| Speaker of the House |
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| Mar 01, 2007 |
; "Image") Choosing speaker systems for theatre can be a daunting task. As a designer, one would hope that design intent would dictate equipment choice, but often we are limited by available equipment, budget or both. Understanding the concepts behind speaker system designs can go a long way in assisting the designer in choosing individual components wisely.
For purposes of this discussion, it is important to make a distinction between individual types of speakers and how those individual elements come together to form a speaker system. Speakers are like Legos™, they come in all sorts of shapes and sizes and can be combined in many different ways.Speaker Types There are two main types of speaker cabinet. One type is your standard speaker cabinet, which encompasses the majority of products (including trapezoidal cabinets), and the other is line array. Standard and trapezoidal cabinets can be used as individual cabinets or in conjunction with other like cabinets to form horizontal, vertical or two-dimensional arrays. Line arrays are meant to be used in a multiple cabinet vertical array configuration. The advantages to standard trapezoidal products are their ability to be used as stand alone or arrayed devices, in both short and long throw applications, while line arrays tend to be a more efficient long throw device. Standard and Trapezoidal Cabinets Trapezoidal cabinets can be used as individual elements in a sound system or combined with other like cabinets to form an array. Speaker manufacturers express the coverage of speakers in degrees. That relates to the average high frequency dispersion of the cabinet. Low frequencies are generally less directional, whereas the high frequencies can be more easily controlled. Speakers with greater horizontal dispersion are better suited to short throw applications (the sound doesn’t have to travel very far to reach a listener and still be intelligible), whereas speakers with narrower dispersion are designed for longer throw applications (the sound will have to travel a longer distance to reach the listener). Manufacturers will often have the same cabinet available in both short and long throw configurations. Line Arrays Line arrays are designed to be used in a vertically stacked arrangement. Line arrays and trapezoidal cabinets are very different in terms of the design of components, especially the high frequency horns. Where the vertical dispersion of a trapezoidal cabinet might range from 30o to 90o, the vertical dispersion of a single line array cabinet is only about 5o. The idea is that the cabinets couple with each other to produce a more efficient array, and the vertical dispersion of the array is determined by the number of cabinets in the array and the angles between them. Line array is not new, but has experienced a resurgence of interest lately. Line arrays are often marketed as the solution for any problem; however, they are better suited to long throw applications, or applications where vertical control takes precedence over horizontal control. A line array is functionally a single speaker whose horizontal coverage is fixed and whose vertical coverage is based on the shape of the array. Line arrays tend to be more difficult to use due to the complexity of the way the individual elements couple with each other based on the angles between the cabinets. Sophisticated tools are required to predict the coverage of a line array system. SYSTEM TYPES If one has limited equipment, time, budget or other resources, one of the best things that can be done is to keep it simple. Placing the speakers in the theatre for maximum audience coverage and minimum spill on the stage, then presenting a mono signal through the system, will often create the best audience experience. Small amounts of panning can go a long way. We’ll call this mono-plus. For example, a little gentle panning of chorus vocals under a lead vocal can really help with the clarity of the lead vocal. The panning should be gentle, keeping in mind that the panning in this situation will vary the mix for each different person in the room and in different ways. Stereo System A stereo system places the speakers on the left and right of the proscenium opening. In the Audio Dictionary, by Glenn D. White, stereo is defined as “…a sound system that provides the listener with an illusion of directional realism…” As far back as 1933, Bell Labs was doing experiments using multiple speaker systems to try and achieve some level of directional realism. The result of these experiments and subsequent patents on the subject was this: to achieve some degree of directional realism, signals from overlapping sound systems must reach the listeners’ ears at roughly the same level and roughly the same time. Altering the level or arrival time of the signals has the effect of artificially shifting the perceived location of the sound. As it turns out, attempting to present a stereo image in a large space is more challenging than one might think. LCR System An LCR system consists of speakers placed left and right of the proscenium with an additional speaker or speakers centered above the proscenium. The LCR system has a lot in common with a stereo system. Sometimes a system with speakers to the left, right and a center cluster is not an LCR system and the three clusters are being used to merely obtain basic coverage for the room. Sometimes the left and right clusters are used together to obtain even room coverage, and the center by itself has whole room coverage. A true LCR system will be designed so that, individually, each of the three clusters of speakers are capable of covering the entire room by themselves. When this is the case, a properly designed and tuned LCR system can be a fantastic tool for the designer. Coupled with a console that offers LCR panning or some clever bussing on a console that doesn’t, sound can be reasonably placed throughout the three speaker systems. Check out the side bar on LCR panning on page 52 for more info. Perhaps the greatest advantage for a theatre is the center cluster. Having the ability to place a speaking part or lead vocal in the center does wonders for clarity if the supporting music, underscore and other elements are placed to the sides. This only works if each of the three clusters cover the entire room, keeping in mind the rules of time and level. If the system cannot be put together to be a true