acoustical parameters - teatro argentino de la plata and teatro margarita xirgu

7/27/2019 Acoustical Parameters - Teatro Argentino de La Plata and Teatro Margarita Xirgu

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

1

ACOUSTICAL PARAMETERS COMPARISON OF TWO HALLS:

TEATRO ARGENTINO DE LA PLATA AND TEATRO MARGARITA

XIRGU

AGUSTN Y. ARIAS1

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

[email protected]

Abstract: This report describes the methods and results from several measurements performed

in the halls TeatroArgentino de La Plata and Teatro Margarita Xirgu to knowthe behavior

of their main acoustical parameters. These parameters are evaluated in detail to classify each

hall according to the degree of their acoustic comfort.

INTRODUCTIONIn the nineteenth century the acoustics becomes a science, with the discoveries of W. C.Sabine, who experimentally demonstrated the relationship between the physical properties of

a room and its reverberation time. The reverberation time is sometimes considered the most

important parameter to evaluate room acoustics. For a great acoustic comfort it is crucial that

the reverberation time be the right-one according to the activity developed within the room

(hall, concert hall, conference room, etc.). However, several researchers have shown that

obtaining an adequate reverberation time is not enough to ensure good sound quality and

then defined various acoustical parameters that serve as indicators of the acoustic

performance of a room, which allows an acoustical categorizing. These acoustical

parameters are indicators that reveal several properties of the behavior of sound waves in an

enclosure and are perceived by listeners in the form of sensations (e.g., degree of liveliness

of the room, apparent sound source size, clarity of voice and music, harmonious, degree of a

correct speech comprehension, etc.).

Thanks to the advancement of research and the rapid development of technology in the

twentieth century, there have been able to define these sensations with measurable and

quantifiable physical quantities described by acoustical parameters. These parameters have

been the result of extensive research conducted by leading researchers of room acoustics.

Today, the individual importance of each of these parameters generates several debates. For

this reason there are different methods of evaluating them in order to quantify the degree of

influence of each one to the contribution to the overall assessment of the room. For this

reason various studies may judge the acoustic of a room in different ways, because many of

them evaluated the importance of each parameter value with different criteria. Because of

this, it still continues to investigate the importance that each parameter has when performing

an acoustical evaluation of enclosures.

This report describes the procedures used to carry out the measurements of the main

acoustical parameters of the halls "Teatro Argentino de La Plata" and "Teatro Margarita Xirgu.

All results are analyzed, trying to justify the behavior of each individually and finally evaluating

the degree of acoustic comfort of each room according to the type of activity (theatrical or

musical).


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ACOUSTICAL PARAMETERS COMPARISON OF TWO HALLS

PART I - Generalities

2

PART IGENERALITIES

Before starting to develop the individual results obtained for each hall, it is important to define

some generalities regarding the measurements and the parameters studied.

To obtain the acoustical parameters defined below, it was implemented the recommendations

and requirements specified in the Standard ISO 3382-20011. The objective of the

measurements process was to obtain the Impulse Response from both halls in each

measurement position. The sound signal used was a Log-Sine sweep. To obtain the results of

each acoustical parameter the signals were recorded and post-processed with Aurora2

plugins

in Adobe Audition 3.0. There were three different types of measurements according to the

parameters studied: Monaural and Binaural Impulse Responses for the measurements

performed in the audience areas and Monaural Impulse Response for the measurements

performed on the stages.

The equipment used was the follows:

Dodecahedral loudspeaker + Subwoofer OUTLINE, used as omnidirectional sound source.

Sound Level Meter Svantek 959 class one with its corresponding calibrator.

Soundfield microphone to perform monaural impulse response in the audience areas.

Earthworks M50 microphone to perform monaural impulse response measurements on

stages (Teatro Argentino de La Plata).

DPA 4060 microphone to perform monaural impulse response measurements on stages

(Teatro Margarita Xirgu).

Dummy Head KEMAR to perform binaural-impulse response measurements in the

audience areas.

Notebooks.Audio interfaces.

Aurora Plugins for Adobe Audition.

The acoustical parameters evaluated in both halls are defined in the ISO 3382-2001 standard.

The complete formulation that defines these parameters can be found in the Annex section of

this report

Monaural Parameters:

Reverberation time. Defined as the time (in seconds) that elapses between the soundsource is stopped until the moment when the sound pressure level falls 30 dB relative

to its initial value, interpolating the decay curve up to 60dB.o Subjective impression:Liveliness of the room.o Parameter evaluated: RTmid.

Early decay time: Defined as six times the time elapsed since the radiated soundsource stops until the sound pressure level drops 10dB.

o Subjective impression: Liveliness of the room (usually gives more reliableresults than the RTmid).

o Parameter evaluated: EDTmid.1

ISO-3382: Acoustics Measurement of the reverberation time of rooms with reference to other

acoustical parameters. 2001.2 Farina, Angelo:Impulse Response Measurements by Exponential Sine Sweeps. Parma, 18 October

2008.


