Most auditoria already have a sound system to reinforce a basic stereo movie soundtrack. Some might even have a center speaker to reproduce the critical voice channel in a film. However, if you are showing a film that was made after the 1980s without a surround sound system, viewers are likely to miss an important dimension of the film that was carefully crafted by the filmmaker. Since the 80s, surround sound has become common and continued to develop and diverge into a plethora of formats that can be confusing for anyone other than experts in film sound technology. It doesn’t have to be difficult to add surround sound to an existing space, but sorting out what to do in your space gets more complicated every year.
Evolution of Surround Sound
Surround sound has evolved in the last 30 years from a single-channel, monolithic ambient sound environment to an immersive sonic environment that can recreate a helicopter as it flies over the theater and onto the screen. Surround sound is anything beyond the “screen channels.” A screen channel is a speaker that is installed behind the screen. Most cinema screens are perforated with tiny, invisible holes to allow sound to travel through the screen, which creates the illusion that sound is emanating from the projected image.
The difference between screen channels and surround is that the surround is the sound of anything “off screen.” For example, you might hear the sound of a car from the side surround speakers before it appears on screen. Today, surround sound can come in a number of different formats. The most popular format in the cinema and the home is a 5.1 system. It includes three screen channels (left, center, right), two surround channels (surround left and surround right) and a subwoofer signal (the “point one”), which delivers very low-pitched sounds. Often in professional theaters, a 7.1 surround system is used, which includes four surround channels (left and right side, and left and right rear channels) plus the subwoofer. Other formats exist, some of which have emerged in the last few years and are summarized below.
In short, if you are planning to add a surround sound system to an existing auditorium, there are some things to consider. First, all rooms where movies are shown should have a significant amount of acoustic absorption, such as heavy drapes, foam or fabric wrapped fiberglass panels on the walls, and acoustically absorptive ceiling panels or tiles. Professional cinema rooms are typically acoustically “dead,” meaning there aren’t a lot of sound reflections in the room. Another way to say this is that they have low reverberation times (the time it takes sound to decay or die away to an inaudible level).
For instance, in a cathedral, it can take as long as 2 seconds for sound to decay. Lecture halls and conference rooms typically have reverberation times of 0.5 seconds to 1.5 seconds. Cinema rooms are recommended to have reverb times of 0.7 seconds or less, depending on room volume, so it may be necessary to add absorption to a typical lecture hall. You may need an acoustical consultant to evaluate your space and make recommendations to achieve the appropriate acoustical environment for cinema.
Secondly, surround speaker layout can be complex, because-typically-multiple surround speakers are mounted on the sidewalls to deliver even sound from the near side seat to the far seat across the room. This means that the speaker needs to be mounted high enough to deliver sound across the room, but not too high to miss delivering sound to the near seats. Ceiling heights can also constrain speaker placement and make them difficult to place where you may need them.
Next, if you plan to create a 7.1 surround sound system, you’ll also need speakers on the rear wall. A 7.1 surround system provides the most flexibility, because you can always play back 5.1 surround onto a 7.1 system, but not vice-versa. Additionally, speaker placement is more complex when you have a balcony. This often means creating separate surround sound zones above and below the balcony. These are often difficult spaces to cover well with sound because of the space constraints above and below the balcony.
Space, Power, and Serving Double-Duty
You may be able to have your surround speaker system serve double-duty as a speaker system for theater sound design but-if you plan to use the system for theater as well as cinema-you’ll want to choose a speaker that is rugged enough for theatrical use. You may also need a special patch bay to connect the surround system to your mixing console.
You’ll need some additional space to house the rack-mounted amplifiers that are needed for the surround speakers. Today’s high-efficiency, multichannel amplifiers can save space and power, but depending on how many speakers you need, it could mean that you need to find a footprint for another equipment rack to house the amps.
Speaking of power, in many cases with higher efficiency audio amplifiers mentioned above, you will not need extra power for your surround system, but it depends on the size of the room, the number of speakers needed and the amount of surplus power you have today in your equipment room. Of course, the cost for adding surround sound to a space can vary widely depending on if you need new conduit and power infrastructure. Typical costs for the equipment alone for a 100- to 500-seat auditorium can range from $7,000- $12,000. For many of these issues it is advisable to consult with a professional AV consultant or integrator as you consider adding surround sound to an existing space.
Surround Sound Playback
You’ll also want to make sure your movie playback system supports surround. Most modern film “decoders,” such as those from Dolby, support 5.1 and 7.1 surround formats. Most professional digital cinema “media blocks” also have surround outputs. Connecting the dedicated surround outputs of these units to the surround speaker amplifiers is a relatively simple matter with which your AV staff or integrator can assist you. Blu-ray players and some PC-based media may have surround-encoded soundtracks too. In the case that you are playing properly licensed material from a Blu-ray Disc Player or PC, you may need a dedicated surround decoder that can be connected to the digital audio outputs of the player and decode the surround signal for connection to the surround speaker amplifiers.
In the case that you need to support surround outputs from multiple playback devices, you’ll also need a way to mix these signals together. Again, depending on your AV staff or consulting with an independent AV integrator is the best option for assistance on complex audio routing systems.
Advanced Surround Formats
In the past five years, the film industry has been experimenting with some new surround formats that haven’t yet been accepted widely, but they are delivering an expanded surround experience to many audiences today. The first to introduce a fully immersive surround format was Dolby with its Atmos system. Atmos, like in all of the new immersive formats, adds a series of ceiling speakers to the room. In 5.1 and 7.1 systems all of the speakers on the left side or right side of the auditorium produced the same sound to the entire audience. In these new formats, each individual speaker delivers a unique sound so that sounds can move from front to back, across the top and around the rear of the cinema very precisely.
Immersive Sound Formats
In addition to Dolby Atmos, immersive surround formats have been introduced by Barco (Aura) and DTS (DTSx). Today, some of the larger cinema complexes have installed an immersive system in at least one of their larger cinema theaters for premieres, and many of the blockbusters from the last few years have supported one or more of these new surround formats.
However, don’t expect to see these immersive systems in university spaces any time soon, because they are complex to design and expensive to install. We may still see these newer formats take off in the near future as highly immersive virtual reality games take hold in popular culture.
A Note on Acoustical Enhancement Systems
Large gathering spaces are scarce in most colleges and universities, and as a result, it is important to be able to use these spaces for as many activities as possible. However, there is at least one issue that can interfere with the use of a space for multiple uses: acoustics. In order to be a good performance space, the acoustics must be lively and support the music being played for the musicians and the audience alike.
In contrast, a room used for cinema playback must be somewhat “dead” in its acoustical characteristic. Sometimes these different requirements can be managed through passive variable acoustics, such as deploying heavy drapery or panels as needed to absorb or reflect sound. In many cases, an existing space may not have the physical infrastructure or the manpower to accommodate such manual variable acoustic systems. As digital audio systems have increased in their capabilities over the last 10 years, several electronic systems have been developed to address this problem.
These systems, referred to as “acoustical enhancement systems,” can change the acoustical environment of a space with the flip of a switch. These systems usually are installed in relatively dead spaces that may support cinema systems nicely. The systems then capture the sounds being made in the room (music, speech, etc.), and artificially reproduce reverberation as as if the room were more reflective.
This is done by placing some microphones in the space together with a large number of loudspeakers located throughout the area to create a natural distribution of artificial reflections in the space. In the equipment room, there is a complex digital signal processing engine as well as many channels of speaker amplification to support the microphones and speakers. These systems can be expensive and challenging to design and deploy – but they often provide the best way to repurpose a space for a wide range of uses.