LCR System, mono may be a better way to go. A/B Systems A/B systems are potentially the most complex, most expensive and time-consuming to set up and tune. An A/B system refers to a system that is essentially two speaker systems placed right next to each other. The most common is to do an LCR or some semblance of an LCR system, and double it. For every speaker coverage area, two speakers are placed to cover the same area. Different signals can be routed to each speaker; rather than mixing those sounds electronically, the sounds are mixed acoustically. It’s a lot of trouble, but there is a point. If the production is using individually miked actors working in close proximity to each other, mixing the signals electronically can produce the unwanted side effect of phasing. The phasing that we hear when two signals are electrically combined is created when one signal gets electrically summed with the same signal, but slightly out of time. Electrically, the two signals are correlated with each other, just out of time with each other. Sending each signal to its own speaker effectively de-correlates the signals, and that de-correlation makes it more difficult to hear the phase problems. A/B systems can also be used to add clarity and definition to a production by using one system for the band or orchestra and the other for the actors or singers. Each P.A. can be tuned differently to accommodate the material it supports. Extended Systems Front Fills The front few rows of seats are usually quite close to the stage. The first row of coverage from the main P.A. could be a few rows behind the front row. An audience member sitting front and center could be looking at an actor a few feet away and hearing that actor’s voice come from 20 feet to the left or right. To combat the problem many designers will install some small speakers on the front edge of the stage to fill in image for the few seats that are missed by the main P.A. Sometimes just a little bit of reinforcement is needed down in front to improve intelligibility and image. Under Balcony Fills Often in theatres with balconies, the P.A. can’t punch all the way to the rear of the sections under the balcony overhang. Also, if a center cluster is used above the proscenium, these speakers can be completely obscured by the balcony. In these situations many designers choose to hang some small speaker cabinets from the ceiling at the front edge of the balcony. These speakers are generally used to reinforce any signals being sent to any of the main speaker positions. They can also be time delayed to make the arrival times of the signals line up. A theatre with a balcony is like dealing with three separate rooms. There is the space in front of the balcony, the space under the balcony and the balcony itself. Compromises usually have to be made when it comes to spaces like this, sacrificing directional realism in favor of even coverage. FX Speakers (Practicals) These can fun. They involve placing speakers on or in the set to provide focus for sounds that should emanate from the performance space. In smaller spaces one can get away with using well placed speakers on the stage for sound the set should make. In larger spaces it is a good idea to supplement onstage FX speakers by folding some of the FX into the main P.A., or perhaps just some parts of the main P.A. This helps avoid having the onstage speaker at unrealistic or ridiculous levels, while helping to maintain the focus of the sound coming from the stage space. Care should be taken when placing special effects speakers in the audience for the same reasons. Surround speakers can also fall into the category of FX speakers. Subswoofers Subwoofers are specialized speaker cabinets designed to produce only low frequency information. They can be used to supplement or extend the low frequency response of a system. They can be part of the main P.A., or they can be used as FX speakers for extra lowend punch on specific sound effects. There is a lot to consider when discussing speakers, types and placement. Having an understanding of the options and concepts can give the designer a real head start toward efficient use of resources. Jason Pritchard is head of audio for Cirque du Soleil’s production of LOVE at the Mirage in Las Vegas. KNOWING WHEN TO INVEST IN NEW SPEAKERS The useful life of an un-abused speaker is actually quite long. Speaker drivers, in and of themselves, haven’t changed much in the past 20 years. What has changed is the way we process signals before they get amplified and sent to the speakers. Powerful digital processing now allows for correction of anomalies in speaker performance that were not correctable a few years ago. Power amplifier technology has also advanced, and many newer speakers have custom processing and amplification built-in. The primary things to look at when considering a new speaker purchase are: • Age – Sometimes they have just reached the end of their useful life. • Function – With the selection of form factors and options available, perhaps there is a product that simply wasn’t available a few years ago that will now solve a specific problem. • If you’re looking to replace a “bad sounding” speaker system, consider that maybe the speakers themselves are OK, and the design or implementation is not quite right. The speakers are part of a much larger entity that is a sound system. Sometimes a simple wiring mistake is hampering the true performance of the system. LCR PANNING Some mixing consoles offer LCR panning as an option. A standard stereo pan pot smoothly moves a signal between two speakers. There are a couple of different types of LCR pan pots. The first pans through all three speakers in turn. With the pan pot turned hard left the sound is panned hard left; when the pan pot is center the signal is routed only to the center speaker; and when panned right the sound is routed only to the right speaker. This type is best used with a true LCR system. The second method begins the same, with a hard left pan being routed only to the left, a center position would route the signal to all three speakers proportionally and hard right routes to only the right speaker. This method is better suited to a system that uses all three speakers to obtain whole room coverage. Different manufacturers have variations on this, so be sure to read about how your particular equipment handles the pan pot. Don't get Stage Directions? Click here to subscribe now!
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