Sound Power Measurements

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Sound power measurements

CHRISTER HEED

SD2165

Stockholm October 2008


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Marcus Wallenberg Laboratorietfr Ljud- och Vibrationsforskning

Sound power measurements

Christer Heed

Approved

Date:

Signature:

SD2165 Acoustical measurements


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ABSTRACT

Three different measurement methods to determine sound power level of a sound source

are compared. Two methods are sound pressure methods. The first one is a comparison

method, using a reference sound source and the second one is a direct method without

using a reference sound source. Third, the sound power level is measured with a sound

intensity method, using scanning. The comparison method yields octave band with thecentre frequency range 125 Hz - 8 kHz and A-weighted sound power level. The direct

method yields only A-weighted sound power level. The intensity method yields octave

band with the centre frequency range 125 Hz - 4 kHz and A-weighted sound power level.

Standards for all methods are used. The comparison method and the intensity method

attain engineering accuracy (grade 2). A few minor derogations are made from them. The

standard deviation should still be about 1.5 dB. The discrepancy of the results of these

two methods is less than 1 dB. The discrepancy is most significant at the 125 Hz and the

500 Hz octave bands. The direct method attain survey accuracy (grade 3) and thus the

standard deviation of the direct method is 4 dB and its determined sound power level is 1

dB higher than for the intensity method and 2 dB higher than for the comparison method.

SAMMANFATTNING (the abstract in Swedish)

Tre olika mtmetoder fr att bestmma ljudeffektnivn frn en ljudklla jmfrs. De tv

frsta r ljudtrycksmtmetoder, med jmfrandet av en referensljudklla respektive utan.

Medan den tredje r en intensitetsmetod med scanning. Intensitetsmetoden ger

ljudeffektnivn i oktavband med centerfrekvenser 125 Hz 4 kHz och den A-vgda. Fr

jmfrandemetoden fs ljudeffektnivn i oktavband med centerfrekvenser 125 Hz 8

kHz och den A-vgda. Fr den direkta ljudtrycksmetoden fs bara A-vgd ljudeffektniv.

Standarder fr alla metoder anvnds. Mest noggrann r jmfrandemetoden och

intensitetsmetoden. Ngra mindre avsteg frn dessa bdas standarder grs, men i princip

r standardavvikelsen 1,5 dB. Skillnaden i resultatet mellan de bda noggrannare

metoderna r mindre n 1 dB. Skillnaden r strst i 500 Hz oktavbandet och i 125 Hz

oktavbandet. Den direkta metoden har standardavvikelsen 4 dB och dess bestmda

ljudeffektniv r 1 dB hgre n den fr intensitetsmetoden och 2 dB hgre n den fr

jmfrandemetoden.


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TABLE OF CONTENTS

1 INTRODUCTION.......................................................................................................3

1-1 Task .....................................................................................................................3

1-2 Test object and its physical description ...............................................................3

2 STANDARDS .............................................................................................................4

2-1 ISO 3747:2000 - Determination of sound power levels of noise sources usingsound pressure -- Comparison method in situ .................................................................4

2-2 ISO 3746:1995 - Determination of sound power levels of noise sources using

sound pressure -- Survey method using an enveloping measurement surface over a

reflecting plane ................................................................................................................4

2-3 ISO 9614-2:1994 - Determination of sound power levels of noise sources using

sound intensity -- Part 2: Measurement by scanning.......................................................4

3 MEASUREMENTS ....................................................................................................4

3-1 Measurement environment ..................................................................................4

3-2 Instrumentation....................................................................................................4

3-3 Data acquisition and settings ...............................................................................5

3-4 Measurement procedure, using ISO 3747:2000 ..................................................53-4-1 Calibration of the measurement system ......................................................5

3-4-2 Measurement of the indicator of accuracy ..................................................5

3-4-3 Measurement of the sound pressure levels ..................................................6

3-4-4 Measuring the background- sound pressure level and noise correction......6

3-4-5 Measurement result .....................................................................................6

3-5 Measurement procedure, using ISO 3746:1995 ..................................................6

3-5-1 Reference box, measurement surface and microphone positions ................6

