Audio Engineering

52 Chapter 2

certain period of time such as 90 dBA for 3.6 s (this could be a series of levels that lasted
seconds, hours, or even days). We can then calculate the decibel level of steady noise for,
say, 1 h that would be the equivalent level of the dBA for 3.6 s. That is, we wish to fi nd
the energy equivalent level for 1 h:

L

P

P

EQ Adt

o

 10

1

3600

2

2

0

36

log

.

s

in decibels

s

∫

⎛

⎝

⎜⎜

⎜⎜

⎞

⎠

⎟⎟

⎟⎟

⎟

(2.28)

where PA is the acoustic pressure, Po is the reference acoustic pressure, and 3600 s is the
averaging time interval.

50 hp siren (100 ft)

Jet takeoff (200 ft)

Riveting machine*

Cut-off saw*

Pneumatic hammer*

Textile weaving plant*

Subway train (20 ft)

Pneumatic drill (50 ft)

Freight train (100 ft)

Vacuum cleaner (10 ft)

Speech (1 ft)

140

130

120

110

100

90

80

70

60

50

40

30

20

10

0

Large transformer (200 ft)

Soft whisper (5 ft)

At a given distance

from the source

Decibels

re: 20 μN/m^2 Environmental

Casting shakeout area

Electric furnace area

Boiler room

Printing press plant

Tabulating room

Inside sport car (50 mph)

Near freeway (auto traffic)

Large store

Accounting office

Private business office

Light traffic (100 ft)

Average residence

Minimun levels – residential

Areas in Chicago at night

Studio (speech)

Studio (sound recording)

Threshold of hearing

*operator position youths – 1000 to 4000 Hz

Figure 2.6 : Typical A-weighted sound levels as measured with a sound level meter.

(Courtesy of GenRad.)