Measurements of Sound Absorption Coefficients

7/27/2019 Measurements of Sound Absorption Coefficients

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Acoustics Instruments and Measurements May 2013, Caseros, Buenos Aires Province, Argentina

MEASUREMENT OF SOUND ABSORPTION COEFFICIENTS

AGUSTN Y. ARIAS 1

1Universidad Nacional de Tres de Febrero, Buenos Aires, Argentina.

[email protected]

1. INTRODUCTIONThe absorption coefficients of a material allow to

know to what extent the incident sound energy is

absorbed. All materials absorb a greater or lesser

extent some of the sound energy incident on them.

This property is of vital importance in the design ofrooms where it is necessary to emphasize the

behavior of the sound field within. Acoustic

parameters such as the reverberation time (RT) are

directly dependent on the absorption properties of the

materials used as termination of the various surfaces

that compose an enclosure. Therefore, the designermust know the absorption properties that each

material has to know which to choose when looking

for the highest degree of acoustic comfort.

This paper describes the procedures undertaken to

assess and obtain the absorption coefficients of a set

of pieces of glass wool, universally used for its

acoustic and thermal performance, based on thespecifications of the ISO-354 standard:

"Measurement of sound absorption in a

reverberation room"[1].

2. GENERALITIES OF ISO-354 STANDARDISO-354 details the procedures and considerations

to perform the tests for determining the absorption

coefficients of the material studied using a

reverberation chamber. Basically this method consists

in comparing the reverberation time of the camera

with and without the absorbent material placed

inside, resulting in differences for each third octave

band between 100 and 5000 Hz. For this, we use the

Sabines RT equation:

A: Equivalent absorption area [m2]

V: Room volume [m3]

T: Reverberation time of the room [s]

c: Sound velocity [340 m/s]

m: Air attenuation coefficient [m-1

]

The Standard also specifies the requirements to bemet by reverberation chamber in terms of size,

volume, diffusion, etc. Mostly it is suggested that thevolume of the chamber is between 150 and 500 m

3.

Furthermore it must be satisfied that:

Imax is the length of the longest straight line that

enters in the chamber.

As for the material under test, the Standard

specifies that it must cover at least an area of 10 m2

and the relationship between the width and length

should be between 0.7 and 1. The material must be

positioned so that the shortest distance between its

edges and the walls is 1 m and also the edges not belocated parallel to any walls.

In addition, the standard requires that the product

of the number of microphone positions and the

number of source positions is of at least 12, being

possible any combination that meets this criterion.The microphones must be separated from each othera minimum distance of 1.5 m, 2 m from the source

and 1 m from the walls. As for the source, each

position used must be at least 3 m away from any

other position.

3. LIMITATIONS OF THE MEASUREMENTSThe measurements should be performed under

certain conditions that do not meet the requirements

imposed by ISO-354.

First, it was not possible to access a reverberation

chamber, so that the measurements were performedin a classroom in the Annex building of the National

University of Tres de Febrero (Figure 1). The volume

of the classroom is 61 m3, forcing a reduction of

almost all distances between the material under test,

microphones and source that defines the standard.Furthermore, under these conditions the surface area

of the material also fails to meet the minimum value

imposed in the standard.


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Figure1: Classroom where the measurements were

performed.

4. PROCEDURE4.1.Background Noise

A background noise evaluation was carried out in

order to meet the acoustics conditions of the

environment in which the reverberation time

measurements were carried out. One minute of

background noise measurement where performed,

obtaining 62.5 dB as Leq result. This result allowed

setting the level of radiation from the sound source so

as to minimize the effects of this noise on the

effective dynamic range of measurements. It can be

observed that the noise is very high, especially at low

frequencies. External condition, such as train andtraffic noise, and the absence of sound insulation

treatment in the classroom adversely affect the results

obtained especially at low frequencies (


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3.95 m2. For both arrangements, the material was

placed on the floor as can be seen in Figure 4.

