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Technical Note

Impact of design decisions on acoustical comfort parameters: Case studyof Dogramacızade Ali Pas�a Mosque

Zühre Sü Gül a,b,⇑, Mehmet Çalıs�kan c

a Dept. of Architecture, Middle East Technical University, 06800 Ankara, Turkeyb MEZZO Stüdyo LTD., Technopolis, KOSGEB TEKMER, No: 112, ODTÜ, 06800 Ankara, Turkeyc Dept. of Mechanical Engineering, Middle East Technical University, 06800 Ankara, Turkey

a r t i c l e i n f o

Article history:Received 8 October 2012Received in revised form 25 December 2012Accepted 26 December 2012Available online 14 February 2013

Keywords:Mosque acousticsWorship acousticsAcoustical designAcoustical measurementAcoustical simulation

a b s t r a c t

Acoustical design of mosques is critical considering speech and music related activity patterns held insuch religious spaces. This paper aims to emphasize the importance of practicing acoustics of mosquesas early as in the concept design stage. Within this framework, design issues of a contemporary mosque,namely, Dogramacızade Ali Pas�a Mosque in Ankara, Turkey are discussed. The edifice known as the tech-nological mosque with its eclectic interpretation has a unique architectural style. The architectural designof the mosque is heavily supported with acoustical design in all phases. Computer simulation is appliedas an acoustical design tool and estimations are held by commercial room acoustics software. Results arejustified with real size field measurements inside the mosque and related objective acoustical parametersincluding reverberation time (RT or T30), clarity (C50 or C80), speech transmission index (STI) and A-weighted sound levels (SPL-A) are assessed. Evaluation of the space indicates that the optimized acous-tical field is proper for intended functions of use in a mosque and satisfies desired tranquil environment.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Mosques being enclosures for Islamic worship are good subjectmatters of room acoustics with their finely defined forms and in-volved activity patterns. Listening as part of group prayer is themajor activity throughout the week in specific times of the day,notably imam’s sermon at minbar on Fridays. Except for the singinghymns and chanting to emphasize the text’s meaning, music is notengaged into the activities held in mosques. In practical means,speech and its intelligibility are the major acoustical consider-ations in acoustical design of a mosque [1]. The desired acousticalcriteria, however, must be incorporated within a visual aesthetic ofclassical – or in contemporary cases with its abstract versions – de-sign for successful mosque which satisfies the spiritual aspects ofworship.

Literature on Mosque acoustics studies can be divided intothree main categories. First group comprises the academic researchwith focus on the analysis of existing buildings. Assessment studiesinclude single mosque cases [2,3], comparisons of mosques toother mosques [4,5] or comparisons of mosques to churches [6–10]. Second group consists of acoustical renovation studies. Some

authors recommend architectural modifications on geometry andmaterials in contrast to traditionally accepted surface finishes orforms. Few cases apply solely electronic sound reinforcement sys-tems with no modifications on interior design, while in some bothapproaches are utilized [11–13]. Third group includes the investi-gations on real or virtual mosques to develop acoustical design cri-teria. Studies under this final group aim either to propose specificarchitectural parameters/features that are effective on the acous-tics of a mosque typology, or to specify acoustical parameter limitsspecifically to be applied for mosque typology [14–17].

Most of the researches utilize scale model and computer simu-lation techniques and/or site measurements. Assessed acousticalmetrics are basically reverberation time (RT60, T30), early decaytime (EDT), clarity (C80), sound pressure levels (SPL-A), speechtransmission index (STI) or articulation index (AI). Case studieson Mosque acoustics reveal some common architectural featuresand corresponding acoustical outcomes. Mosques have all hardand reflective interior finishes for walls and ceilings including cera-mic tile, glass, wood and stone, except for the carpet floor finish to-gether with frequent use of prayer mats and rugs. As a function ofacoustics in order to overcome the large sound attenuation of theimam’s voice over the rows of worshippers on the floor, the imamdelivers his sermon at minbar. Most of the sound from the imam atthe top of the minbar feeds energy into the reverberant sound field.Thus, the worshippers are listening more to reverberation than todirect sound which leads to poor speech intelligibility. Speech

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⇑ Corresponding author at: Dept. of Architecture, Middle East Technical Univer-sity, 06800 Ankara, Turkey. Tel.: +90 312 210 1825; fax: +90 312 210 1826.

E-mail addresses: zuhre@mezzostudyo.com (Z. Sü Gül), caliskan@metu.edu.tr(M. Çalıs�kan).

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intelligibility in a mosque is hampered by reverberation and/orechoes, due to both large sizes of traditional mosques and shelter-ing elements mostly of either domed or vaulted geometry.Churches in compare to mosques have more reverberant acousticalenvironment with the lack of carpet element as a floor finish, thuscorresponding to comparatively worse speech intelligibility levels.On the other hand, the liturgical/music-oriented activities held inchurches compensate the difference in sound fields.

