In most Arab countries there tends to be a concentration of mosques in large residential areas. And as per Islamic law, prayer (“Adaan”) is recited five times a day from these mosques. All mosque’s clocks and audio systems are not synchronized to a centralized broadcast system so it is typical of prayers to be broadcast from several different mosques in dense urban area all at varying times (with delays occuring within seconds or even  minutes sometimes). Furthermore, the “Imam” or person leading the prayers in each mosque exhibits a unique audible range with their recitations (as is only natural given that we are human), whereby some recitations are... pleasant to the ear and others are piercing. 

Given your understanding of acoustics, what are your thoughts regarding an economical and sustainable solution to the above-mentioned problem?  What technologies exist today (or are under R&D for the near future) that can counter “noise pollution” in a dense urban environment (keeping in mind delicate cultural and religious traditions)?  And can you cite examples, if any, of implemented solutions or technologies in cities today that have successfully countered a similar problem(s)?

DragonEars
1) Even if you synchronize the prayers/audio/loudspeakers, the sound will propagate out at the speed of sound. The farther you are from each mosque the longer it will take for the sound to get to you. Each home will hear each mosque at different times even if the broadcasts are all synchronized. Further complicated by the speed of sound varying with temperature, humidity and barometric pressure.

2) Limiting the projection of sound to a specified distance in open space is impossible. You can only reduce the volume to reduce the distance heard (short of ANR, Active Noise Reduction is not practical for large open spaces). Further complicated by temperature,humidity, barometric pressure, surrounding structures, vegetation and water.

3) Imam Idol. :-)

Anonymous
It sounds like the amplified broadcast of prayers at loud volume may be something of a problem in itself. Amplifiers take something away from acoustic sound, and prayer may be especially affected. I believe, an holy utterance of sound is best unadulterated by amplification as prayer does not rattle the perception but resonates from within to summon devoutness. I wouldn't place much faith in technology to create the correct environment for prayer. Internal vs external noise will always be at odds and unless the place of prayer is quiet enough for natural sound to emanate, the breath (spirit) of the prayer will remain obscured. Water fountains may mitigate some types of nuisance noise - and act to absorb distracting energies.

Todd Krieger
They'd have to do some sort of cellular phone line type coordination. Satellite transmission has too much delay... (Although they might be able calculate the "delay" to achieve synchronicity.) I don't know if "land lines" are cleanly transmitted like in the U.S., but if they are, they'd be ideal for urban transmission.

Tomservo
You have several issues. I have been to Egypt and am somewhat familiar with the sounds.I stood outside for some time several evenings just absorbing the sound from all directions. It is difficult or impossible to let electronics decide which sounds are pleasant and which are not. I think if timing would solve part of it, at least that is easier to do. As you suggest a central timer or control would be the way to do it. What technologies exist today (or are  under R&D for the near future) that can counter "noise pollution" in a dense urban environment (keeping in mind delicate cultural and religious traditions)? I think the Muslim inhabitants don't really regard this as "noise pollution". The purpose is that the call for prayer can clearly be heard by everyone.

Travisty
I fear there is no easy solution to your 'issue', not because of the technological issues (a first step might be easy: get five muezzins from five neighboring 'friendly' mosques to gather together each time, but take turns reciting the adaan for the five mosques), but because of the humanissues. The discordance does not bother everyone. And for those it does bother, it does not bother them every day. And where there is 'competition', it has been going on in most residential communities for decades if not centuries. This is not an easy issue. After all, who is going to want to ask an Imam to quiet his muezzin because he does not recite as well as the neighbor's? Does that not question his judgment of what is pious? Of what is appropriate for his congregation? If the Imam thinks his muezzin should be replaced by a younger man (who has not lost touch with his 'gadwal' tone and timbre), he can always ask the older man to teach a new generation of muezzin and everyone will understand. If it were important that all the muezzin in a given community were synchronized, would it not have already been accomplished? This is going to be a prickly subject and I fear there is no way to avoid offending sensibilities.

Variac
A volunteer organization develops and assembles a reasonably priced speaker and amplifier system that is designed to be distributed around the perimeter of the Mosques. Larger mosques have more of the same speakers if they need more coverage. Then: The Imam makes a high quality recording at the organization and it is put on a CD that can be played through the system, which would include a low cost CD player. Or possibly: They could also customize an inexpensive computer that would play back recorded messages, when triggered by a central location.The Imam of each mosque would come to the organization and record his prayers.These would be loaded onto the computer right there, and various people from the mosque could be instructed on the (not-complicated) way to make sure it is operating. A simple program could be written to use different recordings on different days if that is a requirement. The organization would have to provide an on-site repair service to
ensure that all is operating properly.