Ken Kreisel's answer on surrounds
Here's an interesting take on this subject, posted in the amp section earlier...
The Logic Behind Bass Management
O B S E R V A T I O N S by Ken Kreisel
Why use a separate subwoofer?
Why not just have 5 huge speakers?
Engineers mixing multi-channel audio (Stereo, Dolby Surround, 5.1 Dolby Digital, DTS, and other surround-sound formats) are faced with numerous challenges when trying to accurately monitor complex and dynamic material, and determine how this material will sound in its intended playback space. These issues include reproducing 5, 6, 7, or more channels of full range audio, plus an optional Low Frequency Effects channel (LFE), all of which have a bandwidth to 20Hz or below, with very high dynamic range, and achieving consistent bass response from all the channels throughout the control room monitoring area.
Any studio designer will tell you that for a stereo mix environment it is crucial that the left and right monitor speakers, when in their selected studio location, have near identical bass response when measured at the mixer's position. No less is true in multichannel mixing. Proper low frequency equalization and mixing decisions are difficult, if not impossible, unless all 5.1 or more channels have the same bass frequency response at the mixers listening position.
Due to unavoidable room modes, five or more correctly placed full range speakers, (in even the most perfectly designed studio) will produce dramatically different low frequency characteristics at the mix position. This is especially true for the very crucial center channel speaker. Variations of 10 to 20 dB may be measured at frequencies below 80 Hz.
When the bass from all the channels is redirected into a single, PROPERLY placed subwoofer, then each and all of the multiple channels exhibits the identical bass response at the listening position, and gives surprisingly even coverage in virtually every control room. Simply stated, bass management is putting an electronic bass frequency crossover (typically 80 Hz) on all the channels, and redirecting the bass frequencies below 80 Hz from each of the channels to a common subwoofer.
Another good reason for using bass management in the control room is that even the least expensive Dolby Digital consumer decoder, found in millions of homes, has bass management built in, allowing the bass from all channels to be fed to a single subwoofer. Also, all Home THX systems utilize an 80 Hz bass management system. Monitoring with proper bass management in the professional control room insures proper playback translation into the "home theater" environment and into larger spaces such as motion picture theaters.
By combining a carefully designed low frequency acoustic rolloff of all the main speakers, including the surrounds, and matching this up with the circuitry found in the modern receivers mentioned above which have Dolby, THX, and DTS circuitry built in, M&K achieves all the theoretical benefits of a 4th order Linkwitz-Riley filter. In addition, to further create a seamless bass managed system, we use only non-ported sealed cabinet designs on all our speakers and subwoofers, eliminating ported speaker phase anomalies which would prohibit smooth transitions at the bass managed crossover frequency.
It is important to understand that Bass Management is done as a function of the monitoring system and in no way affects the actual mix. Full frequency musical content, is assigned or panned around the room via normal console, Digital Audio Workstation (DAW), or software operations to the various desired subjective locations such as Left, Center, Right, Left Surround, and so on. This full frequency musical data corresponding to each speaker location channel is stored normally on tape, hard drive, etc.
Bass Management psychoacoustically works because the ear-brain mechanism cannot detect direction at low frequencies, but takes its directional cues from the harmonics of the low frequency sound. In the monitoring process, frequencies below 80 Hz are redirected to the subwoofer. Frequencies above 80Hz are sent to the desired speaker. During the hearing process, our hearing mechanism integrates the sound into the correct spatial auditory image.
Bass management is equally suited to all formats from 2-channel stereo to 5.1 and beyond. Indeed, existing control rooms that are doing stereo 2 channel mixes, especially with smaller nearfield monitors placed on the console, can benefit immensely from the correct integration of Bass Management and a subwoofer; because the engineer is now able to hear low frequency anomalies caused by room rumble, microphone stand thumping, breath pops, and other artifacts that cause undesirable actions later in the broadcast, film chain, CD, or DVD Mastering process.
When mixes are intended for theatrical presentation, it is sometimes necessary to use an additional channel for extra low frequency effects or enhancement. This channel is called the LFE or Low Frequency Effects/Enhancements channel. In the control room monitoring environment, the LFE channel, if or when it is used in the mix, is monitored by the same subwoofer used for the regular [L,C,R,LS,RS] channels. In the home playback environment, the LFE channel, when encoded on the playback disc, gets summed with the already bass-managed below 80 Hz program material signal, and is then sent out the subwoofer connector. The home subwoofer, then, just like the control room subwoofer, reproduces everything below 80 Hz from all 5 surround channels and the LFE information, if any.