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Bass Ratio3: Relationship between the reverberation time values obtained at low andmid frequencies.

o Subjective impression: Richness in bass sounds, mellow and soft music.o Parameter evaluated: BR

Brightness3: Relationship between the reverberation time values obtained at high and

mid frequencies.o Subjective impression: Richness in sharp sounds.o Parameter evaluated: Br

Voice Clarity4: Relationship between the sound energy that reaches the listener duringthe first 50ms from the arrival of the direct sound and that comes after the first 50ms,

calculated between the bands from 125 to 4000 Hz.

o Subjective impression:Speech intelligibility degree.o Parameter evaluated: C50(speech average).

Musical Clarity5: Relationship between the sound energy that reaches the listenerduring the first 80ms from the arrival of the direct sound and that comes after the first

80ms, calculated between the bands from 125 to 4000 Hz.

o Subjective impression: Degree of separation between the different individualsounds members of a musical composition.o Parameter evaluated: C80(music average).

Definition: Relationship between the sound energy reaching the listener within thefirst 50ms since the arrival of the direct sound (including the direct sound and early

reflections) and the total sound energy, measured between the octave bands of 125 to

4000 Hz.

o Subjective impression: Speech intelligibility degree.o Parameter evaluated: D50

Strength Factor6: Difference between the total sound pressure level produced by anomnidirectional sound source at a given point of a room and the sound pressure level

produced by the same source in a free field to ten meters away.o Subjective impression: Degree of amplification caused by the room.o Parameter evaluated: Gmid

Center Time7: It is the time of the gravity center of the quadratic impulse response.o Subjective impression: Degree of perceived definition, clarity, or the balance

between clarity and reverberation, in addition to speech intelligibility.

o Parameter evaluated: Ts (average from 125 up to 4000 Hz)Lateral Efficiency8: Relationship between the sound energy reaching a listener laterallywithin the first 80ms since the arrival of the direct sound (excluded) and the received

sound energy in all directions in that time interval.

o Subjective impression: Spatial impression of sound (apparent width of thesound source).

o Parameter evaluated: LFE4

3Beranek, L. L.: Concert and Opera Halls -How they Sound. Acoustical Society of America, 1996.

4Ahnert, W. - Schmidt, W.: Acoustics in cultural buildings). Institut fr Kulturbauten, Berlin 1980.

5Abdel Alim, O.: Dependence of time and register definition of room acoustical parameters with music

performances. Dissertation, TU Dresden 1973.6

Lehmann, P.: On the ascertainment of room acoustical criteria and correlation of the same with

subjective assessments of the acoustic overall impression amkeit. Dissertation TU Berlin, 1976.7

Krer, R.: A simple measuring procedure for determining the "center time" of room acoustical impulse

responses. 7th Intern. Congress on Acoustics, Budapest 1971.8 Barron, M.: Auditorium Acoustics and Architectural Design. Verlag E & FN SPON. London 1993.

Kleiner, M.: A New Way of Measuring Lateral Energy Fractions. App. Acoust., Vol. 27, 321 ff (1989).


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Binaural Parameter:

Interaural Cross Correlation Coefficients9: Cross correlation between the calculatedimpulse responses in both ears in the first 80ms. Indicative of the degree of similarity

existing between the two signals.

o Subjective impression: Spatial impression of sound (apparent width of thesound source). It was demonstrated that this parameter have muchrepeatability than the LFE4.

o Parameter evaluated: 1-IACCE3Stage Parameter:

Room Support10: Parameter to evaluate the degree of acoustic comfort on stage andthe orchestra pit. It is calculated as the ratio of the energy associated with early

reflections (20 to 100ms) provided by the walls and ceiling of the stage, and the energy

received after 20ms, both values obtained at one meter distance from the

omnidirectional sound source located on stage. It is expressed on a logarithmic scale.

o Subjective impression: Musicians capacity to hear themselves and the rest ofthe orchestra.

o Parameter evaluated: ST1midEach parameter has some recommended values that emerged after many years of room

acoustics investigations11

. Thanks to the contributions of Beranek12

, Barron13

and others

experts in room acoustic, it is possible to compare the results obtained for a particular room

and compare them with these recommended values, most of them derived from the greatest

theaters and concert halls in the world in terms of acoustic comfort. They are listed in Table 1.

Table 1. Recommend acoustical parameters values

ACOUSTICAL PARAMETER RECOMMENDED VALUE TYPE OF ACTIVITY

RTmid occupied room0,7< RT

mid< 1,2s Theater

1.8


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In addition, several background noise measurements were performed in order to compare the

results obtained with those recommend by the Argentinian Standard IRAM 407014

. These

recommended values are listed in Table 2. The background noise descriptor chosen is the

Noise Criteria profile. For both halls, the air conditioning systems were turned off.

