Design & Best Practices: Part 2

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Room acoustics are critical because of the strong emphasis on dialogue intelligibility.  

Any “ambience” that the content creator wants us to hear is already mixed into the soundtrack,  

so it’s very important that the room itself not add any additional ambience to the sound. 

Acoustical treatment like draping, acoustical panels, and carpet are  

always recommended, especially along the side and rear walls.  

It’s also critical that the floor area between the front row and the screen be covered with  

some sort of carpet to minimize early reflections from the screen channel loudspeakers.

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.

The room should also provide minimal late reflections that might come from objects like  

glass walls, and metal chairs and concrete floors.

For multipurpose rooms that aren’t intended specifically for the reproduction of cinema sound,  

consider retractable drapes, rolls of carpet, and foam or fabric covers over particularly reflective surfaces.

Just remember, you can’t overdo acoustical treatment.

Now, we’re not going to cover these charts in detail in this video,

but we do want you to know they exist.  

They represent Reverberation tolerances depending on room volume and frequency band.

And, hey, look at that! There’s the symbol that means these and can be downloaded in the Links & Downloads section.

Noise is another major issue.

Unintended sounds from adjacent rooms or noisy urban environments are a major concern to consider. 

For new construction there are a number of construction standards and guidelines to help keep external noise out of the room.  

Some of the critical factors include the composition of adjacent room demising walls, roof decks, and HVAC systems.

Many distracting noises can be easily prevented such as: 

rattling glass in fire extinguisher cases, loose or noisy HVAC vents,

rattling recessed lighting fixtures,

doors that don’t close or seal completely,

You get the point.

Now let’s talk about the audio products and how they’re installed.  

In traditional cinemas, projection and audio electronics are installed in racks in a booth behind and above the theatre.

Most soundtracks require three types of loudspeakers: Screen channels, Surrounds, and subwoofers.

As you can see,  

Screen channel loudspeakers are located behind a perforated screen,

subwoofers on the floor below the screen, and surrounds are mounted along the side and back walls. 

The most basic sound FORMAT is 5.1.  

In this format, there are three full-range screen channel loudspeakers behind the screen.  

Two more channels are dedicated to left and right surround arrays,   

for creating a sense of ambience related to the visual scene on screen, or for special sound effects. 

Then the “point one” channel is a dedicated, band-limited subwoofer channel. 

A seven-point-one system divides the left and right surround arrays into left side,

left backwall, and right side, right backwall loudspeakers.

This enables a greater sense of movement.

7.1 is the most common format used today and any modern theatre should be designed to accommodate this format, at a minimum. 

Each type of loudspeaker has a specific purpose: 

Screen channels deliver the soundtrack’s dialogue and music.  

These are full range, multi-way systems but it’s important to note that their low frequency content does not “crossover” to the subwoofers. 

They receive three discrete channels of audio.  

Left, Center, and Right, with most dialog going to the CENTER channel  

but they all must be an identical make and model since sounds may pan across the screen and should have the same tonality.

Subwoofers receive their own additional track sometimes referred to as LFE or Low Frequency Effects.

The subs do not reproduce content from the main channels and are most often Band-limited from 25 to about 120 Hz.

Surround channels provide “ambience” and special effects, like a rain storm coming in or a plane flying across the screen.

With traditional, movie theatre-style cinema applications, the loudspeakers behind the screen  

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. 

The Left and Right loudspeakers should be located to the outside of the projected image in its WIDEST format, which is usually “Scope” ,

with a width to height aspect ratio of 2.39 to 1.

Screens may use side masking to show content with a 1.85 to 1 aspect ratio, called "Flat".

We’ll cover more on Scope and Flat formats later,

but it’s important to note that different content is still delivered  

in one or the other of these formats, depending on the director’s preference,  

and that your installation needs to be designed to display both.

In actual movie theatres, screen channel loudspeakers are placed permanently on some type of scaffold or platform behind the screen.

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.

In large theatres with a tall fly loft, the screen itself can be flown in with the loudspeakers for temporary use.

Of course there are cases where a perforated screen is not practical.

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.

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.

In every case, it is critical that the Screen channel loudspeakers are identical models.

In a typical room when placed properly at 5/8 to 2/3 height behind the screen,

you’ll get the best coverage possible of the seating area without spending a lot of extra time aiming the loudspeakers.

This ensures good localization of the sound to the correct area of the screen.

If the floor is less steeply sloped, or even flat, some additional downward tilt will be required.

Lets talk about screens for a minute…

While perforated screens are the best way to achieve accurate sound-to-picture localization,

they do create a number of issues on their own that need to be addressed.

Most perforated screens have only about 4 to 5 percent open space, which results in high-end attenuation.

Luckily, the amount of high-end attenuation that typically results in this process

is directly in line with the high frequency roll-off specifications that are recommended for movie theatre screen channel loudspeakers.

This is called the X-Curve and we’ll come back to this a bit later.

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.

So it’s important to place the loudspeakers close to the screen –we recommend about 6 inches.

And to minimize reflected sound we strongly recommend treating the rear wall behind the screen with heavy acoustical absorption.

In addition to Rear wall treatment, let’s talk about baffle walls.

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.

A solid baffle wall attenuates Low Frequency reflections off the back wall, enhances Low Frequency efficiency,

and also will minimize Mid and High Frequency reflections through the screen.

Baffle walls are EXTREMELY important to getting good sound.

Without a baffle wall, Low Frequency energy bends around the loudspeaker and reflects off the rear wall,

combining with first arrival sound, which creates cancellations.

There are plenty of descriptions of how to build an effective baffle wall, but rigid construction is the key.

Even though acoustic treatment material is not effective below about 250 Hz,

it does a good job of attenuating Mid and High Frequency reflections from the back side of the screen.

When a permanent solid baffle wall is just not practical – wings or panels can be attached to the sides of each screen loudspeaker

to achieve some of the benefits of a full baffle wall.

OF course, the front face MUST be lined with acoustic absorption.

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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