UNTREF Ingeniería De Sonido July 2014, Argentina

MEASUREMENT AND COMPARISON OF INTELIGIBILITY AND LATERAL ENERGY FRACTION BETWEEN USINA DEL ARTE

CONCERT HALLS

MARIANO ALVAREZ BLANCO 1 GERMAN HEINZE 1 ARIEL MUSZKAT 1 EMILIANO ROMERO 1

Universidad Nacional de Tres de Febrero, Partido de Tres de Febrero, Caseros, Provincia de Buenos Aires, Argentina.

mariano_alvarez_blanco@hotmail.comgerman.r.heinze@gmail.com

ariel.muszkat@gmail.comemilianoromeromathieu@gmail.com

Abstract – This work presents the measurement process and a comparison of spatial parameters between the concert hall and the chamber music hall built in Usina del Arte complex as well as the comparison and measurement of intelligibility parameters between both halls. The building is located at, La Boca, Buenos Aires, Argentina and was built as multipurpose art complex. Regarding the intelligibility parameters, the speech transmission index (STI) and the rapid speech transmission index (RASTI) were obtained, whereas for the spatial parameters the Lateral Energy fraction (LF), the Lateral Energy Fraction Cosine (LFC), and the four-band Average Lateral Energy Fraction (LE4) are presented. The explained measurement procedure implemented a soundfield

microphone to obtain the spatial parameters. Results shows that STI and RASTI values of 0,48 for male and 0,50 for female, and 0,52 respectively were obtained in the music chamber hall, whereas in the main hall STI and RASTI values of 0.51 for male and 0.25 for female, and 0.56 respectively were obtained. On the other hand LE4 values of 0,419 and 0,424 were obtained in the chamber music hall and concert hall respectively.

1. INTRODUCTION

Usina del Arte complex was built in 2011 as a multipurpose art complex. It is located at La Boca, Buenos Aires city, Argentina. Between the many spaces that conforms this complex a big 1200 - seat concert hall called “Symphonic Hall” is found. In addition, there is a small 400 – seats chamber music hall called “Chamber Hall”. As between other activities these halls are used for musical concerts their acoustical parameters quality result essential 700 personas de capacidad.

International Standard ISO 3382 [1] presents acoustical parameters which may be obtained from impulse responses, whereas intelligibility parameters are presented by many authors as in [4] and [5]. In this work some ISO 3382 [1] and Inteligibillity parameters were measured in both halls. The first group of parameters lies on spatial impression. These are the Lateral Energy fraction (LF), the Lateral Energy Fraction Cosine (LFC), and the four-band Average Lateral Energy Fraction (LE4) [beranek]

[1]. Regarding the Inteligibillity parameters, the speech transmission index (STI) and the rapid speech transmission index (RASTI) were measured. The described parameters were selected in order to make the comparison as they represent two fundamental characteristics of a concert hall in conjunction, of course, of the reverberation time RT30.

Acoustical parameter comparison was made between the Simphonic and Chamber halls.

This report begins with a fundamental concepts section in which the measured acoustical parameters are defined as well as other basic concepts which result fundamental to understand the analysis. Following sections describe the halls and the devices and tools employed to make the measurements. In the final of the report, results are presented and also the conclusions about parameters comparison for both halls.

2. BASIC CONCEPTS

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2.1. Impulse and Frequency Response for a LTI system.

The Unit Impulse Response of a linear system (IR) is defined as its response to an ideal impulsive signal such as a Delta Dirac function. If the system’s IR is transformed by Laplace or Fourier transforms then we obtain the transfer function of the system. By this way, when any signal is introduced to a system its output represented in the time domain could be predicted as the convolution between the input signal and the IR expressed in time domain. If a frequency domain output is required then the mathematical operation between its input signal and the system’s transfer function also expressed in the frequency domain should be a product. Figure one simply shows the previous explanation.

Figure 1: LTI system’s response to an arbitrary signal expressed in time and frequency domain.

The transfer function contain both phase and magnitude information about the system´s IR in the frequency domain. If the magnitude of this transfer function is calculated, then the frequency response of the system is obtained.

Figure 2 shows a representation of a loudspeaker, a room and a microphone as a system, where the loudspeaker reproduces a signal and then it is captured by the microphone in a single position of the room. The input signal is x(t), the output one is y(t) and h(t) represents the complete system’s IR.

