PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Unveiling the acoustics of the Cathedral of Santiago de Compostela
using 3D impulse responses
Francesco MARTELLOTTA1; Angel ALVAREZ CORBACHO2; Lidia ALVAREZ-MORALES3,
Francesca BALESTRA1, Federica CIANI1, Miguel GALINDO DEL POZO2, Juan José GÓMEZALFAGEME4, Pedro Fernando NOGUEIRA LΌPEZ5
1
2

IUACC - Universidad de Sevilla, Av. Reina Mercedes 2, 41012 - Sevilla, España
3

4

DICAR – Politecnico di Bari, via Orabona 4, Bari, Italy

Department of Theatre, Film and Television, University of York, York, UK

ETSIST, Universidad Politécnica de Madrid; Nikola Tesla s/n 28031 Madrid;España

5

Dep. de Física y Ciencias de la Tierra, Universidade da Coruña, Rua da Fraga, 27,15008 A Coruña; España

ABSTRACTThe Cathedral of Santiago de Compostela in Spain is the arrival point of hundreds of thousands of pilgrimsthat every year get to the city along the many paths of the “Camino de Santiago”. Along with the oldest partof the city, the cathedral is listed as UNESCO World Heritage, and several restoration works have been done
and still are under way. Its acoustics is the result of centuries of evolution since the original Romanesquebuilding was started, with contributions from nearly every architectural style. The interior arrangement of thechurch is significantly different from the majority of Spanish cathedrals, where the choir typically breaks themain nave creating many sub-spaces. Here the plan follows the Latin cross shape that allows the pilgrims tolook at the altar from nearly everywhere, allowing source-receiver distances up to 70m. An acoustic surveythat involved different laboratories allowed the collection of monaural, binaural, and B-format impulse
responses in different source-receiver combinations. In the present paper, the results pertaining to B-format
measurements are presented, discussing the spatial distribution of the acoustic parameters as a function of thedirectional pattern of the reflection and of the energy distribution among the different spaces of the church.
Keywords: Worship acoustics, Santiago cathedral, 3D impulse response

1. INTRODUCTION
The architecture of Christian churches, throughout its twenty centuries of history, has been
responding to the needs and activities that developed inside them, becoming the spatial result of theevolution of liturgical practices(1). The first Early-Christian churches were thought as simple meeting
spaces, where the liturgy was considered as the center of the celebration. Later on, churches evolved
according to the different architectural types, regional characteristics, often responding to purely“representative” needs. Thus, the original connection between space and function was often lost in
favour of the splendor of the space “ad maiorem Dei gloriam”.
The role of sound inside such spaces has always been crucial because, even if the Latin language
was the official language of the Church, and most of the faithful were unable to even understand aword, there were preachings and readings that were given in current language, and required better
intelligibility. Inside the presbytery, where all the chapter of the priests stood, good intelligibility was
also requested. In addition, liturgical music and chanting were also part of the liturgy and, for thepreviously mentioned reasons, they played the major role in the involvement and participation of thefaithful to the celebration. And this introduces a further element of complexity which is related to thenatural evolution of the musical language from plain chant to polyphony (2).
The number of mutually connected aspects was therefore quite large, and this, combined with a lackof a clear understanding of the acoustic phenomena, resulted in spaces where the listening experience1

francesco.martellotta@poliba.it

2327

was far from optimal. This is further emphasized after the Second Vatican Council (1962-65) whichgave greater emphasis to the understanding of the whole liturgy (and not just the preaching), by
adopting national languages as a standard.
The number of studies dedicated to church and worship acoustics has been increasing in the last 20
years(3), involving research groups from different countries, and investigating several aspects
spanning from the modelling of sound field variations inside them, the role of occupancy, therelationship with liturgy and music, the definition of rating schemes for optimal listening conditions.
In many cases, given the specific qualities of the spaces (resulting from different shapes andincremental addition of chapels and other sub-volumes, a certain difficulty to generalize resultsappeared, so that detailed investigations are needed in order to acoustically characterize each church.
The Cathedral of Santiago de Compostela, within the context of Spanish cathedrals(4) represents asignificant exception. In fact, its interior arrangement is significantly different from the majority of
Spanish cathedrals, where the choir typically breaks the main nave creating many sub-spaces. Here theplan follows the Latin cross shape that allows the pilgrims to look at the altar from nearly everywhere,according to an interior distribution which is more similar to typical Italian cathedrals. The present
paper describes the results of a measurement campaign that was carried out in the church, outlining inparticular the results obtained by the use of 3D impulse responses.

