The Sound of Space as a Symbol of the Sacred
The sound of a sacred space is a fundamental acoustic experience of people in a society. Accordingly, cultural conventions play a major role in how this is perceived. To the present day in the Christian Occident, churches are widely expected to exhibit a particularly good reverberating sound. Many sacred spaces are crowned by a dome, which symbolises the heavens and the house of God not just visually but also acoustically. In synagogues and in mosques, the need to hear the sermon or prayers clearly has always been and still is of greater importance. However, these spaces – with the holy shrine, the aron ha-kodesh, positioned against the wall that faces Jerusalem or the mihrab, the deep niche for the imam in the qibla wall that faces Mecca – also have a certain numinous quality. The general expectation is that a sacred space will conform to acoustic tradition. One reason for this may be that in the clamour and commotion of the world around us, aural perception has become ever more sidelined to the subconscious. Paradoxically, this makes the architectonic creation of acoustic environments more difficult. However, the greater freedom of formal expression and material design afforded by modern construction has also made new and impressive solutions possible in the field of acoustics.
Architectural Acoustics – The Conflict Between Resonance and Clarity
The history of sacred architecture describes a wide variety of functional requirements and how they have been adapted over time: the functions of prayer, readings from the Holy Scriptures, the sermon and the invocation and worship of the Holy have led to the formation of characteristic spatial shapes and design elements. It is, however, apparent that in certain circumstances the acoustic requirements can contradict one another. For prayer, protection from intrusive noise is required, for readings and the sermon, clarity of the spoken word. The invocation and worship of God are conducted through loud incantation or by a choir with many voices, supported in some religions by an organ or musical instruments.
The acoustics of churches presents one of the most complex cases of architectural acoustics. Traditional churches often consist of one or more zones that are more or less connected with one another and that serve several liturgical functions with different acoustic requirements (nave, side aisles, transept, choir). The sound source (priest, organ, choir) reaches the congregation from changing positions. It can be confusing for readers, singers and audience alike when sound energy from other parts of the space reaches them with a delay. The position of the sound source and the form of the space are the key factors that determine the acoustic interaction of interconnected spaces.
Sound and Sound Propagation
The intensity of a sound source attenuates rapidly with the distance travelled. The intensity of a sound that spreads evenly in all directions reduces by a factor of four as the distance doubles, by a factor of a hundred with a tenfold increase in distance. This makes it clear just how important it is to improve acoustic communication through the use of amplifying sound reflections.
In geometric acoustics, the propagation of sound waves is represented by rays that reflect off the enclosing surfaces of a space according to the same laws of reflection used in optics. Each listener is located in a field of sound waves that arrives in staggered succession, radiating from a sound source at a speed of approximately 340 m/sec and reflecting off wall surfaces and sufficiently large obstacles: the direct sound Dir arrives first, then follows A (reflections from the rear wall of the apse), W (from the nearest side walls), D(reflections from the ceiling), W̓(far side walls), R (rear wall) as well as further reflections from more than one surface such as D̓(reflections from the ceiling and rear wall) and so on. The direct sound serves to locate the position of the sound source, and the interval between the direct sound and the first reflection gives an indication of the size of the room.
Our aural perception combines the direct sound and the successive sound reflections to an overall impression of the sound: we perceive the sound as more lucid and intense the closer the interval between the series of reflections within a time frame of 50 to about 150 milliseconds. Lateral reflections strengthen the impression of spaciousness. They are particularly valuable as our ears are, between 6 to 10 decibels, more sensitive to lateral sound than from other directions. The early reflections strengthen the impression of the sound and improve its clarity. Later reflections are heard as reverberations. Very early reflections (less than 15 milliseconds) and hard and loud late reflections (later than 50 milliseconds) can be perceived as intrusive, overlapping or echoing reverberations. Any alteration in the shape of the space changes the paths of reflection and the sequence in which they arrive at the listener. Likewise, the pattern and sequence of reflections change as the position of the listener or the position of the sound source changes.