+

Figure 2: Combination of a loudspeaker, a room, and a microphone as a complete LTI system.

Although just the room could be considered as a complex system, was decided to include the loudspeaker and the microphone because their own transduction transfer function are modifying the income signal to the hole system as is shown in Figure 3, where s(t), r(t), and m(t) are the impulse

responses of the loudspeaker, room, and microphone respectively. Then it can be interpreted as serial concatenation of single systems where h(t) still represents the complete system’s IR [4].

Figure 3: Serial LTI system.

Equations 1, 2, 3 and 4 represent the fundamental relationship between signals, IR, and transfer function.

y (t )=x (t )∗h (t) (1)

y (t )=x (t )∗s( t)∗r (t)∗m( t) (2)

Y ( f )=X ( f ) . H ( f ) (3)

Y ( f )=X ( f ) . S (f ) . R (f ) . M (f ) (4)

When transfer function H(f) is required then equation 5 is applied. This is the product between the captured signal by the system and the inverse transfer function of the original input signal.

Y ( f ) . [ X ( f )]−1=H ( f ) (5)

Aurora plugins developed by Angelo Farihna is one of the apps that actually allow doing this process to obtain an IR. The software offers different signals and processing options to obtain it. Since the signal x(t) used to get the IR was a logarithmic frequency sweep and its inverse filter function [x(f)]-1 was allowed to reach a very high signal to noise ratio for full spectrum in comparison with other type of signal as impulsive sources [5].

2.2. Uncertainty

In this section the basic concepts of uncertainty and statistics used to validate the microphone calibration are described.

The uncertainty gives information about the validity of a measurement and is represented in the measurement data values units e.g. +/- 0,5 dB. A calibration result, which process includes repetitive measurements instances, isn´t complete if uncertainty isn´t included. When it is included, it conform the confidence interval of the extended uncertainty, interval which provide information about a range in which the true value lies with a certain degree of probability [6]. This will be fully explained below.

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Laplace Transform

X(s) Y(s) = X(s).H(s) H(s)

* x(t) y(t) = x(t) h(t) h(t)

h(t)

y(t) x(t)Spk

RoomMic

h(t)= s(t)* r(t)* m(t)

y(t) x(t) Spk

Room Mic

2.2.1. Types of uncertainty and their calculus.

Two types of uncertainty can be differentiated: type A and type B. The first one is obtained with measured data values and a statistical procedure described in the following paragraphs.

Equation 6 describes Standard Deviation, it shows data’s dispersion around the average value x of measured data values x [7]. The number of measurements is n.

σ n=√∑i=1

n

(x i¿−x)

n(n−1)¿

(6)

Typical deviation is defined as equation 7. Its variables are the same that standard deviation.

σ n−1=[∑i=1

n

( x−x i )2

n−1 ]12

(7)

Typical deviation of the average value is defined in equation 8. It is equal to uncertainty type A S(x) as showed in equation 9 if the number of measurements is more than ten. This last requirement is covered in this work.

σ x=[∑i=1

n

( x−x i )2

n(n−1) ]12

(8)

σ x=S( X ) (9)

Uncertainty type B is obtained from calibration values of measurement equipment, equipment specifications and certificates, older measured data values, and the experience, criteria or knowledge of materials and tools used to measure. A different method rather than statistical analysis is used to obtain this type of uncertainty, not covered in this report.

Usually uncertainty is expressed as a combined expanded uncertainty, where type A and B variables are combined. Equation 10 describes the typical combined uncertainty.

St=√ X2+Y 2+Z2 (10)

Where X, Y, and Z are uncertainties to combine. Equation 11 describes the combined expanded uncertainty where St is multiplied k [8].

Sex ¿−¿+¿ k.S t¿ ¿ (11)

Where k is the coverage factor or security factor, which value is usually 2 for a 95% confidence interval in a normal probability distribution. When uncertainty is obtained from a single measurement point but in many frequency bands, as done in this work, the total combined expanded uncertainty is the highest value between all frequency bands.

