App Note:
An Introduction to Multi-Channel Sound
And How to Use It

When more than one audio channel is used to improve the credibility of sound reproduction we call it “stereophonic.” It is a popular misconception that “stereophonic” means two channels. The term actually refers to any form of multi-channel sound.
Attempts to increase the spatial reality of sound reproduction go back nearly to the beginning of cinema sound. In l931 EMI sound engineer Alan Blumlein filed a British Patent describing one form of stereophonic sound. But it is much easier to place a single soundtrack on the edge of the filmstrip than to reproduce multi-channel sound.

Director
Alcorn McBride France
 
Equipment
Audio Digital Binloop
8-TraXX
Software
Binloop Config

How Many Channels Are Necessary?

The advent of the stereo LP, stereophonic FM radio and stereo magnetic tape recorders democratized stereophonic sound at home in its simplest version: two channels. But for a long while the movies remained largely a monophonic experience.

In 1973, Dolby made its debut in the cinema. The system used an optical reader to reproduce left, right and center audio. Still, most cinema sound remained monaural.

Later advances in technology made it possible for Dolby to separate the various channels and to increase sound quality appreciably. The result was left, right, center, two rear channels and a sub woofer. This speaker layout was nicknamed 5.1, since there were five complete audio channels and one that could only reproduce low frequencies.

5.1 Surround

 

But it was easier to change the sound reproduction equipment than to revamp the amplifiers and speakers. Although the precursors were quickly forgotten, the allocation of channels was retained: left, right and center screen speakers, two rear channels and a sub. This structure is common to Dolby Digital, Sony Digital (SDDS) and DTS. It has also been adopted by multi-channel DVDs.

The number of sound channels in a movie theater (or a living room) is dictated mainly by the limitations of the media, whether it is film or DVD. Contrary to popular notions, 5.1 sound is not the pinnacle of audio quality; it requires drastic compression to fit six audio channels onto the restricted space of media such as a DVD. But an inexpensive medium is a necessity in a movie theater or living room, because the program material is constantly changing.

The situation is very different in a museum or amusement park. The program material changes infrequently, and there are many opportunities for better quality – and better directionality – because the space is designed to maximize the viewing experience of a particular program. Such venues enjoy total flexibility with respect to the number of audio channels, and need not compromise sound quality in achieving the best sound spatial reproduction.

This is why most theme park theaters use Alcorn McBride’s Digital Binloop as the audio playback source. The Digital Binloop makes it possible to deploy as many audio channels as you wish, with audio quality better than a DVD or CD. No matter how many channels are used, the audio is never compressed.

It is thus possible to add channels at will: above the viewer, behind, below, or from a directed point such as a character or display.

Some simple multi-channel applications may be implemented using only roughly synchronized playback. For example, Alcorn McBride’s 8-TraXX provides up to 16 channels. If the various channels are triggered as needed throughout the show, this can provide many interesting point sources. Never try to precisely sync multiple stereo pairs this way, though. The resulting phase interference will be unacceptable.

Theater Block Diagram

 

The Playback System

Since it is possible to add channels at will, multi-channel sound design is very simple: each sound source gets its own channel. There is little incremental cost in this approach, and a large payoff in system performance – and guest enjoyment.

For example, consider a show that includes a video with music, sound effects and two animated characters, one on each side of the set.

We’ll allocate three channels to the video: left, right and center. The center channel will be used primarily for narration. (If the narration were instead mixed into the left and right speakers, there would not be sufficient directionality for viewers located at the edges of the room.)

At least two surround channels will be needed in the room, perhaps more if specific effects must be localized at a precise point in the room: a noise, a simulated interruption by a guest, etc. It is possible that certain channels are used for only a few seconds during the show, but the results are really worth the small additional investment.

The animated characters must each have their own audio channel. Mixing their audio into the screen channels would ruin their realism.

Finally, a low frequency reinforcement channel makes it possible to extend the response of all the other channels into the subsonic range. Because our brain is unable to pinpoint the source of low frequency sounds, one sub woofer is enough for the whole room.

Because the Digital Binloop is a modular audio playback system, it doesn’t really matter how many tracks you choose. The incremental cost of production, reproduction, processing, amplification and reinforcement is minimal relative to the facilities budget. And selection of the Digital Binloop often saves considerable money in other areas, such as control.

The Digital Binloop can be the "Master" of the show, thanks to its internal timecode generator. Connected to one of our Show Controllers, it will give the "signal" to the various show effects at the appropriate times. It can also “Slave” to the video source to guarantee perfect synchronization with the image.

The Mix

It’s important to get the right mix. That’s one of the reasons 5.1 audio is so popular. It can be mixed in a “cookie-cutter” studio and installed with no field testing. Of course, the results of this “one size fits all” approach are as bland as one would expect.

A far better approach is to do the final mix in your facility. With the advent of PC-based multi-channels mixing tools, it is fast and economical for the recording engineer to sit in the finished exhibit space and perfectly balance every channel.

Once finished, the tracks are simply copied directly to the CompactFlash media. The Digital Binloop provides perfect audio playback without any wear or maintenance. Your soundtrack will be reproduced identically for many, many years.

 

Sound Distribution

So the allocation of audio channels is pretty straightforward. The selection of speakers, amplifiers and acoustic materials is actually the harder part of the job. Inappropriate selections can ruin all our efforts to optimize sound quality.

The objective of the sound distribution system – amplifiers and speaker enclosures – is to offer the best possible audio quality to all your guests. This means delivering sound into only the areas occupied by guests and not beyond, so as to avoid reverberation and echoes that will interfere with sound quality and intelligibility.

Most of the time the approach is the same as for a movie theater, particularly if your guests will be seated. There are many high quality manufacturers of sound reinforcement equipment that offer very good systems for both the primary channels and also smaller effect such as our animated characters.

It is sometimes tempting to trim costs by using amplified speakers, particularly in smaller spaces. This is rarely a good idea. The cost of installing power wiring coupled with maintenance and reliability issues when the amplifier is built into the enclosure make this approach impractical. Moreover, since these speaker types are designed for use in close proximity, they are not well suited to audio reinforcement of a large room.

If the budget allows, a digital signal processing system such as Peavey’s Media Matrix should be inserted between the Digital Binloop and the sound reinforcement system. This will allow very fine adjustment of the system’s frequency response and other characteristics, making it easy to correct mixing problems, yet preventing tampering – since such systems have no user-accessible knobs.

Acoustics

If you cannot call upon the services of an acoustics expert for your project you’ll be happy to know that the acoustic design of rooms intended for small attractions is relatively simple.

It is customary to define the reverberation time as the time required for the sound level to decrease to one millionth of its original power, or 60 dB – hence the abbreviation RT60. Your attraction space should have a relatively short reverberation time, say less than 0.8 seconds.

This can easily be achieved using common sound-absorbing materials such as foam or fabric. More acoustic treatment is always better than less; if the room seems too “dead”, you can always add artificial reverberation using the audio processing equipment.

Avoid reflective surfaces such as frameworks around a stage. They create "early reflections" that compromise the localization of the sound sources.

It may be that you cannot alter the acoustics of the room, if your installation is in a historical structure, for example. In this case the audio distribution system will have to be studied with great care in order to achieve optimum results.

Summary

We hope this guide has provided a useful introduction to the design of multi-channel audio systems. Our engineers have many years of experience implementing solutions in all manner of spaces, large and small. We will be happy to assist you in the planning and decision making process. Please give us a call.

 

 




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