Ottoman Mosques in Albania: Building Acoustic Exploration inside Five Case Studies

: The Ottoman mosques of the 15th and 16th centuries represent an architectural typology that has been replicated in territories out of the Middle East for as long as Islam spread its inﬂuence on other countries. This paper deals with the determination of the acoustic properties inside a few Ottoman mosques located in different cities of Albania, speciﬁcally in Elbasan, Berat, Kavaja, Tirana and Shkodra. A comparison between the prayer rooms of these ﬁve mosques has been undertaken in terms of the main acoustic parameters by following the standard requirements outlined in ISO 3382-1. The architectural characteristics of the single-unit mosques chosen for this research study are determined by the domed-square musalla , which has a comparable volume size among all ﬁve mosques. Results gathered by the acoustic measurements undertaken in unoccupied conditions highlight an overall lack of speech understanding given the values of some acoustic parameters fairly acceptable. This outcome is justiﬁed by the geometrical conﬁguration, worsened by the presence of a dome that creates a focusing effect of the sound rays, and by the reﬂecting ﬁnish materials applied to walls and roof that facilitate the build-up defect of echoes. published version


Introduction
The acoustics of the mosques have been already taken in consideration by previous research studies in relation to speech intelligibility [1][2][3][4]. The compositions of its volume, reasonably simple for the single-unit typology and very elaborate for the cross axial and multi-unit design, has been subject to deep analyses for the presence of reflecting surfaces on walls, other than for the presence of semispherical domes that accentuate the unbalanced distribution of the sound energy across all the space [5,6].
The aim of this paper regards the acoustic analysis of five Balkan mosques, built during the 15th and 16th centuries in different cities of Albania. The characteristic feature of being provided with a unique domed-square prayer room is the common key factor of the selected buildings [7][8][9][10][11][12][13][14][15][16]. By undertaking acoustic measurements by using the same equipment and methodology for all the mosques [17][18][19][20][21][22], a comparison of the main acoustic parameters has been performed. The religious functions, also involving the prostration during prayers, contributed to determining the application of absorbing materials on the floor (i.e., carpet) and reflecting surfaces, such as walls and dome (e.g., plaster, stuccos, etc., on stone/masonry structures) [23,24]. Furthermore, the geometry of the prayer room, composed of a semispherical dome on a squared-base parallelepiped, governs the direction of the sound rays to be focused on certain points. This phenomenon is one of the factors that contribute to the lowering of speech intelligibility inside the musalla [25,26].

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Arabesque design, deriving from North Africa and evoking plant configurations; • Arabic calligraphy; • Repetition in modules of a specific geometric pattern.
Other than the musalla (sanctuary), the baseline spaces that are of paramount importance for the constitution of a mosque include an open courtyard for the outdoor activities, a library, workroom, classrooms and a kitchen [7]. These places are organized in such a way to distinguish the ritual from the profane areas [6].
By focusing on the musalla, the plan layout of the sanctuary is usually designed to fulfil the performance of communal worship [6]. The harmony, scale and balance of the musalla should guarantee the straight lines of the congregants without interrupting the religious functions [5]. Therefore, large vaults and open spans represent the preferred structural typologies to enable social inclusion [6,7]. The requirements for salat (prayer) include a clean, carpeted floor where standing, bowing, prostration and sitting acts can be performed [7], as shown in Figure 1.
The investigation undertaken by the authors selects five Albanian mosques comparable volume size and similar dimensions of the squared prayer rooms. T comes highlight a lack of speech understanding, especially at low frequencies [27-ture research studies have been introduced to balance the response across the fr spectrum and apply some acoustic treatments where the simulations will detect face areas that contribute to arise echoes and focalization defects [32][33][34][35][36][37][38].

Holy Meaning of a Mosque
The Islamic mosque is a particular building type of architecture which no est design standards exist for its construction [5]. However, the organization of the can be understood by the activities executed inside [5]. One of the first word that to understand the meaning of this place is masjid (place of prostration) that chara the mosques from churches and synagogues because of the act of prostration [5 other symbol that identifies the mosques from an urban context of a city is the qibl axis) which determines the orientation of the building to be facing Makkah [2]. Th provides no clear rules regarding how the mosque should have the aesthetics; h they are not built by following any of the divine patterns [6]. The three main developed in Muslim art are the following [7]: • Arabesque design, deriving from North Africa and evoking plant configurat • Arabic calligraphy; • Repetition in modules of a specific geometric pattern.
Other than the musalla (sanctuary), the baseline spaces that are of paramo portance for the constitution of a mosque include an open courtyard for the out tivities, a library, workroom, classrooms and a kitchen [7]. These places are orga such a way to distinguish the ritual from the profane areas [6].
By focusing on the musalla, the plan layout of the sanctuary is usually des fulfil the performance of communal worship [6]. The harmony, scale and balanc musalla should guarantee the straight lines of the congregants without interrup religious functions [5]. Therefore, large vaults and open spans represent the p structural typologies to enable social inclusion [6,7]. The requirements for salat include a clean, carpeted floor where standing, bowing, prostration and sitting ac performed [7], as shown in Figure 1. The estimating area to each person is 0.8 × 1.2 m. On this basis, the total oc inside a musalla is generally 500 congregants, against 1000 and 1500 of an urban and an Islamic center, respectively [7].
In Islamic art the circle represents a universal continue, strictly related to the try of the line. The circled domes built at the center of the musalla mirror the ex "God is a circle whose circumference is everywhere and whose center is nowhere" [7]. The estimating area to each person is 0.8 × 1.2 m. On this basis, the total occupancy inside a musalla is generally 500 congregants, against 1000 and 1500 of an urban mosque and an Islamic center, respectively [7].
In Islamic art the circle represents a universal continue, strictly related to the geometry of the line. The circled domes built at the center of the musalla mirror the expression "God is a circle whose circumference is everywhere and whose center is nowhere" [7].