3. ACOUSTICAL PARAMETERS

1.1. ISO 3382 Spatial Parameters

The early lateral energy fraction, LF, is defined as the fraction of energy between the energy received by a figure-of-eight microphone with its null pointing at the source and the energy received by an omni-directional microphone at the same position [Michael Barron]. This is defined in equation 1 as,

LF=∫

0.005

0.08

pL2 ( t )dt

∫0

0.08

p2 (t ) dt

(12)

,where pL is the impulse response measured with the figure-of-eight microphone which is integrated between 5 ms and 80 ms, whereas p is the impulse response measured in the same point as pL with an omnidirectional pattern microphone.The lateral energy fraction cosine, LFC is defined as the fraction between the impulse responses of lateral energy with contributions which vary as the cosine of the angle and the omnidirectional impulse response measured in the same points [1]. This is defined in equation 2 as,

LFC=∫

0.005

0.08

|pL ( t ) . p ( t )|dt

∫0

0.08

p2 (t ) dt

(13)

,where pL is the impulse response measured with the figure-of-eight microphone which is integrated between 5 ms and 80 ms, whereas p is the impulse response measured in the same point as pL with an omnidirectional pattern microphone.This parameter is thought to be subjectively more accurate [2].

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4. INTELLIGIBILITY

The intelligibility of speech is a measure of efficacy in understanding of the spoken word, which quantifies the percentage of the message that is properly understood.There are subjective and objective methods of measuring speech intelligibility. Objective methods make the procedures easier that on subjective measurement but results must finally agree with real conditions and subjective results.One of the most widely used model of prediction is the Transmission Index Voice (Speech Transmission Index - STI) developed by Steeneken and Houtgast (1980). The STI is based on the generation and analysis of an artificial test modulated in amplitude signal, which replaces the signal voice. This method assumes that speech intelligibility is maintained with the acoustics of the room only if the modulation of the artificial signal is transmitted unchanged from the source to the position of auditor. The STI technique is standardized by the IEC 60268-16 standard [3].To calculate the STI, first, we proceed to define the modulation reduction factor m as follows:

m (f m )=( 1

√1+(2 π f mRT

13.8))( 1

1+10(−0.1LS N ) ) (14)

Where, f mis de Modulation Frecuency, RT is the

reverberation time and LSN is the signal to noise ratio level. The apparent signal to noise ratio level and the average apparent signal to noise ratio level are defined as:

LS Napp=10 logm

1−m(15)

LSNapp=∑i=1

7

W i(LSNapp)i (16)

W i Weighting for 7 octave bands (125-8k).

From these values, the STI is defined as:

STI=(LSNapp+15)

30(17)

Figure 4 shows the STI curves for different RT and S/N ratios. For the studied halls, the expected values were presented in annex 1.

Figure 4: STI curves for different reverberation times and signal-to-noise ratio.

The RASTI intelligibility index is a simpler alternative proposal. Its drawback, with regard to STI is that only evaluates two octave bands: 500 Hz and 2 kHz, and does not take into account possible distortions in the system for measure and possible nonlinearities in phase and amplitude.

5. MAIN AND SMALL ROOM

Figure 5: Main room - stage

Figure 6: Main room - audience

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Figure 7: Small room

6. PROCEDURE

For intelligibility measurement one sound source position and seven microphones positions were used in the chamber hall. The equipment consisted in a Dynaudio BM6A speaker and an Earthworks M50 microphone for recording the signal. For the large hall, two source positions and five reception positions were used.

Two signals were reproduced by the loudspeaker: a list of words of Dr. Tato recorded with the Earthworks M50 and a Log Sine Sweep 48kHz sampled from 60 Hz to 12 kHz and 40 seconds duration.

Background noise measurements were also performed and a tone of 1 KHz at one meter from the source was used for sound pressure level calibration.

Figure 8: Setup

With the record of the Log Sine Sweep through the circular convolution process with its inverse filter that generates the Aurora software, the corresponding impulse responses were obtained for each position.

With the impulse responses, the recording of phonetically balanced words of Dr. Tato, measurements of background noise and recording 1 KHz tone, using the plug-in of Aurora, the STI values for bands and global, and RASTI were obtained. Figure 6 shows the software interface for speech parameters calculation.

Figure 9: STI & Octave Band Analysis

For LFC and LF measurements, a sound field microphone was used. The different signals recorded in A-Format and processed by the SPS200 Software Controlled Microphone. Figure 10 shows a scheme of the microphone capture configuration.

