BeBeC-2016-D6

Analysis of sound field variations in concert halls via
visualization and objective parameter comparisonSébastien Barré1 , Olaf Jaeckel1 and Ralf Bauer-Diefenbach2
1 GFaI Gesellschaft zur Förderung angewandter Informatik e.V.

Volmerstraße 3, 12489 Berlin, Germany
2 MMT Network GmbH
Helmholtzstraße 2-9, 10587 Berlin, Germany

Abstract

The application of spherical microphone arrays in the field of room acoustics allows fora more detailed analysis of the behaviour of the sound field characteristics and therefore
it may be a very valuable supplement of standard room acoustic measurement procedures.
With the objective of exemplifying and expanding the application of 3D-beamforming techniques to the field of room and building acoustics, a series of measurements has been performed in real environments. This paper presents the results of an exemplary measurement
session performed at the Werner Otto hall of the Konzerthaus Berlin. The measurement
results demonstrated the ability of the new microphone array techniques to confirm the differences of the acoustic characteristics of the room before and after application of a special
acoustic treatment placed behind and beside the stage.

1 IntroductionT

HIS paper presents the application of a 3D-microphone array system in the field of room
acoustics. The development of correlation and spectral deconvolution methods ([4], [5]) allows
the determination of room impulse responses for every individual microphone within the spherical array. Combined with 3D-beamforming techniques to create acoustic surface maps in theroom, a detailed analysis of room reflections and of further sound field characteristics is provided. In the following, the application of this method is demonstrated using the results of an
exemplary measurement session performed at the Werner Otto hall of the Konzerthaus Berlin.
According to the subjective evaluation given by musicians and by the technical directorate of
the Konzerthaus Berlin, the acoustic of the Werner Otto hall is considered highly absorptive,

1

6th Berlin Beamforming Conference 2016

Barré, Jaeckel and Bauer-Diefenbachwith inevitable corollaries of an unbalance distribution, an insufficient blend of the reverberant
sound field which has unfortunate consequences in orchestral music performances, and the seats
most distant from the stage are acoustically highly disadvantaged. Additionally, in particularroom conditions an audible echo emerges between parallel partitions of the room. The analysisof the microphone array measurement results in combination with classical room acoustic analysis can give a precise description of the acoustic characteristics of the room, and it emphasizes
the benefits of placing wooden standing panels behind and beside the stage. The resulting 3D
acoustic pictures have been matched to the results of a 3D scan of the interior of the concert
hall, the distribution of the acoustic energy in the room has been analyzed, and the results are
given and discussed.

2 Measurement setup
The Werner Otto Saal was built in a modern small-size shoe-box shape configuration, with anelevating system which offers the possibility to adjust the height of the stage and of the audience(as depicted by the lower schematics of figure [1]). During the measurement, the stage was set
at level 0 and the audience was in a gradually elevated floor arrangement.
One source position (on the stage) and two receiver positions were selected for the measurement (one on the stage and one at the fifth row of seats). The receiver positions will be
designated as Stage and Audience in the following. The source and receiver positions as well as
the room configuration are presented figure [1]. Twenty room impulse response measurements
were performed using MLS signal as well as sine sweeps. After a first measurement session at
both receiver positions, RESPA c standing resonators were placed behind and beside the stage,and new measurements have been performed. The RESPA c resonators are wooden made panels which resonate with the sound field of the room. Their development has been inspired by
the technique of the (centuries-old) lutherie craft to enhance the processes of resonance.
One transparent spherical array of 120 microphones together with an Acam Recorder wereused, the room was excited using a Globesource dodecaheder and an RME-Babyface Soundcard. The dodecaheder was supported in the low frequency range by a Meyersound Subwoofer.
Additionally to the measurement with the Acoustic Camera, a type 1 microphone from G.R.A.S.
was used to capture the sound field at the source for the reference signal and a Microtech Gefell
type 1 microphone at the receiver position was used for verification.

2

6th Berlin Beamforming Conference 2016

Barré, Jaeckel and Bauer-Diefenbach

Figure 1: Seating plan of the Werner Otto hall and measurement positions (the receiver positions are indicated by the green marks, the red mark indicates the source position).
c Konzerthaus Berlin.

36th Berlin Beamforming Conference 2016

Barré, Jaeckel and Bauer-Diefenbach

3 Impulse response
The impulse responses of each of the 120 signals captured by the microphones of the array werecomputed. As described in ISO-18233 [1] a linear deconvolution is performed in the frequency
domain (equation 1) between the N signals yi (t) and the original signal x(t) sent to the room.
The measurement and processing principle is presented in the schematics [2].
FFT (yi (t))
hi (t) = IFFT
(1)FFT (x(t))
N - linear deconvolution

N

FFT

N
g1(t)

SD

s(t)

N

IFFT

N

N
gN(t)

d&s
beamforming

FFT

Figure 2: Measurement principle: the loudspeaker excites the room with a sine-sweep signalthat propagates and reaches the N microphones of the array. The linear deconvolution
stage is performed via regularized spectral division (SD) of the Fourier transform(FFT) of the N signals coming from the array by the Fourier transform of the original
signal sent to the room. After an inverse-FT (IFFT), the resulting impulse responses
are processed by delay-and-sum (d&s) beamforming.
The linearity of the deconvolution process preserves the relative delay of arrival between eachsignal of each microphone ([4], [5]) and therefore, by sending the result of the deconvolution
process directly to the beamformer, it is possible to localize the source, as well as the hits of theacoustic waves that arrive at the array on the walls of the room.