The Architectural Features of a Single-Unit Mosque
The five buildings in comparison dealt with in this paper fall into the typology of a single-unit mosque, having the characteristic features reflecting the domed-square structure, typical of the early Ottoman architecture [6]. The walls of the mosques are generally made of cut stones placed between courses of bricks [7]. The solidity of the masonry is broken only by small windows or doors for ventilation and air change reasons [7].
The presence of a dome completely dominates the indoor space leading towards the center [5]. The domed-square structure is usually completed with a porched courtyard and a minaret, characterizing the basic mass of the typical single-unit mosque widely spread in Turkey even today [7]. A typical example of the architectural organization of a single-unit mosque is shown in Figure 2.
Buildings 2021, 11, x FOR PEER REVIEW

The Architectural Features of a Single-Unit Mosque
The five buildings in comparison dealt with in this paper fall into the typolo single-unit mosque, having the characteristic features reflecting the domed-squar ture, typical of the early Ottoman architecture [6]. The walls of the mosques are ge made of cut stones placed between courses of bricks [7]. The solidity of the mas broken only by small windows or doors for ventilation and air change reasons [7] The presence of a dome completely dominates the indoor space leading towa center [5]. The domed-square structure is usually completed with a porched co and a minaret, characterizing the basic mass of the typical single-unit mosque spread in Turkey even today [7]. A typical example of the architectural organizat single-unit mosque is shown in Figure 2. The development of the system, from hemispherical brick dome to the square has gone through the transition from octagonal base to polygonal structure [6]. Th tion of a courtyard before entering the sacred space is considered an evolution of gle-unit layout by leaving intact the domed-square volume, separated from the oth fane places [6].
The spread of Muslim out of the Middle East boundaries developed the const of single-unit mosques also inside the Balkan peninsula, especially when Alba been conquered by Turkish sultans during the 17th century.

Mosque of Naziresha in Elbasan
Naziresha is the oldest mosque built in Elbasan in the 1590s, during the flou period of the Sultan Mehmed II [8]. Elbasan is located in Southern Albania and the m has been built just at the borders of the medieval city in 1599 [7]. It is a purely sing mosque, composed of a prayer hall and a minaret located at the south-west corne been damaged on the upper termination but then entirely restored after a requali of the site. The musalla is a squared planned volume, having internal dimensions m. The semicircular dome has a center height of 13.9 m, with a total volume of a mately 954 m 3 . Figure 3 reports the plan and the elevation of the mosque. The development of the system, from hemispherical brick dome to the squared base, has gone through the transition from octagonal base to polygonal structure [6]. The addition of a courtyard before entering the sacred space is considered an evolution of the single-unit layout by leaving intact the domed-square volume, separated from the other profane places [6].
The spread of Muslim out of the Middle East boundaries developed the construction of single-unit mosques also inside the Balkan peninsula, especially when Albania had been conquered by Turkish sultans during the 17th century.

Mosque of Naziresha in Elbasan
Naziresha is the oldest mosque built in Elbasan in the 1590s, during the flourishing period of the Sultan Mehmed II [8]. Elbasan is located in Southern Albania and the mosque has been built just at the borders of the medieval city in 1599 [7]. It is a purely single-unit mosque, composed of a prayer hall and a minaret located at the south-west corner, it had been damaged on the upper termination but then entirely restored after a requalification of the site. The musalla is a squared planned volume, having internal dimensions of 9 × 9 m. The semicircular dome has a center height of 13.9 m, with a total volume of approximately 954 m 3 . Figure 3 reports the plan and the elevation of the mosque.  The mosque has been recently restored as a whole by taking the occasion of the reconstruction of the upper part of the minaret. Externally the walls are composed of stone until 8.12 m height, while the brick walls called pendentives are those triangular shoulders that make the transition from a cube to a dome [6]. The dome is composed of 0.9 m thick brick structure, covered by metal sheets to be protected from the meteorological conditions. The interior design has plaster on the walls and roof, and carpet on the floor. Figure  4 shows some external views of the Naziresha Mosque.