Figure 10: Sound field mic configuration.

Four signals were provided by the each recording instance. They were processed by the software provided by the microphone manufacturer to get the values required to calculate the LF parameter. Figure 11 shows a software screenshot during the signal processing procedure.

Figure 11: Sound field signal processing.

The signals recording in A-Format should be convert in B-Format because in aurora processing information this signal require.

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7. RESULTSThe results obtained by the program Adobe

Audition with plugin Aurora, while processing the impulse responses, background noise, word list and record for calibration are the descript in Table 1 and 2.

For simplicity sake, only the final results are presented, i.e. the averages for all bands and the global values.

The LFC and LF values are presented in Tables 3, 4 5 and 6.

La figura 13 es el resultado de la medición del tiempo de reverberación en escasas 4 y 3 posiciones con una sola posición de fuente en la sala principal y la de menores dimensiones. Para conocer la dispersión en el ensayo se calcula la desviación estándar, resultando 1,06 segundos en la sala grande y 0,1 segundos en la sala pequeña.

Se planificaron múltiples y diferentes mediciones, pero la ruptura de uno de los equipos limito el tiempo, por lo tanto tambien la información capturada.

Las condiciones de medición no fueron las ideales ya que la sala no estaba desocupada, 20 personas presentes en simultánea en ambas salas presenciaban la medición.

A partir de las mediciones y la obtención del tiempo de reverberación de la sala desocupada descritas en la figura 12 se despejo analíticamente la absorción total (At) del recinto de la fórmula de Sabine. A la misma se le suma la absorción producida por las personas y la cantidad de individuos presentes obtenidas con el procedimiento de Kath y Kuhl [Carrion] según la tabla 7 y las formulas 18 y 19. Utilizando la absorción resultante se calculó nuevamente el tiempo de reverberación con una ocupación del 50% y 100% en ambas salas. Los resultados de los cálculos antes descriptos se presentan en la Figura 13.

Tr (measure )=0,161∗VAt

(18)

At1 = At+N∗Ap (19)

De la fórmula 18 se despeja At, con este parámetro , la cantidad de individuos dentro del recinto(N) y la absorción de una persona con abrigo parada obtenida por la tabla 7 se calcula la absorción total del recinto con personas adentro(At1).

Con la fórmula 20 se calcula dependiendo de la capacidad de cada recinto, el tiempo de reverberación con personas dentro de los recintos.

Tr (Sabine )=0,161∗VAt 1

(20)

Como se acoto el análisis en frecuencias para la obtención del nuevo tiempo de reverberación se presenta nuevamente la dispersión en las medidas siendo 0,007 segundos la desviación estándar para la sala grande y 0,004 segundos la desviación estándar para la sala pequeña.

La tabla 8 muestra parámetros espaciales medidos y el tiempo de reverberación promedio entre 500 [Hz] y 1000 [hz] denominado RT mid de ambas salas.

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Table 1: Results in Main Room

Frec. [Hz] 125 250 500 1000 2000 4000 8000STI: 0,40 0,50 0,52 0,50 0,51 0,51 0,31

STI Male: 0,48 0,50 0,52STI Female: RaSTI:

Table 2: Results in Small Room

Frec. [Hz] 125 250 500 1000 2000 4000 8000STI: 0,52 0,57 0,54 0,53 0,55 0,52 0,28

STI Male: 0,51 0,52 0,56STI Female: RaSTI:

Table 3: Measure LF in format WY Main Room

M=Point S=source

Measure LF in format WY Main Room

Frecuency[Hz] 31,5 63 125 250 500 1.000 2.000 4.000 8.000 16.000IR_WY_M24S1 0,423 0,464 0,475 0,465 0,379 0,351 0,365 0,474 0,342 0,309IR_WY_M25S1 0,437 0,490 0,482 0,436 0,386 0,363 0,335 0,375 0,287 0,215IR_WY_M5S1 0,392 0,473 0,484 0,368 0,380 0,388 0,444 0,399 0,421 0,458IR_WY_M7S1 0,389 0,477 0,492 0,414 0,420 0,416 0,451 0,432 0,417 0,453

