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

A case of archaeological evidence in favour of acoustical intentions
linked with pots in church vaults: Montivilliers Abbey
Jean-Dominique Polack1; Jean-Christophe Valière2; Bénédicte Bertholon3, Pauline Carvalho2
1

Sorbonne Université, UMR CNRS 7190, Institut Jean le Rond D’Alembert, France
2

CNRS – PPRIME, UPR 3346, Université de Poitiers, ENSMA, France
3

CESCM - CNRS - Université de Poitiers, France

ABSTRACT
Montivilliers Abbey near Le Havre in France presents a case where pots in the vault were obviously
employed to act on the acoustics. The archives of the abbey contain a written mention that links pots withacoustics. Originally built in the 13th century, the Romanesque vaulting of the transept crossing was latermodified at an unknown date by the addition of pots. In 1648 the vaulting was lowered down, hiding theoriginal pots. A dozen of new pots were inserted in the new vault. The presentation focuses on the acousticsof the abbey and documents the organization of the pots by the builders in the 1st and the 2nd vaults.
Room-acoustical measurements of the church were also carried out in order to evaluate its acoustical quality.
A major conclusion is that lowering the vault has singled out the church choir where the nuns chanted andwhich is characterized by reverberation times lower than in the nave. Intermediate reverberation times were
measured in the crossing. Reverberation times in octave 125Hz, which includes the resonance frequencies ofthe pots, are marginally lower in the choir and the transept for source in the nave, which could indicate aneffect of the pots.
Keywords:

1. INTRODUCTION
Singing and speaking are fundamental practices in the religious sphere. The texts of the Church
fathers and their commentators are explicit about it. The evolution of singing from monophony topolyphony is also well-documented (1). What is less known are the methods and techniques used bybuilders for modifying spaces to better serve songs and words. Many assumptions exist, but theoreticaltexts from Antiquity (Aristotle, Vitruvius), to the Modern Era (Mersenne, Kircher), even those ofarchitect (Alberti, Ph. De l'Orme), are discrete on these practices (2).
Acoustic pots, spread throughout Europe from Antiquity to the 18th century, are one of the few
identified evidence of efforts to modify sound spaces (2). Rediscovered by scholars in the 19th century
but badly understood due to deficient acoustics knowledge by archaeologists, it was revived in recent(3, 4).
From some 50 churches throughout France and Europe, rules for pot implantation and selection
according to resonance frequencies have been determined by acoustic measurements. More than 1000
pots have been analyzed, and lead to the determination of trends in space according to resonancefrequencies and liturgical uses (5). More recently, effort has focused on particularly significant
buildings such as the Abbey Church of Montivilliers.

2. HISTORICAL ASPECTS
Montivilliers has several unique features. The project aims at better understanding the historicalsequencing of the building modifications and their reasons.
1jean-dominique.polack@sorbonne-universite.fr
jean.christophe.valiere@univ-poitiers.fr
3
b.palazoo@yahoo.com
2295

2.1 Historical sequence
The plan of the Church of Montvilliers (Figure 1) shows the separation between the monastic
Church of Notre-Dame and the parish church of Saint-Sauveur. According to the sources, (3) the nuns
were mainly located in the transept crossing, the very place where modifications related to theacoustics were carried out. Figure 2 sums up the available information.

Figure 1: plan of the Church of Montivilliers with the Abbey part (Notre -Dame) and the parish part
(Saint-Sauveur), probably separated by a gate.
The Abbey was consecrated in 1141 and the Romanesque vaulting was realized in 1251, probably
without pots. Between this date and 1648, pots were added to the vaulting as shown in Figure 3, at an
unknown date. This was probably at the time of the apogee of the Abbey as a result of awareness of
discomfort due to reverberation. The ceremonial of 1626 is quite explicit, as shown in Figure 4. It
recommends that the nuns leave "some small space (...) at the end of the verse before starting theother." Likewise in the responses from one choir to another, "one must n ot take over immediately: but
wait for them to be finished, & the other choir will take over with what it must say..." The nuns wereseparated into two choirs, with the indication that "all the above inclinations must rotate from choir tochoir".