Lead Mosque in Berat
The Lead Mosque has been built by the local feudal Ahmed Bey Uzgurliu in the 16th century (1553-1554) in the center of Berat, specifically about Vakef [5]. Berat, or Arnaut Belgrade, is a city dominated by Byzantine monuments. The mosque has been built with carved stones applied also to the elegant minaret [6]. The prayer hall is a squared planned The mosque has been recently restored as a whole by taking the occasion of the reconstruction of the upper part of the minaret. Externally the walls are composed of stone until 8.12 m height, while the brick walls called pendentives are those triangular shoulders that make the transition from a cube to a dome [6]. The dome is composed of 0.9 m thick brick structure, covered by metal sheets to be protected from the meteorological conditions. The interior design has plaster on the walls and roof, and carpet on the floor. Figure 4 shows some external views of the Naziresha Mosque. The mosque has been recently restored as a whole by taking the occasion of the reconstruction of the upper part of the minaret. Externally the walls are composed of stone until 8.12 m height, while the brick walls called pendentives are those triangular shoulders that make the transition from a cube to a dome [6]. The dome is composed of 0.9 m thick brick structure, covered by metal sheets to be protected from the meteorological conditions. The interior design has plaster on the walls and roof, and carpet on the floor. Figure  4 shows some external views of the Naziresha Mosque.

Lead Mosque in Berat
The Lead Mosque has been built by the local feudal Ahmed Bey Uzgurliu in the 16th century (1553-1554) in the center of Berat, specifically about Vakef [5]. Berat, or Arnaut Belgrade, is a city dominated by Byzantine monuments. The mosque has been built with carved stones applied also to the elegant minaret [6]. The prayer hall is a squared planned

Lead Mosque in Berat
The Lead Mosque has been built by the local feudal Ahmed Bey Uzgurliu in the 16th century (1553-1554) in the center of Berat, specifically about Vakef [5]. Berat, or Arnaut Belgrade, is a city dominated by Byzantine monuments. The mosque has been built with carved stones applied also to the elegant minaret [6]. The prayer hall is a squared planned volume, having internal dimensions of 10.2 × 10.2 m. The musalla is surmounted by a semicircular dome having a center height of 14.4 m. The main volume, that is approximately 1307 m 3 , is lighted by windows located at three orders of height. The geometric transition between the great dome and the cubic hall consists of an octagonal cylinder. The mosque also has a porch with a vaulted roof composed of four little domes of different diameter, the larger placed on the sides while the smaller are at the center of the main axis. Figure 5 shows the plan and the longitudinal section of the mosque. The main volume, that is approximately 1307 m 3 , is lighted by windows located at three orders of height. The geometric transition between the great dome and the cubic hall consists of an octagonal cylinder. The mosque also has a porch with a vaulted roof composed of four little domes of different diameter, the larger placed on the sides while the smaller are at the center of the main axis. Figure 5 shows the plan and the longitudinal section of the mosque. The masonry of the external walls has been built by following the cloisonné technique [7], which consists of two horizontal rows of bricks placed between the rows of stones, all plastered on the inner side. Another material used for indoor is the carpet on the floor.
The minaret of the Lead Mosque of Berat has been built with the same construction technique, terminating at the top with a cauldron as a typical characteristic of all the 16th century mosques [8].
Refurbishment works occurred in 1720, when the prayer room was plastered using a reddish lime [8]. Currently, the interior design follows floral motifs while the exterior is dominated by the structural construction of the cubic volume, as shown in Figure 6.  The masonry of the external walls has been built by following the cloisonné technique [7], which consists of two horizontal rows of bricks placed between the rows of stones, all plastered on the inner side. Another material used for indoor is the carpet on the floor.
The minaret of the Lead Mosque of Berat has been built with the same construction technique, terminating at the top with a cauldron as a typical characteristic of all the 16th century mosques [8].
Refurbishment works occurred in 1720, when the prayer room was plastered using a reddish lime [8]. Currently, the interior design follows floral motifs while the exterior is dominated by the structural construction of the cubic volume, as shown in Figure 6. The main volume, that is approximately 1307 m 3 , is lighted by windows located at three orders of height. The geometric transition between the great dome and the cubic hall consists of an octagonal cylinder. The mosque also has a porch with a vaulted roof composed of four little domes of different diameter, the larger placed on the sides while the smaller are at the center of the main axis. Figure 5 shows the plan and the longitudinal section of the mosque. The masonry of the external walls has been built by following the cloisonné technique [7], which consists of two horizontal rows of bricks placed between the rows of stones, all plastered on the inner side. Another material used for indoor is the carpet on the floor.
The minaret of the Lead Mosque of Berat has been built with the same construction technique, terminating at the top with a cauldron as a typical characteristic of all the 16th century mosques [8].
Refurbishment works occurred in 1720, when the prayer room was plastered using a reddish lime [8]. Currently, the interior design follows floral motifs while the exterior is dominated by the structural construction of the cubic volume, as shown in Figure 6.

