If you set up your two channel system and sit in the “sweet spot”, you will be sitting at a spot that is equal distant from the left and right channel speakers, which are equal distance from each other. The speakers and the listening position form the apexes of an equilateral triangle. If you face forward, using your nose as the center line, you have about 30 degrees to the right of your nose and 30 degrees to the left of your nose, into which the stereo image is perceived within by our ears. This is the “sweet spot”.
Our localization for two channel sound occurs within this 60 degree arc. Stare straight ahead. If you wear glasses, move your eyes from left to right stopping at your glasses edges on both sides. This is the visual of the “sweet spot”. We can resolve differences within this arc of a few degrees. The human localization antenna is very fine tuned and can separate down to degrees within this 60 degree arc. This localization process only works with speakers in front of listener. If you place speakers behind the head or listening position such as in surround sound rear and side channels all this localization ability goes away.
Multiple Mono Sources
When we have multiple mono sources such as in surround sound, we will have just as many different tonality changes as we have speakers emitting energy from behind our side, head mounted ear canals. The speaker positioning in relation to our ears and head causes these tonal changes. If we use a symmetrical approach to these rear speaker positioning, the thinking is that this will yield more accurate surround sound monitoring. Taking the surround sound data and then putting into two channel stereo produces level and frequency issues.
Thomas Holman in his book entitled tells us that 5 channels are not enough to produce the natural ambiances generated by sound that is direct and reflected energy from behind our ears. Holman believes that to really create the energy fields that are ambient sound and non localized by our hearing mechanism requires at least 10 channels of information. Today, we have 5.1. Holman believes we need at least a 10.2 system comprising 10 discreet channels and two LFE or low frequency enhancement channels.
David Bell was another surround sound guy who said that we should position loudspeakers hanging from the ceiling pointed away from the listening position. Speakers positioned in this matter would fire into diffusion treated surfaces. This positioning has the speaker null or back side of the speaker facing the listener and the speaker face firing into a diffusor which spreads the energy directed into it out in a fan like array into the room. This procedure creates the ambiance through diffusion and does not need to rely on the room boundary surfaces producing enough reflections in the right amount and level to create the ambiance required from our surround channels.
Surround Sound Realism
Multiple channel surround sound is always more realistic than two channel surround sound. Thomas Holman proposes dipole rear channel speakers that are placed above and behind the listening position. Dipoles positioned in this matter would present their nulls at the listening position. This would allow for the listener to not hear any direct energy from the side or rear channels but instead allow the listener to hear more of the reflected energy off the room boundary surfaces. This reflected energy creates the ambiance.
Control Room Dipoles
In the control room, the side of the dipole speaker that faces the hardest surface would produce the sound of the highest frequencies. One must be careful in using a dipole arrangement for the surrounds and insure that all reflective surfaces in the control room are evenly distributed. Obviously, this technique would lend itself better to a home environment where these reflective issues would be more evenly distributed. It would also be difficult to achieve a flat response at the monitoring position with so much reflected energy entering into the mix.
Dolby uses multiple sources that distribute the surround energy. The direct energy from mono sources would be heard as direct energy with a tendency to favor the speaker closest to the listener. The direct energy from the speaker closest to the listener would sound brighter because that energy is entering the ear canal first without the benefit of a reflected upon wall surface.
The sound effect of many multiple mono sources spreading their energy across the room surfaces can be very lifelike. The precedence effect would assure that the front main speakers would contain and localize their information and not get smothered and confused with the rear channel data. Vocals would still be centered on the screen and any “front of the house” information would stay in the front of the room closest to the screen. This multiple, discreet channel set up would not be conducive to a home theater environment where space and esthetics are always a factor.
Low Frequency Enhancement
We all know by now that the .1 or .2 behind the number of channels in a surround application stands for the low frequency effects channel information. This information is usually below 80 cycles and is normally produced by sub woofers. With the cut off frequency being 80 cycles, localization becomes difficult which is the intended effect. Below a cut off frequency of 50 cycles, localization is not possible. Positioning of all low frequency effect channels is critical so that room modal responses are not excited.
Proper LFE Placement
To compensate for room modal response excitation, we need to distribute our low frequency generating devices in a way in our rooms that will lend itself to minimizing these resonances. Dolby suggests that one sub woofer be located one third the distance across the room from the side walls and another placed one fifth of the distance across the room from the opposite side wall. This asymmetrical low frequency generating device layout prevents the symmetrical driving of room modal issues.
Surround Sound Perceptions
Surround sound is a blend of many perceptions. We have front channels that must focus vocals and front image issues. We have surrounds that must create the ambiance of the real world in free space and then we must be bring this creation into a small room environment and make it sound realistic. To create this “realism”, we can use dipole or individual discreet channels with the proper amount of acoustical treatment. Care must be taken in locating any low frequency source within the room in order to minimize room resonance issues.