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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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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,

we want to aim the speaker down the audience plane towards the

first row of seats. this covers the whole audience area as evenly as

possible. (See Figure H and Figure I.)

We’ve now designed a surround sound system covering both sides

Figure F

Figure G

Figure D

Figure e

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of the audience and the rear. (See Figure J.) But, we’re not yet truly

immersed in sound. anyone who has ever stood outside in a rain

storm knows that thunder and rain comes from above and not just

the sides. So, we need speakers above the audience too. Let’s look

at how to place two rows of speakers that’ll fire downward. going

back to our pLp position, we can take an angle measurement of the

previously placed side surround speakers from the floor. if we divide

that angle measurement in half and then add 45°, we can identify

the placement of the top row of speakers. (See Figure K.) We aim

the speakers inwards towards the pLp so the two speakers cover the

horizontal audience area. (See Figure L.) in a perfect situation we’d

space the top speakers the same as the side surrounds. this is very

difficult to do in most theatres due to catwalk spacing and available

Figure h

Figure i

Figure J

Figure K

Figure L

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places to hang speakers from. a good compromise would be to place

a top speaker on every other side surround or twice the distance of

the side surround spacing.

Using these techniques, we’ve now created a sound system that

can immerse the audience from all dimensions. (See Figure M.)

in conjunction with the previously designed main proscenium

speakers the system is now capable of creating an environment that

can range from a lively jazz concert to a storm at sea. By adding this

depth to our productions, we can go beyond the fourth wall and

take the audience to the world of the performance. this intensifies

the connection between the audience and the stage and ultimately

improves the audience experience. elevating the artistry of theatre

by utilizing the power of technology will continue to push the limits

of productions into the future. n

Prediction data presented in this article was created using Meyer

Sound Mapp XT software.

Chris Moore i s a Sen ior Consu l tant wi th Theat re p ro jec ts. (www.theat repro jec ts. com) He ho lds mul t ip le degrees and indust ry ce r t i f i ca t ions in sys tem eng ineer ing , aud io-v i sua l des ign , p rogramming, and ins ta l la t ion . Chr i s i s based in Denver, CO and he can be reached at cmoore@theat repro jec ts. com.

Figure M