Kubelie Mosque in Kavaja
The Kubelie Mosque has been built in 1735-1736 by the Ottoman Kapllan Beu in the center of the medieval market [6]. The architectural characteristic of this mosque is the courtyard placed before the prayer room, composed of five arcades in front and three arcades on each side [8]. Originally, this portico should be covered by a sloped wooden roof. The columns with the Corinthian style were built with the purpose of evoking European architecture [8]. The prayer room has a squared layout in plan, having dimensions of 8.6 × 8.6 m. The mihrab is placed on the southern side of the musalla. The geometric transition between the great dome and the cubic hall consists of an octagonal cylinder, as shown in Figure 7.

Kubelie Mosque in Kavaja
The Kubelie Mosque has been built in 1735-1736 by the Ottoman Kapllan Beu in the center of the medieval market [6]. The architectural characteristic of this mosque is the courtyard placed before the prayer room, composed of five arcades in front and three arcades on each side [8]. Originally, this portico should be covered by a sloped wooden roof. The columns with the Corinthian style were built with the purpose of evoking European architecture [8]. The prayer room has a squared layout in plan, having dimensions of 8.6 × 8.6 m. The mihrab is placed on the southern side of the musalla. The geometric transition between the great dome and the cubic hall consists of an octagonal cylinder, as shown in Figure 7.

Et'hem Bey Mosque in Tirana
The Et'hem Bey Mosque has been built in 1793-1794 by Mulla Beut and was completed in 1823 by his son Haxhi Ethen Bey, which is where the name comes from [7]. This mosque also has a minaret and a portico around two sides of the prayer room. The portico The height at the center of the dome is 10.5 m, and the total volume of the musalla is 691 m 3 . The minaret of the Kubelie Mosque has a hexagonal-base pedestal, as shown in Figure 8.

Kubelie Mosque in Kavaja
The Kubelie Mosque has been built in 1735-1736 by the Ottoman Kapllan Beu in the center of the medieval market [6]. The architectural characteristic of this mosque is the courtyard placed before the prayer room, composed of five arcades in front and three arcades on each side [8]. Originally, this portico should be covered by a sloped wooden roof. The columns with the Corinthian style were built with the purpose of evoking European architecture [8]. The prayer room has a squared layout in plan, having dimensions of 8.6 × 8.6 m. The mihrab is placed on the southern side of the musalla. The geometric transition between the great dome and the cubic hall consists of an octagonal cylinder, as shown in Figure 7.

Et'hem Bey Mosque in Tirana
The Et'hem Bey Mosque has been built in 1793-1794 by Mulla Beut and was completed in 1823 by his son Haxhi Ethen Bey, which is where the name comes from [7]. This mosque also has a minaret and a portico around two sides of the prayer room. The portico

Et'hem Bey Mosque in Tirana
The Et'hem Bey Mosque has been built in 1793-1794 by Mulla Beut and was completed in 1823 by his son Haxhi Ethen Bey, which is where the name comes from [7]. This mosque also has a minaret and a portico around two sides of the prayer room. The portico is currently supported by very solid stone columns. Probably the initial design included into Mulla Bey's project would indicate wooden pillars instead, but no documents nor other indications confirm this hypothesis [8]. The musalla has dimensions of 10.5 × 10.5 m and the entire volume is lighted by windows organized onto three different levels. The height at the center of the dome is 14.4 m and the entire volume of the prayer room is 1394 m 3 . The access to the prayer room is given by three doors, while the mihrab has been placed in front of the central door, on the opposite side.
The prayer room, the minaret and the portico are interconnected, as shown in Figure 9. The interior design is composed of plaster on the walls and roof, and carpet on the floor. The minaret was composed of regular stone blocks having at the top the cauldron designed by following the style of 16th century mosques [6]. The whole holy site has been terminated in 1821, confirmed by the inscriptions found on the inner side of the mosque [7]. is currently supported by very solid stone columns. Probably the initial design included into Mulla Bey's project would indicate wooden pillars instead, but no documents nor other indications confirm this hypothesis [8]. The musalla has dimensions of 10.5 × 10.5 m and the entire volume is lighted by windows organized onto three different levels. The height at the center of the dome is 14.4 m and the entire volume of the prayer room is 1394 m 3 . The access to the prayer room is given by three doors, while the mihrab has been placed in front of the central door, on the opposite side. The prayer room, the minaret and the portico are interconnected, as shown in Figure 9. The interior design is composed of plaster on the walls and roof, and carpet on the floor. The minaret was composed of regular stone blocks having at the top the cauldron designed by following the style of 16th century mosques [6]. The whole holy site has been terminated in 1821, confirmed by the inscriptions found on the inner side of the mosque [7]. The interior design is characterized by floral motifs represented on walls and dome surface. The chosen colors, between red and green, contribute to make the holy place warm, as shown in Figure 10.  The interior design is characterized by floral motifs represented on walls and dome surface. The chosen colors, between red and green, contribute to make the holy place warm, as shown in Figure 10. is currently supported by very solid stone columns. Probably the initial design included into Mulla Bey's project would indicate wooden pillars instead, but no documents nor other indications confirm this hypothesis [8]. The musalla has dimensions of 10.5 × 10.5 m and the entire volume is lighted by windows organized onto three different levels. The height at the center of the dome is 14.4 m and the entire volume of the prayer room is 1394 m 3 . The access to the prayer room is given by three doors, while the mihrab has been placed in front of the central door, on the opposite side. The prayer room, the minaret and the portico are interconnected, as shown in Figure 9 The interior design is composed of plaster on the walls and roof, and carpet on the floor The minaret was composed of regular stone blocks having at the top the cauldron designed by following the style of 16th century mosques [6]. The whole holy site has been terminated in 1821, confirmed by the inscriptions found on the inner side of the mosque [7]. The interior design is characterized by floral motifs represented on walls and dome surface. The chosen colors, between red and green, contribute to make the holy place warm, as shown in Figure 10.

