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Virtual reality games fill arcades and the shelves of electronics stores, while technology continues to improve our ability to recreate real-world experiences. One key ingredient to these virtual environments is sound.
With the advent of surround sound in movie theaters in the early 1940s, the experience of sound changed forever. Surround sound brought thunder, revving motors, and explosions right into the theater. These sound simulators take into account not only the human hearing process, but also the shape and size of the room in which the audience sits.
The Basics: Binaural Hearing
Just as two slightly separated eyes produce stereoscopic, or binocular, vision, two separated ears produce binaural sound. Because the ears are separated, sound waves arrive at each ear at different times; these time differences are called interaural time differences (ITDs). Sound waves may also have different intensities, or energies, as they arrive at each ear; these energy differences are called interaural level differences(ILDs). ITDs and ILDs are the cues that we use to determine from which direction a sound arrives.
"The size of the ITD and the ILD both grow as a sound moves to the side of the listener," explains Barbara Shinn-Cunningham, Associate Professor of Cognitive and Neural Systems at Boston University.
We also use spectral content, or frequency information, to determine the location of a sound source.
"Spectral content for a source directly in front of you is different from the spectral content for a source directly over your head," says Shinn-Cunningham.
The listening environment, however, changes the way we interpret spectral content. Sound waves reflect off of some objects and are absorbed by others. The reflections and absorptions distort the original sound waves. People grow accustomed to hearing distorted sound waves because daily environments are filled with objects that obstruct sound. Everything from desks to chairs to cars and skyscrapers affects the way we hear.
"One of the most amazing and interesting things about how we 'perceive’ sound sources is that we are very good at factoring out these distortions," says Shinn-Cunningham. "If you measure the way reflections affect the signals reaching a listener's ear, the distortion is quite dramatic; however, most of the time listeners are not even aware of these effects at a conscious level. Unconsciously, the distortion of reflections actually provides information to the listener--information about the kind of environment they are in."
Furthermore, the shape and size of a room can affect distortion levels. As sound waves reflect off walls, the waves are distorted. Room symmetries and shapes determine what frequencies will resonate, or reverberate. Resonance can distort or overpower the desired signal, so it is important to avoid this phenomenon. Surround sound systems, especially those in movie theaters and concert halls, take into account distortion and resonance.
How Surround Sound Systems Work
The goal of surround sound is to realistically recreate certain hearing environments. Before surround sound, movie sound was controlled by two speakers on either side of the projection screen. The audience received signals from only two speakers, which limited the simulated sound experiences. In fact, recordings for these movies sometimes involved as few as two microphones, or channels.
Surround sound recordings, on the other hand, use many channels. Many speakers are placed strategically throughout the theater, rather than just on each side of the projection screen. As a result of speaker arrangement and multi-channel recording, the audience receives a combination of signals from different parts of the room. When the signals are sent and mixed properly, they create the desired listening experience. Signal mixing brings the sound of thunder, explosions, and distant laughing into theaters for movie-goers.
Both home and theater surround sound systems involve the careful placement of multiple speakers throughout a room. The most basic set-up requires a rear left and rear right speaker, a front left and front right speaker, a potentiometer, and a receiver. The receiver acts as a central speaker to connect the left and right stereo speakers. A television can work as a receiver in a home theater system. Potentiometers, which add different amounts of resistance to a current and reduce voltage, control the volume of the rear speakers.
Good surround sound systems can recreate most of the sound sources in a movie, including planes, leafs, and footsteps. Sound source illusions, however, can exist only in certain parts of the room. These special locations are called “sweet spots.” In the sweet spots, signals mix just the right way to recreate a sound effect. The more speakers used, the more sweet spots there are in a room.
"As you increase the number of loudspeakers, the illusion also becomes more robust in that the 'sweet spot' gets larger," says Shinn-Cunningham. Generally, therefore, the more speakers used in a system, the better the quality of surround sound.
Because the signals from each speaker reach both ears, it is important to know the listener's position with respect to the speakers. If the interaural differences are not correct, then the signals will not properly recreate the desired sound source illusion. Most home theater systems come with illustrations explaining where to position the speakers.
The Ideal Room Shape
Because room shape can change sound waves, there are certain requirements a room should meet before the installation of a surround sound theater system.
“In general, you [want] your room to be as large as possible,” says Mendel Kleiner, Director of the Architectural Acoustics Program at Rensselaer Polytechnic Institute. “This will reduce the sound reflections off the walls.”
Rooms using a surround sound system should also be symmetrical, according to Trevor Cox, Professor of Acoustics at the University of Salford, UK. Symmetry ensures that the sound sources simulated by each of the speakers match.
Not all symmetrical shapes are desirable, however.
“The room should not be round, because it will focus sound like a concave mirror,” says Cox. Square and cubic floor plans are also undesirable because they will lead to low frequency resonances. Rectangles are the most basic desirable shape because they distort sound the least, but generally a home theater system can be adjusted to work in most rooms. The key is to have the distances between the speakers proportional, so the proper signal mixing occurs.
Though the term "surround sound" was coined by Dolby Laboratories, which created the well-known multi-channel sound system, the term is now used for virtually any multi-channel home or theater sound system.
Developments in surround sound are increasingly being aimed towards home theater. As home living rooms are small compared to stadium-seating movie theaters, some companies are trying to adapt the multi-speaker system into just one or two units. The Digital Sound Projector™ by 1Limited, for example, simulates an 8-channel audio system using just a single panel that connects to a CD or DVD player. Signals from the projector bounce off of objects in the room, creating the necessary surround sound effects for movies and music. A remote sensor detects walls and furniture to specially adjust the unit to individual rooms.
Dolby recently released Dolby Pro(R) Pro Logic(R) IIx, which can be used not only for movies and music, but also to enhance the sound experience while playing video games. The system can also mix up to seven signals, though this means it requires seven speakers.
Another advancement in surround sound technology is THX. THX, sometimes considered the next generation of surround sound, was created at Lucasfilm, where George Lucas wanted to improve the quality and stability of his movie sound. THX systems use filters to stop signals from going to the wrong speaker. THX technology also includes a device to increase the number of “sweet spots” in the room.
As surround sound technology continues to improve, movie enthusiasts can look forward to enjoying a theater experience right in their own homes, minus perhaps stadium seating and a 30-foot screen.