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Ad info:http:// foh.hotims.com 46 Theory and Practice Generators: Size Counts By When forced with the option to use portable generators for live production, there are aspects that most sound engineers do not understand. While install power connections are reasonably lightly loaded for the audio watts provided, the nature of the current demands will vary with the music intensity of the performance and needs to be considered with portable power. Yes, there are stories of success using small generators designed for the RV and household emergency power markets; these scenarios are very conscious of the limitations and the gear is minimal to do the gig. A typical rock ‘n’ roll show, even with local talent production standards and modest audience, can draw tens of thousands of “peak” watts during the course of the event. No gennycart I know of would survive that kind of show; and if the constant breaker tripping doesn’t get you, the possibility of equipment damage due to under voltage and under frequency is likely. Understanding the Load T P So, how do you size a generator? Well, a fair and conservative assessment of the power “loading” needs to be done. Audio equipment like mixing consoles, signal processing and wireless equipment are the easy items because they mostly draw consistent current at 120 volts AC. But audio power amplifiers are where things get difficult. The good news is that the higher frequency audio signals are filtered by the power supplies within the amplifiers and just the amplitudes of the signals are brought back as power supply demand. So, the amplifiers will draw based on the loudness demands throughout the show. For instance, subwoofer power amplifiers will draw power in impulses as each kick drum hit occurs and bass guitar note attack sounds. The power distribution will see an ampere or two continuously with 10 ampere or more impulse draws to replenish in the internal amplifier power supply as the source material demands. The midrange and high frequency power amplifiers also have the same crescendo demands, but typically the impulses are wider and less intense. Given all the amplification with each amp’s internal power supply needing replenishment with the source audio, the power distribution and portable generator peak/average demands vary a lot. Genny Design T P Generator designers are not dumb engineers, and they do understand that some load tasks will require a lot of peak current demand with lower average demand. To accomplish the provision for peaks, the electric AC generator transfers the electrical demand back as resistance to shaft rotation of the generator’s armature. To counter this variable mechanical problem, a beefy “flywheel” is NOVEMBER 2008 www.fohonline.com employed. The flywheel stores rotational energy and keeps the generator shaft moving at constant speed (nearly) as the generator varies its torque demands. There are two ways to deal with kind of loading. One is to design AC generator systems for “show power,” in which variable loading is a given and the sufficiently oversized flywheel is provided to ensure the voltage and frequency are stable for the event. The other way is to just choose a larger-than-required AC generator to oversize the flywheel and everything else. A portable generator also uses a diesel or gas engine as the energy source. But just like driving a vehicle using the accelerator pedal, engine response can be hundreds of milliseconds, which is slow compared to the load demands. That is why the flywheel effect is essential to power regulation while waiting for the engine to sense shaft speed slowing and to add fuel to speed up back to normal. To be honest, the best practice on genny selection is to do both items when specifying a generator for a gig. Ask for both a “showpower” generator, and more power capacity than is required to produce the show. Besides, you may get stage lighting and concession loading if the promoter is not aware. Practice T P In practice, a good rule of thumb is to compute your audio loading at MarkAmundson maximum capability and then double that value. For example, my rig offers about 10 kW at <strong>FOH</strong> and a similar value to the stage monitors, but I will specify a 45 kW to 50 kW showpower generator or more for outdoor gigs. Not only do most showpower generators have extra engine muffling for quiet backstage operation, but the design is conservative for live audio work. Stage lighting can work with you or against you if tacked on to the same generator. Today’s actively lit shows routinely go from very dim to very intense lighting (e.g. audience blinders) to accent the performance. If you are on the edge on either generator capacity or using a generator with not much ability to keep voltage or frequency stable, then adding some constant stage wash lighting will help lower the peak loading with respect to average loading. My advice is to always error on the side of plenty of genny power. STAGING • LIGHTING • SOUND Your #1 Source for continuing education. Exploring Sound Reinforcement A practical guide to understanding PA systems, applications and operation, from Yamaha Corporation of America. Exploring Sound Reinforcement is a major work produced by Keyfax NewMedia in conjunction with Yamaha Corporation of America that looks at all aspects of buying, assembling, and operating sound reinforcement systems. www.fohbookshelf.com