Lead Mosque in Shkodra
Shkodra is considered the most important city of Northern Albania because of its long history. The mosque of Lead is one of the best preserved in Albania, showing evident features of the typical Ottoman style [7]. The design of this mosque evokes the early 15th centuries Muslim buildings realized in Turkey, especially for its geometry of the plan and the volumetric composition of all the complex. Erected in 1774, the musalla of the Lead Mosque is a domed volume having a squared base of 9.2 × 9.2 m, while the total height taken at the center of the dome is 14.7 m. The hemispherical dome is made of a brick masonry and rests on an octagonal cylinder. Three windows are open on each side of the lateral walls and three doors give access to the sanctuary. The total volume of the prayer hall is approximately 1051 m 3 . Figure 11 shows the plan and the side elevation.

Lead Mosque in Shkodra
Shkodra is considered the most important city of Northern Albania because of its long history. The mosque of Lead is one of the best preserved in Albania, showing evident features of the typical Ottoman style [7]. The design of this mosque evokes the early 15th centuries Muslim buildings realized in Turkey, especially for its geometry of the plan and the volumetric composition of all the complex. Erected in 1774, the musalla of the Lead Mosque is a domed volume having a squared base of 9.2 × 9.2 m, while the total height taken at the center of the dome is 14.7 m. The hemispherical dome is made of a brick masonry and rests on an octagonal cylinder. Three windows are open on each side of the lateral walls and three doors give access to the sanctuary. The total volume of the prayer hall is approximately 1051 m 3 . Figure 11 shows the plan and the side elevation. The indoor finish materials of the musalla are plaster on the walls and roof, and carpet on the floor. The courtyard represents the mandatory path to cross before entering the musalla. The courtyard is composed of 12 little domes, corresponding to each arcade having same width. Figure 12 shows the views of the mosque, from outside the entire complex and inside the courtyard.

Summary of the Architectural Features
In the previous subsections all the five selected mosques have been described in detail. Only the Lead Mosque of Shkodra is particularly complex due to the presence of a courtyard. The other four mosques are only composed of the prayer room and an arched portico on the access side. A comparison between all the mosques has been made in terms of architectural characteristics of the prayer room, as summarized in Table 1. The indoor finish materials of the musalla are plaster on the walls and roof, and carpet on the floor. The courtyard represents the mandatory path to cross before entering the musalla. The courtyard is composed of 12 little domes, corresponding to each arcade having same width. Figure 12 shows the views of the mosque, from outside the entire complex and inside the courtyard.

Lead Mosque in Shkodra
Shkodra is considered the most important city of Northern Albania because of its long history. The mosque of Lead is one of the best preserved in Albania, showing evident features of the typical Ottoman style [7]. The design of this mosque evokes the early 15th centuries Muslim buildings realized in Turkey, especially for its geometry of the plan and the volumetric composition of all the complex. Erected in 1774, the musalla of the Lead Mosque is a domed volume having a squared base of 9.2 × 9.2 m, while the total height taken at the center of the dome is 14.7 m. The hemispherical dome is made of a brick masonry and rests on an octagonal cylinder. Three windows are open on each side of the lateral walls and three doors give access to the sanctuary. The total volume of the prayer hall is approximately 1051 m 3 . Figure 11 shows the plan and the side elevation. The indoor finish materials of the musalla are plaster on the walls and roof, and carpet on the floor. The courtyard represents the mandatory path to cross before entering the musalla. The courtyard is composed of 12 little domes, corresponding to each arcade having same width. Figure 12 shows the views of the mosque, from outside the entire complex and inside the courtyard.

Summary of the Architectural Features
In the previous subsections all the five selected mosques have been described in detail. Only the Lead Mosque of Shkodra is particularly complex due to the presence of a courtyard. The other four mosques are only composed of the prayer room and an arched portico on the access side. A comparison between all the mosques has been made in terms of architectural characteristics of the prayer room, as summarized in Table 1.

