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KANI KARACA isn鈥檛 used to working in these conditions, but he gives it his best shot. The 75-year-old singer, one of Turkey鈥檚 best known, is accustomed to hearing his songs resonate around Istanbul鈥檚 grand mosques. But today he鈥檚 in a studio with sound-absorbing walls, and his voice sounds deadened and uninspiring. Karaca鈥檚 not about to throw a tantrum, though. His recording could be the key to preserving some of Turkey鈥檚 finest buildings. The singer is involved in a project to prevent the mosques of Mimar Sinan, the master builder of the Ottoman Empire, from losing their splendid acoustics.

Sinan lived from 1489 to 1588 and designed some of Ottoman Turkey鈥檚 grandest mosques, including the stunning S眉leymaniye in Istanbul, or Constantinople as it was officially called back then. His mosques boast ornate facades and lavishly decorated interiors, but there鈥檚 more to them than just visual beauty. Sinan also took unprecedented care over the acoustics so that songs, prayers, preaching and recital鈥攁ll central to the life of a mosque鈥攚ould carry.

Unfortunately, restoration efforts over the years have taken their toll on these wonderful acoustics. Sound has never been a consideration in conservation, says Zehran Karabiber, an acoustics researcher at the Yildiz Technical University in Istanbul. Some efforts to preserve and restore Sinan鈥檚 mosques have inadvertently corrupted the acoustics.

Take the S眉leymaniye mosque, for example. It鈥檚 Istanbul鈥檚 second largest mosque and was Sinan鈥檚 tribute to his sultan, S眉leyman the Magnificent. Sinan used a Roman technique to enhance the mosque鈥檚 acoustics. He set jugs into the walls and domes, with their mouths open to the air. 鈥淭he air circulation between the jugs and the room has an effect on the sound absorption,鈥� Karabiber says. Or it would do, if some slap-happy plasterer hadn鈥檛 covered the holes at some point during the mosque鈥檚 450-year history.

The plastering was done too long ago to lay the blame at anyone鈥檚 feet, but Karabiber wants to stop such things happening again. She believes that ignoring the desecration of a mosque鈥檚 acoustics is worse than allowing lumps of plaster to fall from its walls. So she has assembled a team of researchers to thwart acoustic damage before it can happen. Their plan is to try out restoration jobs in a virtual world, before anyone even touches a hammer or a bucket of plaster. That way botched jobs can be rectified at the click of a mouse. They hope to find out what conservation teams should avoid.

Karabiber鈥檚 team has already built virtual versions of three of Sinan鈥檚 mosques, S眉leymaniye, Sokullu and Selimiye, as well as some of the older Byzantine churches which inspired him, Hagia Sophia, St Sergius and St Bacchus, and St Irene. It was no mean feat: their computer model of the S眉leymaniye mosque, for instance, contains 4677 surfaces and 7123 corners, all translated from architectural drawings into computer code. Add to that information about the acoustic properties of the material used on each surface, and the scale of their task begins to emerge.

But the effort is worth it. When they play the acoustically dead recording of Karaca鈥檚 singing 鈥渋nside鈥� these models, it reverberates off every virtual surface. The researchers can then put themselves anywhere inside the virtual mosque and hear exactly what someone standing in that spot would hear. Most importantly, they can change the acoustical properties of the virtual building.

That means they can experiment with different materials and listen to the effect they would have. 鈥淲e can hear what it would sound like if we removed the carpets, put plaster on the walls, or any kind of restoration you want,鈥� says Jens Holger Rindel of the Technical University of Denmark in Lyngby, who leads the Danish side of the project. If they wanted they could do the most appalling restoration job, replacing marble columns with concrete, for instance.

No real conservation team would be quite so reckless, but restoration is fraught with danger. Even the subtlest of alterations can alter the acoustics. 鈥淓ven changing the details of the plaster鈥攊ts grain size, for example鈥攃an make an audible difference,鈥� Rindel says. So the team plans to measure the acoustics of the mosques and churches as they are now and try to recreate their original sound. Then they can lobby future restoration teams to make sure that鈥攁s far as possible鈥攖he acoustics end up as Sinan intended.

The main task is to measure the 鈥渞everberation time鈥� of each building. To acousticians, this is the most important property of a building. It is a measure of the way sound fades away, and is affected by the volume of air involved and the materials used on the floor, walls and ceiling. Reverberation time is defined as the time it takes for a sound in the room to drop to one-millionth of its original intensity鈥攔oughly equivalent to an orchestral crescendo fading to nothing. Researchers usually measure reverberation time using a reference sound at 500 hertz.

A low reverberation time makes speech easier to understand, so lecture halls tend to have reverberation times of less than a second. Play music in these spaces, however, and it sounds empty and dull. So concert halls are designed with longer reverberation times鈥攁round two seconds鈥攁nd cathedrals, which are designed for flamboyant organ and choral music, are more reverberant still. Notre-Dame in Paris has a reverberation time of 8.5 seconds, and London鈥檚 St Paul鈥檚 a whopping 13.5 seconds.

Until now, no one had measured the reverberation time of Sinan鈥檚 mosques. Karabiber鈥檚 team found that the S眉leymaniye mosque currently has a reverberation time of around 8 seconds, similar to Notre-Dame. Of course, no one knows whether Sinan intended the reverberation time to be this long. But Karabiber鈥檚 team now has a means to decide what sounds best. They plan to get a team of Muslim and non-Muslim acoustics experts to listen to the simulation and give their opinions on different set-ups. 鈥淲e still don鈥檛 know the optimum reverberation times for mosques鈥攐ne of the aims of the project is to put values on these,鈥� Karabiber says.

If they can pin down the ideal reverberation time for a mosque, it could influence future building projects all over the world. 鈥淭he acoustics are really bad in some recently built mosques,鈥� Karabiber says. 鈥淚t will help these designs if we can discover why the acoustics of Sinan鈥檚 mosques were so good.鈥�

But the conservation goal remains the most difficult. The researchers believe they will have to fight to get restorers to accept acoustic considerations as an important part of their work. The idea is completely novel, and restorers might balk at adding extra dimensions to an already difficult job.

Even if all else fails, however, the project will at least provide computer restorations of the mosques so tourists can experience their original splendour on screen. Researchers on Karabiber鈥檚 team from the Swiss Federal Institute of Technology in Lausanne are working on a virtual tour of the mosques on CD-ROM. Visitors will be able to move through an Ottoman crowd, watching and listening to an imam lead his congregation. The sights and sounds should be much as S眉leyman the Magnificent would have experienced them. That should prove an especially interesting experience for Kani Karaca. After all, not many people get to hear themselves singing four hundred years before they were born.