2. METHODS
2.1 Building description
The Cathedral of Santiago, was originally built as a small Roman mausoleum of the first century, inwhich the remains of the Apostle Santiago (AD 44) were buried in the year 813. After the discovery of
the apostle’s relics, a first chapel was built of stone and terracotta. Being too small to accommodate thefaithful, in 899 it was replaced by a temple which was later destroyed by the Muslims and rebuilt in
1003, in a pre-Romanesque style, which converted it into a great pilgrimage cathedral. The final
aesthetic revolution, arrived in 1860 with the facade of the Obradoiro in Baroque style. Currently it has
a volume of around 42,000 m3 , a floor area of 2,511 m2 , an overall surface of about 18,000 m2 , and acapacity of 620 seated people.
Its interior is finished in materials such as granite (floor, walls and chorus), plaster (all the temple)
glass (windows), wood (pews and confessionals) typically characterized by relatively low absorption
coefficients, with the exception of wood and glass that are more absorbing in the low frequencies.
This somewhat suggests that the expected reverberation time will be long and strongly dependent on
the presence of the faithful who, on the other side, are characterized by a high sound absorption.

a)b)

Figure 1 – a) Facade of the Obradoiro, b) interior view of the Cathedral of Santiago de Compostela
2.2 Measurement procedure
The measurements were carried out during the night, in order to minimize background noise, andthe requirements of ISO 3382-1 (5), as well as according to the specific prescriptions established forthe churches (6,7). With reference to the arrangement of sources and receivers, as the cathedral ofSantiago moves away from the typical configuration of the Spanish churches (7), the source was2328

located at the Main altar (S1), on the pulpit (S2) and at the modern altar (S3). On the pulpit, the source
was placed as close as possible to the balustrade to simulate the natural position of the speaker. In all
the cases source height was 1.70 m. As the cathedral of Santiago, with Latin cross plan, is almost
perfectly symmetrical, the receivers were located in the main nave in only one half of the church,placing only three control receivers in the other half, in the transept braces, and in the presbytery. All
the microphones were located at a height of 1.2 from the floor (Figure 2).
The impulse response acquisition process, from which all acoustic parameters were determined (inaccordance with ISO 3382-1 (5), was carried out by different researchers teams, with their own tools,
in order to guarantee a complete coverage of the whole church and the detection of all acoustic
monaural and binaural parameters. Sound sources included an AVM DO-12 dodecahedron with B&K
amplifier 2734, a B&K 4296 source, and a self-built source. The recordings were made using different
microphones, including a B-format Soundfield ST350, an Audio-Technica AT4050 / CM5 microphone
in its omnidirectional and figure-of-eight configurations, a Head III binaural head (Head Acoustics)
connected to a B&K 2829 signal conditioner, and two microphones AKG (omnidirectional andfigure-of-eight shaped).
The methodology used by the three research groups was developed with the aim of being the most
optimal so that the data could be easily shared and comparable. Impulse responses (IRs) were recordedby exciting the source with a sine sweep spanning from 63 Hz to 16 kHz. The length of the sweep was
kept at about 10 s so that the impulse-to-noise ratio (INR) was at least 45 dB at each band for every
source receiver combination. Different software tools were used to manage impulse response
acquisition. In one case (Bari) the microphone was connected to a four-channel portable recorder(Tascam DR680), and the measurements were carried out in open loop configuration, being
subsequently post-processed. The recorded room responses were deconvolved and the resulting IRs
were then used to calculate all the acoustical parameters, calculated according to ISO 3382-1(5).

