Boxoffice-July.1997

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74 RfiVfifFU-F. — SPECIAL REPORT: Sound Motion IF THEY KNEW WHAT YOU WERE MISSING: Part Two WHY TODAY'S MOTION PICTURE SOUND SYSTEMS FAIL TO MAKE THE GRADE AND WHY THEY CAN'T SOUND BETTER picture sound is critically important in setting the emotional atmosphere of a Him. Sauna alone is often the determining factor in an audience knowing W a scene is a comedy or tragedy. Obviously, Hie better the theatre's sound presentation, the better the film works. If sound is, as some insist, fully half the show, how can exhibitors best exploit its potential? BOXOFFICE contributing writer John F. Allen continues his important series of articles exploring the current state of movie theatre sound and providing some valuable auidance. Part one vâs published in the April 1997 issue of BOXOFFICE/ parts three and four will follow later this year. Part one of this series was devoted to a detailed analysis of the current state of the sound systems in today's movie theatres and some of the many complex reasons that preclude them from being able to reproduce digital soimdtracks. Much of the article covered speakers used behind motion picture screens and explained why the majority of these loudspeakers are simply too small. Although all the visual action takes place on the screen, the sound system is installed throughout the theatre. The all-important surround speakers are often overlooked as one of the most vital components of the experience that attending a theatre is suppxjsed to be. In this installment, I shall explain why the surrounds usually fail to deliver what they should, as well as discuss subwooferand power amplifier deficiencies that audiences are also to likely to endure. WHAT IS BAD SOUND? Perhaps 1 should once again state what 1 believe should qualify as "bad sound." Understandable dialogue is cef: tainly essential for telling a story, but it is hardly enough. Any sound reproduction that is unnatural or colored in tone, or is lacking in fullness or dynamic range, or that is unfaithful to the recording in any way, should not be regarded as acceptable. Pre,sentations often encountered in which music or background sounds are ttxj loud, too dull or altogether inaudible should not be regarded as acceptable. Phony, distorted and inadequate bass is widely prevalent in this industry. (Just think of all the subwoofers that have failed.) Finally, it must be undersiotxl that the typical "honky" dialogue sound so prominent in movie theatres for so many years is bad sound. Voices simply don't sound that way unless someone is speaking through a hose. SURROUND SPEAKERS It's hard to imagine a mistake one could make in designing or installing a surround array or sunound speakei^ that is not commonplace in today's movie theatres. Many—âll too many surround speakers have required replacement, sometimes repeatedly, over the years. It would almost seem that we are now quite accustomed There ought to be no debate that a safety margin of some amount is mandatory. But there is debate as to how much the safety margin should be. to hearing of such failures with virtually every opening of a new action picture, particularly those with a digital stereo track. Surround speakers serve a critical role by bringing the audience into the film. Surrounds create the atmosphere. In addition, films like "Twister" and "Independence Day" use the surrounds for some enormous sound effects. These effects can demand bass levels that exceed those pixxluced at the same lime by the subwoofers. Yet surround speakers typically fail to produce anything beyond some kind of noise during the loudest scenes. The rest of the time they hardly sound like they're on at all, even though they are supposed to be. (How often has that complaint been heard?) The sound ofthe surrounds should match the sound from the screen. Of course, the final issue that must be faced is that, due to poor placement among other things, most surround systems don' t ultimately surround anything. Listeners all too often localize to, and can even feel attacked by, the surround speaker or group of speakers to which they're closest. (See "Why Surround Systems Don't Usually Work," BOXOFFICE, March 1983, p. 68.) Returning to the theme that most theatre speakers are too small, the vast majority of surround systems installed today are also lacking in size and performance. For theatres equipped for either Dolby SR or digital, 1 would advise that surround speakers with anything smaller than a 12-inch woofer should not be considered. Beyond that, surround presentations cannot improve until far more attention is paid to speaker placement The goal should be that no locali?.ation to individual speakers or wall of speakers should occur finom any seat in the house. Due to their height restrictions, theatres with stadium seating and balconies require a far greater level of skill and understanding of coverage geometries than is generally available to this industry at this time. The usual practice of locating surround speakers in theatres by habit and mere guessing needs to go the way of the hand-cranked projector. The bottom