3-5-2 Estimation of environment correction .........................................................7

3-5-3 Measurements and result .............................................................................7

3-6 Measurement procedure, using the intensity method, ISO 9614:1994................8

3-6-1 The intensity method by scanning ...............................................................8

3-6-2 Calibration ...................................................................................................8

3-6-3 Scanning ......................................................................................................8

3-6-4 Requirements...............................................................................................9

3-6-5 Calculations, measurement results and accuracy ........................................9

3-6-6 Derogation ...................................................................................................9

3-6-7 Results .........................................................................................................9

4 DISCUSSION............................................................................................................10

4-1 Comparison of the results ..................................................................................10

4-2 Comparison of the results in octave bands ........................................................10

4-3 Measurement improvements .............................................................................114-4 The importance of calibration............................................................................11

4-5 Advantages ........................................................................................................11

5 REFERENCES..........................................................................................................11

6 APPENDIX ...............................................................................................................12

6-1 Measurement, results and calculations ..............................................................12

6-1-1 The comparison method, ISO 3747...........................................................12

6-1-2 The direct method, ISO 3746 ....................................................................13

6-1-3 The intensity method, ISO 9614................................................................14

6-2 Important parameters of measurement accuracy [4] .........................................14


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1 INTRODUCTION

1-1 Task

The task is to practice and compare different sound power measurement methods. Three

different methods are being used to determine the sound power level of a sound source.

First, the sound power level is measured with a sound pressure method by using a

reference sound source. Second, sound power level is measured with a sound pressuremethod without the reference sound source. Third, the sound power level is measured

with a sound intensity and scanning method. These methods are further described in

sections 2 and 3.

1-2 Test object and its physical description

The test object that is used as a sound source for the measurements is a grinding machine,

to the left in figure 1. It is mounted on a wooden board.

Figure 1:To the left, the test object is a grinding machine and to the right the reference

sound source from Brel & Kjaer that is used in the first method, ISO 3747:2000.

The technical data of the test object are as follows:

Dimensions (reference box):o Length: 0.36 mo Width: 0.25 mo Height: 0.28 m

Weight (estimated): 5 kg


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2 STANDARDS

To determine the Sound Power Level, the following three standards are used. The

methods and requirements are specified in sections 3-4 to 3-6 respectively.

2-1 ISO 3747:2000 - Determination of sound power levels of noise sources

using sound pressure -- Comparison method in situThis standard describes how to determine the sound power level with a comparison sound

pressure method by using a reference sound source, [1, 2].

2-2 ISO 3746:1995 - Determination of sound power levels of noise sources

using sound pressure -- Survey method using an enveloping measurement surface

over a reflecting plane

This standard describes how to determine the sound power level with a direct sound

pressure method, i.e. without using a reference sound source [1, 3].

2-3 ISO 9614-2:1994 - Determination of sound power levels of noise sourcesusing sound intensity -- Part 2: Measurement by scanning

This standard describes how to determine the sound power level with an intensity method

by scanning acoustic intensity of the object, [1, 4].

3 MEASUREMENTS

3-1 Measurement environment

The environmental data and date are as follows:

Measure location: Room MWL 75, KTH in Stockholm Swedeno Dimensions (LWH): 6.7 m 6 m 4.5 m

Date: 25 September 2008 Air temperature: 22 C Relative humidity: 44 %

3-2 Instrumentation

The following equipment where used for the measurements:

Frequency analyzer: HP 3568Ao Serial No.: 3442A00389

Sound level calibrator: Brel & Kjaer type 4230o Level.: 94.4 dB at 1000 Hz

Microphone: MK225, Pre-polarized condenser microphoneo Serial No.: 970486

Reference sound source (RSS): Brel & Kjaer Type 4204o Serial No.: 955287

Sound intensity calibrator: GRAS 51ABo Serial No.: 29559

Intensity probe: Brel & Kjaer 2681o Serial No.: 1894856

o Microphone capsules: Type 4181, Pre-polarized condenser microphone Serial No.: 1963773 (Part 1)

Sensitivity: 10.05 mV/EU (Calibrated)


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Serial No.: 1863773 (Part 2)

Sensitivity: 10.01 mV/EU (Calibrated)o Spacer: 12 mm UC5269

Data processing software: Microsoft Excel

The microphone is mounted on an ordinary microphone stand and is connected to the

frequency analyzer. When the Intensity probe is used it is connected to the frequencyanalyser input 1 and 2. The data is transferred to Excel by hand and the calculations are

available in the appendix. Further pictures of the instrumentation are available from the

author on request.