Figure 2: historical sequence of acoustics-related interventions.

296

2.2 Text mentions in “ceremonials” and “daily registers”
The 1626 ceremonial traces the experience and practice of the years or even centuries before.
Polyphony further complicates the perception of voices and of overall harmony, but we do not known
if it was in use at Montivilliers. In any case, this type of mention is not common in ceremonials. But it
can be set in relation with the description of very reverberant places ("circumsonens") by t he Romanarchitect Vitruvius, described as"those where, by the fact that [the voice] spins around, only producing reduced effects andmaking the words ring without their end terminations, it dies out leaving their meaning
uncertain"

Figure 3: pot number 14 in Romanesque vaulting.

Figure 4: extract from the ceremonial (page 9)

In his 1787 "dictionary of music", Meude-Monas recommends to the chapel masters to play only
slow music in large spaces:"Chapel masters [...] do not pay enough attention to the space the sound has to cross, andconsequently to the size of the space. They should know that lively pieces executed in a vast andsonorous place, produce only confusing noise, not a clear and detailed execution. It is therefore
necessary, in a large space, to only sing pieces of an easy, noble and posing character.”
The sound question is also present at ceremonies hosting large audiences. According to the daily
register, on 30 November 1643, the nuns were visibly embarrassed by the hubbub and "one wasobliged to draw the curtain to sing with more devotion" (Figure 5).

Figure 5: extract from the daily register for 30 November 1643
The curtain seems to satisfy two functions: refocusing the nuns on their prayers by limiting thespace (visual aspect); damping space a little (auditory aspect). This interpretation could seem arbitrary
if it did not appear 5 years earlier in the daily register on the day where the vaulting was delivered (29November 1648, Figure 6), where it is written that the embellishment of the choir is above all made"for the relief of voices and singing"

Figure 6: extract from the daily register for 29 November 1648 (file 349)
In the 5 years between these two notes, there is no mention of building works in the choir itself,297

only in the refectory and the dormitory in 1647: they are adjacent to the south transept. The registervery rarely mentions building decisions nor the discomfort they cause, but only their reception. On
delivery, the nuns seemed reassured because the Abbess had them sing the Te-Deum in the presence of
the workers "to have preserved them during this very risky work", a sign that it did not take place
without disturbance for the community.
Though difficult to prove an intention to act on the acoustics when the pots were inserted in the
Romanesque vaulting, it seems that lowering the vaulting in 1648 was undertaken to act on thesonority of the space, the space was not very comfortable acoustically for the choir of the nuns.

