Handbook for Sound Engineers

Loudspeaker Cluster Design 655

measures the reduction in modulation depth of a
specialized test signal which replicates the burst nature
of real speech. The STI scale ranges from 0 to 1, where
1 represents perfect intelligibility. STI is considered the
most accurate of the intelligibility measures.

18.7.3 Architecture and Room Acoustics

18.7.3.1 Reverberation

Reverberation is the persistence of sound in a space
after the original sound has been removed.

RT60 is the measure for reverberation, and it is
defined as the amount of time required for the average
sound energy density in a space to decrease from it’s
original value by 60 dB after the original sound has
stopped.

The Sabine equation relates RT60 to the volume of a
room with it surface area and the absorption coefficients
of the materials applied to the surfaces.

As room volume increases relative to surface area
and absorption coefficients, the RT60 increases.

As surface area and absorption increase relative to
room volume, RT60 decreases. It is this persistence of
sound that interferes with our comprehension of conso-
nants and contributes towards degrading intelligibility.

18.7.4 Line Arrays

Figs. 18-14 to 18-16 show the direct sound coverage of

various loudspeakers in a sanctuary 100 ft × 65 ft. The

chancel adds 20 ft to its length. The roof peaks at 52 ft.

The room volume is roughly 250,000 ft^3. The room has

plaster walls, wood ceiling, terrazzo floors, and empty

wooden pews. This produces a RT60 of about 3.5 s.

Notice the high SPL levels on the walls and ceiling in

the flown-horn array simulation. The high frequency

beaming of the mechanically tilted column array

prevents good coverage of the front of the audience

area. The digitally steerable column array covers only

the audience area and has very little coverage on the

walls and no coverage of the ceiling. Only the steered

column array has acceptable (good to fair) intelligi-

Evaluation STI %ALcons

Bad 0.20 to 0.34 24.3 to 57

Poor 0.35 to 0.50 11.3 to 24.2

Fair 0.51 to 0.64 5.1 to 11.2

Good 0.65 to 0.86 1.6 to 5.0

Excellent 0.87 to 1.00 0.0 to 1.5

Copied from The Audio System Designer Technical Reference by

Peter Mapp and published by Klark Teknik.

Table 17-1. Intelligibility Comparison Chart

RT60 < 1 s Excellent intelligibility can be achieved.

RT60 1 to 1.2 s Excellent to good intelligibility is possible.

RT60 1.2 to 1.5 s Good intelligibility can be achieved.

RT60 >1.5 s Careful system design is required.

RT60 >1.7 s Limit for good intelligibility in large spaces.

RT60 >2 s Very directional loudspeakers are required,

intelligibility can have limitations.

RT60 >2.5 s Intelligibility will probably have limitations.

RT60 >4 s Highly directional loudspeakers will be

required to achieve acceptable intelligibility.

Figure 18-14. Flown large format horn array.

Figure 18-15. Mechanically tilted four meter column array.

Figure 18-16. Digitally steered column array.