Computer programmes use three-dimensional ray tracing to calculate the geometric propagation of sound. Such calculations are valid only for a particular combination of the position of the sound source and the listener. With an appropriately chosen cross section, two-dimensional geometric analyses can provide general information about the dissipation of sound taking into account higher order reflections from two, three or more surfaces. Although the attenuation of sound energy is not directly visible, such geometric analyses can provide useful information for the planning of the form of the space. Acute angles and niches lead to localised sound accumulation, which can be intrusive if that is where listeners are located, or if they lead to the delayed relaying of sound to the listener. Right angles and rectangular spaces provide the most even distribution of sound. As with mirrors, concave surfaces (such as the inward curve of a wall or a barrel vault), concentrate sound in a focal point, whereas sound reflecting off convex surfaces is dispersed. Depending on the distance between the sound source and the reflective surface, the effect of a surface can change entirely.
Reflection and Diffraction
In order to be effective, a reflector must be significantly larger than the wavelength of the sound. For frequencies of between 20 and 20,000 hertz, this can range from 17 metres to 17 millimetres. If an obstruction is smaller than the wavelength, this portion of the frequency of the sound is ignored. For this reason, acoustic shadows can occur behind pillars and under galleries in sacred spaces as low frequencies pass by obstructions while high frequencies are reflected.
As with light, the edges of reflecting surfaces and surface structures in the average dimension wavelength can result in sound diffraction. A surface with a modulated structure with projections and recesses in the range of a centimetre to a decimetre can be used to reduce hard reverberations and reduce flutter echoes between parallel surfaces. It can also serve as a means of absorbing high frequencies. This rule of reflector dimension and surface structure can have aesthetic implications for the design but can also be integrated into the architecture of the space. For example, this mixture of absorption and diffuse reflection explains the good acoustic properties of baroque stucco ornamentation or the ornamental surface reliefs in mosques.
Reverberation Time and Absorption
After the source of the sound falls silent, the sound persists for as long as it takes for all sound waves that have not been absorbed to reach the listener. The room volume, V, absorption surface area, A, and reverberation time, RT (time required for the sound pressure level to fall by 60 decibels), are connected by a simple formula: RT (sec) = 0.163 V/A, whereby A = α 1 S 1 + α 2 S 2 + α 3 S 3 ...
However, this formula devised by Wallace C. Sabine is only valid for enclosed spaces in which sound can distribute rapidly and evenly. It can be used in the design phase to calculate the reverberation time when the absorption coefficient αx and surface areas Sx of the respective materials used are known. The reverberation time RT has to be determined for every frequency, and is therefore represented in the form of a reverberation decay curve. An individual value without qualifying frequency generally denotes the mean value of the reverberation time for frequencies between 500 and 1000 hertz. The size of the surface area of absorption required for a predetermined reverberation time can be derived for a given volume of the space with the help of a table. If precise laboratory measurements are not available, α-values from technical datasheets can be used for the calculations (for example the α-value tables in Robert E. Mickadeit (et al): Building Construction. Materials and Types of Construction, 5th edition, New York, 1981). Sufficient reserves should be incorporated to ensure that fine adjustments to the room acoustics can still be made. Typically the reverberation time is measured on-site once the shell of the building has been completed to determine the actual acoustic properties of the construction. A final measurement after the principal elements of the interior fittings have been installed allows one to verify the acoustics and, if necessary, to make final corrections.
Because the sound signal of speech changes rapidly, clarity is particularly important. Relatively long reverberation times can nevertheless be tolerated when reverberations decay sufficiently rapidly and evenly and, as far as possible, consistently for all frequencies. This has been verified by measurements taken in sacred spaces with particularly good concert and vocal acoustic properties. Further quality factors include the degree of syllable intelligibility, tonal colour of the initial reverberation, as well as values determined by the geometry of the space such as clarity, acoustic transparency, stereoscopic impression and laterality. For a mean reverberation time of 500 to 1000 hertz in historical and modern sacred spaces, recommended empirical values exist, which, depending on the architectural style, vary between 1.5 and 7.5 seconds for empty spaces with a volume of between 500 and 20,000 cubic metres. Even in large sacred spaces, the reverberation time rarely exceeds 15 seconds. In spaces with a lack of effective reflective surfaces, the reverberation time can also be surprisingly short, as is the case in St Peter’s in Rome where the mean reverberation time is only 3.5 seconds. This is an indication that the unfavourable acoustic properties, which are well known, begin to approach those of being out in the open.