3-3 Data acquisition and settings

The frequency analyzer, see section 2-2, is used to integrate and average the sound

pressure for the two sound pressure methods. For the intensity method the frequency

analyser is also used to integrate and average the sound intensity over a well defined area

and calculates the sound power level in octave bands. It is also used to calculate the

accuracy of the measurements as described in 3-6-5. For the analyzer settings andmanagement, see the reference [1] side 110 and forward.

3-4 Measurement procedure, using ISO 3747:2000

The test object is located on the floor approximately in the middle of the room. The

reference sound source (RSS) is placed next to it, see figure 1. This comparison method

gives octave band and A-weighted sound power levels. All measurements are made in

octave bands with the centre frequency range 125 Hz 8 kHz from which the A-weighted

is calculated.

Four microphone positions in different directions and heights are chosen in such a way

they can reflect the influence of the environment, that is, they are positioned rather close

to a wall. The microphone distance could be seen in table 1 in appendix 6-1-1 where the

measurement results are presented.

3-4-1 Calibration of the measurement system

The first step is to calibrate the microphone. For this the sound level calibrator and the

HP-analyzer are used. The sound pressure level of the calibrator (94.4 dB) is entered to

the HP-analyzer. The microphone sensitivity is 53.8 mV/Pa, which is close to the

sensitivity supplied by the manufacturer. This is done in the measurement environment

before the actual measurements take place.

3-4-2 Measurement of the indicator of accuracy

The sound pressure level( )'pi RSSL is recorded in octave bands into the analyser while the

RSS is running. The indicatorf

L is calculated in excel according to [1]. The value of

fL decides the achievable accuracy of the measurements. To attain engineering

accuracy (grade 2),f

L have to be larger than 7 dB. The upper value of the standard

deviation of reproducibility for the A-weighted sound power level is then 1.5 dB, [2]. The

microphone position is changed (farther away) till the indicator meets the requirement of

grade 2 accuracy. Due to the low humidity, fL is to low for the 8 kHz octave band.


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Figure 2: The reference box (grey) is located inside the measurement surface. Thegrinding machine can fit exactly in the reference box. The red squares mark the

microphone positions, 1 m from the reference box in the middle of each side. The fourth

position is at the front side opposite to the second position, but it would be inconvenient

to mark it in the figure. All units are in meters and the figure is not to scale.

3-5-2 Estimation of environment correction

An approximate method is used to determine the environment correction. The approximate value of

the mean sound absorption coefficient of the room is 0.25 as suggested in the standard [3]. Theenvironment correction is then calculated to K2A= 3.8, appendix 6-1-2.

3-5-3 Measurements and result

The set-up of the instruments is the same as for the previous method. The background noise ismeasured and then the sound pressure levels are measured at each of the five microphone positions.

Now the average A-weighted sound pressure level over the measurement surface could be

calculated. The background noise correction K1A is also calculated, but no correction is needed asthe background noise is more than 10 dB lower than the measured sound pressure level. The

measurement results and calculations are presented in appendix 6-1-2.

The A-weighted sound power level is being calculated to 70.3 ( )WA

L dB A= .

2.36

1.28

2.250.36

0.28

0.25

1.00

1.00

1

5

0.64

3

0.64

1.00

2

0.64


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3-6 Measurement procedure, using the intensity method, ISO 9614:1994

3-6-1 The intensity method by scanning

The intensity measurement method used for this measurement is scanning. By integratingthe sound intensity over a well known surface that embraces the measurement object, the

sound power is computed. The test object is located on the same place on the floor as in

the previous methods. For easier scanning with the intensity probe, the test object is inthis case embraced by four stands with strings attached parallel to the floor, which defines

the measurement surface as a parallelepiped box with the dimensions (LWH) 0.71 m 0.65 m 0.52 m. The surface is then S = 1.88 m2.