Most of investigated edifices have enclosures with poor soundinsulation characteristics and located in noisy city environments.Therefore, all are suffering from noise intrusion which reducesthe signal-to-noise ratio and disrupts intelligibility. Intruding exte-rior noise due to low transmission loss of exterior wall systems,glazing systems and doors commonly installed is a major soundinsulation problem. For new generation mosques operating fansindicate very high background noise levels that cause interferencewith speech communication. Owing to the existence of noise gen-erated by the ventilation systems, sound amplification systems be-come a necessity. Considering the conditions of historical mosquesby then, less environmental noise as traffic and the absence of elec-tro-mechanic systems depicts the applied exterior shell details tobe functioning in their times. As a final point, electronic soundreinforcement approach should not be regarded as an exclusivesolution for inappropriate acoustical comfort levels of a worshipspace, without a proper architectural background. All studies sup-port the idea that wisely detailed building envelope and properinterior design are the key elements for the high-quality acousticperformance of mosques as well as churches.

Literature on the topic reveals the increasing interest on theworship space acoustics. Most of the current studies in literaturefocus on the analysis of existing mosques. Few studies incorporateacoustical interventions for enhancing the acoustical qualities ofpre-designed and built mosques’ interior environment, while al-most none present cooperative, ground-up architectural acousticsdesign. The aim of this research is to document the fluid interac-tion of two fields of expertise, namely, acoustics and architecture,in conceptualizing and realizing a sound space that is both aesthet-ically and acoustically viable. Acoustical design of DogramacızadeAli Pas�a Mosque, which is a contemporary case, is held using com-puter simulation technique. Basic acoustical design considerationsin this study are defined to be shaping and sizing of the shelteringdome over the main prayer hall, optimization of acousticallyabsorptive, reflective and diffusive surfaces, functioning sound iso-lation details and HVAC design assessment in frame of backgroundnoise limits. Acoustical field measurements are utilized for com-parison with acoustical simulations, and for upmost as a feedbackfor design steps in completion of acoustically functioning mosque.

2. Case study: Dogramacızade Ali Pas�a Mosque

Dogramacızade Ali Pas�a Mosque, granted by Dogramacı Foun-dation in Ankara, is inaugurated for community use in September2008, in Bilkent, Ankara. The mosque has a unique architecturalstyle in such that the Architect differentiates classical understand-ing of mosque image by his eclectic imitation with an aim of bring-ing more quality to the classical model (Fig. 1). The edifice is alsoknown as the technological mosque with its fully automated com-puter controlled lighting and HVAC systems. The worship buildingis planned to serve for Christian and Jewish uses as well. ArchitectErkut S�ahinbas� of SFMM, Ankara, designed the building complex inclassical Ottoman külliye context. The complex houses multi-func-tional facilities including concert, conference and exhibition uses.With its exclusive architecture, technology and respect towardsdifferent beliefs and religions, the structure brings a new under-standing to the Mosque typology.

2.1. Design philosophy and architectural features

According to Aran traditional values are constantly replacedwith the new ideas and it follows that architecture changes itsform and philosophy [18]. The author defines Dogramacızade AliPas�a Mosque as iconoclast and admires the architect in his eclecticinterpretation. Within this commentary the convention is imitatedwith a decorative artful style in depicting the Mosque as an art ob-ject. Symbolic elements of classical mosque image are converted toappropriate presentations with today’s material and technology.

Horizontally emphasized composition of the structure embod-ies the simplified form of Ottoman Mosque plan layout. The tradi-tional mosque, herein the model, has a square plan layout with thepyramid upper structure sitting on eight piers creating an octago-nal prayer area. Thus, the model is a square plan (22 m � 22 mfrom the perimeter of interior octagon) with structural piers. Thearchitect first revision is the two story height subsidiary space atthe ceiling level surrounding the interior space defined by thefooted pyramidal shelter with the daylight flowing through theglazed surfaces. This metamorphosis is an innovation in mosquearchitecture. Traditional dome element is redefined with stainedglass dome in modest sizes supported from exterior by a steelframe structure (Fig. 2). The painted glass with the sunlight flowingthrough different directions during the day creates the intendedholy sky effect. Walls surrounding dome are decorated with spe-cially crafted gold handwritings.

The second abstraction is that, except for a stained glass domeproviding natural light to the interior, all lighting is in form of re-cessed coves on wall surfaces rather than real windows opening upto the outside world. The third revision is the pyramidal upperstructure sitting on eight piers viewed from the interior as anoctagonal, yet from exterior as a quadrangular pyramid, not dis-similar to the traditional model. This quadrangular pyramid andstained glass dome with its metal structural framework refer tothe pyramidal shape of the traditional mosque roof structure withvaults and semi domes, while adequately abstracting itself fromthe same model. In this abstract symbolism the architect imitatesthe classical mosque dome form with certain isolation so thatthe traditional reappears with the new and inventive [18].