The Number of People
The people present in a sacred space together form a sound-absorbent area which, depending on the shape and arrangement of the space, can represent a large proportion of the interior surface area, particularly when the height of the space, is less than half the width. In high churches and halls, the effect of people present in a space is reduced due to the large area of the flanking walls but it nevertheless remains an important element. The amount of seating, the covering of the seats and their arrangement are therefore some of the most important determining factors for the acoustics of a space. In order to achieve really good acoustic conditions for speech and music for all listeners without the help of electronic amplification with today’s density of seating, the maximum amount of seating should not exceed 1800 to 2000 seats.
The application of absorbent materials not only reduces the intensity of sound reverberation but can also avoid the accumulation of sound energy that can lead to undesirable echoing effects. Generally speaking, secondary areas that adjoin the main space should have more sound absorption than the main space. For spaces that can be subdivided and used separately, the acoustics need to be optimised for each part of the space as well as for the space as a whole.
When hard and heavy materials are used which absorb very little sound energy in the frequency range of 100 to 200 hertz (for instance stone and concrete, but also laminated multiplex wood panelling that is firmly bonded to a substrate without a cavity or pronounced joins), the absorption of disruptive low frequency sound requires careful planning and complex corrective measures. Most effective in such cases are panel absorbers (soft panels with an enclosed cavity that are applied over hard materials with a heavy mass) and hollow chambers clad inside with absorbent material, which can be adjusted to match the frequencies that need absorbing. To achieve a well-balanced effect, it is important to distribute the absorptive surfaces regularly in alternation with the reflective surface – for example stone paving for the open areas of the floor and hollow wooden floors laid beneath the seating.
The reduction of higher and medium frequencies is usually achieved by using porous materials such as mineral and organic fibres, by people in the space, and through the use of structured surfaces. Ornamental plasterwork, coarse plaster, clay plasters, exposed brickwork and rough-hewn wood or stone blocks all have favourable acoustic qualities as they reduce hard sound reflections. The absorptive range of curtains can be extended from high frequencies to medium frequencies, the heavier the curtain material and the greater the distance to the wall. Carpets, the most common floor material used in the prayer hall of mosques, are also used in synagogues and churches to improve the acoustics of empty spaces and to deaden the effect of footsteps.
Special Aspects of Acoustics in Sacred Spaces
The tradition of sacred spaces has produced pragmatic solutions to dealing with the problem of acoustics (for example the narrow choir space that is acoustically connected to the nave via sound reflections from the ceiling), which today can be realised in new forms thanks to modern construction methods and measuring techniques. The architectural acoustics should be considered throughout the entire design and building process – whether the spatial distribution of functions or the design of the interior fittings, the formal design of the space or the choice of materials, all aspects contribute to the design of the acoustics. As a result, acoustic conditions and qualities become a part of the overall concept. Through the appropriate choice of standard materials and elements for the interior furnishings and the careful planning of the shape of the space, it is almost always possible to achieve the desired acoustic conditions.
