Audio Engineering

56 Chapter 2

and the noise combined ( LSN ). Then take another reading of the noise alone (the source
having been shut off). The second reading is designated LN. Then

LS

LLSN N





10 log 10 10 1010

⎛

⎝

⎜⎜

⎜⎜⎜

⎞

⎠

⎟⎟

⎟⎟

⎟

. (2.33)

To combine the levels of uncorrelated noise signals we can also use the chart in
Figure 2.8.

Table 2.9 : Combining Pure Tones of the Same Frequency but Differing

Phase Angles

Signal 1

amplitude,

LP (dB)

Signal 1

phase, in

degrees

Signal 2

amplitude,

LP (dB)

Signal 2

phase, in

degrees

Combined

signal amplitude,

LP (dB)

(^90 0)  90 0 96.02
(^90 0)  90 10 95.99
(^90 0)  90 20 95.89
(^90 0)  90 30 95.72
(^90 0)  90 40 95.48
(^90 0)  90 50 95.17
(^90 0)  90 60 94.77
(^90 0)  90 70 94.29
(^90 0)  90 80 93.71
(^90 0)  90 90 93.01
(^90 0)  90 100 92.18
(^90 0)  90 110 91.19
(^90 0)  90 120 90.00
(^90 0)  90 130 88.54
(^90 0)  90 140 86.70
(^90 0)  90 150 84.28
(^90 0)  90 160 80.81
(^90 0)  90 170 74.83
(^90 0)  90 180