Summary of the Architectural Features
In the previous subsections all the five selected mosques have been described in detail. Only the Lead Mosque of Shkodra is particularly complex due to the presence of a courtyard. The other four mosques are only composed of the prayer room and an arched portico on the access side. A comparison between all the mosques has been made in terms of architectural characteristics of the prayer room, as summarized in Table 1. The interior design of all the mosques was very similar: carpet on floor, plaster on walls and minimal furniture, as shown in Figure 13.  The interior design of all the mosques was very similar: carpet on floor, plaster on walls and minimal furniture, as shown in Figure 13.

Measurements
A few campaigns of measurements have been undertaken inside the five mosques in order to analyze the acoustic behavior of these architectural typology constructions. The following equipment has been used: • Balloon pops [14,15]; • Omnidirectional microphone (BRAHMA) [18]9.
This is a digital four channels microphone that allows to covert the recording in an Ambisonics B-format through the use of the software Brahmavolver [9,10]. The composition of the four microphones is arranged in a tetrahedral array and it allows converting the native format into orthogonal sound records during the post-processing [16].
The balloon pops represent a type of sound source affected by the uncertainties of repeatability and directivity [17][18][19]. Furthermore, the balloon diameter should be wide enough to abundantly cover the low frequencies. The other concern riced by the use of balloon pops is the directivity, resulting in quite a robustness in some directions. Although the excitation signal has a spectrum bandwidth more limited than any electronic signal

Measurements
A few campaigns of measurements have been undertaken inside the five mosques in order to analyze the acoustic behavior of these architectural typology constructions. The following equipment has been used:

•
Balloon pops [14,15]; This is a digital four channels microphone that allows to covert the recording in an Ambisonics B-format through the use of the software Brahmavolver [9,10]. The composition of the four microphones is arranged in a tetrahedral array and it allows converting the native format into orthogonal sound records during the post-processing [16].
The balloon pops represent a type of sound source affected by the uncertainties of repeatability and directivity [17][18][19]. Furthermore, the balloon diameter should be wide enough to abundantly cover the low frequencies. The other concern riced by the use of balloon pops is the directivity, resulting in quite a robustness in some directions. Although the excitation signal has a spectrum bandwidth more limited than any electronic signal (e.g., Exponential Sine Sweep (ESS), etc.), the survey has been carried out in accordance with the standard requirements outlined by ISO 3382-1 [20].
In each mosque, the sound source location has been selected to be close to the mihrab, where the balloons were burst at the height of 1.3 m from the floor, while the microphone was placed in different positions across the musalla at the height of 1.3 m [21]. The receiving positions were five in each mosque, as shown in Figure 14.
Buildings 2021, 11, x FOR PEER REVIEW 10 of 16 (e.g., Exponential Sine Sweep (ESS), etc.), the survey has been carried out in accordance with the standard requirements outlined by ISO 3382-1 [20]. In each mosque, the sound source location has been selected to be close to the mihrab, where the balloons were burst at the height of 1.3 m from the floor, while the microphone was placed in different positions across the musalla at the height of 1.3 m [21]. The receiving positions were five in each mosque, as shown in Figure 14. All the acoustic measurements have been carried out in unoccupied conditions. A brief description of the selected acoustic parameters is given below:

•
Early decay time (EDT), measured in seconds, is the time corresponding to 10 dB from the decay curve of the sound pressure level measured after the interruption of the noise source. The EDT is particularly sensitive to the microphone position. All the acoustic measurements have been carried out in unoccupied conditions. A brief description of the selected acoustic parameters is given below:

•
Early decay time (EDT), measured in seconds, is the time corresponding to 10 dB from the decay curve of the sound pressure level measured after the interruption of the noise source. The EDT is particularly sensitive to the microphone position. • Similar to EDT, the reverberation time (T 20 ) is defined as the time required for the sound pressure level to drop by 60 dB after the noise source has been switched off. The value of this parameter is in function of the room volume and absorption of the surface area. Since the absorption of materials varies with frequency, the reverberation time varies according to the considered frequency bands.

•
Clarity index (C 50 ) is a characteristic parameter of speech goodness in a hall. It is a descriptor obtained from the correlation of the emitted energy that reaches the listener within the first 50 milliseconds (including the early reflections) and the energy that arrives in the following instants. • Definition (D 50 ) is a parameter related to the goodness of speech understanding in a room. It is obtained from the correlation of energy that reaches the receiver in the first 50 milliseconds and entire signal energy built up in the room until its total dissipation. The early reflection energy added to the direct sound gives a positive contribution to speech understanding. • Speech transmission index (STI) represents the degree of amplitude modulation in a speech signal and refers to the distortion of a speech caused by reverberation, echoes and/or background noise. STI values can be comprised between 0 and 1: if the result is >0.5, favorable speech conditions occur.