When a combination of spatial zones is discarded in favour of a simple rectangular space, then it is the task of the interior furnishings and fittings to create suitable conditions for the different functions in the space. The necessary stillness for prayer can be provided by an anteroom or by a zone that is more strongly sound insulated with appropriate materials. Traditionally a dome or cupola served as acoustic amplifiers for the invocation, singing of praises and instruments. In spaces without a dome, a sufficiently long reverberation time can serve the same basic purpose. Longer reverberation times do not compromise clarity of speech if they are linearised for all frequencies and when care is taken to avoid speech setting off reverberations. A reflector positioned above the speaker can be used to effectively exclude the acoustic volume of the space above. For the sermon and reading it is sufficient to raise the speaker to ensure the necessary direct sound radiation to the congregation: a step or raised pulpit, as is typical of the lecterns in mosques as well as synagogues, create the important direct visual and aural connection between speaker and listener. A niche or appropriately placed flanking wall surfaces can serve to create early reflections that strengthen the intensity of sound. By contrast, singers, organists and all other musicians are dependent on reverberation to transmit the sound and are therefore best placed on a raised gallery not too far from the ceiling. The now omnipresent use of electroacoustic amplification can help to amplify weaker voices, but their effect is that much better when the space is heavily sound insulated. However, the excessive use of sound insulation causes sacred spaces to lose one of their most essential qualities: the sense of otherworldliness, the atmosphere that creates the conditions for transcendent experience.
To illustrate the relevance of form, material and surface structure for architectural acoustics, a selection of interiors from the project examples have been analysed based on the plans and details given.
Eight of the project interiors have a simple rectangular plan form (see Chapel of St Ignatius, Donau City Church, House of Silence at Königsmüster Benedictine Abbey, Cistercian Monastery Our Lady of Nový Dvůr, Dresden Synagogue, Funeral Chapel, Maulburg Cemetery). All of the remaining plans are more complex. Many of them derive from a rectangular space – with one or more spatial projections or indentations – or are a rare combination of overlapping spatial elements with a rectangular plan (see St Clara Church, Höör Chapel, Rudolf-Alexander-Schröder House, St Ireneo Church, Our Lady of the Pentecost Church, Church of St Francis, Kärsämäki Church, Christ Church, Church of Reconciliation. Where wavelengths are the same length or larger than the depth of the projections, a smoothed out line along the wall (ignoring the projections) serves as the line of reflection. Only a few of the spaces exhibit a plan form that fans outwards from the altar (see Padre Pio Pilgrimage Church, Duisburg Jewish Community Centre, Chapels of Rest, Am Fließtal Cemetery), which for large spaces (for example Cathedral of Our Lady of the Angels) can result in insufficient lateral sound reflections from the side walls leading to poor acoustic reception in the central seating area of the space. To a certain degree, one can compensate for this disadvantage by choosing a slightly convex ceiling form with sufficiently high side walls.
In spaces with concave curving side walls (segments of a circle, other curves or angles with upright wall surfaces), sound can concentrate in the area of the congregation (see Saint Joseph’s Church, Church of Santa Teresa de Jesús, Maranatha Moluccan Church, Christ Church, Church of St Maraia Magdalena: mirrored zigzag walls, Chapel of St Mary of the Angels and Hill of the Winds Crematorium: irregular octagon).
Cross Section and Longitudinal Section
Very little can be done to improve the acoustics of wide spaces with low ceilings in comparison to the width of the space. Insufficient lateral sound reflections that strengthen the impression of the sound mean that what the congregation hears lacks clarity and transparence. A well-established historical form of low hall that does, however, exhibit relatively good acoustics for speech is the hypostyle hall in mosques, which like the tree trunks in an open forest without undergrowth, distribute high frequencies diffusely. The numerous pillars, the strongly structured ceiling and array of small cupolas help to distribute sound in the medium and high frequency range (see Mosque of Rome).
In very long spaces with parallel wall surfaces, the interval between the direct sound and the wall reflections can be more than 50 milliseconds (a difference in path length of more than 17 metres), resulting in echoing. One can counteract this by splaying one of the surfaces, by giving it a surface structure or by applying absorbent materials (see Cistercian Monastery Our Lady of Nový Dvůr: chapel with semi-circular apse). It is also necessary to treat one of the other parallel surfaces accordingly to avoid a strong imbalance in the tonal colour of the reflections.
Funnel-like battered walls (Cymbalista Synagogue) direct reflections increasingly upward towards the ceiling, returning to ground level only after they have reflected off one or more further surfaces. The effect is similar to that of a semi-circular vault: the reverberation time is lengthened.
