This article gives the the results of a research project studying the acoustical properties of several Ottoman mosques designed by Mimar Sinan in the 16th century. The results of measurements concerning several of these mosques are provided and analysed. Important conclusions are deduced from this carefully conducted analysis, demonstrating the innovative designs of in-built acoustical systems.
A long term  research project has been started to define acoustical properties of the mosques designed by the famous architect Mimar Koca Sinan (died in Istanbul in 1588). Seven of them have been selected as pilot mosques for the beginning. But, continuation of the restoration activities in Sehzade Mosque forced us to work on the remaining six which are:
1. Mihrimah Sultan (Üsküdar-Istanbul),
2. Süleymaniye (Istanbul),
3. Cenabî Ahmet Pasha (Ankara),
4. Rustem Pasha (Istanbul),
5. Sokollu Mehmet Pasha (Kadirga-Istanbul),
6. Selimiye (Edirne).
The reason for selecting Süleymaniye and Selimiye Mosques is due to their importance among the mosques designed by Sinan. The planning geometry of the other four is the basic reason of the selection, and ceramic finishing of Rustem Pasha and Sokollu Mehmet Pasha are also effected. This selection gave us the chance of comparing and discussing the effects of planning geometry and also finishing materials on properties of the mosques.
1. Reverberation Time Measurements
At the beginning of the project the reverberation time of the mosques were measured which gave us surprising results . The measurements were carried out under the rules of ISO recommendation 354 and TS 1476 and the equipment used were the production of Brüel and Kjaer (Figure 1). Although the measured reverberation times of Süleymaniye and Selimiye were prolonged at low frequency region and long in the middle frequencies, we still got the proper results in the other four mosques (Figure 2). These results led us to estimate the reverberation times for those four mosques fully occupied by adding congregation factor to the obtained results (Figure 3). After having the perfect reverberation curves by estimation, it became necessary to investigate the acoustical technology applied by Sinan.
Figure 1: General scheme of the measurement of sound reverberation experiments.
Figure 2: The measured reverberation times of the mosques.
Figure 3: Estimated reverberation times of the fully occupied mosques.
1.1. Interpretation of Reverberation Time Curves
Before investigating the applied technology the question, asking the reasons for the differences between the acoustical performances of two big mosques and the others, must be answered. A short study in Süleymaniye and Selimiye Mosques showed us, the acoustical systems on the boundaries of these mosques had been spoiled during the restoration activities done previously.
The measured reverberation times gave proper curves for the other four mosques, especially for Rustem Pasha and Sokollu Mehmet Pasha where the walls are finished with ceramics. Although reverberation curves for the other two mosques fluctuate a little at low frequencies and increase a little at middle frequency region, they still are acceptable and decrease smoothly with increasing frequency. These values are generally above the recommended values , depending on room volume (Figure 4). But, the estimation gave us the perfect reverberation time curves especially for Rustem Pasha and Sokollu Mehmet Pasha Mosques with agreeable recommended values.
Figure 4: Recommended reverberation times for mosques, results of measurements are indicated with red figures for empty rooms and green figures for occupied rooms (○500 Hz, □ 1000 Hz, Δ 2000 Hz).
2. The Technology used by Sinan
After having the result mentioned above, we investigated the technology applied by Sinan to understand his solutions for Acoustical problems. The investigation showed that the applied acoustical systems were the production of long term estimations and design, and also proved the abundance of his knowledge and applied technology on acoustics.
2.1. The homogeneous sound distribution
The analysis of plans and sections geometry showed that the mosques were designed so that the sound energy would diffuse homogeneously in the rooms, which is also a starting point of today's room acoustics. Sinan used all building elements for this purpose, such that;
i. He avoided using regular forms like a perfect square or cube, and used niches, buttresses, galleries, etc. to break regularity he even designed regular forms (namely; Istanbul Bali Pasha Mosque, Diyarbakir Behram Pasha Mosque . The same elements were also used to break the continuity of big parallel surfaces in the rooms. These applications need knowledge on eigen frequencies and, standing waves which hardly ruin the intelligibility.
ii. In a rectangular room oblique eigen frequencies are seen on the axis of diagonal corners . The use of stalactites at the corners disperses the reflected sound energy which avoids the standing waves. This application can also be seen in Sinan's Mosques.
iii. Dissipative forms were given to all structural elements to disperse the reflected sound energy.
iv. Sinan gave special care to the dome to prevent reflections from the dome which would cause eco and prolongation especially at low frequencies which will be mentioned later.
It must be mentioned that, the element said above have other functions than acoustics, like static and aesthetics. Sinan superposed static, acoustics and aesthetics in his mosques which explains his greatness in the history of architecture.