Measured Results
The recorded data of the IRs have been edited at the beginning by using the software Audacity by having a sample rate equal to 44 kHz. Thereafter, the signals have been processed by using the software Dirac. Figures 15-18 report the main acoustic parameters in the octave frequency bands comprised between 125 Hz and 4 kHz, which were considered as the average results of all the measuring positions. Figure 15 shows a similar curve trend for all the mosques. The difference at low frequency between the minimum and the maximum value is up to 1.5 s, particularly between Kavaja that shows the lowest values and Shkodra having an EDT equal to 3.0 s at 125 Hz. This difference is considerably reduced at high frequencies where the values range between 1.0 s (Berat and Tirana) and 1.3 s (Shkodra). The lowest values at low frequency are those related to Kavaja, which is the mosque having the smallest room volume, while the highest values across all the frequency bandwidth are related to the Lead Mosque of Shkodra, although it does not have the biggest room volume among all of them. By considering the optimal values of EDT ranging between 1.8 s and 2.6 s [22], the outcomes indicate that these values have been achieved between 125 Hz and 1 kHz only for Berat, Elbasan and Tirana; Shkodra has been found to be exceeding the upper range limit of up to 0.5 s, while Kavaja has the EDT value at 125 Hz slightly below the lower range limit. At high frequency none of the mosques has EDT values within the optimal range, since all of them are below the lower limit range.
In terms of T 20 , Figure 16 shows that the trend lines of the mosques are slightly dissimilar each other. In particular, Elbasan has the highest value at 125 Hz, equal to 3.3 s, and the lowest at 4 kHz, equal to 1.1 s. Tirana and Berat, by having comparable room volumes, result in a similar line trend. The flattest response of T 20 across all the frequency bandwidth is given by Kavaja, to be around 1.8 s averaged through all frequencies.
In addition, there are few factors that contribute to the T 20 results, although these do not emerge from the graph; in particular, not only should the room volume size be considered as the main influence but also the wideness of the squared plan, the height of the dome and the grade of reflectivity of the finish materials applied mainly to the walls and roof [23,24]. Therefore, the focus of the sound rays localized at the center of the musalla is considered an acoustic defect typically found in other rooms having similar geometry and architectural organization [25,26]. across all the frequency bandwidth are related to the Lead Mosque of Shkodra, although it does not have the biggest room volume among all of them. By considering the optimal values of EDT ranging between 1.8 s and 2.6 s [22], the outcomes indicate that these values have been achieved between 125 Hz and 1 kHz only for Berat, Elbasan and Tirana; Shkodra has been found to be exceeding the upper range limit of up to 0.5 s, while Kavaja has the EDT value at 125 Hz slightly below the lower range limit. At high frequency none of the mosques has EDT values within the optimal range, since all of them are below the lower limit range. In terms of T20, Figure 16 shows that the trend lines of the mosques are slightly dissimilar each other. In particular, Elbasan has the highest value at 125 Hz, equal to 3.3 s, and the lowest at 4 kHz, equal to 1.1 s. Tirana and Berat, by having comparable room volumes, result in a similar line trend. The flattest response of T20 across all the frequency bandwidth is given by Kavaja, to be around 1.8 s averaged through all frequencies. In addition, there are few factors that contribute to the T20 results, although these do not emerge from the graph; in particular, not only should the room volume size be considered as the main influence but also the wideness of the squared plan, the height of the dome and the grade of reflectivity of the finish materials applied mainly to the walls and roof [23,24]. Therefore, the focus of the sound rays localized at the center of the musalla is considered an acoustic defect typically found in other rooms having similar geometry and architectural organization [25,26].
By using the software Dirac, it is possible to analyze the IRs obtained with the measurements. In particular, Figure 17 shows an increase of sound energy after the first reflections such that the trend of the decay curve is not linearly pointing downward but presents a double slope with an initial increase of energy pointing upward and then decreasing to the background noise levels. In terms of T20, Figure 16 shows that the trend lines of the mosques are slightly dissimilar each other. In particular, Elbasan has the highest value at 125 Hz, equal to 3.3 s, and the lowest at 4 kHz, equal to 1.1 s. Tirana and Berat, by having comparable room volumes, result in a similar line trend. The flattest response of T20 across all the frequency bandwidth is given by Kavaja, to be around 1.8 s averaged through all frequencies. In addition, there are few factors that contribute to the T20 results, although these do not emerge from the graph; in particular, not only should the room volume size be considered as the main influence but also the wideness of the squared plan, the height of the dome and the grade of reflectivity of the finish materials applied mainly to the walls and roof [23,24]. Therefore, the focus of the sound rays localized at the center of the musalla is considered an acoustic defect typically found in other rooms having similar geometry and architectural organization [25,26].
By using the software Dirac, it is possible to analyze the IRs obtained with the measurements. In particular, Figure 17 shows an increase of sound energy after the first reflections such that the trend of the decay curve is not linearly pointing downward but presents a double slope with an initial increase of energy pointing upward and then decreasing to the background noise levels.  Figure 18 shows the results related to the speech clarity index, which are found to be below the optimal target (>+3 dB) [28] for all the mosques. This means that the intelligibility of words is fairly poor. The worst C50 has been found in Elbasan, to be up to 13 dB below the lower range limit, especially at 125 Hz and 1 kHz. The mosques of Tirana, Berat and Kavaja are indicated to have a similar response in terms of C50, showing beneficial peaks at 125 Hz, 500 Hz and 4 kHz. The results found in Shkodra draw a line trend to be closer to Elbasan at 125 Hz and close to the other mosques for mid-high frequencies.  By using the software Dirac, it is possible to analyze the IRs obtained with the measurements. In particular, Figure 17 shows an increase of sound energy after the first reflections such that the trend of the decay curve is not linearly pointing downward but presents a double slope with an initial increase of energy pointing upward and then decreasing to the background noise levels.
This acoustic behavior has been reported only for the Naziresha Mosque of Elbasan, but it is a common aspect found in the other four mosques.
This paper in mainly focused on the evaluation of the acoustic parameters to be in accordance with the ISO 8233-1; investigations on domes' geometry would be developed in a future research study with the support of digital models.
On this basis, the speech transmission index (STI) has been analyzed given the main activity developed inside the musalla centered on prayers and sermons. As such, the STI has been considered as the parameter indicating the degree of amplitude modulation in a speech signal [27]. The STI can be assessed as the level of distortion in speech signals caused by reverberation, echoes and background noise. Although the index can assume values comprised between 0 and 1, the measured average values in all mosques are found to be fair for speech understanding, based on the STI scale [29] as reported in Table 2. In order to understand the STI values, the background noise levels (L Aeq ) have been summarized in Table 3 below, related to all the mosques.  Figure 18 shows the results related to the speech clarity index, which are found to be below the optimal target (>+3 dB) [28] for all the mosques. This means that the intelligibility of words is fairly poor. The worst C 50 has been found in Elbasan, to be up to 13 dB below the lower range limit, especially at 125 Hz and 1 kHz. The mosques of Tirana, Berat and Kavaja are indicated to have a similar response in terms of C 50 , showing beneficial peaks at 125 Hz, 500 Hz and 4 kHz. The results found in Shkodra draw a line trend to be closer to Elbasan at 125 Hz and close to the other mosques for mid-high frequencies.
In relation to speech clarity, other than the STI parameter, the definition D 50 can be also evaluated based on the measured results indicated in Figure 19. Although previous research has determined a good speech definition (D 50 ) for values higher than 0.5 (50%) [29], the outcomes related to all the mosques show that the values across all the frequency bands fluctuates around 0.2 (20%), which is considered poor. In relation to speech clarity, other than the STI parameter, the definition D50 can be also evaluated based on the measured results indicated in Figure 19. Although previous research has determined a good speech definition (D50) for values higher than 0.5 (50%) [29], the outcomes related to all the mosques show that the values across all the frequency bands fluctuates around 0.2 (20%), which is considered poor.