Figure 4. Absorbent material distribution. Top: groupedmaterial. Bottom: Dispersed material

4.4.Measurements analysisOnce all sound recordings of Log Sine-sweep

were obtained (36 records total) it was proceeded to

analyze them. Firstly, the impulse responses were

obtained at each measurement point using the

convolution process in AURORA plugins, between

the recorded signal and the inverse filter of the sine-

sweep (Figure 5).

Figure 5. AURORA interface to obtain the ImpulseResponse.

Then, using the software Dirac 3.0 of B&K brand,

they were obtained the values of the T30 reverberationtime in 1/3 octave bands (100-5000 Hz) from the

impulse responses for the three measurement

conditions. This reverberation time were obtained by

averaging the 12 values obtained in each set. Then it

was used the equation of Sabine RT (Eq. 1) to obtainthe "equivalent sound absorption area" A1 (emptyroom), A2_g (room with absorbent material grouped)

and A2_d (room with absorbent material dispersed). It

was supposed that the temperature and the humidity

were almost constant during the measurements, so no

account was taken of the term . The value of the

speed of sound was set .

Figure 6. Dirac interface to obtain the average RT value in

1/3 octaves band.

Finally, the absorption coefficients were

obtained by the formula:

= g, for the material grouped

= d, for the material dispersed

All these calculations were performed on each 1/3

octave band between 100 and 5000 Hz.

5. RESULTSThe following are the results obtained from the

measurements.

5.1.Reverberation timeIn Figure 7 it can be appreciated the reverberation

time values for each of the three measurement

conditions. It is observed that for the two conditions

of the absorbent material arrangement, RT values are

similar, with a maximum difference of 0.17 seconds

in the band of 1250 Hz. Also, the values of the RT forthe grouped material condition are higher than those

of the dispersed material, except for the band of 315

Hz. At 200 Hz it can be seen a drop of the RT for the

empty room condition. This effect may be associatedwith a modal influence of the room becuadse this


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effect is present in three microphones positions for

any source position. The maximum values of the RTfor both material arrangements were obtained in 160

Hz.

Figure 7. RT results for the three measurements conditions.

5.2.Absorption coefficientsFigure 8 shows the values of the absorption

coefficients for both arrangements of the

absorbent material.

Figure 8. Absorption coefficients obtained for both material

arrangements.

There is a clear increase in the value of

(grouped absorbent material) in the band of 315 Hz

of 0.26 sabines regarding the value of (dispersed

absorbent material). This difference justifies the RT

variation between these arrangements for the same

frequency band. Moreover, the rest of the absorption

coefficients tend to be very similar. Excluding the315 Hz band, the biggest difference between the two

curves is 0.09 sabines in the 500 Hz band.

5.3.Comparison with the results obtained inlaboratory

To see how the results adjust to the official values

given by the manufacturer (ISOVER) it wasperformed a comparison table that evidences the

difference between both measurements (Table 1) [3].

Table 1. Absorption coefficients comparison.

Frequency band [Hz] 125 250 500 1000 2000 4000

Results obtained for this

report0,14 0,43 0,58 0,80 0,80 0,72

Results given by the

manufacturer (ISOVER)*0,10 0,32 0,55 0,66 0,79 0,77

Difference 0,04 0,11 0,03 0,14 0,01 0,05

*ISOVER Panel "PL-156". Thickness 3mm. According to AC3.D11.78

essay of the Instituto de Acstica (Centro de Fsica Aplicada

It can be seen that the highest difference is 0.14

sabines for the 1000 Hz octave band. There is not a

great difference between the results, so it can beassumed that the measurements performed for this

report gives estimative values of the absorption

coefficients.

6. OTHER IMPORTANT ANALYSISIn addition to the measurements performed and

the corresponding results obtained, there are other

important criteria to take account.

