Video Transcript
Design & Best Practices: Part 2
9m 27s
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Ready for more? Let's get going.
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Room acoustics are critical because of the strong emphasis on dialogue intelligibility.
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Any “ambience” that the content creator wants us to hear is already mixed into the soundtrack,
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so it’s very important that the room itself not add any additional ambience to the sound.
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Acoustical treatment like draping, acoustical panels, and carpet are
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always recommended, especially along the side and rear walls.
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It’s also critical that the floor area between the front row and the screen be covered with
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some sort of carpet to minimize early reflections from the screen channel loudspeakers.
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Rooms should be acoustically neutral if at all possible. That means the room itself and the items in the room, should not brighten or darken the sound.
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The room should also provide minimal late reflections that might come from objects like
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glass walls, and metal chairs and concrete floors.
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For multipurpose rooms that aren’t intended specifically for the reproduction of cinema sound,
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consider retractable drapes, rolls of carpet, and foam or fabric covers over particularly reflective surfaces.
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Just remember, you can’t overdo acoustical treatment.
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Now, we’re not going to cover these charts in detail in this video,
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but we do want you to know they exist.
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They represent Reverberation tolerances depending on room volume and frequency band.
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And, hey, look at that! There’s the symbol that means these and can be downloaded in the Links & Downloads section.
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Noise is another major issue.
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Unintended sounds from adjacent rooms or noisy urban environments are a major concern to consider.
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For new construction there are a number of construction standards and guidelines to help keep external noise out of the room.
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Some of the critical factors include the composition of adjacent room demising walls, roof decks, and HVAC systems.
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Many distracting noises can be easily prevented such as:
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rattling glass in fire extinguisher cases, loose or noisy HVAC vents,
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rattling recessed lighting fixtures,
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doors that don’t close or seal completely,
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You get the point.
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Now let’s talk about the audio products and how they’re installed.
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In traditional cinemas, projection and audio electronics are installed in racks in a booth behind and above the theatre.
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Most soundtracks require three types of loudspeakers: Screen channels, Surrounds, and subwoofers.
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As you can see,
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Screen channel loudspeakers are located behind a perforated screen,
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subwoofers on the floor below the screen, and surrounds are mounted along the side and back walls.
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The most basic sound FORMAT is 5.1.
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In this format, there are three full-range screen channel loudspeakers behind the screen.
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Two more channels are dedicated to left and right surround arrays,
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for creating a sense of ambience related to the visual scene on screen, or for special sound effects.
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Then the “point one” channel is a dedicated, band-limited subwoofer channel.
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A seven-point-one system divides the left and right surround arrays into left side,
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left backwall, and right side, right backwall loudspeakers.
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This enables a greater sense of movement.
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7.1 is the most common format used today and any modern theatre should be designed to accommodate this format, at a minimum.
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Each type of loudspeaker has a specific purpose:
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Screen channels deliver the soundtrack’s dialogue and music.
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These are full range, multi-way systems but it’s important to note that their low frequency content does not “crossover” to the subwoofers.
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They receive three discrete channels of audio.
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Left, Center, and Right, with most dialog going to the CENTER channel
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but they all must be an identical make and model since sounds may pan across the screen and should have the same tonality.
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Subwoofers receive their own additional track sometimes referred to as LFE or Low Frequency Effects.
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The subs do not reproduce content from the main channels and are most often Band-limited from 25 to about 120 Hz.
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Surround channels provide “ambience” and special effects, like a rain storm coming in or a plane flying across the screen.
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With traditional, movie theatre-style cinema applications, the loudspeakers behind the screen
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are positioned so that the acoustic center of the Mid to high frequency section is elevated to about 5/8 to 2/3 the screen’s height.
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The Left and Right loudspeakers should be located to the outside of the projected image in its WIDEST format, which is usually “Scope” ,
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with a width to height aspect ratio of 2.39 to 1.
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Screens may use side masking to show content with a 1.85 to 1 aspect ratio, called "Flat".
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We’ll cover more on Scope and Flat formats later,
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but it’s important to note that different content is still delivered
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in one or the other of these formats, depending on the director’s preference,
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and that your installation needs to be designed to display both.
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In actual movie theatres, screen channel loudspeakers are placed permanently on some type of scaffold or platform behind the screen.
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If the application is a multi-purpose room the screen loudspeakers can be rolled into position on a mobile cart, or lowered from a fly loft above the stage.
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In large theatres with a tall fly loft, the screen itself can be flown in with the loudspeakers for temporary use.
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Of course there are cases where a perforated screen is not practical.
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In these cases, screen loudspeakers can be positioned over the screen or with the Center loudspeaker over the screen, and Left and Right to the sides.
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In either of these solid screen situations, it is critical to keep the loudspeakers as close to the screen perimeter as possible, and in the same plane as much as possible.
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In every case, it is critical that the Screen channel loudspeakers are identical models.
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In a typical room when placed properly at 5/8 to 2/3 height behind the screen,
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you’ll get the best coverage possible of the seating area without spending a lot of extra time aiming the loudspeakers.
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This ensures good localization of the sound to the correct area of the screen.
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If the floor is less steeply sloped, or even flat, some additional downward tilt will be required.
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Lets talk about screens for a minute…
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While perforated screens are the best way to achieve accurate sound-to-picture localization,
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they do create a number of issues on their own that need to be addressed.
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Most perforated screens have only about 4 to 5 percent open space, which results in high-end attenuation.
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Luckily, the amount of high-end attenuation that typically results in this process
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is directly in line with the high frequency roll-off specifications that are recommended for movie theatre screen channel loudspeakers.
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This is called the X-Curve and we’ll come back to this a bit later.
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Also, much of the sound delivered by the loudspeakers is actually reflected directly off the rear of the screen and heads straight back to the wall behind the loudspeakers.
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So it’s important to place the loudspeakers close to the screen –we recommend about 6 inches.
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And to minimize reflected sound we strongly recommend treating the rear wall behind the screen with heavy acoustical absorption.
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In addition to Rear wall treatment, let’s talk about baffle walls.
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A baffle wall is essentially a false wall into which the screen channel loudspeakers are flush-mounted and is typically the same size as the screen.
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A solid baffle wall attenuates Low Frequency reflections off the back wall, enhances Low Frequency efficiency,
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and also will minimize Mid and High Frequency reflections through the screen.
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Baffle walls are EXTREMELY important to getting good sound.
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Without a baffle wall, Low Frequency energy bends around the loudspeaker and reflects off the rear wall,
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combining with first arrival sound, which creates cancellations.
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There are plenty of descriptions of how to build an effective baffle wall, but rigid construction is the key.
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Even though acoustic treatment material is not effective below about 250 Hz,
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it does a good job of attenuating Mid and High Frequency reflections from the back side of the screen.
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When a permanent solid baffle wall is just not practical – wings or panels can be attached to the sides of each screen loudspeaker
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to achieve some of the benefits of a full baffle wall.
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OF course, the front face MUST be lined with acoustic absorption.
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A mini-baffle or “bafflette” will never be as effective as a full baffle wall, but they are definitely better than nothing.
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Let's pause right there. Click the next video when you're ready!
Lesson Description
Design & Best Practices: Part 2
9m 27s
Get tips on how to control room acoustics and learn the types of loudspeakers used in cinema and their proper placement. Cinema screens and baffle walls are also covered.
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Design & Best Practices: Part 2
9m 27s