Figure 3: Time representation of the impulse response measured at the stage position.

46th Berlin Beamforming Conference 2016

Barré, Jaeckel and Bauer-Diefenbach

Finally, the acoustic map delivered by the beamformer is superimposed on a 3D-model ofthe room under investigation to localize the direct path at the source and the early reflections
on the floor, the walls, and on the ceiling of the concert hall. The first 400 ms of the impulse ispresented figure [3]. Figure [4] and [5] illustrate the results of the mapping.

Figure 4: Mapping of the acoustic energy in the room, direct path (left) and first reflection on
the standing resonator behind the stage (right).

Figure 5: Mapping of the acoustic energy in the room, third reflection on the first step of theaudience area, and forth reflection on the ceiling with RESPA c panels.

56th Berlin Beamforming Conference 2016

Barré, Jaeckel and Bauer-Diefenbach

4 Third octave analysis and energy decay
To analyze the reverberant field in the room more precisely, the energy decay curves in thirdoctave bands are computed from the impulse responses. This provides us with a frequency aswell as time domain decomposition of the acoustic energy in the room. Figure [6] depicts the
3rd octave frequency bands 630 Hz and 800 Hz, figure [7] the 1600 Hz and 2000 Hz frequency
bands. The red curves depict the measurements performed with the acoustic treatment, the blue
those without treatment.

Figure 6: Energy Decay curve at the stage measurement position without and withRESPA c panels (blue and red curves respectively) left: 630 Hz , right: 800 Hz.

Figure 7: Energy decay curve at the stage measurement position without and with RESPA c resonator (blue and red curves respectively) left: 1600 Hz , right: 2000 Hz.
Using this type of analysis and averaging over the microphones of the array, the flutter echo
is clearly visible. The corrections effected by the acoustic treatment are also well visible at thelater part of the energy curve: the slope of the energy decay remains constant and homogeneous;

66th Berlin Beamforming Conference 2016

Barré, Jaeckel and Bauer-Diefenbachwhereas high peaks, corresponding to the reflections, have totally disappeared. To be able tolocalize the reflections responsible for the flutter echo, the 3D mapping of an energy peak of the2000 Hz third-octave band (depicted figure [8], left) is computed. The mapping of the same timeinterval of the measurement with the wooden panels is also depicted for comparison (right). As
expected, the energy distribution in the room is much more homogeneous after the acoustic
treatment. As a comparison, the pictures figure [9] present the mapping of the acoustic energy
in the room without treatment. The selected time range is taken at the exact position (left) andjust after a peak (right) appearing at the very beginning of the flutter echo. On the second map,the energy appears well distributed on the walls of the room. This confirms that at that time, theacoustic energy did not start to echo back and forth between the parallel walls of the hall.

Figure 8: Mapping of the acoustic energy in the room energy peak of the 2000Hz 3rd octave
band, without and with RESPA c panel.

Figure 9: Mapping of the acoustic energy in the room without treatment. Selected time range atthe exact position (left) and just after a peak (right) appearing at the very beginningof the flutter echo.

76th Berlin Beamforming Conference 2016

Barré, Jaeckel and Bauer-Diefenbach

5 Late reverberation: spacial distribution of the acoustic energy
The spacial distribution of the acoustic energy in the room is analyzed with a mapping of the
600 ms following the early reflections (98 - 698 ms of the impulse response). The picture on
the left, representing the sound field in the room without the acoustic resonators, reveals thehigh energy reflections between the front and back walls of the hall. In comparison, thesereflections are attenuated by the treatment as shown in picture [10] on the right. This analysis
can be refined using a differential acoustic photo in which the level difference at each pointof two acoustic maps is computed and presented on a color scale. Figure [11] depicts thedifference between of the acoustic maps at the stage position: the minuend being the map withtreatment, the subtrahend the map without treatment. The color scale extends from -1.2 dB
to 1.2 dB. The predominance of the reflections on the back wall of the hall and the positiveeffect of the treatment can be observed at the green and blue areas (the negative values): theydelimit the areas at which the energy diminished between the two measurements. Inversely, thepositive values (yellow-red-magenta areas) delineate the areas at which the acoustic reflections
increased. Their absolute strength did not necessarily increase but with the flutter echo being
eliminated, their relative strength at the measurement position increased and the sound field
homogenized.
The frequency distribution of the differential energy can be determined from the result andrepresented graphically (figure [12]).