The frequency analyzer integrates the intensity over the area of the parallelepiped and the

sound power level is calculated and presented in octave bands and A-weighted. To be

able to compare the different measurement methods a 12 mm spacer is used, with whichthe effective frequency range is 100 Hz 5 kHz, [1]. The measured frequency range in

octave band will be 125 Hz 4 kHz.

3-6-2 Calibration

The first step is to calibrate the two microphone capsules in the intensity probe

separately. Then the intensity calibrator from G.R.A.S. is used to get the pressure residualintensity index pI measured in dB, this parameter is used to determine the degree of

accuracy, se section 3-6-6. This is done in the measurement environment before the finalmeasurements.

3-6-3 Scanning

One side of the surface is scanned at once, and is then paused in the analyzer. In thissetup it is not possible to use the remote control of the probe. Scanning is done by the two

orthogonal patterns in figure 3, i.e. first one segment is measured by first scanned withthe left pattern, and then without pausing the analyzer, the segment is scanned with the

right pattern. That is, the two measurements are averaged into a single spectrum. Then theanalyzer is paused and the measurement continues for the rest of the segments. The total

integrating time is set to 300 s and the recording is interrupted manually on the analyzerwhen the scanning is finished.

Figure 3:The two different scanning pattern that is used. The patterns is figurative and

not necessarily to scale.


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3-6-4 Requirements

The standard 9614, [4], requires that the integration time should be at least 20 s persurface segment and that the speed of scanning should be between 0.1 and 0.5 m/s. These

requirements are some of the requirements that should be fulfilled for the terms in 3-6-5.

In this case 30 s of integration time per segment is used and the average speed ofscanning is about 0.2 m/s.

3-6-5 Calculations, measurement results and accuracy

The sound intensity level as well as the sound pressure level for the measurement is

calculated by the analyzer in octave bands, A-weighted and linear. The measurementresults are given in table 3 in the appendix 6-1-3.

There are three different degrees of accuracy for sound power level measurements with

the intensity method. For this measurement, engineering accuracy (grade 2), is chosen. Ifthe standard is followed the standard deviation for the result is between 1.5 and 3 dB in

the octave bands (given in table 2 in appendix 6-1-3) and 1.5 dB for the A-weighted sincethe total A-weighted sound power in the bands outside the range 400 Hz to 5 kHz doesnt

exceed the total, according to [4], appendix 6-1-3. The true value of the A-weighted

sound power level is expected with a certainty of 95 % to be in the range of 3 dB [4].For grade 2 accuracy, the bias error factor, K, should be selected to 10 dB.

The most important parameters used to decide if the desired degree of accuracy isacquired in this case are: Pressure Residual Intensity Index - pIand Surface Pressure-

Intensity index - FpI. These are further explained in the appendix 6-2.

The results of the pressure residual intensity index - pI, together with the calculations of

the dynamic capacity index -LDare presented in table 3 in the appendix 6-1-3. The soundpower level is then calculated in Excel, appendix 6-1-3.

3-6-6 Derogation

Two important derogations from the standard is that only criterion 1 and not criterion 2

and 3 are used, i.e. the limit on negative partial power and the partial power repeatabilityis not checked, which is required according to attain engineering accuracy [4]. These

criterions are explained in the appendix 6-2-1. This could affect the accuracy of the

measurement results. The reason for the chosen method is because the data has to betransferred to the computer by hand and much time is saved this way.

3-6-7 Results

The Sound power level is 69.1 ( )WA

L dB A= and the sound power level in octave band is

presented in figure 5 in section 4-2.


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4 DISCUSSION

4-1 Comparison of the results

The discrepancy of the sound power level determination of the comparison method and

the intensity method is less than 1 dB and the deviation for each of them is 1.5 dB.Comparison of the results of all three methods is presented in figure 4.

