In addition to sound waves' interactions with the human body, virtual surround-sound speakers use a number of tools and techniques to create the illusion of 5.1-channel sound. Some systems, particularly digital sound-processing systems, physically reflect sound waves off of the walls of the room. In this case, some of the sound seems to come from behind you because it is bouncing off of the wall behind your head. Systems like these often require you to provide the dimensions of your room or to calibrate the speakers using a microphone. Otherwise, the reflections may happen at the wrong angles or in the wrong places.
Many two-speaker systems also incorporate crosstalk cancellation. This is basically the creative use of destructive interference to eliminate unwanted interference between the sound you should hear with your left ear and the sound you should hear with your right. This makes it less likely that your ears will pick up on one another's cues, ruining the illusion of five-speaker sound.
The algorithms and crosstalk cancellation protocols require the help of a computer processor, usually found in a receiver/amplifier. This device includes a sound-processing chip that can apply the algorithms to the sound waves in real time. As with other amplifiers, it receives the sound information from a source, like a satellite box or a DVD player. It applies the algorithms and makes any other adjustments to volume or sound quality before sending the signal to the speakers. In some systems, this receiver/amplifier is built in to the speaker units.
The biggest drawback to virtual surround-sound systems is that their immersive effect is an illusion rather than the product of multiple speakers. Maintaining this illusion requires you to sit in the right spot and to look directly at the television screen. Moving too far to the left or the right of the sweet spot can disrupt the sensation of real surround sound, placing you outside of the directed sound field. Sometimes, sounds that move from one side of the room to the other or from in front of you to behind you seem to be interrupted or sound unnatural. Since the sound waves themselves are only coming from two speakers, the sound field often has less power and impact than one from a full set of speakers.
In addition, there are a few points to keep in mind if you are shopping for a virtual surround-sound system:
- Room size and shape: Since they tend to take advantage of sound reflections, digital sound projection systems often do not work very well in extremely large, open rooms or in rooms with irregularly-shaped walls.
- Desired effect: If you're looking for full, room-filling sound, a two-speaker system may not be able to live up to your expectations.
- Subwoofer: 2.1 systems include a subwoofer. Many digital sound-projection systems don't, but you can purchase one if you want additional bass sound.
- Setup requirements: Many virtual surround-sound systems allow you to simply plug in your speakers and go. However, some require a calibration step that may involve measuring the room.
- Price: Some 2.1-surround systems can be an affordable alternative to 5.1- or 7.1- surround systems. However, high-end digital sound-projection systems can cost more than $1500.
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