Through the use of contemporary material and techniques, thearchitect creates a form shift in minaret design as well. The scaleand detailing of the dome and the minaret regenerate the tradi-tional design, encouraging the viewer to embrace a personalunderstanding of the timeless and classical model. Further revi-sions from the model satisfy varied aspects of the human spirit.The Mosque courtyard opens to the outside environment (Fig. 3);the ‘last prayers’ area is below the ground level as opposed to beingan arcade adjacent to the entrance of the main prayer hall open atone side to the courtyard; benches are located around the centralprayer zone so prohibit the elderly worshippers from having tosit and pray on the floor [18].

The complex is located on a hill with a natural inclination. Thedirection of this inclination is also utilized as a design element.Surrounding greenery is enriched and redefined suitably for theuse of pedestrian and vehicle traffic. Lighting concept is anotherfeature of the Mosque that is heavily exercised in emphasizingthe architectural language of the structure and in defining differ-ent spaces within the complex. On the exterior skin there is aplain harmony of naked concrete and travertine surfaces. For cre-ating the desired effect, the color and texture of the stone as a fa-cade finish material is specifically selected to be in unity with thelighting design. At first glance the outer shell appears as rigidlyintroverted mass, ripped up vertically at certain points in orderto let the flow of controlled sunlight to the interior. In this waylight emphasizes the center and invites one into to the prayerzone [19].

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2.2. Acoustical design

Acoustics tops the list of major comfort parameters in mosquedesign together with lighting and environmental factors. Speechintelligibility is the primary acoustical concern and necessitatessuitable designation of volume, geometry of the main space andappropriate use of acoustically absorptive and diffusive materialsas finish surfaces. In view of that, acoustical design is integratedinto the architecture in the very early stages of concept design ofDogramacızade Ali Pas�a Mosque. During schematic design, majorgeometric forms are generated and material selected to solve aes-thetic and acoustical concerns. As previously mentioned, the domeis the symbolic shelter element of traditional mosque model. In thecase of Dogramacızade Ali Pas�a Mosque, the architect specified amodest-sized stained glass dome to embrace architectural aesthet-ics and minimize probable acoustical defects.

As it is well known, concave surfaces are mostly problematic forthe behavior of sound and intelligibility. Depending on the distanceof the sound source from the wall – or the dome in this case – andthe radius of curvature, focusing or spreading effects can occur inanalogy to optical curved mirrors. If the distance from the soundsource to the so-called hard surface is greater than half the radiusof curvature, a focusing point beyond the center of curvature isformed. Such sound energy concentration is perceived as detri-mental when the focusing point falls within a region of the audi-ence/worshipper [20]. Being the combination of infinite numberof concave planes, the dome as the common ceiling form for reli-gious structures can generate focused reflections. These long-de-

layed reflections are particularly unpleasant for the talker/imam.If the delayed reflection is above certain pressure level, it can actas a severe impediment to normal speech. Hence, locating the talk-er and listener near the center of curvature should be avoided.Curved walls or the dome surfaces could be treated with absorp-tion; however, since the normal-incidence absorption coefficientsare lower than random-incidence values, there may still be re-flected levels high enough to be of concern [21].

The Ottoman Architect Sinan’s Mosques are renowned withtheir architectural grandeur and acoustical impression. Acousticalfield measurements of his unoccupied mosques vary greatly withother standard building interior volumes, particularly in theirexcessive reverberation times. For instance, Süleymaniye Mosquehas a mid frequency reverberation time average of 7.3 s, RüstemPas�a Mosque has an average of 4.2 s and Cenabi Ahmet Pas�a Mos-que has an average of 3.6 s [5]. In the acoustical analysis of Dogra-macızade Ali Pas�a Mosque, Ottoman era mosques are considered tobe role models. In traditional mosque architecture the dome ele-ment is mostly associated with acoustical defects as previously dis-cussed. It is believed that Sinan the Architect’s design called forceramic pots to be inserted within holes of the dome interior toact as Helmholtz resonators to increase sound absorption at spe-cific frequencies. Thus, acoustical elements introduce asymmetryto treat an acoustical defect caused by the geometric symmetryof the dome.

In the case of Dogramacızade Ali Pas�a Mosque the scale of theglazed dome is intentionally adjusted to maintain certain geomet-rical limits in order to keep the unwanted reflections away from

Fig. 1. Exterior views of Dogramacızade Ali Pas�a Mosque, Ankara.

Fig. 2. Interior and exterior view of the dome element, Ankara.