3. ANALYSES OF THE POT SYSTEMS
What distinguishes Montivilliers is not several distinct series of acoustic pots nor the addition ofpottery on pre-existing vaulting, but that these successive devices concerned the same part of the
Church: the crossing of the transept, traditionally reserved for singing. However, in spite of the texts
evoking the acoustics difficulties encountered and the associated site works, no mention of pots has
been found until now in the texts of Montivilliers.
A comprehensive international study of acoustic pots has led to four characteristics (5):Point1. Pots are chosen in relation to their resonance frequency, mostly in the range of the singing
or spoken voice.
Point 2. This choice is organized:- the set of pots is homogeneous in frequency (unimodal type)- two groups of pots are chosen and tuned to the fourth or the fifth (bimodal type)- continuity of frequencies over an octave approx. (continuously distributed type)- discrete distribution most often over an octave (discontinuously distributed type)
Point 3. Pots are organized in space according to:- a high/low principle- a north/south principle- both sometimes
Point 4. Pots are positioned:- above the monks/nuns or in the liturgical choir- in the entire space
Characteristic 4 (point 4) is evident in Montivilliers, as in many monastic buildings. But inMontivilliers with its two liturgical spaces, only the monastic space is concerned. The other threecharacteristics differ for the Romanesque and the modern vaulting. The points 1, 2 and 3 is analyzed in
the following.
3.1 Romanesque vaulting
There are originally 18 acoustic potteries. 16 were installed at the corners and 2 on the piers
supporting the arches. Niches were practiced in the masonry for inserting the pots (figure 3). They
were sealed with mortar shaped in form of reflector around the opening of the pots. Today, 17 pots arein place, and one is kept in a Museum in Rouen. We have carried out acoustic measurements of the pots
as well as non-invasive acoustic and archaeological observations. Some pots are cracked and several
types of pots have been used: globular belly, conical belly, round belly pot without collar.
The analysis of the measurements follows the preceding classification.
3.1.1
Choice of pots (Point 1)
The number 18 is not a surprise, but symmetrical distribution would lead to 24. There is no trace on
the walls that there have been so many. Only 10 pots give reliable frequency measurements, and 17
reliable geometric measurements.
Figure 7-a and 7-b respectively present the frequency distribution of the 10 non-broken pots and thedepth distribution of 17 geometrically measurable pots. Pots are numbered clockwise from 1 to 18
starting from the northwest pillar. The frequencies are rather evenly distributed from 130Hz to 252Hz
or approximately from C2 (130 Hz) to C3 (260 Hz), i.e. one octave. These frequencies are quitecharacteristic of what is observed in France, with an average at Montivilliers of 177 Hz and adispersion of 23%. For a monastery of nuns, the frequencies correspond to the range of female spoken
voices, but not to singing voices: it is globally too low, corresponding to the lower range of current
Alto voices.

298

300

Depth vs. pot number

Resonance frequency vs. pot number

250

40
30

200

R² = 0,9233

20

150

10

100

R² = 0,9582

0
13

9

14

16

15

7

4

8

10

3

3 6 17 5 2 8 10 4 18 1 11 15 7 13 16 9 14

Figure 7-a : frequency distribution ofnon-broken pots

Figure 7-b: depth distribution of 17 pots

3.1.2
Frequency organization (point 2)
Grouping does not resist analysis. In the classification, it corresponds to an extended frequency
distribution. As shown in Figure 7-a, linear regression gives a very high correlation coefficient. The
distribution of the lengths of the pots is even more linear and cannot be due to chance.
Correlation between frequency and length for the 10 measurable pots is -0.95, which is statisticallysignificant. We expect a minus sign as frequency is inversely proportional to the square root of thevolume, which increases with length. Correlations between resonance frequency and neck length, resp.
neck diameter, are as low as -0.2 and 0.25. This indicates that choice is made on the volume, and thatthe shape of the pots must be relatively homogeneous, conical or oblong, as observed by theceramologist. Indeed, if parts of the pots were globular, the correlation with the length would not be as
strong.
Pots were thus chosen in quasi-linear fashion covering an octave, a "continuous distribution" in our
terminology. But this choice is difficult to explain, as it does not correspond to the range of the singing
voice.
3.1.3
Spatial organization (point 3)
Frequencies are on average higher on the north wall and lower on the south wall, inversely to thegeneral trend.
3.2 Modern vaulting
Acoustic measurements were carried out with a nacelle. There are 25 pots, 6 on each pillar at thebottom of the vaulting, except the North-West one with 7. The rule of 6-grouping is almost respected.
Cochet (9) mentions that a pot was taken down in his time, but direct observation does not confirm it.
Unlike the pots of the Romanesque vaulting, all pots were installed during the construction of the newvaulting, which is confirmed by the fact that some pots are partially masked by the ribs.
After cleaning all pots but one that was not accessible, resonance frequencies and geometrical data
were systematically collected. What strikes is the great homogeneity of pot shapes: long and straightnecks, sometimes flared, sometimes with a flat bottom. It could be a model of pot for culinary use(carafe). The mean resonance frequency is 117 Hz with 7.5% dispersion, and the mean height of thepots 33 cm with 6.8% dispersion (Figure 8). Necks vary most with average external diameter 79.4mm(7.7% dispersion), length 143mm (nearly 13% dispersion) and average narrowing 62.5mm(12%dispersion).
3.2.1
Choice of pots (point 1)
The configuration differs from the Romanesque Vaulting. The two difference are: a lower meanfrequency of 117 Hz, corresponding to the spoken male voice, certainly not to a female voice; a narrow
range of frequencies (104 – 136 Hz, about G#1-C#2), that is, 7.5% variation, just slightly larger thanmanufacturing randomness for pots which is in the order of 5%.
3.2.2
Frequency organization (point 2)
The 3 pots standing out of the main group have an average frequency of 133.7Hz (2.63%
dispersion) and the main group 114.4 Hz (5.45% variation) and 133, 67Hz (2.63%), slightly more than
one tone, which is hardly interpretable in terms of musical theory and relevan ce.