The effect of the plan and walls must always be considered in conjunction with the ceiling. When the height of a space is the same or less than the radius of the dome crowning it, the sound will be concentrated by the dome; in all other cases, the dome reflects sound out into the congregation below.
Tapering and rectangular rooflights can cause sound to accumulate before relaying it with a delay into the space below. Depending on the choice of material and the size of the area affected, this can be intrusive. Such areas should be made sound absorbent.
A 90-degree gable provides a favourable transmission of acoustic reflections between the side walls; a steeper gable (such as a pointed vault) leads to sound accumulation and an accompanying delay in the reflected sound; a more shallow gable (similar to a shallow vault) leads to reflections that travel along the gable before arriving with a delay on the opposite side. Bulging gables that are curved in a longitudinal direction (see St Florian’s Church, Dunaújváros Church; Chapel of St Ignatius) produce a similar effect to shallow vaulting (see for example Korean Presbyterian Church). One-sided pitched roofs always lead to sound accumulation in the sharp corner at the upper edge (see Brother Claus Church, Tornbjerg Church, Rudolf-Alexander-Schröder House). Butterfly or M-shaped gables divide the space acoustically into two more or less separate halves resulting in an effect much like two parallel vaulted naves (see Enghøj Church, St Peter’s Chapel). For spaces defined by such concave surfaces, the resulting effect depends on the position of the sound source and of the listeners as well as the absolute volume of the space.
A General Rule
As the distances involved and the volume of a space increase, spaces that disperse sound unevenly become more problematic. Conversely, the smaller the dimensions of a space, the greater the risk of standing waves, which can also occur diagonally across several surfaces.
Mosque of Rome, Paolo Portoghesi, Vittorio Gigliotti, Sami Moussawi, 1995
In this prayer hall accommodating 2000 believers in an area of 40 by 40 metres, the large dome distributes the voice of the Imam back into the rear three-fifths of the square hall. The imam is located beneath a small cupola at the front centre, which functions as a reflector to distribute sound both lengthways as well as laterally across the entire space of the prayer hall. The small cupola on the opposite side next to the entrance wall gathers and reflects the direct sound, amplifying it for the rear of the space. The small cupolas along the side walls have a similar effect. These compensate for the decreasing intensity of the direct sound with distance, which is important for a room of this size. The 32 pillars, each made of four intertwined shafts that open outwards at the top and the decorative ribbing of the dome, whose slender dimensions only affect high frequency sounds, distribute the sound reflections diffusely in all directions, improving the intelligibility of the prayer between the columns. The soft carpet counteracts hard reflections off the floor. Despite the hardness of the concrete, this combination produces a calm and lucid acoustic atmosphere.
Dresden Synagogue, Wandel Hoefer Lorch Hirsch, 2001
The inner sanctuary is a space within a space: a relatively narrow rectangular volume that stands at a slight angle to the outside walls and is enclosed on all sides by a curtain made of a diaphanous brass textile. The metal textile bands are in acoustic terms almost entirely transparent. The solid reflecting elements of the inner space consist of oiled oak panels and extend upwards about a quarter of the height of the space. The “inner” lateral reflections result only from these wooden surfaces, which enclose the torah wall and raised podium like a U-shaped bracket. At the opposite end, a similar construction with the same reflective surfaces houses the women’s gallery. The massive external walls of the “outer” envelope represent the limiting acoustic surfaces.
It is important that undesirable acoustic effects do not occur in the outer 26 by 24 by 24 metre cuboid form. The rotational torsion of the walls avoids such effects. Each course of blockwork is rotated slightly so that in all from top to bottom the walls are inclined by 5 degrees. The resulting twist and the surface structure of the opposite walls avoids the creation of flutter echo and standing waves, and dissipates high frequency sound waves. Likewise, the coffered concrete ceiling eliminates reverberations that are too hard.
The raised position of the reading lectern, the bimah, directly in front of the centre of the space and the U-arrangement of the pews around it ensure a direct visual and acoustic intimacy between the reader and the listeners. The parallel side walls and their surface structure as well as the 1:1 height to width ratio of the outer space provide good acoustic conditions for the spoken word.
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