Table 2. Recommend background values according to IRAM 4070 standard.BACKGROUND NOISE DESCRIPTOR RECOMMENDED VALUE TYPE OF ACTIVITY

NC (Noise Criteria)NC-25 Theater

NC-20 Concert Hall

MICROPHONES AND DUMMY HEAD CALIBRATION

To calibrate the Soundfield and Earthworks M50 microphone and the Dummy head, they were

located at 3m from the sound source and connecting each one to their respective Audio

interface. The Sound Lever Meter was also located between them. Then, the sound source was

turned on and reproducing Pink noise it was adjusted the signal level in order to measure a

sound pressure level of 94dB. Once this value was reached, the audio interfaces gains wereadjusted to obtain no levels difference between the microphones inputs.

STAGE MEASUREMENTSThe microphone positions to perform the stage acoustical parameters measurements were

located at 1 meter of the sound source (for both source position) covering a circular array at

0o, 90

o, 180

oand 270

oaround the geometrical center of the dodecahedron, as can be seen in

Figure 1.

Figure 1. Microphone positions for the stage acoustical parameter (ST1mid) measurements.

14IRAM 4070: Ruidos Procedimiento para su evaluacin utilizando los perfiles NC y RC. 2008.

0o

90

180o

270o

Sound Source


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PART IITeatro Argentino de La Plata

6

PART IIACOUSTICAL PARAMETERS OF THE

TEATRO ARGENTINO DE LA PLATA

ABOUT THE HALL:The project of the Center for the Performing Arts Teatro Argentino de La Plata has been

started as the result of a fire incident in the hall of the same name on August 18, 1977. After

several discussions and controversies as it should be its reconstruction, the idea that prevailed

was not only replace but add beside the lyric hall a series of additional and complementary

rooms namely, a Prose, a Micro-Cinema multipurpose, rehearsal rooms for Scene and

Orchestra where they could test the set-up with the decorated of each lyrical presentation,

Choir rehearsal Halls, Auxiliary Halls, Ballet rehearsal Halls, Individual rehearsal Halls and many

others. All of this putted together with restaurants, cafes, garages, TV and Radio Studio. Thus,

the hall constitutes a true cultural complex with a covered area of over 60,000 m2.15

Figure 2. Stage view

ROOM GEOMETRYIn Table 3 are listed the surface and volume data of the hall.

15 Snchez Quintana, R.: Diseo de la sala de opera del Teatro Argentino de La Plata. TecniAcstica

Congress. Madrid, 2000.


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Table 3. Surfaces and volumes.

Area Total Surfaces

AudienceSeats Covered area (m2) Hallways Walls Ceilings Sills Drapes

2200 1089 469 1114 1812 458 222Pit 108

Stage 1365

Proscenium Arch 451

Stage Opening 198

Volumes (m3)

Stage 25200

Hall 15030

Table 4. Main dimensions.

Average Height Average Width Length Maximum Distance

20,5m 27m 22m 37m

MEASUREMENTS POSITIONSFigures 3 to 5 show the sound source, microphones and dummy head positions to perform the

IR measurements. For each measurement position the sound source was placed in two

positions (Figure 3): A (stage) and B (sunken pit). As can be seen in Figure 2 there are three

central balconies and six lateral balconies. The measurement positions showed in Figure 3

correspond to the ground floor, Figure 4 correspond to the first floor (1 central balcony and

1 lateral balcony) and to the second floor (2 central balcony and 3 lateral balcony) andFigure 5 correspond to third floor (3 central balcony and 5 lateral balcony).

Figure 3. Ground floor. () Microphone positions () Source positions

123

456

B

A


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Figure 4. First and Second floor measurement positions. Position 7: 1 Central balcony. Position 8: 1

Lateral balcony. Position 9: 2 Central balcony. Position 10: 3 Lateral balcony. () Microphone

positions.

Figure 5. Third floor measurement positions. Position 11: 3 Central balcony. Position 12: 5 Lateral

balcony. () Microphone positions

Figure 6. Left: Orchestra pit. Right: Stage

7 - 9

8 - 10

11

12


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As it can be seen, a total number of twelve microphones and dummy head position and two

sound source position were used to perform the measurements. The symmetry along the

longitudinal axis of the hall allows locating the measurement positions on one side, estimating

an invariance of the acoustical behavior for the other half of the hall.

Figure 6 shows the Orchestra pit and the Stage.

MEASUREMENTS PROCEDUREOnce the microphones and dummy head were calibrated, the Log-Sine sweep signal was

recorded in each measurement position. This signal had the following characteristics:

Frequency range: 20-20000 [Hz]

Length: 15 [s]

Fade-in: 0.1 [s]

Fade-out: 0.1 [s]

Number of repetitions: 1

During the Log-Sine sweep signal recording process, ten background noise measurementswere performed, each one in different areas (ground floor, first floor, second floor, third floor,

boxes and stage). Then the average value was calculated to obtain the Noise Criteria profile of

the hall.

