Designing immersive sound - Theatre Projectstheatreprojects.com/files/pdf/immersive-sound.pdf ·...
Transcript of Designing immersive sound - Theatre Projectstheatreprojects.com/files/pdf/immersive-sound.pdf ·...
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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