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Volume 11, Issue 5: November 2004
How It Works
Surround Sound Systems are Bringing the Theater Home
Tiffany-Rose
J. Sikorski, News Editor
Astronomy
and Physics, Boston University
sikorski@jyi.org
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
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Interaural time difference (ITD). The distances between a sound
source and each ear allow us to detect the sound’s location. Source: Rice University, Sarah Coppin,
Kin Daniel, Jeremy Pearce, Chris Rozell, and
Yasushi Yamazaki.
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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.
Distortion Sources
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
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The “poor
man’s” surround sound setup. This is the minimum equipment
needed to create a surround sound system. Source: HowStuffWorks, Tom
Harris.
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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.
The Future
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® Pro Logic® 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.
References
and Suggested Reading
“How
Surround Sound Works”. http://entertainment.howstuffworks.com/surround-sound.htm.
The site includes a diagram of the most basic surround sound system
arrangement.
Home
theater tutorials. http://timefordvd.com/tutorial/index.shtml
Compare
prices for home surround sound theater systems. http://consumerguide.bizrate.com/buy/products__cat_id--11120000,keyword--Surround%20Sound%20Systems.html
Information
on the history of movie sound and articles by movie sound designers. www.filmsound.org
Information
on the Dolby Pro® Pro Logic® IIx.
http://www.dolby.com/Consumer/Technologies/PLIIx/
Information
on the Digital Sound Projector™ by 1Limited. http://www.1limited.com/tech/sp/index.html
Information
on the THX Home Theater by THX Ltd. http://www.thx.com/mod/products/homeTheater.html#about
Journal of Young
Investigators. 2004. Volume Eleven.
Copyright © 2004 by Tiffany-Rose Sikorski and
JYI. All rights reserved.
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