RESULTS AND COMMENTS

BACKGROUND NOISE RESULTSFigure 7 shows the spatial-average values in octave bands of the background noise. From these

results, the corresponding NC profiled assigned to the hall is NC-35. This value is greater than

the suggested, but there are some situations that could affected the measurements. Firstly,

during the measurements, several stage-technicians were working in the back side of the

stage, moving stage machinery (mobile platforms) and chatting to each other. In some

situations, the backdoor of the stage (direct exit to the street) was opened. These activities

affected directly the background noise measurements, so they are not much reliable.

Figure 7. NC references curves and background results


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ACOUSTICAL PARAMETERS RESULTSThe results obtained for each parameter were calculated with the Aurora plugins in Adobe

Audition. In the following paragraphs the detailed observation of these parameters are

described and discussed. All the acoustical parameters results for each microphone or dummy

head and source positions are listed in Tables 6 to 10, except for the Reverberation Time

(RTmid), Bass Ratio (BR) and Brightness (Br) parameters, which deserves a discussion regardingthe spatial distribution:

Reverberation Time [RTmid]

The T30 values in each octave frequency band can be represented as the space-average values

from all the microphones and source positions. To justify this assumption, Table 5 shows the

standard deviation between the results obtained in each frequency band for each microphone

position.

Table 5. T30 standard deviation

T30 Standard Deviation [s]

Frequency Band [Hz]Source Position 125 250 500 1000 2000 4000

A 0,26 0,14 0,09 0,10 0,18 0,04

B 0,22 0,07 0,05 0,08 0,09 0,05

Figure 8. Average T30 and error bars.

As it can be seen, the major deviation occurs at 125 Hz for both sound source positions. The

greater values of the T30 at 125 Hz were found in the balconies positions (all of them) being

approximately 2,5s. In the ground floor, this value is approximately 2s. For the other frequency

bands, there are not considerable differences. For this reason, the spatial-average value of the

T30 showed in Figure 8 represents the global value for the hall.

According to Beranek, the RTmid value is obtained as the average T30 in 500 and 1000 Hz octave

bands. The result obtained was the following:

RTmid = 1,79 s.

Considering the acceptable invariance of the T30 results, the Bass Ratio (BR) and Brightness (Br)

parameters can be evaluated from the RTmid value:

BR = 1,20 - Br = 0,82


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The RTmid value obtained is slightly lower than the minimum value recommended, but it is

important to remark that the references values defined in the Part I of this report were obtain

for full audience occupancy, so it is necessary to consider people's absorption, which will

reduce the T30 (and therefore the RTmid) value, especially at mid-high frequencies. For this

reason, the reverberation time of the hall is not optimal for concert presentations.

The BR value is greater than the minimum recommended values, but the Br is not. So, theseresults suggest that the reverberation time of the hall should be longer at high frequencies, to

get optimal RTmid and Br values for concert presentations. It will produce a flatter T30 spectrum,

which is highly recommended for concert halls. Below are listed the results obtained of the

others acoustical parameters evaluated.

Ground floor results

Table 6. Acoustical parameters results. Ground floor.

SourcePosition

Position

Acoustical ParameterEDTmid

(s)C50(Speech

average) (dB)C80(Music

average) (dB)D50(%)

Gmid(dB)

Ts (ms) LFCE4 IACCE3

A

1 1,77 -0,44 1,72 52,9 6,72 106,4 0,17 0,54

2 1,47 -3,58 0,28 34,6 3,59 111,2 0,24 0,49

3 0,74 0,93 5,43 54,8 0,33 82,7 0,34 0,40

4 1,72 1,70 3,53 61,8 5,96 87,3 0,29 0,47

5 1,39 0,28 3,01 50,8 3,38 97 0,37 0,35

6 1,01 0,02 3,98 49,7 -2,06 92,8 0,34 0,30

B

1 1,72 -4,51 -1,87 42,6 3,88 116,6 0,20 0,56

2 1,41 -11,51 -3,56 52,1 2,02 126,4 0,27 0,29

3 1,37 -5,78 -1,53 36,2 -6,85 116,5 0,39 0,25

4 1,56 -4,73 -0,69 53,5 3,73 116,3 0,34 0,30

5 1,19 -7,12 -2,35 34,6 3,69 113,9 0,38 0,28

6 1,41 -3,88 -0,92 70,5 -5,59 117,8 0,41 0,21

First floor results

Table 7. Acoustical parameters results. First floor.

SourcePosition

PositionAcoustical Parameter

EDTmid(s)

C50(Speechaverage) (dB)

C80(Musicaverage) (dB)

D50(%)

Gmid(dB)


A7 1,26 -1,91 3,11 42,6 3,45 93,9 0,26 0,34

8 1,42 1,22 2,81 52,1 0,77 82 0,26 0,49

B7 1,06 -7,04 -0,35 14,7 4,22 116,5 0,34 0,35

8 1,55 0,02 2,01 42,2 1,62 93,6 0,28 0,42


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Second floor results

Table 8. Acoustical parameters results. Second floor.

