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Transcript of Designing immersive sound - Theatre Pro · PDF file Designing immersive sound systems for...

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    60 Spring 2019

    Open any trade magazine, and you can pretty much

    guarantee that you’ll find an article or advertisement about virtual

    reality, augmented reality, or immersive technologies. it’s all the

    buzz these days. the idea of immersive sound systems is nothing

    new and technologies of today have a lineage going back decades.

    the technology of surround sound for creative intent was first used

    in 1940 when disney Studios released Fantasia. in one scene of the

    film, the surround effect of a bee flying around the theatre was used.

    the sound technology developed during the creation of that film

    was aptly named Fantasound. many of the techniques and processes

    developed during the creation of Fantasound formed the base for

    all the processes, technologies, and techniques still used today.

    Since 1940 there have been countless iterations of surround and

    immersive sound systems and just as many standards to describe

    them. trying to cover all the different technologies and techniques

    that have been developed since 1940 would take a whole book to

    cover and there are already several really great books on the topic of

    spatial sound and immersive systems. We’ll be taking a more high-

    level approach on how to begin designing immersive sound systems.

    this article is a continuation of a previous article in the 2019

    Winter edition of Protocol titled “Speaker placement and techniques

    to preserve Sound image Quality.” i encourage readers to absorb the

    information in that article before continuing.

    What is the purpose of an immersive sound system? according

    to the Webster Dictionary, the definition of immersive is: providing,

    involving, or characterized by deep absorption or immersion in

    something such as an artificial environment. So simply put, we’re

    creating an artificial environment to simulate the way we experience

    sound in real environments. in the preceding article, we defined

    what sound imaging is by imagining sitting in the audience of a jazz

    club watching a trio perform acoustically with no amplification.

    Closing our eyes and using only our ears, we created a spatial

    image of the trio onstage. We could tell where the instruments were

    on stage both left and right as well as near and far in relation to

    ourselves. that is where we stopped and where we’ll pick up now.

    We established the sound image of the jazz trio on the stage, but

    we stopped at the stage edges. So, let’s describe the rest of the room

    around us. if the band we’re there to see is any good there will be

    other patrons around us. these people would be talking, moving

    around, clinking glassware, and generally making noise. the room

    itself may have a loud air-conditioning system that introduces a

    low-level rumble. if the club is next to a street, we may hear the

    Designing immersive sound systems for theatre By Chris Moore The elliot Galvin Trio at the Cheltenham Jazz Festival 2017

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    occasional car or truck drive by. all of these sounds immerse us in

    the environment or soundscape of the club. Our goal is to design a

    sound system that allows us to recreate that soundscape.

    Before we get to speaker placements, we should cover what i refer

    to as the sound system resolution. most everyone knows what video

    resolution is and how resolution effects the quality of the image we

    see. all you have to do is take a popular video game system from

    1987 with a resolution of 256 pixels x 240 pixels and compare it to

    a popular game system of today with a resolution of 1920 pixels x

    1080 pixels. Obviously, the more pixels in the image, the clearer the

    picture is. We can think about immersive sound systems the same

    way. if we have two speakers, we can create a stereo image. if we

    have three speakers, we can create a left, center, right system and pan

    sound between the three speakers. as we continue to add speakers,

    we give ourselves a higher resolution sound image. With this line

    of thinking, a system designer may attempt to create the ultimate

    spatial imaging sound system and line up 1,000 speakers next to

    each other, so the resolution is as high as possible. But let’s look

    at what happens when those speakers interact with each other. in

    Figure A we see one speaker at 2 kHz. We then add a second speaker

    close in proximity. We add a few more to get to five speakers. you

    can clearly see the interaction between the speakers. the lobing and

    phasing issues you see will cause degradation of the quality of the

    sound due to speaker interaction. We can truly see that this is too

    much of a good thing.

    How do we determine how many speakers we need in our system,

    and how do we arrange them so we can achieve a high resolution

    while minimizing speaker interaction? Since this is a continuation

    of the previous article, we’re not going to concentrate on the

    main speakers around the proscenium. For the purposes of this

    article, we’ll use the left, center, right (LCr) main speaker design

    we designed previously and will now concentrate on the speakers

    around the audience area. it’s important to note that many of the

    new immersive and spatial sound systems available today utilize

    multiple main speakers located around or above the stage, providing

    a higher resolution sound image for the stage beyond the classic

    stereo or LCr system designs.

    For the next design calculations, we’re going to borrow techniques

    from cinema system designers. Let’s start with a base point in the

    audience area called the primary Listening position (pLp). the pLp

    is considered the optimal listening position in a cinema and the

    point from which a lot of calculations are done. the pLp position

    is determined by finding the centerline of the room and measuring

    the distance from the downstage stage edge to the rear wall and

    then calculating 2/3rd the distance (see Figure B). an audience

    area that is 35' from the stage to the rear wall would have a pLp of

    23.31' (35 x 0.666 = 23.31) on the centerline of the room. For our

    next calculation, we draw a horizontal line across the room passing

    through the pLp from edge to edge of the audience area. We can

    note the total distance for later calculations, but right now we’re

    interested in the distance between the closest listener to the wall and

    the wall itself. We find a measurement of 4'. By taking the distance

    of 4' and calculating 120% of that distance, we can determine the

    spacing of our surround speakers. So, our spacing is 4.8' (4 x 1.2 =

    4.8). We have an acceptable variance of +/- 10% so we can round

    up to 5' spacing to make things nice and easy. (See Figure C.) We

    want to cover as much area as possible per speaker so our speaker

    horizontal coverage should be greater than 90°. the 120% rule

    works for many situations. By calculating the distance to the nearest

    listener from the surround speaker we can determine spacing for

    most seating configurations.

    Figure A

    Figure B

    Figure C

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    62 Spring 2019

    next let’s look at the mounting height of the surrounds. there are

    lots of techniques when it comes to surround mounting heights, so

    i’m going to explain a hybrid technique that borrows from several

    other approaches. there are many ways to approach this, so i would

    recommend learning several techniques and utilizing the best

    method for the situation. Our goal is to cover the audience evenly so

    a speaker with a medium vertical dispersion is recommended, 60° of

    vertical coverage or less. We need to find the minimum variance line

    in the vertical coverage. refer to Figure D for how to calculate the

    minimum variance line for a speaker. in order to achieve optimum

    coverage—or to get as close as to possible—we should begin by

    rotating the speaker downward vertically until the minimum

    variance line is parallel with the audience plane. next, we should

    raise the speaker in elevation until the centerline of the speaker is

    pointing at the 2/3rds distance of the audience length. (See Figure

    E.) Once we set the height of one speaker at the pLp position, then

    we can match the height on all other surround speakers in relation

    to the audience floor from the front to the back of the theatre. (See

    Figure F.) Looking back at our view of the theatre, we now have our

    surround speakers spaced horizontally and vertically around the

    theatre. (See Figure G.)

    Similar to the side surround speakers, we need to place rear

    surrounds extending our sound image all the way around the

    audience. ideally, we’d want the rear speakers spaced the same as

    our side surrounds and at the same height as the last side surround

    in the back of the theatre. When deciding how to aim the speakers