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the receiver/prayer zone. The effective area of the dome curvatureis shown in Fig. 4. The height of the dome from finish floor is19.23 m. The radius of the dome section is 5.60 m, making a diam-eter of 11.20 m. The center of the dome falls above the two thirdsof the full height. Possible sound focuses due to dome form fallsinto the shaded range (Fig. 4). The dome does not cause a particularfocusing effect while adds volume and provides spaciousness tothe space as a positive subjective acoustical indicator in the caseof Dogramacızade Ali Pas�a Mosque.

Apart from the geometrical development of the mosque dome,the finish materials to be applied on interior surfaces are studied.

10 mm thick woven wool carpeted floor surface generates the larg-est absorptive area being effective on the high end of the frequencyrange. Mid frequency range is optimized by the partial application(130 m2 in total) of perforated wood on inclined pendentive sur-faces, while the remainder is left as solid wood surface. Perforatedwood boards have 200 mm air gap behind filled with 50 kg/m3

mineral wool. The remaining wall areas are of travertine and so-lid/imperforated wood (maple) boards. Stone wall surfaces aresandblasted for increasing the scattering characteristics and pro-viding even distribution of sound. Detailed information on absorp-tion and scattering coefficients of materials are given in followingsections.

Related acoustical parameters to be assessed are T30, C30, STIand SPL-A. Recommended ranges for acoustical parameters arelisted in Table 1 [22–26].

3. Methods

3.1. Acoustical simulations

Computer aided simulations and acoustical analysis areadopted as major design tools. The use of computer simulationsin the analysis of acoustical properties of a space in design phaseprior to the actual construction has proven practical and reliableas to be an integral part of the architectural design process. Acous-tical simulations of Dogramacızade Ali Pas�a Mosque are carried out

Fig. 3. Courtyard, Dogramacızade Ali Pas�a Mosque, Ankara.

Fig. 4. Shaded area: effective acoustical dome area of Dogramacızade Ali Pas�aMosque (C: center, r: radius, f: sample focal point).

Table 1Room acoustic parameters and allowable limits (occupied spaces).

Parameter Recommended range (for givenvolume)

Just noticeabledifference

T30 (for 500–1000 Hz)

1.15 s (for speech only) to 1.96 s (forreligious music)

About 0.1 s

C80 (for 500–1000 Hz)

0–9 dB (for speech + music) �2 dB to2 dB (for music)

1 dB

STI Greater than 0.60 0.05SPL-A Minimum variations in SPL < 10 dB 2 dB

Fig. 5. 3DOpenGL view of the Mosque interior (top), ray tracing view of acousticalmodel (bottom).

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using ODEON Room Acoustics Software (v10.13). Calculationmethod of the software depends on a hybrid process combiningray tracing and image source methods specifically applied foreither early or late sound reflections. A statistical approach isadopted for modeling the higher order reflections. ODEON usesvector-based scattering in its prediction algorithms by which thereflection from a surface is modified from a pure specular behaviorinto a diffuse behavior for more accurate results [27,28].

3.1.1. Room informationAcoustical model of Dogramacızade Ali Pas�a Mosque is obtained

using CAD software after simplifying the graphical model suppliedin 3DStudioMAX file format. In this process of modification/simpli-fication of the geometry and dimensions, the graphical model coor-dinates are preserved in maximum allowable extent. Acousticalmodel is then imported into ODEON v10.13. The simplified acous-tical model is made up of 3-D face elements and comprises 748plane surfaces (Fig. 5). Ray tracing is an important feature of roomacoustics software. The room has to prove that there is no soundleaking locations so as to create a soundproof and a closed rever-

berant environment. Acoustical volume of the interior prayer zoneis estimated to be 6840 m3.

An omni-directional point source is assigned in front of mihrabat 1.50 m above floor surface corresponding to imam’s mouth po-sition (Fig. 6).

Effective praying/receiver area is the floor surface within theoctagonal plan layout on the ground floor and the women’s mahfilias a coupled gallery to the main space on the first floor. Receiverarea is divided into grids of 0.90 m � 0.90 m indicating each personpraying. In order to get the results of different acoustical parame-ters and their distribution throughout the mosque, the maps andcumulative distribution graphs for calculated parameters are ob-tained for receiver surfaces.

3.1.2. Material informationMaterials with different sound absorption and scattering coeffi-

cients are assigned to all surfaces in accordance with previouslymentioned acoustical design decisions. Related acoustical informa-tion is listed in Table 2.