SourcePosition


EDTmid(s)



D50(%)

Gmid(dB) Ts (ms) LFCE4 IACCE3

A9 1,00 -2,38 2,96 36,2 1,83 88,9 0,29 0,28

10 1,37 1,49 2,56 53,5 -0,98 82,3 0,34 0,38

B9 1,05 -3,22 0,69 25,5 3,17 103,9 0,27 0,33

10 1,41 -2,18 1,44 40,3 -1,21 105,6 0,39 0,33

Third floor results

Table 9. Acoustical parameters results. Third floor.

SourcePosition


EDTmid(s)



D50(%)

Gmid(dB)


A11 1,33 -2,41 2,65 34,6 1,03 97,1 0,39 0,32

12 1,06 3,49 5,40 70,5 4,12 78,5 0,45 0,58

B11 0,99 -1,72 1,93 37,2 4,29 99,3 0,27 0,35

12 1,01 -0,20 2,93 49,5 3,88 85,9 0,38 0,60

Stage and Orchestra Pit

Table 10. Acoustical parameters results. Stage and Orchestra pit.

Source PositionAcoustical Parameter

ST1mid (dB)

A (Stage) -20,2

B (Orchestra Pit) -10,5

Early Decay Time [EDTmid]

The EDT results are strongly related to the sound source and microphone positions, so it would

not be correct to take an average value of this parameter.

Figure 9 shows three EDT results for both sound source positions. The EDTmid values tend to

decrease significantly in the measurement positions under the balconies in comparison with

the other positions. Audience areas located under the balconies are isolated from a large

number of reflections arriving from the top of the room. While lower the balconies, the

greater the amount of reflection losses, especially in the areas close to the rear wall. This

results in that for these areas, the room seems drier.

It has been found also that the source position influence significantly the position 11 results.


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Figure 9. EDT comparison between three measurements positions for the two sound source locations.

Speech Clarity [C50(speech average)]:

The C50 (speech average) parameter results have interesting considerations. Firstly, the

values measured in each position are greater for the sound source position A, especially for

the ground floor measurements position. This is because direct energy from the source located

at position "B" is considerably reduced by the side surfaces of the orchestra pit.

Secondly, in the audience areas located under the balconies it was perceived a slight increase

in this parameter value due to the decreased late-energy caused by less reflections, as

explained above.

Music Clarity [C80(music average)]:

The results for the C80(music average) parameter have the same behavior that those for the

C50 parameter regarding the sound source position and the audience areas under balconies.But it is remarkable that for the balconies position, especially for those in the Side balconies,

the values increase significantly for both sound source positions. This suggests that the music

played by the orchestra will be appreciated better in these positions.

Definition [D50]

Since this parameter is strongly related to the C50, there is no appreciable difference between

the results behavior in both cases. But, it is important to remark that only in the measurement

position 12 the D50 reaches the minimum suggested values. This reaffirms that the theatrical

activities, where an optimal speech comprehension is required, do not have a high degree of

acoustical comfort.

Measurement

position N2

Measurement

position N3

Source Position: A Source Position: B

Measurement

position N12


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Center Time [Ts]:

According to Hoffmeier16

, the higher the center time is, the more spatial is the acoustic

impression at the listener's position. The maximum achievable center time Ts is based on the

optimum reverberation time. The results obtained lies on the recommend margin (between 70

and 150ms). It is interesting to note that the values obtained for each measurement position

are slightly higher for the sound source position B.

Lateral Fraction [LFE4]:

The LFE4 values in the listener positions near the longitudinal axis of the room (Ground floor)

are lower than those located in the sides, as it can be expected. In the balconies there are

subtle differences between the measurements positions because all of them were located

near to a reflective surface (walls and balconies for positions 8 and 10, walls and ceiling for

positions 11 and 12 and balconies for positions 7 and 9).Positions 1 and 2 are very

distant from any reflective surface and for that reason the LFE4 values there are the lower ones

and not reach the minimum value recommended. The Side balcony positions exceed the

minimum recommended value of 0,19. It is important to remark that there are notconsiderable differences in the result according the sound source position. In the previous

parameters [C50(speech average) and C80(music average)], the direct sound of the sound

source placed in the position B (pit) affect significantly the results, especially in the audience

area of the ground floor.

Interaural Cross Correlation Coefficients [1-IACCE3]:

The first interesting results obtained for this parameter refers to the difference between the

positions of the sound source for the audience area of the ground floor. As it was defined, the

greater the 1-IACCE3 value, the greater the apparent sound source width. The results obtained

for the sound source position B are more resemble to the suggested values than those

obtained for the sound source position A. It indicates an optimal sound amplification of theorchestra due to the pit location.

The third floor side balcony (position 12) showed the worst IACCE3 results for both sound

source positions. It may be due to angle of incidence of the reflections. Ando demonstrated

that the most important reflections that arrive to the listener are those between 35 and 75

angle of incidence (lateral reflections). For the three side balconies positions there are not

near reflective surfaces to generate reflections in those directions. For this reason, in these

positions the IACCE3 results were the worst.