Possible reasons of the discrepancy for the direct method are that it only attains survey

accuracy, and the standard deviation is thus 4 dB. The environment correction is

calculated to K2A= 3.8 dB (appendix 6-1-2), but according to the standard, it is calculatedfrom an approximate value of the mean sound absorption coefficient, = 0.25, that is

meant for basically an empty room with some furniture. In this case there were a lot ofabsorbing people present during the measurements. If is bigger, the correction will be

smaller, so this would make the sound power level even bigger.

In the comparison method the background noise was too high at the 125 Hz octave band

for microphone position 3 and could not be used. This should be a minor problembecause of the relatively small contribution to the A-weighted sound power level from

that frequency band. The sound absorption in the air is due to the low humidity a problem

for high frequencies. The accuracy indicatorf

L is too low at the 8 kHz octave band,

appendix 6-1-1.

Finally, for the intensity method there were two derogations from the standard whichcould have minor affect to the uncertainty of the measurement.

67

67,5

68

68,5

69

69,5

70

70,5

71

SPL (dB)

ISO 3747 ISO 3746 ISO 9614

A-weighted

Linear

Figure 4: Comparison of the A-weighted and linear sound power level for the threemethods: LW, ISO 3747 = 68.4 dB(A), LW, ISO 3746= 70.3 dB(A) and LW, ISO 9614= 69.1 dB(A).

4-2 Comparison of the results in octave bands

When analyzing the sound power level in octave band in figure 5, it is obvious that thereis a difference in the 125 Hz band between the two engineering accuracy methods. The

higher value for ISO 3747 is due to the high background noise, section 3-4-4. There isalso a rather big difference in the 500 Hz band. That could be due to problems with a

directive tone, environment changing or bad scanning technique, confer to section 3-6-6.

When A-weighting is made, the contribution from the 500 Hz band will be bigger then

from the 125 Hz band. The 8 kHz octave band, only measured in ISO 3747 is uncertaindue to the air absorption, mentioned in 4-1-1. One should also consider that the effectivefrequency range of the 12 mm spacer doesnt cover the 125 Hz- and 4 kHz octave bands

completely, confer to section 3-6-1.


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0

1020

30

40

50

60

70

SPL (dB)

125 250 500 1000 2000 4000 8000

Centre frequency (Hz)

ISO 3747

ISO 9614

Figure 5: Comparison of the sound power level from the two engineering accuracy

methods in octave band. Notice the difference in the 125 Hz and 500 Hz band. The 8 kHz

band is only measured with the comparison method, ISO 3747.

4-3 Measurement improvements

For the comparison method, the lower frequencies could be improved if the background

noise is reduced. If the humidity is raised the high frequencies gets better. It is alsopreferable with a more reverberant room. The direct method is difficult to improve since

the standard was fulfilled. The intensity method could be improved if all of the criterions

in the standard are followed and if the scanning technique could be improved. There isalso guidance in [4] to increase the grade of accuracy.

4-4 The importance of calibration

If the microphone is not calibrated before the measurements, one has to use the sensitivity

supplied by the manufacturer. For the sound pressure methods, the same microphone is

used, so they would be comparable without calibration. The value of the A-weightedsound power level however would not be correct. In this case though, the sensitivity wasrelatively close to the one supplied by the manufacturer. For the intensity method the

calibration is very important since it measure both pressure and intensity from two

opposite directions at the same time. Without the calibration it would not be comparableto the other methods due to that it uses the capsules instead of the microphone. Further

more, phase calibration has to be done for both capsules in order to get good accuracy.

4-5 Advantages

The major advantage with the intensity method is that it (in theory) does not depend on

the environment. There is no need to use expensive acoustic measurement rooms to get

high accuracy measurements [1]. Practically though, an acoustic room helps to getcontrol of the environment and thus the measurement accuracy could be improved.

This intensity measurement method is also a rather quick method due to the easyarrangement and rapid calculations of the sound power level.

5 REFERENCES

[1]: Leping Feng, Acoustical measurements, Lecture notes, TRITA-AVE 2007:07,

ISSN 1651-7660, 2nd

print (2008)

[2]: International Standard ISO 3747:2000, ICS 17.140.01[3]: International Standard ISO 3746:1995, ICS 17.140.01[4]: International Standard ISO 9614-2:1996(E), ICS 17.140.01


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6 APPENDIX

6-1 Measurement, results and calculations

All calculations are made in Microsoft Excel. The tables are showing the measuredvalues and the calculated values according to [1], side 110 119.

