3.1 SOUNDLEVEL METERS

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The basic parts of most sound level meters include a microphone, amplifiers,

weighting networks, and a display indicating decibels. Typical sound level

meters are shown in Figs 3-1 and 3-2. The microphone acts to convert the

input acoustic signal (acoustic pressure) into an electrical signal (usually

voltage). This signal is magnified as it passes through the electronic pream-

42 Chapter 3

FIGURE 3-1 Sound level meter. This sound level meter provides manual operation

and storage of results at the end of each run. The output is displayed on a screen on

the meter, or the data can be downloaded to a PC. (By permission of Casella CEL

Instruments Ltd.)

Copyright © 2003 Marcel Dekker, Inc.

plifier. The amplified signal may then be modified by the weighting network

to obtain the A-, B-, or C-weighted signal. This signal is digitized to drive

the display meter, where the output is indicated in decibels. The display

setting may be ‘‘fast’’ response, ‘‘slow’’ response, ‘‘impact’’ response, or

‘‘peak’’ response. Unless one is interested in measuring rapid noise fluctuations,

the ‘‘slow’’ response setting is usually used. An output jack may be

provided to record or analyze the signal in an external instrument system.

Sound level meters are rated in the following categories, based on the

accuracy of the meter: (a) type 1, precision; (b) type 2, general-purpose; (c)

type 3, survey; and (d) special-purpose sound level meters. The type 1 or

Acoustic Measurements 43

FIGURE 3-2 Sound level meter. This meter is a handheld modular precision sound

analyzer system. (By permission of BruЁ el and Kjaer.)

Copyright © 2003 Marcel Dekker, Inc.

type 2 sound level meter is required for OSHA noise surveys, and is specified

in most community noise ordinances.

There are several items of auxiliary equipment that are used with

sound level meters, including a calibrator and a windscreen. Many sound

level meters have output ports for connection to a PC for post-processing of

data.

The calibrator, shown in Fig. 3-3, is a portable, battery-operated

instrument that is used to calibrate the sound level meter. The microphone

on the sound level meter is inserted into one end of the calibrator, and the

calibrator generates a pure tone at a frequency of 1 kHz and a known level

(such as 114 dB). The reading of the sound level meter is compared with the

known output of the calibrator, and the sound level meter is adjusted to

match the calibrator.

There are two general types of calibrators for sound level meters: the

loudspeaker type and the pistonphone type. The loudspeaker type contains

a small loudspeaker that produces known sound pressure levels at several

frequencies, such as 125 Hz, 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz. The

pistonphone calibrator consists of an air cavity, in which the microphone

is placed at one end and cam-driven pistons are located at the other end. The

oscillation of the pistons changes the volume of the cavity and produces a

known variation of the instantaneous acoustic pressure in the air cavity at

the microphone diaphragm. The pistonphone usually provides calibration at

one frequency, such as 250 Hz, for example.

44 Chapter 3

FIGURE 3-3 Acoustic calibrator. (By permission of Casella CEL Instruments Ltd.)

Copyright © 2003 Marcel Dekker, Inc.

A windscreen should always be used when making sound level measurements

outdoors. The windscreen consists of a spherical piece of opencell

foam material that can be fitted over the microphone of the sound level

meter, as shown in Fig. 3-4. The windscreen minimizes the effect of wind

turbulence over the microphone. Generally, sound level measurements are

not effective when the wind speed exceeds approximately 12 mph or 19 m/s

(SkoЁ de, 1966). For acoustic measurements in ventilation ducts where the

direction of the flow of air is constant, nose cones are usually attached to the

microphone to alleviate the wind noise effect.

A precision sound level meter may include an impulse network or

software package to measure impulsive sounds, or sounds in which the

pressure level rises rapidly for short periods of time. The impulse feature

has an output of maximum rms sound level or the maximum peak level (or

both) for the impulsive sound. The readings may be denoted by dBA(I) or

dBC(I), depending upon the weighting network used with the impulse

feature.

Acoustic Measurements 45

FIGURE 3-4 Sound level meter with windscreen in place on the microphone. (By

permission of BruЁ el and Kjaer.)

Copyright © 2003 Marcel Dekker, Inc.

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