Room Support [ST1mid]:

The ST1midresults suggest that the musicians capacity to hear themselves and the rest of theorchestra is better in the Orchestra pit (position B), as it can be expected. However, the

result obtained for this position is higher than the maximum recommended value indicating

that there is a slightly excess of early reflections (those who reaches the musicians positions

between 20 and 100ms) which may produce a low perception of orchestral clarity. To improve

this result it is necessary to add absorbing acoustical materials to the pit walls, or any other

sound absorbing treatment in order to decrease the early reflections energy.

The extremely low result obtained in the position "A" indicates a lack of early reflections due

to the large distances between the sound source and the surfaces that make up the stage (side

walls, back wall and ceiling).

16 Hoffmeier, J.: Investigations on the influence of room timbres on speech definition. Technical

University of Denmark. Thesis for a degree at the TU Dresden, 1996.


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15

GLOBAL ASSESSMENT OF THE HALL

The main hall of the "Teatro Argentino de La Plata" is notable for its high degree of acoustic

comfort for musical performances. But for theater activities, we have seen that the parameters

related to speech intelligibility are not optimal, although the room can be used, by its

dimensions, to accommodate a large number of audiences.

For musical activities there are two failures:

1) Short reverberation time, which causes the room does not get the optimal degree ofliveness.

2) High background noise, although we mentioned that the results were affected by humanactivity itself within the room. In return, Quintana

15performed some measurements of

background noise and obtained better results. Specifically, he ranked the room with the

profile NC-20, which meets the requirements suggesting by the IRAM 4070 standard.


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PART IIITeatro Margarita Xirgu

16

PART IIIACOUSTICAL PARAMETERS OF THE

TEATRO MARGARITA XIRGU

ABOUT THE THEATER:

Margarita Xirgu Cultural Complex, is located in the historic San Telmo neighborhood of Buenos

Aires. Inside is the "Casal de Catalunya", a building built in the late nineteenth century was the

center of various entities of the Catalan community in Buenos Aires. This building represents a

true sample of the historical tradition of the Spanish architecture of this region.

Since the sixties, the theater room of "Casal de Catalunya" is named Margarita Xirgu, in

homage to the Catalan actress. With the advent of the new millennium, the theater became a

true cultural complex, designed as a new space for creative meeting. Many operas have been

brought to the stage of this theater during the last years.

In addition to the plays presented in the main hall of the theater, it also offers workshops,

seminars and permanent art exhibitions are organized. The incorporation of these proposals

reflects the intention to strengthen ties with the community, bringing them closer workshops

and courses offered by excellent professionals.

The building was built by Luis Castells and Sivilla, in 1886. The library room, the restaurant, the

grand staircase and the characteristic facade, dating back to 1909, when it was made the first

major reforms, sought to respect the original architectural language derived from the Catalan

Modernism. Further reforms took place between 1928 and 1936, but this time they adopt a

neo-Gothic design in Barcelona version. Even today, the interiors show medieval and

modernist details in the woodwork, the capitals of the columns, in the windows and ceilings.17

Figure 10. Stage view

17 Alternativa Teatral: http://www.alternativateatral.com/espacio68-teatro-margarita-xirgu-espacio-de-

arte


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17

MEASUREMENTS POSITIONS

Figures 11 and 12 show the sound source, microphones and dummy head positions to perform

the impulse response recordings. For each measurement position the sound source was placed

in two stage positions (A and B). As can be seen in Figure 10 there are two stories of thebalcony seats.

Figure 11. Ground floor. () Microphone positions () Source positions.

Microphone position N4 is located in the side box.

Figure 12. First and second floor. Positions 5: First floor. Position 6: Second floor. () Microphone

positions.

As it can be seen, a total number of six microphones and dummy head position and two sound

source position were used to perform the measurements. The symmetry along the longitudinal

axis of the hall allows locating the measurement positions on one side, estimating an

invariance of the acoustical behavior for the other half of the hall.

Figure 13 shows a first floor and a second floor audience views.

5

6

B

A

1

2

3

4


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Figure 13. Top: Second floor view. Bottom: First floor view.

MEASUREMENTS PROCEDUREOnce the microphones and dummy head were calibrated, the Log-Sine sweep signal was

recorded in each measurement position. This signal had the following characteristics:

Frequency range: 80-20000 [Hz]

Length: 30 [s]

Fade-in: 0.1 [s]

Fade-out: 0.1 [s]

Number of repetitions: 1

During the Log-Sine sweep signal recording process, fourteen background noise measurements

were performed, each one in different areas (ground floor, first floor, second floor, boxes and

stage). Then the average value was calculated to obtain the Noise Criteria profile of the hall.


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RESULTS AND COMMENTS

BACKGROUND NOISE RESULTSFigure 14 shows the spatial-average values in octave bands of the background noise. From

these results, the corresponding NC profiled assigned to the hall is NC-25. This value is the

same than the suggested, so the hall can be considered quiet enough for theatrical

presentations.

