Audio Engineering

54 Chapter 2

LDN (day–night level), CNEL (community noise level), and so on all follow similar
schemes with variation in weightings for differing times of day, etc.

It is of interest that shooting a 0.458 magnum 174.7 LP (peak) for 2.5 ms translates into

LEQ 



10

10 0 0025

3600

113 12

log

174.7 10

s

s

dB

⎛

⎝

⎜⎜

⎜⎜⎜

⎞

⎠

⎟⎟

⎟⎟

⎟

of steady sound for 1 h. OSHA allows only 15 min of exposure to levels of 110–115 dBA.
As Howard Ruark’s African guide, Harry Selby, remarked after Ruark had accidentally
set off both barrels at once of a 0.470 express rifl e while being charged by a Cape buffalo,
“ One of you ought to get up. ”

2.13 Combining Decibels ................................................................................................

2.13.1 Adding Decibel Levels

The sum of two or more levels expressed in dB may be found as follows:

LT

LL LN

 10 10 10 10

12

log^1010 K^10.

⎛

⎝

⎜⎜

⎜⎜⎜

⎞

⎠

⎟⎟

⎟⎟

⎟

(2.29)

If, for example, we have a noisy piece of machinery with an LP  90 dB and wish to turn
on a second machine with anLP  90 dB, we need to know the combined LP. Because both
measured levels are the result of the power being applied to the machine, with some percentage
being converted into acoustic power, we can determine LT by using Eq. (2-33). Therefore

LT 



   



10 10 10

10 10 10

10 2 10

93

(^90109010)
99
9
log
log
log

()

()

()

dB.

Doubling the acoustic power results in a 3 dB increase.