299Modern Vault

Number of pots

6
4
2
0

100 104 108 112 116 120 124 128 132 136 140
Hz (1/s)

Figure 8: histogram of pots per resonance frequency
3.1.3
Spatial organization (point 3)
No clear trend could be singled out.

4. ACOUSTIC MEASUREMENTS
4.1 Hypotheses to check
The texts mentioned that the acoustics of the Church could be cumbersome for the voices, andcorrective actions were clearly undertaken throughout the centuries. Acoustic measurements wereplanned to verify whether the pots have an effect in the 125Hz octave band, as the 17th century pots
resonate in this band.
4.2 Measurement method
The acoustic measurements were carried out by the LAM team of the Institut D'Alembert at
Sorbonne University, using an omnidirectional source "outline GSR", a "Tannoy VS10" subwoofer ,and an "Ambisonics® SoundField ST250" microphone. The measurement softw are is the "Aurora"plug-in (6-8) installed on "Adobe Audition", which drives a "motu traveller" sound card. The protocol
consists in transmitting in the church a sine-sweep of 20 to 20kHz and 10s duration, and to record themicrophone signals simultaneously on the computer.
There were 3 source positions and the 10 microphone positions. The source positions are imposedby the configuration of the Church, with its well-delimited choir and nave volumes, and the transition
through the transept crossing. The same applies to microphone positions, which are distributed in eachvolume.
4.3 Results
Only the omnidirectional responses (output W of the Ambisonics ST250 microphon e) and thelateral responses (output Y, figure-of-8 response) were analysed.
4,00
3,50
3,00
2,50
2,00
1,50
1,00
0,50
0,00

EDT(s)
RT30(s)

125

250

500

1000

2000

4000

8000 16000

Figure 9: Mean reverberation times (RT) and EDT in Montivilliers.
Figure 9 shows the mean reverberation times (RT) in Montivilliers. In fact, they vary slightly
depending on the source position
RTs at Montivilliers are surprisingly short for a 14000m3 church: around 3 seconds at meanfrequency, which is rather good for intelligibility. But the critical distance is only 20 m, and speech
will not be really intelligible beyond that distance. Sounds emitted in the choir will hardly reach the300nave, justifying the installation of the choir of the nuns at the transept crossing.
Detailed analysis of the RT reveals 3 coupled volumes: the choir, the transept with its crossing, andthe nave and its aisle. Indeed, RTs are the same throughout the church for the source in the nave But
RTs increase for the source in the choir when the receiver moves from the choir to the transept and to
the nave. Lowering the vaulting of the transept crossing certainly improved the acoustics, but only thatof the choir by reducing and confining its volume. For the other parts of the church, it has probably notchanged much.
The average levels in the nave are significantly higher for a source placed in the transept crossing
than in the choir, and the inverse for the average level in the choir. This corroborates independent
coupled volumes, and the assumption that lowering the vaulting of the crossing has improved theacoustics of the choir, but only in the choir.
As for an effect of acoustic pots on reverberation, it is not visible since there are virtually no RT
differences between the transept crossing and transept, the and that RTs are significantly lower in the
choir.