Figure 2 – Plan of the church with source (S) and receiver (R) placement

2.3 3D sound field visualization
Although a thorough description of the procedure to obtain spatial visualization of sounddistribution can be found elsewhere(8), it may be useful to quickly remind the basic concepts behind it.
Ambisonic microphones provide four signals identified as W, X, Y, and Z. The first one represents theomni-directional response of the microphone at the centre of the microphone array. The others
correspond to the 1st order components and provide a figure-of-eight response oriented along each oneof the three Cartesian axes. So, they provide the sound pressure multiplied by the vector of the sounddirection along that axis. In other words, considering that particle velocity (u) is a vector quantity
oriented along the direction of sound propagation, and that (at least for plane waves) u is proportional
to sound pressure (p) through characteristic impedance Z0 (u=p/Z0 ), X, Y, and Z may also be assumed
as the Cartesian components of the particle velocity and used to determine sound intensity properties.

2329

All this stated, considering the relationship between sound intensity, sound pressure and particle
velocity, the instantaneous intensity components may be expressed as:
𝐼 = 𝑝 ⋅ 𝑢 = 𝑤 ⋅ 𝑥/𝑍
𝐼 = 𝑝 ⋅ 𝑢 = 𝑤 ⋅ 𝑦/𝑍
𝐼 = 𝑝 ⋅ 𝑢 = 𝑤 ⋅ 𝑧/𝑍(1)

Where w, x, y, and z are the output signals of the B-format microphone. At this point, given the I x ,I y, and I z components of the sound intensity, the direction of arrival of the sound at a given time may beeasily calculated from the following equations:𝜃 = atan𝐼
𝐼𝐼; 𝜙 = atan

(2)

𝐼 +𝐼

So, combining together the direction of sound with its intensity, sound field properties may be
reproduced using a multichannel loudspeaker system or, as in the present case, used to obtain a spatial
map of the sound intensity as a function of time.

3. RESULTS
3.1 Mean values of acoustical parameters
First of all the spatially averaged values as a function of frequency and source position wereanalyzed. For reverberation time (T30) no substantial differences appeared as a function of source
position (Fig. 3a), while in terms of frequency, substantially stable values appeared up to 500 Hz, thenrapidly decreasing as a consequence of air absorption. When early decay time (EDT) was considered(Fig. 3b), the shortest values appeared when source S3 was used, while the longest appeared when thesource was on the pulpit. However, a significant scatter in data was observed, with standard deviations
as large as 2 s in the frequency bands below 1 kHz and very short values observed close to the source.
Center time (Ts) and clarity (C80) both showed similar variations as a function of frequency andsource position (Fig. 4), with the lowest clarity values (and the longest Ts values) observed whensource was on the pulpit. The explanation of this rather counter-intuitive result stands in the fact thatthe receivers included in each average differ slightly as a function of source position, so that a larger
number of receivers closer to the sound source may lower the mean values, but this does not imply thatpoints at comparable distance behave differently.
6.0

6.0
S1

5.0

S3

3.0

S3

3.0

2.0

2.0

1.0

1.0

0.0

S2

4.0

EDT [s]T30 [s]

4.0

S1

5.0

S2

0.0
63

125

250

500

1k

2k

4k

8k

Octave Band Frequencies (Hz)

63

125

250

500

1k

2k

4k

8k

Octave Band Frequencies (Hz)Figure 3 – Plot of spatially averaged T30 (a) and EDT (b) values measured as a function of frequency andof source position

2330

350

5.0

300

4.0

S2

3.0

S3

2.0

C80 [dB]

Ts [ms]

250

S1

200
150

1.0
0.0
-1.0

S1

-2.0

100

S2

-3.0

50

S3

-4.0

0

-5.0
63

125

250

500

1k

2k

4k

8k

63

Octave Band Frequencies (Hz)