3.2. Field measurements

Acoustical field measurements are held in accordance with ISO3382-2:2008 [29]. B&K (Type 4292-L) standard dodecahedronomni-power sound source is used in acoustical signal generationwith B&K (Type 2734-A) power amplifier. The impulse responseat various measurement points is captured by B&K (Type4190ZC-0032) microphone, incorporated into the hand held ana-lyzer (B&K-Type 2250-A). Sampling frequency of the recorded mul-ti-spectrum impulse is 48 kHz, covering the interval of interestbetween 100 Hz and 8000 Hz. DIRAC Room Acoustics SoftwareType 7841 v.4.1 is used for both generating the broadband whitenoise signal and postprocessing of the measured impulse responsedata for each receiver position (Fig. 7).

Field tests are held in the main prayer hall on the ground floorin empty/unoccupied condition of mosque. Omni power soundsource is located in front of mihrab at 1.50 m height adjusted byB&K Type UA-0801 Tripod mimicking the position of standingImam. Octagonal plan layout is divided into grids of3.2 m � 4.5 m. 26 microphone/receiver (R1–R26) positions are lo-

Fig. 6. Source position (P1) in acoustical model, plan view (top), section (bottom).

Table 2Sound absorption and scattering coefficients of materials.

Material 63 Hz 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz 8 kHz Scatteringfactor

Sand blasted travertine 0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.02 0.15Solid timber door 0.14 0.14 0.10 0.06 0.08 0.10 0.10 0.10 0.25Double glazing 0.10 0.10 0.07 0.05 0.03 0.02 0.02 0.02 0.10Single pane of glass 0.08 0.08 0.04 0.03 0.03 0.02 0.02 0.02 0.10Ceramic tiles 0.01 0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.152 � 13 mm gyps. board on frame, mineral wool backing in 50 mm cavity 0.15 0.15 0.10 0.06 0.04 0.04 0.05 0.05 0.1012 mm wood on studs 0.28 0.28 0.22 0.17 0.09 0.10 0.11 0.11 0.10Peforated MDF panel-TopAkustik 5/3T, thickness (1.6 cm) with mineral wool

backing in 200 mm cavity (130 m2 in total) or its equivalent0.20 0.30 0.80 0.95 1.00 0.80 0.60 0.60 0.10

Fig. 7. Measurement set-up of field tests.

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cated at intersections (Fig. 8). For each receiver location micro-phone is placed roughly at the height of the head of a seated personon floor, 0.70 m, adjusted by Holder Tripod (B&K – UA 0801).

4. Results

4.1. Simulation results

Two different scenarios are studied to verify the use of perfo-rated wood panels on pendentive surfaces. The first series of sim-ulations are held for the use of solid wood panel as a pendentivefinish material in unoccupied and occupied condition of mosque.Second series include application of perforated wood on penden-tive surfaces together with solid wood panels for unoccupied andoccupied mosque. The required amount of perforated wood areais given in Table 2.

4.1.1. Reverberation time (RT, T30)Reverberation time in general scientific terms is defined as the

time required for the average sound energy density to decay by60 dB (to one-millionth of its original value) from an equilibriumlevel after stopping a sound source. Since Sabine first studied thephenomenon in 1900, RT has been used as the paramount tech-nique for discovering the acoustic characteristics of room’s envi-ronment [30] and, in the case of the holy mosque, clues onintelligibility and sound aesthetics. In Dogramacızade Ali Pas�aMosque acoustical simulations of T30 over frequency range areestimated as a global average of the interior volume and presentedfor each receiver location in the form of distribution maps.

Results of global estimates indicate that perforated wood appli-cation on inverse triangular pendentive surfaces are effective bothfor unoccupied and occupied conditions of mosque, particularly inimprovement of lower to mid frequency range (Fig. 9). Being theselected alternative design solution, all other simulation resultsare presented only for perforated wood application on pendentivesurfaces. Average mid frequency T30 for unoccupied mosque withacoustical treatment on pendentive surfaces is 1.94 s and averagelow frequency T30 is 2.07 s. With treatment, average mid fre-quency T30 for occupied mosque is 1.34 s and average low fre-quency T30 is 1.86 s. Warmth expressed in terms of bass ratio(BR) should be greater than 1.20 for music performances [23].Although speech is the leading activity in a mosque, the low fre-quency and spiritual context of the male imam’s voice prove ratherdifferent than the ordinary speech, making it advisable to have aBR that is closer to optima for music. In the case of DogramacızadeAli Pas�a Mosque, BR is 1.06 for unoccupied mosque and 1.38 foroccupied mosque. Worshippers’ corporeal presence attribute tofurther absorption in mid to high end frequency range of the occu-pied mosque space, creating a warm acoustical environment.