By DanDaley Scientific protocol calls for lots of testing before a product is finally assembled. However, in the highly subjective and opinionated realm of loudspeakers, testing has become not so much an afterthought as an afterthe-fact proposition in some instances. The transducer and cabinet assembly of each component in a line array are subjected to plenty of testing as each element is developed and connected, but the dynamic and incrementally minute nature of the line array itself tends to get its final checks in situ, flying above the crowd. Routine Systems Testing BIZ JBL has historically been a major proponent of systems testing — during the development of the VerTec line array system at the turn-of-the-century rigs were routinely set up in the parking lot and on the roof to check ground planes and other splays, and in one instance, an entire system was set up in an airplane hangar at Van Nuys airport near the Harman Industries’ headquarters. “This enabled JBL engineers to validate correlations of predictive software tools with actual measurements of large-scale arrays, which is especially important with line-array type system formats,” recalls David Scheirman, JBL’s vice president of tour sound. “The issue with very large sound systems is that the bigger they get, the farther away you have to measure them from and it can reach the point where it becomes less and less practical to test physically,” comments Paul Bauman, JBL’s director of tour sound engineering. Tips ‘n’ Tricks Combating Feedback Even though monitor placement is simple, it is very important. Where you place your monitors can affect the perceived tone and be an important tool in combating feedback. If you ask your worship band players or singer where they want their monitors placed you may find that they have some very specific ideas about placement. I like to have my wedge set at about 10 o'clock to my left. I play 12-string acoustic guitar at my church and if I place the monitor directly in front of me, there is a good chance my guitar will feed back. At 10 o'clock, I can hear the wedge and I am off axis for resonance-induced feedback from my guitar. — Jamie Rio from his “Sound Sanctuary” column in the October 2008 issue. It’s a pragmatic and logistical problem, but one rooted in ideology as much as science. “Back in the early days of EASE there was a debate over angular resolution,” says Bauman. “The analogy to that today is whether you can accurately extrapolate far-field data from near field measurements.” A Small and Interesting Niche BIZ This situation, along with the proliferation of line arrays being used in applications ranging from massive arenas to small clubs, has created a small and interesting niche, one that more and more systems designers and component manufacturers are coming to rely on. In the 1960s and ‘70s, Ron Sauro used to work at NASA, measuring trajectories that would help put Voyager, Pioneer and Apollo spacecraft into space. One of the human manifestations of the peace dividend, Sauro now parses data about speakers in his lab in Santa Clarita, Calif., where his company NWAA Labs tries to bring some order to a combination of tricky measurements and marketing-speak. “It used to be that you could measure key aspects, like magnitude and phase, of a speaker in a system fairly easily when most AP systems took the form of clusters,” he says. “Once the line array became popular, it was slowly realized that it couldn’t be measured accurately the same way. With a cluster, you could take all of the far-field information and trace it back to a single point in space. But with line arrays, where the point sources are multiple and highly variable, you have to measure more and from farther away to create a meaningful set of data.” www.fohonline.com NWAA accomplishes this with its MACH (Multi-Angle Computerized High- Speed) testing system, which uses a proprietary array comprised of 19 matched Earthworks M-30 microphones and a Linear X precision turntable capable of 0.1-degree accuracy. It simultaneously samples impulse responses within an anechoic chamber and uses these to create loudspeaker radiation “balloons” (a graph of a sound radiation pattern) in a fraction of the time used by other testing methods. This data is gathered by a multi-channel version of EASERA and is then converted to EASE V4, EASE V3, CLF 1 and 2 formats for transmission to the client or user. If you were to apply the older methodology of taking 2,500 [impulse response] measurements to a line array, it would take you a week,” he says. Using MACH, he says they can do it in less than an hour, yet still measuring for every possible configuration of LF, MF and HF boxes at any and all possible angles, by rotating the box around the axis of the driver. Cutting Through the Clutter BIZ While some of his clients are systems designers seeking to cut through the clutter of marketing materials masquerading as spec sheets, the majority are actually the speaker makers themselves, including JBL, Meyer and EAW, seeking to provide their customers a more valid way to compare performance characteristics before a system is hung. What becomes an enormous amount of measurement data is codified into a program called Generic Listening Library (GLL), that seeks to document and ar- The Biz Testing, Testing… chive performance records of all types of speakers. The message, though, should be less about the data than how it is used, says Pat Brown, president of Synergetic Audio Concepts (Syn-Aud-Con) in Greenville, Ind., and its testing division, Electro- Acoustic Testing Co., one of a handful of independent testing facilities in the sector. “There is good agreement on the required data set for simple loudspeakers with modeling as the stated objective, [but]… it is more important that the data be appropriate for the device being tested,” Brown cautions. “Understanding loudspeaker data is not trivial, and comparing different loudspeakers with a few ‘one number’ metrics such as sensitivity or power handling can be completely meaningless. Unfortunately, this is often how the buying decision is made, and this fact can affect how loudspeaker specifications are determined and published. Collecting the data is a much easier task than conveying to the end user what it means and how to use it in a meaningful way.” Testing at this level is likely to remain a niche, albeit an intense one. “There’s something to be said for having an independent, impartial laboratory do these kinds of measurements,” says Bauman. “It’s reassuring to clients like sound designers who deal with issues like allocating the resources and time needed to do it right, which is often not practical even for large manufacturers anymore.” Dan Daley can be reached at ddaley@ fohonline.com 2008 NOVEMBER 47 Ad info:http:// foh.hotims.com
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