Tiilv. 1997 75 OR equaUzations cannot be ignored. So, when designing a sound system, one must factor in the typical amount of equalization anticipated by the speakers employed and increase the ampUfier power accordingly. SO HOW LOUD DO MOVIES GET? TOTAL LOW FREQUENCY RADIATING AREA = 16 SQ FT MAXIMUM OUTPUT AT 50 FT / 16.5 M = 121 line is this: If you're in a theatre where you can "feel surrounded" only in the center seats, the surround system in that theatre, by definition, fails to surround everyone not seated in the center. This is, of course, virtually the entire audience for every show, every day, week after week. Surround speakers of sufficient size, output and quality, properly placed for zero localization, can and do significantly—almost magically—enhance the motion picture experience for all films, large and small. SUBWOOFERS As noted earUer, the number of subwoofer failures this industry has experienced is very high indeed. Frankly, there is no excuse for this. If a sound system is properly designed and equipped, its loudspeakers should never fail. I repeat, never. As with both the screen and surround speakers, most theatres are presently outfitted with terribly inadequate subwoofer systems—how inadequate depends on the theatre. We do know that subwoofers are required to deliver peak levels of at least 1 1 5 dB sound pressure level (SPL) in the center of a theatre. To do this in, say, a 100-foot-long theatre, the subwoofer system must be able to deliver 139 dB SPL at 1 meter, according to the inverse square law and actual measurements done in a theatre. Adding a 6 dB safety maiîn is a good design practice. This protects the drivers from damaging overloads and also allows for realworld signal conditions that wiU actually push the subwoofer higher than the theoretical peak levels. This brings the total output requirement at one meter to 145 dB SPL in a theatre 1(X) feet long. Subwoofer ampUfiers must also be equipped with cUpping protection or limiting for those occasions when even this isn't enough. BeUeve me, it does happen. Because typical subwoofers quit at around 124 dB, theatres need more than one or two to do the job. In fact, a 500-seat, 100-foot-long theatre requires 10 standard subwoofers or two high-efficiency subwoofers. Either approach will provide the power output required. The radiating area of 10 standard subwoofers is 16 square feet vs. more than 20 for the two highefficiency units, but they are at least closer. Figure 1 TOTAL LOW FREQUENCY RADIATING AREA = 20.5 SQ dB SPL (See Figure 1, above.) If one is using direct radiator subwoofers, systems with 18-inch drivers would make a better choice. Most theatres are equipped with only one or two standard subwoofers if they have them at all, so the shortfall in subwoofer power in these theatres is truly severe. (See "Digital Subwoofers," BOXOinCE, June 1995, p. 48.) If a sound system is properly designed and equipped, its loudspeakers should never fail. I repeat again: Never! AMPLIFIERS Another of the more important factors contributing to the lack of performance in motion picture theatre sound systems is the pwwer ampUfiers. As with most of the installed loudspeakers, the amplifiers often found driving them are too small for their task. Determining the power required for any channel in any sound system is a very straightforward exercise. Using the speaker's one watt, one meter sensitivity, the maximum peak SPL of the program material involved and the distance at which that level must be deUvered, we can quickly calculate the power in watts required. We then add 6 dB, or multiply the watts by four; this adds a safety margin so that the ampUfiers never run out of power and cUp. This is critically important in assuring a long Ufe for the speakers as well as, obviously, keeping the sound clean. There ought to be no debate that a safety margin of some amount is mandatory. But there is debate as to how much that safety margin should be. Some have said it can be as Uttle as 3 dB, which is twice the power demanded by the peak levels. In the real world, however, this is not enough, for at least two good reasons. The first is that, due to its complex nature, real program material can frequently use more power than theoretical calculations indicate. Second, and just as important, is that we are dealing with equalized sound systems. If the equaUzation requires the bass control to be turned upjust 3 dB, as is often the case, that 3 dB immediately doubles the amplifier power required. Some common surround speakers need a 6 dB bass boost. These Peak levels for digital motion picture soundtracks are 105 dB SPL for each screen channel, 102 dB SPL for each surround channel and, as stated earUer, II 5 dB SPL for the subwoofer channel. Adding 6 dB brings the design critaia up to 111 dB SPL, 108 dB SPL and 121 dB SPLrespectively, as measured in the center of the theatre. Sound systems built to these specifications will not clip and can be considered reUable. Unfortunately, these common-sense practices are seldom foUowed. As an example, one 500-seat theatre I visited advertised its sound as "state-of-the-art." In some sense, I suppose, you could say it was, as it just poured out distortion with films Uke "Twister" and "Independence Day." Doing the math, I calculated that the cUpping levels for this sound system were 102 dB SPL for the screen channels, 99 dB SPL for the surround channels and 103 dB SPL for the subwoofer. At a minimum, this meant that the screen and surround channels were 50 percent too smaU. The subwoofer channel was a whopping 94 percent too smaU. If one were to include a 6 dB safety margin, these shortfalls become four times worse. Clearly, it is an appalling situation for this industry when such a system can be sold as a quaUty digital-ready package. One reason this continues to happen has been a reluctance on the part of some to own up to the real power demands of digital stereo—even though anyone with a calculator could figure it out for themselves. On separate occasions, two manufacturers have stated to me their feeUng that if they told the truth "no one would buy" their processors. I strongly disagree. The industry is far better served by the most honest and forthcoming advice. Without it, everyone on both sides of the boxoffice loses out. The following text is offered as a readyto-use amplifier power specification for exhibitors and odiers responsible for instaUing motion picture sound systems. And, pubhshed for the first time anywhere. Tables 1, 2 and 3 provide a quick reference for the minimum amplifier fX)wer requirements for cinema sound systems. AMPLIFIER POWER SPECIFICATION The power ampUfier requirements in [name of theatre circuit] 's sound systems shall be calculated solely using the inverse square law. Tests have shown that, in the center of the theatre, the peak sound pressure level (SPL) for a single screen channel is 105 dB SPL, 102
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