Distribution maps are only presented for 500 Hz–1000 Hz cor-responding to dominating speech frequencies. Average T30 for500 Hz for unoccupied mosque is 2.02 s, while minimum value atmain hall is 1.93 s and maximum is 2.13 s. Average T30 for1000 Hz for unoccupied mosque is 1.80 s, minimum value at mainhall is 1.72 s and maximum is 1.95 s. Distribution map results areconsistent with global estimates in unoccupied state of the mos-que. Average T30 for 500 Hz for occupied mosque is 1.52 s, whileminimum value at main prayer zone is 1.40 s and maximum is1.67 s. Average T30 for 1000 Hz for occupied mosque is 1.28 s, min-imum value at main prayer zone is 1.14 s and maximum being1.62 s (Fig. 10). Global estimate results rest in between minimumand maximum values calculated for receiver locations in grid lay-out for the occupied condition of mosque.

4.1.2. Clarity (C80)The technical description for the clarity is the ratio of early

sound energy arriving within 80 ms of direct sound-to-late, orreverberant, sound energy arriving 80 ms after the direct sound.In subjective terms it is the property of sound to be clear in a closedspace and the degree to which individual notes are distinct orstand apart in a musical content [24]. Regarding mosques, clarityof fine details/words from Ezan/invitation to prayer and ability tohear and participate in the recital of the musical versions of theHoly Quran is the key.

C80 distributions for 500 Hz and 1000 Hz are given in Fig. 11 forunoccupied and occupied conditions of mosque. Average C80 for500 Hz for unoccupied mosque is �0.25 dB, while minimum valueat main hall is �6 dB and maximum is 6 dB. Average C80 for1000 Hz for unoccupied mosque is 0.9 dB, while minimum valueat main hall is �5.5 dB and maximum is 6.5 dB. Average C80 for500 Hz for occupied mosque is 2 dB, minimum value at mainprayer zone is �3.5 dB and maximum is 7.6 dB. Average C80 for1000 Hz for occupied mosque is 4 dB, minimum value at main

Fig. 8. Plan view of mosque showing measurement positions and source location(S: Source, R: Receiver).

Fig. 9. Estimated global reverberation times (T30) over frequency range, (a) solidwood on pendentives, for unoccupied mosque, (b) perforated wood together withsolid wood on pendentives, unoccupied mosque, (c) solid wood on pendentives, foroccupied mosque and (d) perforated wood together with solid wood on penden-tives, occupied mosque.

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prayer zone is �2 dB and maximum is 10 dB. C80 estimations indi-cate that the results in average are within parameter criteria limitsfor unoccupied mosque. Values are little higher than the limits forspeech-only purposes when the mosque is fully occupied, whilefitting well with the tolerable range for the rooms to be used forboth speech and music performances.

4.1.3. Speech transmission index (STI)Speech is the primary activity in a mosque, and intelligibility of

the speech is the major concern, particularly when imam conveyshis messages at Friday’s sermon from above minbar and/or forimam or preacher of his informative speech from the pulpit. STIhas been promoted for predicting speech intelligibility through im-pulse response. It accommodates both the signal-to-noise ratio andthe impulse response aspects that affect intelligibility. The idea be-hind this measure is that for good speech intelligibility the enve-lope of the speech signal should be preserved. It allows fordifferent contributions of the various frequency bands to speechquality, and also for the mutual masking between adjacent fre-quency bands occurring in our hearing process [24]. Reverberationtime and background noise levels are inversely proportional withSTI.

NC30 is used in defining the background noise as it is the givenlimit appropriate for HVAC system noise of spaces as mosques. Asound power level of 87 dB is assigned to the omni-directionalsource representing the naked voice of ‘imam’ preaching. Distribu-

tion maps of STI for occupied and unoccupied mosque are given inFig. 12. Average STI for unoccupied mosque is 0.54, while mini-mum value at main hall is 0.44 and maximum is 0.72. AverageSTI for occupied mosque is 0.60, while minimum value at mainprayer zone is 0.50 and maximum is 0.75. STI estimations indicatethat, on average, the parameter is satisfied with Good rating [22]for both unoccupied and occupied condition of the mosque.

4.1.4. A-weighted sound level (SPL-A)Sound pressure level is the most commonly used indicator of

the acoustic wave strength, correlating with human perception ofloudness. The A-weighted sound level (dBA) is the single numbermeasure of loudness that ignores low-frequency sound energycontent similarly to the human ear [21]. In a closed acoustical envi-ronment as in mosques’ prayer hall, the distribution of SPL-A is sig-nificant both in terms of indicating the strength of sound in eachreceiver location and also in illustrating how evenly sound is dis-tributed throughout the space.

Distribution maps of SPL-A for occupied and unoccupied mos-que are given in Fig. 12. Average SPL-A for unoccupied mosque is70.8 dBA, while minimum value at main hall is 68.2 dBA and max-imum is 75.8 dBA. Average SPL-A for occupied mosque is 68.5 dBA,while minimum value at main prayer zone is 66.7 dBA and maxi-mum is 75.5 dBA. SPL-A difference is no more than 9 dB withinall receiver locations.