5. SYNTHESIS OF RESULTS AND PERSPECTIVESThe Abbey of Montivilliers is a very special case where during 4 centuries it is possible to identify
building works related to acoustical purposes.
Local texts from the 17th century are particularly explicit regarding both the discomfort felt by thenuns when praying and the building works undertaken to relieve their voices. The bu ilding sequence
long, with a first lowering of the vaulting at the crossing, the insertion of pots in the Romanesquevaulting, the use of a curtain to separate the parish part of the church form the monastic part, and afurther lowering of the vaulting with new insertion of pots.
In this paper, we have presented two superposed systems of post. In the first system, the pot
resonance frequencies are regularly spread in frequency on an octave in the range of a female spoken
voice, but also in the very energetic 250Hz band. The second system of pot is much more homogeneouswith respect to sizes and frequencies, all of them in the 125 Hz octave band which is difficult to relate
to the voice. The question therefore arises whether in the 17th century knowledge concerning pot
selection had been lost or whether this frequency range was a deliberate choice whereas pots are tuned
on voices in the majority of buildings in France. From a point of view of Architectural Acoustics ,current knowledge justifies this choice; but there is no evidence that the builders of that time have had
this knowledge and practice.
Measurements were carried out to find out whether the current church has particularly difficult
acoustics. However, the pots of the 17th century vaulting were not cleaned prior to the measurements,
and the curtain does not hang any longer. Measurements reveal a rather low TR (2 to 3 sec.) for achurch of this size. The wooden vaulting of the nave certainly plays an important role in absorption,
but no evidence supports it. Secondly, TRs vary significantly with source and receiver positions,indicating that the church consists of a series of coupled volumes (nave, choir, transept). Most
significantly, the transept crossing with its lowered vaulting due to the 17th century’s modification by
the nuns presents a lower TR. But no definite effect of the pots was observed in the 125Hz band.

ACKNOWLEDGEMENTS
The authors wish to thank the French Ministry of Culture for financial support, as well as theircolleague Laurent Philippon for his help with the various measurements.”

REFERENCES
1. Palazzo E., Fiala D., Saulnier D., private communication
2. Valiere J.C., Dupuy E., Bertholon B. "From Methodology to Archeoacoustics in the Time of Scripture:
Complex Dialogue Between Archaeological Evidence, Texts from Scholars and Written Mentions", 23rd
International Congress on Acoustics, 9-13 September 2019 in Aachen, Germany
3. Carvalho P., Palzzo-Bertholon B., Valière J.C. “La filiation entre les echea antiques et les pots
acoustiques médiévaux”, in “Archéologie du Son”, ref. (4) 2012.
4. Palazzo-Bertholon B., J.-C. Valière J.C (Ed.), « Archéologie du Son : les dispositifs de pots
acoustiques dans les édifices anciens », Suppl. to Bulletin Monumental 5, SFA, 2012, Paris,(http://archeoacoustique.labo.univ-poitiers.fr/)
5. Valière, J.-C., Palazzo-Bertholon, B, “Towards a history of architectural acoustics using archaeological301

evidence: Recent research contributions to understanding the use of acoustic pots in the quest for soundquality in 11th—17th-century churches in France”, in “Worship Place” Lavandier C. and Guillebaud
C. (Ed.), Rootledge, 2019, London.
6. Farina A. Simultaneous Measurement of Impulse Response and Distortion with a Swept-Sine
Technique. In: Audio Engineering Society Convention 108. Audio Engineering Society, 2000.
7. Farina A. Advancements in Impulse Response Measurements by Sine Sweeps. In: Audio Engineering
Society Convention 122. Audio Engineering Society, 2007:21.
8. Figueiredo F.L., Polack J.D., Acoustical measurements in occupied rooms in Paris. In: ICA 2010 - Sydney –2010
9. Cochet J.-B.-D., « Notes sur les poteries acoustiques de nos églises », Précis analytique de l’Académie desSciences, Belles-Lettres et Arts de Rouen, 1864, p. 181-191.

302