You should consider any information you find on the subject as suggestions or guidelines, and try out different options to find what works best for you in your system.