Fig. 10. (a) T30 distribution maps for 500 Hz (left) and 1000 Hz (right), unoccupied mosque and (b) T30 distribution maps for 500 Hz (left) and 1000 Hz (right), occupiedmosque.

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4.2. Field measurement results

4.2.1. Background noiseThe recommended noise criterion interval for religious spaces

as churches and mosques is the NC25-30 [31]. In acoustical designof Dogramacızade Ali Pas�a Mosque this upper limit range is consid-ered in HVAC design, exterior skin wall section and glazed domepoint detail studies. Before field measurements, background noiselevels are measured for making sure that obtained signal to noiseratios (SNRs) during the measurements are not causing a loss ofinformation in recorded signal. Recorded background noise levelsin octave bands in comparison to NC25 and NC30 curves are pre-sented in Fig. 13. Resulting background noise level is measuredto be 23.7 dB [32]. It should be noted that for the minimum inter-ference to the recorded impulse the HVAC system is shut downduring the measurements. NC level and recorded background noisecomparison are given for informative purposes.

4.2.2. RT, C80, STI, SPL-AThe measurement results of acoustical parameters including RT,

C80, STI and SPL-A for the 26 measurement/microphone points arepresented in Tables 3 and 4. Given data are for the unoccupied con-dition of the Mosque.

Distribution of sound throughout the prayer zone could be con-sidered even within certain maxima and minima. The field is freefrom echoes or hot and/or dead spots. Minbar has a shadowing ef-fect when both distribution maps and field measurements are ob-

served. Shadowing behind the architectural element occurs mostlyin parameter values of C80 and STI. SPL-A with a 5 dB difference inbetween the closest and furthest prayer spot shows a modest dis-tribution of sound pressure levels. Even in an unoccupied mosque,STI average still shows Good rating. During Friday sermon whenspeech intelligibility is key, the mosque attendance is maximized,thus increasing absorption and a drop of T30 in speech frequenciesand, consequently, an increase in overall STI as shown in simulateddistribution maps for occupied space.

5. Discussion

Three main criteria in mosque acoustics are satisfying the limitsfor reverberation time, intelligibility of speech and even distribu-tion of sound at prayer/worshipper zone. Related acousticalparameters are assessed initially in design phase by acoustical sim-ulations for unoccupied and occupied conditions of mosque. Afterthe construction is complete field tests are held. For practical pur-poses real-size measurements can be held in unoccupied conditionof the assessed space as a standard application. Comparison graphsof simulation and measurement results for unoccupied conditionof mosque are given in Fig. 14.

Reverberation time averages of prayer zone of simulation overoctave bands are within 10% of discrepancy of average field results.This gives confidence for the use of simulation results in assess-ment of occupied condition in real environment. T30 over fre-quency is relatively flat in comparison to mosques with big

Fig. 11. (a) C80 distribution maps for 500 Hz (left) and 1000 Hz (right), unoccupied mosque and (b) C80 distribution maps for 500 Hz (left) and 1000 Hz (right), occupiedmosque.

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domes and large volumes indicating balanced sound absorptionover octave band spectrum and better intelligibility of speech. Inoccupied condition the maximized number of worshippers attend-ing Friday prayer significantly affects the total sound absorptioninside the mosque and the duration of reverberation time becomesmore controlled. Average mid frequency T30 is 1.34 s for occupied

space, balancing need for a tranquil environment with speechintelligibility requirements for given volume.

Good values of clarity indicate an adequate distribution of earlyreflections in receiver locations. For a satisfactory acoustical spacethe musical details should be heard since the orders conveyed byImam in praying mode are more of a musical call than ordinaryspeech. Daily praying activities should be considered in unoccu-pied state of the mosque as fully occupied condition would com-monly refer to the Friday’s sermon. For unoccupied mosque theparameter is within design criteria limits with 1.61 dB averagefor mid frequencies. Numerically, in comparison to T30 values,C80 results of simulation and field measurements show a higherdivergence (Fig. 15). However, the parameter’s optimum rangecovering a 10 dB scale should be indicative. Thus a maximum3 dB of divergence is tolerable in comparison of C80 measurementsand simulations.

STI is directly related to speech. To ensure good speech intelli-gibility, the envelope of the signal should be preserved, allowingdifferent octave bands to contribute to speech quality. This re-quires optimum reverberation duration with minimum interfer-ence of background noise. Apart from the shadow zone rightbehind the minbar most of the prayer locations are satisfying Goodintelligibility criteria. It should be noted that STI simulations andfield measurements are held for single source only depicting Imamspeaking with natural voice. In the presence of SRS, which will bethe case in real environment as in most other mosques or even inhalls for certain amount of amplification, STI range could easily be

Fig. 12. (a) STI distribution maps, unoccupied (left) and occupied (right) mosque and (b) SPL-A distribution maps, unoccupied (left) and occupied (right) mosque.

