Handbook for Sound Engineers

Stadiums and Outdoor Venues 205

8.5 Techniques for Achieving High Acoustic Pres-

sures

In a previous calculation it was shown that for a

200 meter path length, the source must achieve a level

of 143 dB at a distance of one meter even in the absence

of atmospheric absorption. Including compensation for

air losses, the required level can easily rise to 150 dB.

Horn throat pressure leading to this level at one meter

from the horn mouth would be significantly higher than

150 dB and would suffer from a serious amount of

nonlinear distortion. Such pressures are usually

achieved by using a coherent array of multiple devices.

Typical medium and long throw devices have coverage

angles of 40° vertical by 60° horizontal and 20° vertical

by 40° horizontal, respectively, these being the angles

between the half pressure points of the devices. The

long throw angles required in a stadium are usually

narrow in the vertical and wide in the horizontal so such

devices are stacked to form a vertical array with the

axes of the individual devices being parallel. This

arrangement for two devices is depicted in Fig. 8-5.

Consider for the moment that the devices are iden-

tical point sources that are driven in phase with equal

Figure 8-3. Absorption of sound in air at 20°C and one
atmosphere for various relative humidities.

10

6

10

5

10

4

10

3

10

2

10

1

100

101

102

Absorption coefficient–dB/m

101 102 103 104 105 106

Frequency–Hz

0% Classical

100%

10%

Figure 8-4. Absorption of sound for different frequencies

and values of relative humidity.

Figure 8-5. A vertical array of two stacked devices

100K 50K 20K 10K 5K 2K 1K 500 200 100

0.001

0.002

0.005

0.001

0.002

0.05

0.1

0.2

0.5

1.0

0.02

0.01

0.005

Frequency–Hz

Attenuation constant–dB/m

Temperature—20°C

20% RH

40% RH

60% RH

80% RH

r 1

r 2

r

d Q