125

250

500

1k

2k

4k

8k

Octave Band Frequencies (Hz)Figure 4 – Plot of spatially averaged Ts (a) and C80 (b) values measured as a function of frequency and ofsource position
3.2 Effect of source receiver distanceIn order to better understand some of the behaviors observed before, the parameters were plotted asa function of source receiver distance for selected frequency bands. In case of EDT (Fig. 5a) the plot
showed a quite varied condition, with significantly shorter values at receivers close to sound sources.
However, distance proved not to be the only influencing factor, as receivers 10 and 16 behaved quite
differently when source S1 (which is at nearly the same distance of about 9 m) was used. Receiver 16
had an EDT very close to the spatially averaged T30 value, while receiver 16 was shorter than 2 s. The
latter was located inside the presbytery area, where double slopes were clearly detected by means ofBayesian analysis (9). In fact, Figure 5b showed that at R16 the first decay process was characterized
by a reverberation time of about 1 s, while the second one, appearing after about 0.5 s, clearly had thesame value of the mean T30.
The mid-frequency values of Ts and C80 were also plotted as a function of source receiver distance(Figure 6). In this case, in order to provide a reference, predicted values obtained by means of the“revised model”(10-12) were also plotted. Values were calculated assuming the geometric parameters
given before and assuming the constant terms s=0.2 (suitable when the source is raised and surroundedby reflecting surfaces), and k=2.8 (corresponding to a space with a richly articulated volume). It isinteresting to observe that values pertaining to sources S1 and S2 fitted well with predicted values,
showing the typical exponential variation as a function of distance. Conversely, results pertaining tosource S3 appeared shifted, with clarity which was 3 dB higher compared to points at the same
distance, and Ts which was about 75 ms shorter than points at comparable distance.
6
5

EDT,mid [s]Relative level [dB]

R10

4
3

S1

2

R16

S2

1

S3

0
0

20

40

60

Source_Receiver Distance [m]Figure 5 – a) Plot of mid-frequency values EDT as a function of source receiver distance and source
position. b) Results of Bayesian analysis applied to S1-R16 at 1 kHz.

2331

10

450

8

S1

400

6

S2

350

4

S3

300

2

Revised model

C80,mid [dB]

Ts [ms]

500

250
200

S1

150

S2

100

S3

50

Revised model

-2
-4
-6
-8
-10

0
0

a)

0

20

40

Source_Receiver Distance [m]

0

60

b)

20

40

60

Source_Receiver Distance [m]Figure 6 – Plot of mid-frequency values of a) Ts and b) C80 as a function of source receiver distance andsource position. Revised model was applied by assigning the characteristic parameters s=0.2 and k=2.8.

A substantially similar behavior was observed also at other frequencies, with the significant
exception of the lowest bands, where, as already observed, source S2 showed markedly lower C80
values and higher Ts. Realistically, the position of source S2, although raised, offered little support tolower frequencies which were diffracted by the pillar rather than reflected. Conversely, source S3
which stood in a well isolated sub-volume (and during measurements the presence of scaffolding on
the top of the presbytery caused a further reduction of the available space), benefitted of increased
early reflections that realistically caused the observed variations. In order to better clarify such aspects
3D mapping offered a significant support.
3.3 3D sound maps
Following the observations made in the previous section it was interesting to compare different
source receiver combinations corresponding to similar distances in terms of directional distribution ofreflections. As a first example combinations S1-R02 and S3-R01 were considered (Figure 7),
corresponding respectively to 21.1 m and 23.8 m. In the second case, even though the distance wasbigger, the direct sound was louder, and stronger early reflections, mostly coming from the presbytery
area were observed. Conversely, in the first case, reflections arriving in the first 100 ms after direct
sound were more diffuse, arriving mostly from the front and the sides, but definitely less loud. Such
difference in the direction of arrival was also proved by the different J LF values, which were 0.34 in thefirst case and 0.19 in the second.