Figure 14. NC references curves and background results

ACOUSTICAL PARAMETERS RESULTSThe results obtained for each parameter were calculated with the Aurora plugins in Adobe

Audition. In the following paragraphs the detailed observation of these parameters are

described and discussed. All the acoustical parameters results for each microphone or dummy

head and source positions are listed in Tables 12 to 15, except for the Reverberation Time

(RTmid), Bass Ratio (BR) and Brightness (Br) parameters, which deserves a discussion regarding

the spatial distribution:

Reverberation Time [RTmid]

The T30 values in each octave frequency band can be represented as the space-average valuesfrom all the microphones and source positions. To justify this assumption, Table 11 shows the

variance between the results obtained in each frequency band for each microphone position.

As it can be seen, the major deviation is 0,16 s and occurs at 125 Hz for the sound source

position A. This difference is somewhat small to be taken into account. For the other frequency

bands, the differences are also negligible. For this reason, the spatial-average value of the T30

showed in Figure 15 represents the global value for the hall.


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Table 11. T30 standard deviation

T30 Standard deviation [s]

Frequency Band [Hz]

Source Position 125 250 500 1000 2000 4000

A 0,16 0,04 0,07 0,04 0,05 0,03

B 0,11 0,09 0,06 0,10 0,11 0,05

Figure 15. Average T30 and error bars.

The RTmid value is obtained as the average T30 in 500 Hz and 1000 Hz octave band. The result

obtained was the following:

RTmid = 1,12s.

Considering the acceptable invariance of the T30 results, the Bass Ratio (BR) and Brightness (Br)

parameters can be evaluated from the RTmid value:

BR = 1,21

Br = 0,83

The RTmid value is within the recommended values, but it is important to remark that the

references values defined in the Part I of this report were obtain for full audience occupancy,

so it is necessary to consider people's absorption, which will reduce the T30 (and therefore the

RTmid) value, especially at mid-high frequencies. Considering the audience areas sizes, it can be

estimated that the RTmid value for full audience occupancy will still remain between the

recommended values margins. For this reason, the reverberation time of the hall is optimal for

theatrical presentations.

The BR and Br value suggests that the reverberation time of the hall at low frequencies is

acceptable for theatrical presentations, although generally it is recommended a flat T 30

spectrum to avoid possible speech intelligibility failures. This allows that the hall may be used

for small concert presentations with a reasonable reverberation field behavior.

Below are listed the results obtained of the others acoustical parameters evaluated.


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Ground floor results

Table 12. Acoustical parameters results. Ground floor.

SourcePosition


EDTmid(s)



D50(%)

Gmid(dB) Ts (ms) LFCE4 IACCE3

A

1 1,17 1,07 2,81 57,7 14,59 77 0,18 0,44

2 1,04 2,25 4,59 61,8 12,67 70,5 0,25 0,34

3 0,75 4,38 6,28 71,2 16,01 61,4 0,32 0,21

4 1,05 2,33 4,26 58,9 11,84 67,1 0,30 0,28

B

1 1,48 2,64 3,36 63,0 9,79 73,8 0,13 0,60

2 1,28 0,95 2,98 52,8 8,42 76,7 0,21 0,51

3 1,03 2,01 4,07 58,5 8,95 73,4 0,27 0,30

4 1,23 0,07 1,78 47,4 7,83 78,5 0,21 0,42

First Floor results

Table 13. Acoustical parameters results. First floor.

SourcePosition


EDTmid(s)



D50(%)

Gmid(dB)


A 5 0,81 4,78 7,23 69,6 16,04 54 0,31 0,26

B 5 0,66 5,43 7,71 75,4 14,49 51,4 0,27 0,35

Second Floor results

Table 14. Acoustical parameters results. Second floor.

SourcePosition

Position

Acoustical ParameterEDTmid

(s)C50(Speech

average) (dB)C80(Music

average) (dB)D50(%)

Gmid(dB)


A

6 1,20 2,05 3,97 57,7 13,66 76,3 0,29 0,36

B 6 1,35 -3,36 0,30 29,2 6,29 109,9 0,23 0,31

Stage and Orchestra Pit


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Table 15. Acoustical parameters results. Stage.

Source PositionAcoustical Parameter

ST1mid (dB)

A (Stage) -14,9

B (Orchestra Pit) -12,5

Early Decay Time [EDTmid]

The EDT results are strongly related to the sound source and microphone positions, so it would

not be correct to take an average value of this parameter.

The EDTmid values obtained for the sound source position B are about 0,17 s greater than those

obtained for the sound source position A. It may be related to an increase of the reflections

energy coming from the back wall of the stage (sound source position B is closer to that wall

regarding the A position). Another important fact is that the EDTmid values decrease as the

microphones/dummy head position moves away from the sound source positions. So, the

lower values were obtained in the measurement positions 3 and 5. In addition, the

measurement position 5 is located below the second floor balcony, which reduces the amount

of reflections coming from the ceiling and therefore produces a considerable decrease of the

EDTmid value. This phenomenon tends to decrease the subjective sensation of liveliness for any

musical activity, but it will not affect considerably the theatrical presentations.

