3.7 NOISE MEASUREMENT PROCEDURES

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As in any data-taking situation, the engineer should carefully define the

purpose of the experimental study before taking data (Figliola and

Beasley, 1991). The reason for making the measurements (hearing damage

considerations, community reaction to noise, reduction of machinery noise,

etc.) will determine, to a large extent, the type of instrumentation required.

It is good practice to make in initial visual and aural survey of the

environment to be studied. This preliminary survey should answer such

questions as (Beranek, 1971):

(a) What are the suspected or obvious sources of noise?

(b) What are the operating characteristics of the noise source?

(c) What is the physical size of the noise source?

(d) Does the source operate continuously or intermittently?

(e) What are the directional characteristics of the noise source?

(f) Are there any special environmental considerations?

The more familiar the engineer becomes with the problem prior to making

measurements, the more likely he will arrive at an optimum choice of measuring

instruments and obtain worthwhile data to help solve the noise problem.

After the preliminary survey has been completed, a specific strategy or

plan for the experimental program should be developed. This plan should

ensure that the acquired data meets the objectives for the test: a well-developed

plan for the experimental program eliminates many ‘‘surprises’’ and

errors of omission.

Acoustic Measurements 69

Copyright © 2003 Marcel Dekker, Inc.

During the planning stage, consideration of the accuracy level required

by the instrumentation and data acquisition procedure to meet the experimental

objective should be made. An uncertainty analysis should be used to

select an effective measurement method and instrumentation.

Portable battery-operated instruments are generally used for field measurements:

Before commencing, it is important to check the batteries in all

instruments. Sound level meters usually have a ‘‘battery check’’ function

key. Spare batteries should be included with the instruments, and the batteries

should be checked again when the instrumentation is set up in the

field.

The sound level meter should be calibrated prior to taking measurements

and the calibration checked again after taking all measurements. The

‘‘electrical noise floor’’ of the instrumentation should be checked to determine

the lower limit on the levels of signals that can be measured accurately.

This check may be accomplished by replacing the microphone with an

equivalent electrical impedance or by reference to the specifications provided

by the instrument manufacturer.

When the initial equipment check has been completed, several environmental

factors should be considered. Barometric pressure and ambient

temperature should be recorded, because some instruments are sensitive to

ambient pressure and/or ambient temperature. Some calculations require

knowledge of ambient air density, which can be determined from pressure

and temperature readings. It is good practice to measure the ambient air

humidity, because some microphones, such as condenser microphones, are

sensitive to moisture in the ambient air. If the measurements are to be taken

outdoors, a windscreen should be used on the microphone.

All microphones exhibit directivity effects at high frequencies, and the

microphone should be positioned with these effects in mind. For example, if

the response of the microphone is ‘‘flat’’ (very little change in gain as frequency

of the noise input is changed) at 08 sound incidence, then the microphone

should be positioned such that the microphone diaphragm faces the

sound source. On the other hand, if the unit has ‘‘flat’’ response at 908 sound

incidence, the microphone should be oriented with the microphone axis

perpendicular to the source direction.

The sound level meter should be positioned such that the meter and

the operator have a negligible effect on the sound field being measured.

Obviously, the operator should not stand between the noise source and

the microphone. As a rule of thumb, the meter should be placed at least

500mm (20 in) from the operator’s body or a tripod support should be used

to minimize reflections from the operator.

When making measurements outdoors, the microphone should be

located 1.20—1.50m (48–60 in) above the ground and at least 3.50m

70 Chapter 3

Copyright © 2003 Marcel Dekker, Inc.

(12 ft) from any reflecting surfaces, if possible. Indoor sound measurements

should be taken with the microphone located 1.20–1.50mabove the floor, at

least 1.00m (40 in) from walls, and 1.50m (60 in) from any windows. To

overcome the effect of standing waves in indoor measurements, one should

average at least three readings made at positions about 500mm(20 in) apart.

If at all possible, the background noise level in octave bands should be

measured with the source of noise turned off. If the background noise level

is 10dB or more below the noise level produced by the source, the error due

to neglecting background noise is less than 0.5 dB. If the background noise

level is 20dB or more below the source noise level, the error is less than

0.1 dB and background noise will have negligible effect on the noise measurement

of the source. If the difference between the measured and background

levels is less than about 3 dB, the noise level from the source alone

becomes quite difficult to measure accurately.