Figure 7 – 3D sound map at 1 kHz octave band of: a) S1-R02; b) S3-R01. In all the cases a 100 ms timeinterval is considered, starting 5 ms after direct sound2332

The second pair of IRs that were analyzed were S2-R05 and S3-R04 (Figure 8), withsource-receiver distances respectively of 46.3 m and 49.7 m. It was interesting to notice that, in thiscase, most of the reflections were focused along the longitudinal axis, with a clear dominance offrontal reflections. In particular, when the source was located on the pulpit the lack of large reflecting
surfaces (and the significant diffraction and scattering effects) caused amore circumscribed
distribution of the reflections around the source-receiver direction. Conversely, when the source wasclose to the main altar, the direction of arrival of the early reflections spread over an angle of ±30° andcontributions were generally stronger. This difference in angular distribution of reflections could befound also in J LF values, equal to 0.126 for S02-R05 and to 0.153 for S03-R04.
So, taking into account both the previous results, what emerges clearly is an important role of themain altar and of the surrounding surfaces (it is important to remember that quite unusually, the spansof the presbytery are closed with thick glass surfaces, that ensure further strong reflections comingfrom that area), in increasing the number and magnitude of early reflections. When the source is at themodern altar or on the pulpit the reflections appeared weaker and scattered.
In order to better clarify the role of the surfaces around the presbytery, combination S3-R16 was
finally analyzed, taking into account different time intervals. As shown in Figure 9, the earlyreflections clearly arrive from the closest and hardest surfaces (like the floor and the glazed elementsthat close the bays), and they continue arriving from within the same subspace up to about 0.5 s. Afterthat time, diffuse sound reflections coming from the main church volume start arriving and they
contribute more and more to the late sound decay.

Figure 8 – 3D sound map at 1 kHz octave band of: a) S2-R05; b) S3-R04. In all the cases a 100 ms timeinterval is considered, starting 5 ms after direct soundFigure 9 – 3D sound map at 1 kHz octave band of S3-R16: a) considering a 100 ms time interval, starting 5
ms after direct sound; b) considering a 0.5 s starting from 1 s after direct sound arrival2333

4. CONCLUSIONS
The results of the measurements carried out in the Cathedral of Santiago de Compostela have beenpresented in the paper. The church represents a significant exception among the Spanish cathedrals
because of the lack of the choir opposite to the presbytery that typically closes the central nave. Thissolution, adopted for the benefit of the many pilgrims that attend the celebrations, makes the church
more similar to Italian models. The analysis of the acoustical parameters pointed out significant
variations as a function of source and receiver placement. In particular, presbytery area which ischaracterized by a significant amount of reflecting surfaces (bays are closed by glazings and ascaffolding reduced the available height at the time of the measurements) affected significantly both
source and receivers located inside it. With reference to the source, the 3D sound maps confirmed thatextra reflections provided by the surfaces contributed to increase magnitude and number of earlyreflections, so that a 3 dB improvement in clarity (and a decrease of 75 ms in center time) values wasfound as a function of distance. Comparison with predictive models, also showed that values
pertaining to other sources were in good agreement with the expected behavior, while the source at thehigh altar provided quite unusual results. With reference to receivers inside the presbytery, they
showed double slopes, independent of the source position (but clearly emphasized when the source
was inside the same space). Again 3D sound maps clearly showed that up to about 0.5 s reflections
arrived mostly from surfaces inside the presbytery, while after that time the dominant contribution
arrives from diffuse reflections from the main nave. A more detailed analysis in under way to better
clarify the role of source and receiver placement with reference to other combinations (e.g. located in
the transept), as well as in order to provide a more thorough analysis including the results of all theteams participating to the project.

ACKNOWLEDGEMENTS
Authors are grateful to the Santiago Cathedral Foundation for giving us access to the acoustic andarchival study of this magnificent place, and to Jose Carlos Alonso Seoane, parish priest of the Churchof Santa Cruz de Oleiros (A Coruña) for his contribution to the organization of the measurements.

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