Fig. 13. Measured background noise levels in octave bands in relation to NC25 andN30 noise criterion curves.

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raised over Good to Excellent levels of intelligibility rating for all re-ceiver/prayer positions.

Within the mosque sound distribution throughout the usableprayer zone/floor is found to be clear of hot and dead spots. Distri-bution is even and parameter value drops/increases are consistentwith the distance from the sound source. Dome as the dominant

architectural figure and acoustical element is not causing any det-rimental effect due to its intentionally adjusted dimensions andheight above the floor. When the dome is completed to a spherethe line of the circumference is far above the receiver surfaces.Therefore, none of the prayer/receiver positions fall into the prob-lematic focusing volume.

Two important subjective acoustical parameters for the mosquefunction are envelopment and warmth. Envelopment is both re-lated to the lateral reflections and spaciousness, which is providedby the increased volume due to the additional space of dome struc-ture at the upper zone. Warmth is provided by the longer low fre-quency content that is necessary to create desired spiritualatmosphere for religious recitals. The latter necessitates low RTvalues at mid to high frequencies allowing optimal speech intelli-gibility for Imam’s Friday’s sermon when enabling both warmthand high intelligibility BR over 1.3 is recommended [33]. In Dogra-macızade Ali Pas�a Mosque for occupied state an estimated of 1.38BR would augment the male voice and complement the acousticalenvironment for hymn singing while providing speech intelligibil-ity limits for given volume.

6. Conclusion

Considering all the studied parameters it could be stated thatDogramacızade Ali Pas�a Mosque is an optimum space for mosqueactivities. The present literature on mosque acoustics indicate thatmost of the old/ancient or even contemporary mosques haveacoustical problems due to focusing dome geometry, excessive vol-ume, thin facade and shelter/roof elements with low sound insula-tion values, and/or improper SRS and HVAC systems. In contrast,the main prayer hall of Dogramacızade Ali Pas�a Mosque is a welldesigned, modestly sized dome structure with appropriate absorp-tive and reflective interior finish materials and a strong outer shell,all creating a significantly more comfortable and acoustically supe-rior interior environment.

The study highlights the necessary interaction of different pro-fessions in creation of habitable spaces. Architectural and acousti-cal decisions should be made simultaneously at the very earlystages of design, not only in mosques with limited activity pat-terns, but in all venues specifically designed for speech and/or mu-sic related performances. This type of an approach saves material,effort and time over the life of the building by preventing any fu-ture acoustical renovations or major architectural replacements.

Acknowledgments

Authors are indebted to Mr. Erkut S�ahinbas�, the architect of themosque for sharing architectural design philosophy and permis-sion for the release of visual material. They also gratefullyacknowledge the permission granted by Mr. Ramiz Akgün ofDogramacı Foundation for acoustical measurements. Specialthanks are extended to Dr. Semra Arslan for her help in the acous-tical design. For their support in field measurements, authors arethankful to Mr. Erinç Odabas� and Ms. Seda Karabulut of MEZZO

Table 3Measured RT, C80 over octave bands; spatial averages for 26 receiver locations, minimum and maximum values.

Parameter 125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz 8000 Hz

RT (s) Minima 1.52 (R14) 1.90 (R1) 1.89 (R23) 1.63 (R2) 1.49 (R4) 1.03 (R14) 0.71 (R24)Spatial average 1.97 2.14 2.06 1.81 1.56 1.13 0.77Maxima 2.24 (R25) 2.35 (R15) 2.23 (R16) 1.96 (R13) 1.65 (R18) 1.22 (R16) 0.82 (R26)

C80 (dB) Minima �2.38 (R10) �7.27 (R10) �6.58 (R10) �6.59 (R10) �6.12 (R10) �4.93 (R10) �2.17 (R10)Spatial average 2.24 2.27 0.83 2.40 3.19 6.26 10.63Maxima 9.64 (R5) 8.22 (R1) 8.70 (R1) 9.60 (R1) 10.6 (R1) 14.26 (R1) 24.69 (R1)

Table 4Measured STI, SPL-A; spatial averages for 26 receiver locations, minimum andmaximum values.

Parameter Minima Spatial average Maxima

STI 0.44 (R10) 0.59 0.79 (R1)SPL-A (dBA) 87.4 (R26) 90.9 92.3 (R1)

Fig. 14. Reverberation times (T30) over octave bands, averages for receiver points,unoccupied mosque; (a) field measurements and (b) computer simulation.

Fig. 15. Clarity (C80) over octave bands, averages for receiver points, unoccupiedmosque; (a) field measurements and (b) computer simulation.

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Stüdyo Ltd. Final appreciation is to Elisa Berger for her invaluablecontribution of the context and structure of the paper.

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