2.2. The decay of sound energy
After having diffused sound field in a room, the optimum reverberation time must be realized to get sufficient intelligibility. For this purpose the sound energy decay is controlled with absorption at the surfaces of the room boundaries and the elements in the room. It is seen by the investigation that excessive sound energy is absorbed by the surfaces and the elements in the mosques. These are typical examples of acoustical systems; like minbars and balcony parapets made of carved marble (perforated plates). There are three specifically which are seen as more important on the controlling of the sound energy decay. Those are:
i. Plaster: Two more different types of plaster made from horosan mortar was seen besides the traditional one. First one was tow added horosan mortar and basically used for sound absorption. By addition of tow, which give a spring affect, the system works like a panel resonator efficient at low frequencies. The change on mass and stiffness of the system depending on necessary absorption were also seen. In general, the plasters at the domes were more damped than those used at walls. Obviously any reflection from the high dome like the dome of Rustem Pasha and Sokollu Mehmet Pasha caused an eco and prolongation of the reverberation time. This problem was solved by Sinan. But, the use of gypsum plaster instead of an absorbent plaster during restorations causes such problems. The results of the resembling faults were seen at Suleymaniye and Selimiye Mosques.
The second type of plaster was also made from horosan mortar in 4 mm thickness. It was soft and elastic like mastic asphalt. It is thought that this plaster was used to fix the ceramic tiles, which also gave a chance of using vibrating panel resonators finished with ceramics tiles. This succession explains the success of the acoustical design in Rustem Pasha and Sokollu Mehmet Pasha. This subject will be investigated in future.
ii. To prevent standing waves, caused by eigen frequencies, terracotta (earthenware) vessels were used as cavity resonators. In general they were used in the domes with openings through the inside, and great numbers of them were seen. The cavity resonators are one of the systems in acoustics with dimensions less than the wavelength of sound. They have a scattering and absorption affect on incident sound. Figure 5 gives physical definitions for cavity resonators .
Figure 5: The physical definition of the cavity resonators.
Unfortunately, we have not been able to reach the domes of the pilot mosques for detailed inspection of such resonators. But, continuation of restoration in Sehzade and Sultan Ahmet Mosques gave us a chance of such an inspection. There are 35 resonators; each has a perforated lid to increase the internal resistance of the system, at the wall of Sehzade Mosques. They are certainly acoustical systems to control standing waves in such a big room. Their effect on lightening the weight of structure is negligible and must not be confused with urns  which do not have openings through the inside volume. At Sultan Ahmet Mosque, after getting in touch with the site engineer, blocked and plastered resonators were found. Now there are 75 resonators on three rings at the dome. A group of them have small opening with radius 1.5 cm, and the others 3 cm (Figure 6). An approximate estimation gave 100-120 Hz for resonant frequency of the resonators with small radius, and 180-200 Hz for the others. On the other hand, it is mentioned that a great number of the resonators at the dome of Selimiye were blocked with bricks and then plastered with gypsum during the last restoration finished recently, which explains the fast prolongation of reverberation times at low frequencies. It is the same also for Süleymaniye even though 64 resonators were counted at the dome.
Figure 6: A group of resonators.
iii. The third element is carpet floor coverings. While the reverberation times of empty rooms are in acceptable limits, fully occupied mosques give perfect result. It proves that the effect of congregations on acoustics has been calculated, and to reduce the effect of congregation absence the floors were finished with carpet, which is also one of the principles in acoustics today (Figure 7-10).
Figure 7: Reverberation times of Mihrimah Sultan.
Figure 8: Reverberation times of Cenabî Ahmet Pasha Mosque.
Figure 9: Reverberation times of Rustem Pasha Mosque.
Figure 10: Reverberation times of Sokollu Mehmet Pasha Mosque.
The acoustical systems, treatments and reverberation time curves prove that acoustical performances of the mosques were planned before construction. It is seen that Sinan had been able to realize the optimum acoustical performances for different planning geometry and finishing materials, which also proves that these performances were results of detailed estimations and design. Used acoustical systems are highly developed and need extensive knowledge and technology. On the other hand, extension of the spectrum of the research become necessary with the increase of subjected mosques and also including sound distribution and intelligibility measurements in research program.
 This article was first published at Uluslararasi Mimar Sinan Sempozyumu Bildirileri (Ankara, 24-27 October 1988), Ankara Kultur Merkezi Baskanligi, Ankara: Turk Tarih Kurumu Basimevi, 1996, pp. 171-175. See also Mutbul Kayili, Acoustic Solutions in Classic Ottoman Architecture, published online (2005).
 M. Kayili, "Mimar Sinan'in Camilerinde Akustik Verilerin Degerlendirilmesi", Mimar Koca Sinan 'in Yasadigi Çag ve Eserleri, Istanbul, 1988, pp. 545-554.
 Ibid, pp. 545-554.
 A. Kuran. Mimar Sinan, Istanbul 1986.
 L.E. Kinsley and A.R. Frey, Fundamentals of Acoustics, New York 1967.
 U. Ingard, "On the Theory and Design of Acoustic Resonator", J.A.S.A., 25 (1953), pp. 1037-1061.
 G. Goodwin, A History of Ottoman Architecture, Oxford 1987.