Speech Clarity [C50(speech average)]:

The C50 (speech average) parameter results have interesting considerations. Except the

measurement positions 4 and 6 (for the sound source position B), the C 50(speech average)

values are acceptable for theatrical presentations. Due to the shape of the hall, and the soundsource position (B), the measurement position 4 may be located at a high lateral reflections

receiving point, which produce lowers values of this parameter. The same consideration is

applied to the measurement position 6. But, in any case, these results must to be investigated

further.

Music Clarity [C80(music average)]:

The results for the C80(music average) parameter have the same behavior that those for the

C50 parameter regarding the sound source position and the audience areas under balconies.

The failures noted in the previous section for measurement positions "4" and "6" now are

beneficial for musical clarity. But, this also leads to that in the rest of the measurement

positions, the C80 (music average) values are too high (excessive early sound energy) which

suggests that the performance of the room to musical activities is not satisfactory.

Definition [D50]

Since this parameter is strongly related to the C50, there is no appreciable difference between

the results behavior in both cases, and the conclusions obtained for the C50(speech average)

parameter can be also applied to the D50 results.

Centre Time [Ts]:


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A relationship between the (subjective) syllable intelligibility VS and the center time Ts is given

by Krer18

by the following correlation rule:

Ts in ms.For a syllable intelligibility of VS 80 % the results according to the above -mentioned rule are

Ts 130 ms. So, as it can be observed from the results, the Ts measured are optimal in all

measurement positions.

Lateral Fraction [LFCE4]:

The LFCE4 values in the measurement position 1 (near the longitudinal axis of the room -

ground floor) are lower than those positions located in the sides, as it can be expected. It is

interesting to note that in the measurement position 5, the LFCE4 values are the higher, even

when this position was located near to the longitudinal axis of the room. Again, the shape of

the room may produce reflections focusing at that point.

It is important to remark that there are considerable differences in the result according the

sound source position. For the sound source position B, the results are greater. It is because

that position was displaced from the longitudinal axis of the room, so the lateral energy

arriving to the measuring points increases. The results obtained are optimal, but is not enough

to classify the room according to its musical performance as it is described in the previous

parameters evaluated.

Interaural Cross Correlation Coefficients [1-IACCE3]:

The 1-IACCE3 results obtained have the same behavior of those for the LFCE4 regarding the

sound source and microphones positions. The conclusion is the same: even though the results

are optimal, the room does not offer good acoustic comfort for musical activities.

Room Support [ST1mid]:

There is an important consideration regarding the results obtained for the ST1mid parameter. If

we compare these results with those recommended we can conclude that the musicians

capacity to hear themselves and the rest of the orchestra is optimal. However, the

recommended values were obtained for big concert halls, which have noticeable constructive

differences regarding the hall volume, audience area size, stage dimensions, etc. The

Margarita Xirgu theater is considerably small in comparison with that kind of concert halls

(cannot accommodate large orchestras) so it is not correct to use the recommended values to

make a global assessment of the hall. May be, for a small chamber orchestra we can say that

the results obtained are optimal.

GLOBAL ASSESSMENT OF THE HALL

The theater "Xirgu Margarita" has a high degree of acoustic comfort for theatrical

presentations. This was evident from the results obtained for the reverberation time and the

parameters that define the quality of speech intelligibility. On the contrary, the room does not

have good performance for musical activities (except those using electroacoustic amplification

systems, which can improve the performance of the room using, for example, artificial reverb).

18 Krer, R.: A simple measuring procedure for determining the "center time" of room acoustical

impulse responses. 7th Intern. Congress on Acoustics, Budapest 1971.


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This is compounded by the small size of the stage for this type of activity, although it can be

developed presentations of small chamber orchestras.


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ANEX

25

ANEXFORMULATION OF ACOUSTICAL PARAMETERS

19

Monaural Parameters:

Reverberation time (s):

Early decay time (s):

Bass Ratio:

Brightness:

Voice Clarity (dB):

Musical Clarity (dB):

19Dr. Wolfgang Ahnert and Dr. Wolfgang Schmidt: Fundamentals to perform acoustical

measurements. EASERA user manual.


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ANEX

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Definition (%):

Strength Factor (dB):

: Reference sound pressure level measured at 10 meters in free field.

Central Time (s):

Lateral Efficiency:

: Impulse response instantaneous sound pressure measured with a microphone with figure-

eight pattern.

Binaural Parameter:

Interaural Cross Correlation Coefficients:

Left ear impulse response.

Right ear impulse response.


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ANEX

Stage Parameter:

Room Support (s):

acoustical parameters - teatro argentino de la plata and teatro margarita xirgu

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