Sound Quality

Henry Corrado

A word on Sound Quality

Just like in a movie soundtrack, a great theme park sound field includes music, voice and sound effects. Legitimately, one may wish that the music is as natural as possible, as dialogues and announcements can be easily understood, while sound effects are thundering. We will see that, just as in a movie soundtrack, these elements require specific, sometimes conflicting, constraints from the sound system.

How to evaluate the sound quality, and choose an audio system? What differentiates a professional sound system from their living room counterparts? This is what we will try to review in the following lines.

Almost like home
The sound reproduction amateur seeks a neutral system that reproduces sound with the greatest accuracy possible. In the pursuit of this quest, purists are willing to spend astronomical amounts on their stereo systems. Different approaches are possible for each element of the chain, from the source to the speakers via the mains cables. Lovers defend their beliefs with a fanatical zeal.
Things are not very different in the professional world. Reproduction as close as possible to reality should satisfy the constraints mentioned above. However, the situation gets a little a little more complicated in a public place, which differs widely from the living room. While the goal of a domestic installation is to provide quality sound at a specific location, professional installations must provide consistent performance over a large area. In other words, the spatial characteristics of the speakers, which are a secondary concern in high fidelity, are fundamental in the professional field. Another difference is related to efficiency. For the same amount of electrical power, some speakers provide more acoustic energy than others, that is to say the least efficient need more powerful amplifiers to provide the same sound energy. This implies more expensive equipment and extra power consumption. In order to cut equipment and power bills, parks have to shop for efficient speakers.

As we have seen, the spatial characteristics of a loudspeaker are at least as important as its sonic performance, for three reasons:

- The visitors are likely to be scattered across a large space. It is important that both the level and the quality of sound are consistent. Perfect reproduction at a single point is of interest only to those who occupy it. Experts who control systems or mixi on attraction soundtracks can often be seen roaming around the area to assess the sound performance in the worst locations.

- Only the audience needs sound reinforcement. Reflections and echoes on walls and ceiling should normally be avoided.

- Unlike a home stereo system?, it is likely that several speakers have to work together, either to increase power or to broaden the dispersion. In the latter case, multiplying speaker locations, or assembling speakers clusters are common approaches. These approaches correspond to specific applications: the constant soundfield of a queue line is obtained by multiplying the number of speakers, while in a show, the sound must come from the source (this aspect is so obvious that it is sometimes forgotten).

Directivity control
So controlling how sounds propagate "spread" is critical for professional speakers. However, controlling what we call directivity is tricky and we must be prepared for serious concessions. Without going into abstruse explanations, we must keep in mind that we cannot control the directional characteristics of wavelengths which are smaller than the object emitting them. For example, the wavelength of a 100Hz sound is 3.40 m, which results in a humongous enclosure should we wish to have some control of the dispersion of frequencies that low ? Even when large sound objects are used such as columns or speaker clusters, they are mostly composed of individual elements whose directivity tends to increase with frequency.

Several speakers working next to each other generate interferences. From a certain frequency which is proportional to the size of the transducer, the sound will combine and subtract successively, transforming the hopefully flat frequency response into a series of dips and peaks. This is what we call "comb filter". It has a disastrous effect on sound quality. As several transducers are required in most installations, this problem must be addressed. Without dwelling on this vast subject, there are ways to optimize spatial coverage of the speakers or groups of speakers. They rely on physical tricks, such as arranging individual transducers in arrays of certain sizes or shapes, and the use of electronic processing.

The soundtrack of an attraction is most likely to be composed music, sound effects and dialogs. Our brain decodes these elements by linking them to past memories. What is true for sounds is also used true of decoding spoken messages. Once it has enough elements, our brain recognizes words and the ideas associated with them. It is possible to force our attention to build words from a relatively small amount of data, but this process is quite tiring (a chopped cell phone conversation in a nightclub is a good example). For Western languages, it is accepted that the consonant sounds are tags that allow us to reconstruct complete words. Thus their alteration affects intelligibility. A good sound system should not alter the 'STI' (the Speech Transmission Index), which is not necessarily the case of a very musical one, especially if it is able to reproduce very low frequencies.

Choosing the right speakers for an attraction
As we have seen, professional speaker systems share little with their home counterparts. The selection process should also be different. Listening to music on a pair of professional speakers out of context can give a general idea of their overall performance, but nothing more. In our world, the result has to be heard in the real life context of an attraction. As in a kitchen, success is based on using the right components to assemble a system aligned by a seasoned professional. At Tejix we often spend hours tweaking the settings of our sound systems until they suit us perfectly. We are very proud when the realism is such that visitors do not even notice the presence of the sound system.