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Be an “Ace of Bass” - Part 3

Wed 22 Dec 2010

Be an “Ace of Bass” - Part 3

In the last two weeks we’ve looked at the basics of sound and why bass reflections can be useful. This week we’ll move up the frequency spectrum and look at the crossover frequency.

Before home cinema became popular it was normal to use just two speakers for music playback, one for left and one for right. Both speakers had to cope with the full frequency range from the very lowest bass to the very highest treble. When home cinema arrived the need for better bass reproduction (to convey the non-musical low frequency effects present in movies) meant that dedicated subwoofers became popular. As an upshot this also meant that the low frequencies in the “main” speakers (left, right, centre, right surround and left surround) could also be diverted to the subwoofer if they were not particularly good at reproducing bass.

Sub/Sat systems

With this arrangement physically smaller (i.e. more domestically acceptable) main speakers could be used.  This configuration became known as the “sub/sat” system (“sat” for “satellite”) and remians popular to this day.

Modern “5.1” and “7.1” soundtracks found on DVD, BluRay and some digital TV broadcasts have a dedicated LFE (Low Frequency Effects) channel in the soundtrack. This is addition to the main channels and its signal goes straight to the subwoofer along with any bass frequencies mixed in from the main speakers.

Bass Management

To make these systems work we need to divide up the sound so that only bass (but from all channels) is sent to the subwoofer while sounds from other channels (minus the bass) are sent to their respective satellite speakers. This process is called “bass management” and is normally carried out digitally in the surround processor or AV receiver. The frequency at which the divided happens is called the “crossover” frequency.

So what’s “large” and what’s “small”?

Looking inside the setup menu of a typical home cinema processor or receiver you’ll find a choice of “small” or “large” for the speakers you’ve connected. At first glance this sounds like a simple choice based on the physical size of the speaker. However, making this choice actually determines if bass stays in the connected speaker or is diverted to the subwoofer. If we choose “large” any bass will remain in the main speaker, if we choose “small” it gets diverted.

In many cases speakers should be set to “small” even if they floor standing, physical large types. This is to make sure all bass from all channels sounds the same. It also means that a device designed specifically for the job sounds low frequencies. Centre speakers should almost always be set to “small” as it is very rare to find a centre speaker that can properly handle low bass.

Crossover Frequency

The crossover frequency is set in the AV processor or receiver and is usually variable to some degree. A range of 50-150Hz is typical but the normal default frequency of 80Hz is a good starting point. The smaller the speaker the higher the crossover frequency will need to be. However, it’s worth knowing that sound becomes directional at approx. 100Hz. Setting the crossover point above 100Hz may result in the subwoofer sounding “disconnected” from the main speakers, especially if it's placed some distance away.

So, that’s it for this week. Next week we’ll look at placing a subwoofer in a typical listening room. As always, if you have any comments on this post or any others in this series feel free to let our Webmaster know!