6.1.RepeatabilityThe repeatability is defined as the value below

which the absolute difference between two single testresults obtained with the same method on identical

test material, under the same conditions can be

expected to lie with a probability of 95% [4]. This

value can validate to some extent the measurementprocesses and the results obtained. The formula to

calculate the repeatability is:

n: number of measurements (in this case n=12)

t = Student distribution factor (for n=12, t=2.18)

This analysis was performed over the twelve

measurements of the grouped material condition.

Figure 9 denotes the results obtained.

It can be seen that for the 315 Hz band, the

repeatability is not very much efficient. It is due to

the considerable RT variations that occurred in thatband, where the difference between the min value

and max value is 1.01s as can be seen in Table 2. At

low frequencies, the RT values also differ between

different positions, but it is understandable because ofthe background noise influence in the T30 calculus.


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For this reason, the repeatability is less efficient in

low frequencies in comparison with the othersfrequencies bands (except for 315 Hz, as mentioned

above).

Table 2. RT Variations for the 315 third-octave band

RT results for 315Hz third-octave band

N of measurement Reverberation Time

1 1,81

2 1,58

3 1,65

4 1,16

5 1,85

6 1,97

7 1,63

8 0,96

9 1,63

10 1,43

11 1,32

12 1,68

Figure 9. Repeatability results for the grouped material

condition.

6.2.Edge absorptionThere is an "extra" absorption produced by the

edges of the material. This effect known as "edge

effect" can cause that some absorption coefficients

are greater than 1 [5]. It occurs due to the sound

diffraction phenomena that occur at the edges of the

material. Generally, this effect is most evident if the

edges are rigid. In addition, the effect increases withdecreasing frequency, decreasing specimen size,

increasing aspect ratio, and increasing sound

absorption coefficient. The glass wool pieces used for

this work do not show a significant presence of this

effect. As shown in section 5.2, no values of theabsorption coefficients ( and ) is greater than 1

in all tested frequency bands. Anyway, an analysis

was made of the effect. The values obtained, are

the sum of the effective value of the absorption

coefficient and the absorption value obtained by the

edge diffraction, as shown in eq. (5):

: Measured value.

: Absorption coefficient considering an infinite

sample material.

: Edge absorption given exclusively by

edge diffusion.

: Wavelength.

The values of may vary according to the criteriaused. Using the criterion of Ten Wolde [7], takes

the values shown in Figure 10. The analysis to the

edge effect, results in that it does not affect

significantly the values of obtained in the

measurements. The maximum deviation is 0.008,

calculated for in the 500 Hz frequency band.

Figure 10. values according to the Ten Wolde criteria.

7. CONCLUSIONSThe results of the measurements are estimative,

because it did not meet all the guidelines in the ISO-

354. However, these results do not differ much with

those supplied by the manufacturer, so that the

method used in these measurements can be employed

if it is not possible to access a reverberant chamber to

obtain absorption coefficient values similar to those

obtained by standard tests.

The main problems of this method are: noise

(mainly affects the low-frequency range), the

influence of the room (eigenmodes), walls-ceiling-floor absorption (this effects are minimized in a


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reverberant room) and the heterogeneous material

employed.

8. REFERENCES[1] ISO-354, Measurement of sound absorption in a

reverberation room.[2] Farina, Angelo. Impulse Response

Measurements by Exponential Sine Sweeps. Parma,

18 October 2008.

[3] ISOVER, Isolation handbook.

[4] ASTMC423, Standard Test Method for Sound

Absorption and Sound Absorption Coefficients by the

Reverberation Room Method. 2002, ASTM

International.

[5] A. de Bruijn, The edge effect of sound absorbing

materials revisited, NAG 2007.

[6] Marshal, A. H., Meyer, J. The Directivity and

Auditory Impressions of Singers.

[7] Ten Wolde, T., Acmtica 18, 207-212,(1967).

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Measurements of Sound Absorption Coefficients