Conclusions
The Ottoman mosques herein studied are active places of cult and prayers. The intention of the authors is to highlight the grade of speech understanding of the existing conditions, as this is the main activity performed inside these environments [30,31].
To increase the level of speech definition and move the speech clarity index to be within the optimal range limits, the authors propose acoustic treatments to be applied mainly on the walls [32][33][34][35][36]. A limitation of this study is related to the utilization of the balloon pops, given a certain directivity and a limited spectrum width. In addition, this type of sound source has an unpredictable spectral response, as well as a poor repeatability. All these variabilities, if not properly controlled, can give a low value of signal-tonoise ratio (S/N), even if this technique is still widely used, especially for small room volumes as it could be a musalla.
In order to avoid invasive or irreversible detriments to these historical buildings, removable transparent micro-perforated panels could represent a solution with a minimal visual impact [37]. As such, the transparency of the new panels would not conceal the beauties of the walls' decorations and, in addition, a specific cavity gap to be left between the panel and the back wall will be also considered.
Future research studies will be focusing on the acoustic simulations with the presence of the proposed absorbing panels, in such a way to improve the listening conditions especially by absorbing the low frequencies and restoring a tonal balance across all the frequency bandwidth [38].

Conclusions
The Ottoman mosques herein studied are active places of cult and prayers. The intention of the authors is to highlight the grade of speech understanding of the existing conditions, as this is the main activity performed inside these environments [30,31].
To increase the level of speech definition and move the speech clarity index to be within the optimal range limits, the authors propose acoustic treatments to be applied mainly on the walls [32][33][34][35][36]. A limitation of this study is related to the utilization of the balloon pops, given a certain directivity and a limited spectrum width. In addition, this type of sound source has an unpredictable spectral response, as well as a poor repeatability. All these variabilities, if not properly controlled, can give a low value of signal-to-noise ratio (S/N), even if this technique is still widely used, especially for small room volumes as it could be a musalla.
In order to avoid invasive or irreversible detriments to these historical buildings, removable transparent micro-perforated panels could represent a solution with a minimal visual impact [37]. As such, the transparency of the new panels would not conceal the beauties of the walls' decorations and, in addition, a specific cavity gap to be left between the panel and the back wall will be also considered.
Future research studies will be focusing on the acoustic simulations with the presence of the proposed absorbing panels, in such a way to improve the listening conditions especially by absorbing the low frequencies and restoring a tonal balance across all the frequency bandwidth [38].