seat absorption coefficients measurement

7/27/2019 Seat Absorption Coefficients Measurement

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

SEAT ABSORPTION MEASUREMENT USING THE KATH & KUHLMETHOD

AGUSTN Y. ARIAS 1

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

[email protected]

1. INTRODUCTIONThe absorption coefficients of a seat arrangement

allow estimating the absorption area of such seats

when installed in the room for which they were

chosen. There are several methods to measure theabsorption coefficients of a seating array, but one of

the most recognized and used is the Kath and Kuhlmethod. The aim of this report is to explain themethodology used to measure the absorption

coefficients of a seat arrangement and extrapolate the

results obtained to estimate the absorption that would

produce these seats if they are installed on the ground

floor of the Teatro Argentino de La Plata andTeatro Margarita Xirgu, both located in the

province of Buenos Aires, Argentina.

2. KATH & KUHL METHODThe Kath and Kuhl method involves placing the

seating in the corner of the reverberation chamber in

rows with their intended row spacing. The exposed

edges are covered with barriers for some of the

measurements. The barriers need to be massive and

stiff to reduce low frequency absorption. The barriers

should be at least as high as the seating, and higher ifany audience is present for an occupied measurement.

The set-up is schematically shown in Figure 1.

Though it seems that the array is mirrored in the

adjacent walls of the chamber, thus effectively

increasing its size, it is not effectively infinite as Kath

and Kuhl thought. Diffraction effects are still presentand so the measured absorption coefficient may still

vary with sample size. The concept is to separately

measure three absorption coefficients by carrying out

measurements with and without barriers [1]:

For an infinite array with no edges, yields an

absorption coefficient , with side and front

barriers in place.

For the front edges, , by measuring with the

side barrier only in place and combining the

result for .

For the side edges, , by measuring with the

front barrier only in place, and combining with

the result for .

Figure 1. Set-up for Kath and Kuhl seating absorptionmeasurement.

The following equations are used to obtain ,

and :

and are the length, width and height of the

barriers respectively.

A1, A2 and A3 are the total absorption of the

sample with both panels, front panel only, andside panel only, respectively.

Finally, if the areas of the front edges Sf, side

edges Ss and plan area Sp of the hall are known, the

absorption coefficient of the audience block is given

by:


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3. LIMITATIONS OF THE MEASUREMENTSThe requirements that must be accomplished for

the reverberation chamber are defined in the ISO-354standard: "Measurement of sound absorption in a

reverberation room" [2]. However, it was not

possible to access a reverberation chamber, so the

measurements were performed in a classroom in the

Annex building of the National University of Tres deFebrero (Figure 2). Furthermore, under these

conditions only six seats could be used to perform the

measurements, which is a very small amount to

extrapolate the results to a hall.

Figure 2. Classroom used as reverberation chamber.

Microphones and source positions.

4. PROCEDURE4.1.Background Noise

Two background noise measurements were

carried out in order to meet the acoustics conditions

of the environment. Each measurement has duration

of 30s. The Leq result was 62.5dB which allowedsetting the level of radiation from the sound source so

as to minimize the effects of this noise on theeffective dynamic range of measurements. External

condition, such as train and traffic noise, and the

absence of sound insulation treatment in the

classroom adversely affect the results obtained

especially at low frequencies.

4.2.Reverberation time measurements.Four sets of measurements of the reverberation

time (T20) of the room in different conditions were

used:

Empty Room.Room with chairs grouped and both panels.

Room with chairs grouped and Side panel.

Room with chairs grouped and Front panel

For these four conditions, an omnidirectional

source (Dodecahedral loudspeaker) brand

OUTLINE with its respective subwooferwas used.

From this source a "Log Sine-Sweep" wasreproduced, generated with the AURORA plugins

package in Adobe Audition [3], with the following

configuration:

Figure 3. Log-Sine sweep configuration

This signal meets the flat spectrum requirements

and maintains a high signal/noise ratio.Four Earthworks M50 microphones were used to

record the signals reproduced by the source. So, four

microphones in fixed positions and three sound

source positions were used for all the measurements

giving a total of 12 measurements for each

arrangement.

Figure 4. Seat arrangement.

As it can be observed in Figure 4 there were two

rows of three seats. Figure 5 shows the dimensions ofa single seat.


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Figure 5. Seat dimensions.

The area occupied by the seats is 2 m2,

representing 8.9% of the area of the room (31.2 m2).

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

were obtained (48 records total) those were analyzed.Firstly, the impulse responses were obtained at each

measurement point using the convolution in

AURORA plugins, between the recorded signal and

the inverse filter of the sine-sweep (Figure 6).

Figure 6. AURORA interface to obtain the ImpulseResponse.

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

the reverberation time (T20) in 1/3 octave bands (125

- 4000 Hz) were calculated for the three measurement

conditions. These reverberation times were obtained

by averaging for the twelve values measured in eachset. Then the equation of Sabines RT (Eq. 5) was

used to obtain the equivalent sound absorption area

Ae (empty room), A1, A2 and A3.

A: Equivalent absorption area [m2]

V: Room volume [m3]

T: Reverberation time of the room [s]

c: Sound velocity [343 m/s]

m: Air attenuation coefficient [m-1

]

It was supposed that the temperature and thehumidity were almost constant during the

measurements, so the term was not taken

account of. The speed of sound was set at

.

5. RESULTSThe following are the results obtained from the

measurements.

5.1.Reverberation TimeReverberation time results are shown in Table 1.

These results allow obtaining the equivalent soundabsorption area of each measurement condition.

Table 1. Reverberation time results

Frequency

[Hz]

RT empty

room (s)

Rt with both

panels (s)

RT with Front

Panel (s)

RT with Side

Panel (s)

125 1,92 1,68 1,91 1,80

160 2,41 2,12 2,10 2,08

200 2,60 2,28 2,16 2,19

250 2,68 2,20 2,17 2,18

315 2,79 2,16 2,16 2,17

400 2,65 2,20 2,04 2,04500 2,61 2,07 2,00 1,95

630 2,54 1,93 1,94 1,83

800 2,58 1,94 1,91 1,88

1000 2,56 1,96 1,89 1,85

1250 2,62 1,96 1,92 1,93

1600 2,66 2,03 1,95 1,91

2000 2,54 1,96 1,89 1,84

2500 2,35 1,83 1,78 1,78

3150 2,11 1,66 1,64 1,62

4000 1,90 1,52 1,51 1,50

From these results, A1 can be calculated

performing the subtraction between the Aempty_room

and Aboth_panels. The same procedure is applied toobtain A2 and A3 for each set.

5.2.Teatro Argentino de La PlataFigure 7 shows the results of the absorption

coefficient associated with the ground floor seat area

of the theater applying the method of Kath & Kuhl.

To obtain these results the surfaces listed in Table 2were used.


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[3]Farina, Angelo. Impulse Response Measurements

by Exponential Sine Sweeps. Parma, 18 October2008.

seat absorption coefficients measurement

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