The noise level measurements may be ‘‘corrected’’ for background

noise by subtracting (in decibel fashion) the background from the total

noise measurement:

LрcorrectedЮ ј 10log10Ѕ10LрmeasuredЮ=10 _10LрbackgroundЮ=10_ р3-48)

The readings may also be corrected using:

LрcorrectedЮ ј LрmeasuredЮ_A_ (3-49)

The factor A_ is a function of the difference between the measured and

background noise levels. Values for this factor are given in Table 3-4.

If the background noise level is excessive, there are several approaches

that can be used to obtain better measurements of the noise generated by the

source. The measurements may be taken at a time when the background

noise is lower (at night, for example). Moving the microphone closer to the

noise source may increase the difference between the source and background

noise levels, and thereby allow more accurate measurements of the source

noise.

The following list includes the more important items of data that

should be recorded when making most acoustic measurements. Many of

the items of data are automatically recorded by some acoustic analyzers

and programmable sound level meters:

(a) Date and time of measurement.

(b) Types, models, serial numbers, and other identification for all

instruments and equipment used. This data allows one to replicate

the data with the same instrument, if needed.

Acoustic Measurements 71

Copyright © 2003 Marcel Dekker, Inc.

(c) Description of the area where measurements were made. This

data is important when writing the report on the experimental

study.

(d) Description of the noise source, including dimensions, type of

mounting, location within the room or relative to other objects,

nameplate data, speed and power ratings, etc. Auxiliary information,

such as surface area of the noise source, etc., may be determined

from this data.

(d) Description of secondary noise sources, including location, type,

dimensions, etc. This information is helpful when assessing the

accuracy of the data, effect of background noise, etc.

(f) Location of observers during the time of the measurements.

(g) Orientation of the microphone axis relative to the direction of the

source from the microphone.

(h) Barometric pressure, ambient temperature, wind speed and direction,

and relative humidity. It is much more accurate to measure

this data at the time of the experiment than to try to reconstruct

the data from weather records several days after the measurements

were taken.

(i) Measured frequency-band (1/1 or 1/3 octave data) levels at each

microphone position.

(j) Measured frequency-band (1/1 or 1.3 octave data) levels for the

background noise.

72 Chapter 3

TABLE 3-4 Background noise

correction factors:

LрcorrectedЮ ј LрmeasuredЮ _ A_

_L ј LрmeasuredЮ _ LрbackgroundЮ

_L, dB A_, dB _L, dB A_, dB

1.0 6.9 6.5 1.1

1.5 5.3 7.0 1.0

2.0 4.3 7.5 0.9

2.5 3.6 8.0 0.7

3.0 3.0 9.0 0.6

3.5 2.6 10 0.5

4.0 2.2 12 0.3

4.5 1.9 14 0.2

5.0 1.7 16 0.1

5.5 1.4 18 0.1

6.0 1.3 20 0.0

Copyright © 2003 Marcel Dekker, Inc.

(k) Results of calibration tests, and the fact that the calibration tests

were made prior to making noise measurements. This step is

important in establishing credibility of the experimental data.

With previously mentioned data at hand, one can usually make an

effective analysis of the noise problem and suggest one or more approaches

that can result in a solution of the problem.

Example 3-6. The measured overall sound pressure level around a fan is

83dB. The measured overall sound pressure level for the background (ambient)

noise in the room where the fan is located is 77 dB. Determine the

overall sound pressure level produced by the fan alone.

First, let us determine the fan sound pressure level from Eq. (3-48):

LpрcorrectedЮ ј 10log10Ѕ108:3 _107:7_

LpрcorrectedЮ ј 10log10р1:4941_108Ю ј 81:7 dB

Next, let us determine the fan sound pressure level from Eq. (3-49) and

Table 3-4. The difference between the measured and background levels is:

_L ј LpрmeasuredЮ_LpрbackgroundЮ ј 83_77 ј 6:0 dB

From Table 3-4, we find that: A_ ј 1:3 dB. The corrected fan sound pressure

level is:

LpрcorrectedЮ ј LpрmeasuredЮ_A_ ј 83_1:3 ј 81:7 dB

We do obtain the same answer by using either method.

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