6-1-1 The comparison method, ISO 3747

Table 1a Microphone position 1, distance r = 2.8 m

Centrefrequency(Hz) ( )( )W RSS L dB ( )

'

( )pi RSSL dB ( )fL dB ( )

'

piL dB ( )' ( )pi BL dB ( )piL dB ( )f dB

125 78.2 73.0 14.74316 49.6 45.8 47.25766 3.8

250 80.7 69.8 9.043161 46.1 33 45.88191 13.1

500 81.4 72.1 10.64316 54.9 27.3 54.89245 27.6

1000 85.1 75.8 10.64316 52.9 25.9 52.89133 27.0

2000 85.3 73.3 7.943161 51.5 19.8 51.49706 31.74000 82.7 71.6 8.843161 41.5 14.8 41.49071 26.7

8000 79.3 65.7 6.343161 33.0 11.6 32.96842 21.4

Table 1b Microphone position 2, distance r = 3.48 m


'

( )pi RSSL dB ( )fL dB ( )

'

piL dB ( )' ( )pi BL dB ( )piL dB ( )f dB

125 78.2 69.5 13.13158 53.2 45.8 52.32769 7.4

250 80.7 67.3 8.431585 43.2 33 42.76409 10.2

500 81.4 69.9 10.33158 51.1 27.3 51.08186 23.8

1000 85.1 71.9 8.631585 54.0 25.9 53.99327 28.1

2000 85.3 72.3 8.831585 48.7 19.8 48.6944 28.94000 82.7 68.8 7.931585 40.0 14.8 39.98686 25.2

8000 79.3 63.6 6.131585 34.0 11.6 33.97494 22.4

Table 1c Microphone position 3, distance r = 4.39 m


'

( )pi RSSL dB ( )fL dB ( )'

piL dB ( )'

( )pi BL dB ( )piL dB ( )f dB125 78.2 64.9 10.54929 45.7 45.8 Not valid! -0.1

250 80.7 67.7 10.84929 47.2 33 47.03167 14.2

500 81.4 72.0 14.44929 53.8 27.3 53.79027 26.5

1000 85.1 79.0 17.74929 50.8 25.9 50.78592 24.9

2000 85.3 70.9 9.44929 49.3 19.8 49.29512 29.54000 82.7 68.2 9.34929 45.0 14.8 44.99585 30.2

8000 79.3 62.2 6.74929 32.6 11.6 32.56537 21.0

Table 1d Microphone position 4, distance r = 4.44 m


'

( )pi RSSL dB ( )fL dB ( )'

piL dB ( )'

( )pi BL dB ( )piL dB ( )f dB125 78.2 63.7 9.447659 14.74316 45.8 53.28725 8.2

250 80.7 67.5 10.74766 9.043161 33 45.88191 13.1

500 81.4 68.2 10.74766 10.64316 27.3 50.78056 23.5

1000 85.1 70.0 8.847659 10.64316 25.9 48.67715 22.8

2000 85.3 69.7 8.347659 7.943161 19.8 46.79133 27.0

4000 82.7 65.8 7.047659 8.843161 14.8 38.58186 23.8

8000 79.3 60.4 5.047659 6.343161 11.6 28.71645 17.2


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Tabell 1e Calculated sound power levels

Centrefrequency(Hz) ( )WL dB ( )nA dB ( )( )

0.110 W n

L A + ( ), ( )W AL dB A ( )0.110 WL 125 64.0 -16.1 60966.94662 47.9 2483673

250 58.3 -8.6 93412.17964 49.7 676711.4

500 63.5 -3.2 1083038.359 60.3 2262788

1000 63.9 0 2449068.312 63.9 24490682000 62.9 1.2 2570554.228 64.1 1949965

4000 56.2 1 526704.5887 57.2 418376.3

8000 48.6 -1.1 56519.33747 47.5 72811.01

Table 1a-e:The results from the measurements and calculations. The background sound

pressure level in table 1a-d, '( )pi B

L is red marked when' '

( )pi pi Bf L L = < 15 dB and is

then corrected topi

L according to [1].