Figure 10: Mapping of the late reverberation at the stage measurement position. Left no treatment, right with treatment.

6 Room acoustics parameters
The subjective impression given by the acoustic field in the room is usually characterized withthe help of acoustical parameters. Those which have acquired the widest acceptance and havebeen subject to standardization are the reverberation time T20 , the early decay time EDT , Clarity
C80 and Definition D50 (ISO-3382 [2]).
The decay the sound energy undergoes after the impulse excitation is characterized by T20 . It
is obtained by calculating the slope of the energy curve after it has been integrated starting from8

6th Berlin Beamforming Conference 2016

Barré, Jaeckel and Bauer-Diefenbach

Figure 11: Difference photo of the acoustic energy (level difference at each point of two
measurements).

Figure 12: Frequency analysis of the difference photo.
the end (Schröder integration [6]). The EDT parameter is similar to the T20 with the difference
that the slope is determined by the first 10 dB decays of the curve instead of 30 dB for the T20 .
The first part of the impulse response is essential for the transmission quality of the information
between the source (speaker or music instrument) and the receiver (the audience). This is what
the Clarity and Definition parameters attempt to characterize (Kuttruff, [3]). Their definitions
are given equation [2].
Definition:

Clarity: 80
2g(t) dt 
C80 = 10 log  0 ∞ dB
2g(t) dt

RR

 R 50
2g(t) dt 
D50 =  R0 ∞
 100%

80

0(2)

g(t)2 dt

Their computation was performed on the averaged data collected by the microphones of
the array. Figure [13] presents the results thereof before (blue curves) and after the acoustic
treatment (red curves) for the Stage and Audience measurement position. As expected, theparameters’ overall tendency will follow the corrections brought by the acoustic treatment: theEDT and reverberation time are slightly lowered at the frequencies at which the flutter was
most prominent because the energy decay becomes more homogeneous. It is to be noted thatthe Definition and Clarity parameters are much more sensitive to the corrections.

96th Berlin Beamforming Conference 2016

Barré, Jaeckel and Bauer-Diefenbach

Figure 13: Room Acoustic parameters of the Werner Otto Saal, stage and audience measurement positions, before (blue curves) and after treatment (red curves).

7 Conclusion
The application of the beamforming technology with a spherical microphone array in combination with room impulse response measurements by deterministic signal can deliver a detailed
insight of the sound propagation mechanisms. Confronting the results given by beamforming
the conventional frequency decomposition as well as the acoustic parameters shows the coherence of the analysis. The system also includes differential acoustic map computation which
offers the possibility to visualize the effects of local modifications and treatment applied to aroom, and illustrate the results of an optimization. It is a very powerful tool that provides theacoustician with visualizations of the sound propagation in a room and can assist them in theirwork of room and building acoustic optimization.

10

6th Berlin Beamforming Conference 2016

Barré, Jaeckel and Bauer-Diefenbach

Acknowledgment
We thank MMT Network GmbH for their very professional support of the standard room acoustic measurements. Thanks also go to our colleagues at gfai tech GmbH for their active help withthe measurements and to the technical directorate of the Konzerthaus Berlin for giving us accessto the measurement venue.
This research work has been funded by the German Federal Ministry for Economic Affairs
and Energy (Bundesministerium für Wirtschaft und Energie, BMWi) under project registration
number MF130141.

References[1] ISO-18233:2006. “Acoustics – application of new measurement methods in building
and room acoustics.” ISO ISO-18233:2006, International Standard Organisation, Geneva,
Switzerland, 2006.
[2] ISO-3382:2008. “Acoustics – measurement of room acoustic parameters – part 2: Reverberation time in ordinary rooms.” ISO ISO-3382:2008, International Standard Organisation,
Geneva, Switzerland, 2008.
[3] H. Kuttruff. Room Acoustics, Fourth Edition, chapter 7 Characterisation of subjective effects, pages 190–233. Taylor & Francis, 2000. ISBN 9780419245803.
[4] S. Müller and P. Massarani. “Transfer-function measurement with sweeps.” Journal of
the Audio Engineering Society, 49(6), 443–471, 2001. URL http://www.aes.org/
e-lib/browse.cfm?elib=10189.
[5] Neugebauer, Barré, and Döbler. “Application of correlation analyses in 2d and 3d beamforming.” In Proceedings on CD of the 5th Berlin Beamforming Conference, 19-20 February 2014. GFaI, Gesellschaft zu Förderung angewandter Informatik e.V., Berlin, 2014.
ISBN 978-3-942709-12-5. URL http://bebec.eu/Downloads/BeBeC2014/
Papers/BeBeC-2014-06.pdf.
[6] M. R. Schroeder. “New method of measuring reverberation time.” The Journal of the
Acoustical Society of America, 37(3), 409–412, 1965. doi:http://dx.doi.org/10.1121/1.
1909343. URL http://scitation.aip.org/content/asa/journal/jasa/
37/3/10.1121/1.1909343.

11