Average 0,410 0,476 0,483 0,421 0,391 0,380 0,399 0,420 0,367 0,359

Table 4: Measure LFC in format WY Main Room

M=Point S=source

Measure LFC in format WY Main Room

Frecuency [Hz] 31,5 63 125 250 500 1.000 2.000 4.000 8.000 16.000IR_WY_M24S1 0,598 0,656 0,672 0,657 0,536 0,496 0,516 0,670 0,483 0,437IR_WY_M25S1 0,619 0,693 0,682 0,616 0,546 0,513 0,474 0,530 0,406 0,305IR_WY_M5S1 0,554 0,669 0,685 0,520 0,538 0,548 0,628 0,565 0,596 0,648IR_WY_M7S1 0,551 0,675 0,695 0,586 0,594 0,588 0,638 0,611 0,590 0,641

Average 0,581 0,673 0,684 0,595 0,554 0,536 0,564 0,594 0,519 0,508

Table 5: Measure LF in format WY Small Room

M=Point S=source

Measure LF in format WY Main Room

Frecuency[Hz] 31,5 63 125 250 500 1.000 2.000 4.000 8.000 16.000IR_W_M1S3 0,295 0,452 0,450 0,456 0,424 0,414 0,430 0,435 0,451 0,433IR_W_M2S3 0,312 0,457 0,474 0,365 0,401 0,365 0,410 0,413 0,420 0,382IR_W_M3S3 0,290 0,462 0,494 0,391 0,411 0,442 0,431 0,474 0,461 0,409

Average 0,299 0,457 0,473 0,404 0,412 0,407 0,424 0,441 0,444 0,408

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Table 6: Measure LFC in format WY Small Room

M=Point S=source

Measure LFC in format WY Main Room

Frecuency[Hz]

31,5 63 125 250 500 1.000 2.000 4.000 8.000 16.000

IR_W_M1S3 0,417 0,640 0,636 0,644 0,600 0,585 0,608 0,615 0,638 0,612IR_W_M2S3 0,441 0,647 0,671 0,517 0,567 0,517 0,580 0,584 0,594 0,541IR_W_M3S3 0,410 0,654 0,699 0,553 0,581 0,625 0,609 0,670 0,652 0,578

Average 0,423 0,647 0,669 0,571 0,583 0,576 0,599 0,623 0,628 0,577

Table 7: Absortion measure with Kath y Kuhl method.

Table 8 : Comparison different parameters in both Rooms

Mid -Rt Unoccupied

Mid-Rt 50% Occupied

Mid-Rt 50% Occupied

LFE4 LF Mid

Main 2,031 1,404 1,080 0,419 0,390Smal

l1,400 1,102 0,906 0,424 0,414

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31.5 63 125 250 500 1000 2000 4000 8000 160000.000

0.500

1.000

1.500

2.000

2.500

3.000Tr Main Room

Tr Small Room

Frecuency [Hz]

Rev

erb

erat

ion

Tim

e [S

econ

ds]

Figure 12.Reverberation time measure in 4 positions with 1 source at both Room Unoccupied.

125 250 500 1000 2000 40000.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

1.600

1.800

Small Occupied 50%

Small Occupied 100%

Main Occupied 50%

Main Occupied 100%

Frecuency [Hz]

Rev

erb

erat

ion

Tim

e[Se

con

ds]

Figure 13 Reverberation Time calculates for Sabine and Kath y Kuhl in different room occupied.

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8. Discusion

Si el patrón polar del microno soundfield presentan distorcion en los gráficos de direcctividad en alguna banda,las mediciones de parámetros de espacialidad en la misma debera ser justificada o comparada con los medidos utilizando otra configuración propuesta por farina en el software Aurora ,entre ellas 2 microfonos omnidireccionales o un omnidireccional y otro figura de 8 ya que los resultados pueden no ser concluyentes .

Este trabajo no llego a realizar estas comparaciones por que el tiempo estipulado no fue el del programado por la ruptura de un fusible en la de alimentación de la fuente omnidireccional.

Mencionamos que el método de cálculo Rasti para la obtención de la inteligibilidad es una aproximación veloz, por procesar menos información, que el STI. Más allá de esta posible ventaja, es obsoleta su implementación en ciertas aplicaciones y debe ser contemplado.