( )( )( )0.1,The A-weighted sound power level is calculated as 10 log 10 68.4 ( )W nL AW AL dB A += =( )( )0.1The linear sound power level is calculated as 10log 10 70.1WLWL dB= =

6-1-2 The direct method, ISO 3746

A-weighted sound pressure levels

Microphone position, figure 2 ( )' , ( )pA iL dB A

1, Top 61.9

2 63.03 59.94 63.55 58.6

( )' , ( )pA averageL dB A 61.8

Table 2:The average of the measurements of the A-weighted sound pressure levels.

The environment correction is calculated to ( )( )2 10log 1 4 / 3.8 ,AK S A dB= + = where Sis the surface of the reference box, figure 2 and A is the total absorption of the room.

VA S= , where is the approximate value of the mean sound absorption coefficient of

the test room = 0.25 and SV is the total surface of the test room. The background noise

correction is1

0 ,A

K dB= so the A-weighted sound power level is:

( )', , 1 2 010log / 70.3 ( ),W A pA average A AL L K K S S dB A= + = where the reference surface

area is S0 = 1


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6-1-3 The intensity method, ISO 9614

Centrefrequency (Hz)

0( )

pIdB

( )I

L dB

( )P

L dB

( )WL dB

( )pl

F dB

( )dL dB

( )nA dB

( )WAL dB

125 15.5 55.5 59.3 58.2 3.8 5.5 -16.1 43.2

250 18.6 56.8 59.5 59.5 2.7 8.6 -8.6 50.9

500 26.0 64.9 66.9 67.6 2.0 16.0 -3.2

1000 22.7 61.5 63.1 64.2 1.6 12.7 0

2000 20.9 59.6 61.3 62.3 1.7 10.9 1.2

4000 25.0 52.8 54.2 55.5 1.4 15.0 1

( )( )WAL dB A 21.9 66.4 68.2 69.1 1.8 11.9( )WL dB 26.5 68.1 70.1 70.8 2.0 16.5

Table 3:The measurements results, both accuracy and sound power levels.

The pressure residual intensity index0pI

is calculated in the frequency analyzer. It should

be at minimum 16 dB when the frequency is equal or higher than 250 Hz in order to fulfil

the IEC requirements for class 1 instruments [1]. In this case it is fulfilled. The dynamiccapacity index is checked, see section 6-2. The total A-weighted sound power level from

the octave bands 125 Hz and 250 Hz is calculated to be able to check the accuracy of thetotal A-weighted sound power level, confer 3-6-5.

Finally0

10lognW I

SL L

S

= +

, where IL is the measured sound intensity level, S is the

measured surface and S0is the reference area (1 m2).

6-2 Important parameters of measurement accuracy [4]

The most important parameters used to decide if the desired degree of accuracy isacquired when using the standard ISO 9614 are:

Pressure Residual Intensity Index -0pI

is defined as the difference between the measured

sound pressure level and the measured sound intensity level when the probe is placed in a

sound field such that the intensity is zero. Thus this parameter is a measure of the qualityof the probe.

Surface Pressure-Intensity index - FpI is defined in [1]. FpI is an index of the quality of

the sound field. If FpI 0 the sound field is free and if FpI Lp the sound field is

diffuse, where Lpis the sound pressure level.

The dynamic capacity index - LDmust exceed FpIin all frequency bands and LDis given

by: LD =0pI

K, there K is the bias error factor, selected to 10 dB (criterion 1 in [4]).

The negative partial power indicator F+/-(criterion 2 and the definition in [4]), is an index

of the amount of absorption inside the integrated area. F+/-< 3dB is the criterion and inthis case it is not checked.

Partial power repeatability (criterion 3 in [4]) is the difference between two orthogonalscanned measurements. The difference must not exceed the standard deviation of 1.5 - 3

dB depending of the frequency band. The standard requires that this should be done onesegment at a time and then the total sound power level is calculated from the partial

sound powers. This is not checked in this exercise.

Date post:	02-Jun-2018
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Sound Power Measurements

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