Q & A: Digital Bass Management Basics
The down-low on LFE
Everyone loves the big bass experience that subwoofers offer in a well-designed home theater system. Bass management is critical in your home theater system, but they’re often misunderstood. Home theater receivers and processors include a variety of setup options, but mistakes are easy for novices to make, even with auto-calibration systems. Incorrect digital bass management settings in the A/V receiver (AVR) can result in poor sound quality, and in some cases can even damage the loudspeakers. While a mistake in bass management or subwoofer setup isn’t a life or death crisis, why settle for less than the quality experience you paid for. Ed Mullen, Director of Technology and Customer Relations, at SVS shared some of his insights on bass management.
Does bass only come from a subwoofer?
A Blu-ray DVD typically contains seven discrete speaker channels, and also a Low Frequency Effects (LFE aka .1) channel. It is a popular misconception that the LFE channel is the only channel on a DVD which contains deep and powerful bass. In reality, depending how a given DVD is mixed, there can also be deep, powerful bass in any of the speaker channels.
Why do I need a separate speaker just for bass?
In a modern home theater system, the loudspeakers are typically not designed to handle deep bass, and can be overloaded (or even damaged) by incorrect bass management settings in the receiver. The goal of proper digital bass management is to ensure all of the deep bass present in the source material is directed to the subwoofer, which leaves the loudspeakers to handle the mids and highs.
What setting do I need to make in my receiver?
The speaker size setting in an AVR can be confusing or misleading, because they are often labeled ‘Large’ or ‘Small’, and in reality these terms have little to do with the actual physical size of the loudspeaker itself.
A Small setting doesn’t necessarily mean the loudspeaker in that channel is physically small – it just means the bass in that channel is being digitally managed by the AVR. A Small setting redirects the bass in that channel to the subwoofer, below the crossover frequency selected/specified in the AVR. While historically a Small setting typically meant a crossover in the 80-100 Hz range, modern AVRs now commonly offer crossover frequencies as deep as 40 Hz, which covers even physically large and bass-capable speakers like towers.
With the possible exception of truly full-range tower speakers, a Small setting is recommended for all speaker channels, since this will ensure that the speakers are not sent bass which is below their safe and effective limits. This improves the sound quality of the speakers, and it also frees up additional power in the AVR, thereby improving overall system dynamics.
A Large (aka Full-Band) setting means that speaker channel will receive a full-range signal from the AVR. Again, with the possible exception of truly full-range tower speakers, a Large/Full-Band setting is generally not recommended, since it might overload and/or damage the speaker, and it also can consume additional power from the AVR, thereby reducing dynamic headroom.
How do I know where to set the crossover?
The optimal crossover frequency for each speaker channel varies with the bass extension and dynamic output capabilities of the connected loudspeaker. Below is a general guideline for recommended speaker/subwoofer crossover frequencies.
• Tiny ‘satellite’ speakers: 150-200 Hz.
• Small center, surround, bookshelf: 100-120 Hz.
• Mid-size center, surround, bookshelf: 80-100 Hz.
• Large center, surround and bookshelf: 60-80 Hz.
• Very large center, surround, bookshelf: 40-60 Hz.
• Tower speakers with 4”-6” woofers: 60 Hz.
• Tower speakers with 8”-10” woofers: 40 Hz or Large/Full-Band (i.e., full-range).
If you are unsure of the best crossover frequency for each speaker channel, double check with the loudspeaker manufacturer.
If you have towers in the front but small speakers in the rear. Do they all get the same crossover?
Some AVRs allow a crossover frequency to be assigned to each individual speaker channel that is being bass-managed. This is a nice feature because it allows the crossover frequency to be tailored to the bass extension and dynamic output capabilities of each loudspeaker.
Other AVRs only offer one global crossover frequency which applies to all channels being bass managed. This single ‘compromise’ crossover frequency may not be optimal for all the speakers, particularly if there are tower speakers in the system.
What’s the difference between LFE and LFE+mains in an AVR’s menu?
The AVR Subwoofer Mode menu options will vary depending on the sophistication of the bass management circuit, and also the speaker size selected. Below is a description of common AVR subwoofer modes:
• LFE (aka Normal)
• The LFE channel is sent to the subwoofer
• The redirected bass from all speaker channels being bass-managed (Small) is sent to the subwoofer below the specified crossover frequency
• The bass from any speaker channel being sent a full-range signal (Large/Full-Band) is not sent to the subwoofer
• LFE+Mains (aka Double Bass, Plus, Sub+Mains)
• The LFE channel is sent to the subwoofer
• The redirected bass from all speaker channels being bass-managed (Small) is sent to the subwoofer
• The bass from any speaker channel being sent a full-range signal (Large/Full-Band) is duplicated and also sent to the subwoofer. The low-pass (cut-off) frequency for this duplicate signal depends on the sophistication of the AVR bass management circuit.
Can I just use the automatic calibration system in my AVR?
Nearly all modern AVRs offer an auto-set-up routine with a plug-in microphone and an on-screen GUI with easy to follow steps. The four core functions of the auto-set-up routine are:
• Level matching the speaker channels and the subwoofer so they all play at the same relative volume, which results in a balanced sound field
• Time-aligning the speaker channels and the subwoofer so they all arrive at the listening position at the same time, which is important for convincing panning effects across multiple channels
• Equalizing the speaker channels and the subwoofer for the best sound quality
• Selecting the best digital bass management settings for each loudspeaker channel and the subwoofer
Auto-set-up routines occasionally make non-optimal choices (or even outright mistakes), particularly in the digital bass management settings. For example, an AVR might assign a crossover frequency which is too deep for a given loudspeaker, or it might even set a loudspeaker channel to full-range when it should be bass-managed instead. So it’s best to review the auto-set-up results and consult with the subwoofer manufacturer to ensure all settings are optimal for your particular loudspeakers and subwoofer model.
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