9. Conclusions

A medida que la sala comienza a tener más gente dentro, la diferencia entre el tiempo de reverberación de las altas frecuencias y las bajas en la sala grande se incrementa. Las bajas frecuencias enmascaran las altas frecuencias, por esta razón pierde inteligibilidad el mensaje a medida que es mayor el número de individuos dentro de la sala [Carrion]. Este mismo fenómeno impacta con menor medida en la sala más chica.

Cuando la sala grande está vacía su RT mid cumple con las condiciones recomendadas para ser una sala de conciertos de música de cámara y sinfónica. A medida que la misma incrementa su población va tendiendo a ser apta para ejecutar solo música de cámara en ella[Carrion].

La sala pequeña por su Rt mid se recomienda para música de cámara. A medida que aumenta su población dentro baja el RT-mid y tiende a ser una sala polivalente.

La figura 14 muestra la relación del tiempo de reverberación y las preferencias de las cualidades de 40 salas del mundo en un ranking [Beranek]. Se compara el rt mid obtenido en la sala grande con dos salas: el Leverpool Philharmonic Hall (LV) 1,5 segundos en la posicion 31 y Washington opera house (29) 1,55 segundos en posición 25.Se eligieron esta salas por su volumen similar a la sala principal de la Usina del Arte. Si la sala estuviera al 50% de ocupación obtendríamos un Tiempo de reverberación similar a las salas bajo análisis.

El Rt-mid es muy bajo con la sala mayor al 100%. Si no tenemos en cuenta el volumen y vemos la evolución del tiempo sin ocupantes desde 2 [Seg] hasta el 50% de ocupación el RT-mid se encuentra dentro del rango de las mejores salas rankiadas .

La sala pequeña tiene tiempos más bajos, en cuanto a las cualidades subjetivas analizadas en relación con el tiempo de reverberación que denota el grafico se prefieren tiempos más altos, por lo tanto en la sala mayor es mejor cuando se compara cualidades subjetivas y Tiempo de reverberación.

Figure 14.Mid Frecuency reverberation time for 40 concert hall, measure with full

occupancy,plotted versus the subjective ranck orderings of acousticl quality.

Comparando los resultados de STI de las mediciones con los propuestos en el anexo g de la IEC 60268-16,

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tendrían que ser mayores y llegar a un STI de 0,62 como lo recomienda la norma para concert hall.

Según la tabla del anexo 1 , recomienda un STI y RASTI mayor a 0,65 por lo tanto se repite la situación anterior. La sala pequeña está más cerca de cumplir el objetivo recomendado [IEC60268][anexo1].

La figura 15 muestra el ranking de las mejores salas en relación a su LF. La medición de ambas salas de la usina mostro un LF resultante que supera el valor máximo de cualquier sala de las mostradas.

Figura 15.Plot of the lateral early fraction verssus the quality ratings of 22 concert hall.

La tabla del anexo 1 recomienda Un LF mayor 0,19 y la sala mejor rankiada tiene un LF de 0,17 es una situación ambigua .Por lo tanto la sala está dentro de lo recomendado pero no se la puede comparar con

ningún otra sala de las propuestas por la figura 15 por su gran diferencia.

Farina compare diferent transducter for measurement special parameters and conclude with “It can be concluded that actually no available microphone system can be used for assessing reliably the values of spatial acoustical parameters such as LE, LF or LFC”.[Farina].This postulate could concluded with result are incorrect.

1. REFERENCES

[1] International Standard ISO 3382, “Acoustics -- Measurement of room acoustic parameters”, 2009.

[2]Kleiner, M. A., “New way of measuring lateral energy fractions on spatial impression”. 6th International congress of acoustics, Tokyo, 1968.

[3] IEC 60268-16, Sound system equipment – Part 16: Objective rating of speech intelligibility by speech transmission index. 2003.

[4]Sala, L, P “Una nueva Mirada sobre las listas de palabras fonéticamente balanceadas”, 2012.

[5] Onieva, R, O, “Diseño acústico de un sala multifunción mediante empleo de paneles móviles”, 2013.

[6]Beranek,L, “Concert hall and Opera House”.Second edition.Springuer.USA 2004.

[7] Farina A., “Advancements in impulse response Measurements by sine sweeps technique”. 2007

[8] Carrion,A. “Diseño Acústico de espacios arquitectónicos”.Universidad Politécnica de Catalunya. EdicionsUPC. Mexico200

Annex 1: Recommended Values

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Annex G: IEC 60268

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