Cutting the reverb
Published: MEA
With a lack of absorbent surfaces, mosques can present a major reverberation challenge for sound designers. This is the situation Soliflex faced at the Qatar State Mosque, as James Ling discovers
One of the challenges regularly faced by sound designers across the world is how to deal with long reverberation times so that audio can be as intelligible as possible. While not exclusively a problem for mosques, the nature of their design with extensive use of hard surfaces, such as marble and glass, makes these buildings particularly susceptible to this problem. This was the challenge faced by Swedish audio consultants Soliflex at its recent project for Qatar’s new State Mosque in Doha.
Designed in the traditional Qatari style, the State Mosque has 28 major domes over the main prayer hall and 65 minor domes around the perimeter as well as a 65m high minaret. The 20,000 sq-m mosque is divided over two floors and features an external courtyard – giving it the capacity for around 20,000 -30,000worshipers, ofwhich 13,000 can be hosted in the main prayer hall. While visually beautiful, the building itself presented a number of acoustic problems.
With a history of working on high-profile projects in the region, Soliflex was initially brought in by local distributor and installer AV-Tech to handle the Ease modelling for the system design in the main prayer hall. ‘We did that work and gave them the simulation that showed the speakers were not correctly placed,’ explains Jan Setterberg, senior advisor at Soliflex. ‘The column speakers were shooting at around 6.5m above the heads of the audience.’ This work led Soliflex and Mr Setterberg to be brought in to partner with AV-Tech to design the sound system for the entire mosque.
Designing the system
On his first visit to the site, Mr Setterberg immediately saw the challenge he would be facing for this project. ‘I could see we were up against an enormous reverb time here – the calculation gave a result peaking at around 9s,’ he says. Further to this, the initial Ease simulations had a very low speech transmission index (STI) of around 0.3 rather than the 0.5 that was required. ‘I explained that if they wanted intelligible sound, they would need to re-do the whole system.’
The first task was to use dampening treatments to get the acoustics under control. The initial stage was to use around 6,000 sq-m of dampening material from Fellert to bring the reverberation time to around 4.2s. This material was chosen because not only was it able to be effective at a low thickness, it was also possible to get a smooth surface that would fit in with the architect’s demands. The same treatment was also used on the rear wall of the ladies prayer room and under the mezzanine, where the ceiling and cupolas were dampened due to the severe focusing of reflective sound. ‘I couldn’t do anything about the lower-end, but I was able to get it under control in the frequencies of the spoken word,’ says Mr Setterberg. The acoustic treatment also aided the STI, with an average of 0.55 achieved over 18 points in the main prayer room.
The next stage was to design the sound system itself with speakers that would be able to work with the dampening treatment to further improve the room acoustics. Mr Setterberg believes that due to the difficult acoustic environment in a mosque, it is important to use loudspeakers with well-defined dispersion characteristics as this will give the designer the tools to avoid problematic reflections and dispersions in the building. To achieve this at the Qatar State Mosque, Mr Setterberg opted for Renkus-Heinz Iconyx speakers. The main speakers are 3m tall IC24R columns, six of which have been fitted on the 120m long main wall in the prayer hall, three either side of the Qiblah. Further to this, there is a 16m deep space underneath the mezzanine floor which is served by six IC8Rs for delays that have been fitted in spaces carved into the columns. Above this on the mezzanine floor is the ladies prayer hall which has a further six IC8Rs fixed on the columns.
Additionally there is a 120m long marble corridor behind the back wall of the main prayer hall, which is covered by another six IC8Rs and two other corridors around the courtyard which are served by three and two IC8Rs respectively. These solid marble corridors are open to the courtyard and have no acoustic treatment at all, but with a very careful setting of the directions and EQ of the speakers, Soliflex was able to achieve a STI of 0.45 - 0.48 – a figure that Mr Setterberg was so astonished by he insisted the test be run again to ensure it was correct.
In the courtyard, a further four IC Live speakers were installed on columns in front of the main prayer hall. The IC Live speakers were chosen due to their higher capacity of sound level.Completing the speaker installation are four Community Professional R2-52Z horn speakers on the minaret which aim outside the mosque powered by a Lab.gruppen C28:4 amp. While these are intended to fire over the mosque, some of the sound still enters the courtyard. ‘These speakers are used for the call to prayer, but are also on during the worship. I calculated the distance from the centre point of the courtyard to the IC Lives and the minaret and focused on having perfect phase in the middle of the courtyard. It was a fair compromise, I can’t see any better solution,’ says Mr Setterberg.
There were several reasons behind Soliflex’s decision to go with Renkus-Heinz speakers for the project. ‘First, I know the speakers and I wanted something I would feel safe with,’ explains Mr Setterberg. ‘I’m sure there are many other good suppliers, but you can’t know them all!’ The need for a column speaker with beam steering capabilities was also of importance for this installation. ‘AV-Tech is an importer of Meyer Sound, and were very keen on using Meyer speakers for the project, but we needed steerablecolumn speakers and at the time Meyer did not offer this option.’
At the other end of the signal chain, the team opted for wireless microphones from Sennheiser, specifically the EW135 and EW122. ‘We used Sennheiser for the high pass filter cut in the low-end because it takes care of the extra low frequencies generated due to proximity effects,’ explains Mr Setterberg. Other signal sources for the system come from Tascam and Denon. Tascam has supplied a CD-01U Pro CD player, CD355 CD changer and double cassette player while Denon was used for the radio tuner.
Completing the system is a Fostex RM-2 stereo rack monitor for audio monitoring plus four Sonifex RB-DA6 rack-mount distribution amplifiers for splitting the source to various outputs.
When it came to testing the system, one challenge faced by Soliflex was that it could only use pink noise and as well asa CD with three different Imams preaching – audio that Mr Setterberg was unfamiliar with. ‘I had in my test instrument music I know very well, but it was western-style music and not allowed, so I had to rely on my partners to tell me if the system sounded correct, but everyone was happy with it.’
Networked audio
The audio system at the State Mosque runs over a digital optical network with all cabling from Klotz. The sound system is controlled by a Yamaha DME64N DSP using D-Link DGS-1216T with DEM-311T gigabit switches to convert from optical to digital CobraNet. ‘We opted for Yamaha for the control because we were looking for something that was simple and stable,’ Mr Setterberg explains. The network consists of four D-Link switches and is connected through an optical ring for extra security.
On top of this there is a traditional analogue network to act as an alternative should there be a problem with the digital network. ‘For each speaker the main feed is CobraNet, then there is an extra analogue feed for redundancy,’ explains Mr Setterberg. ‘I can hear a slight difference, which I think is due to the long cable runs – I think the analogue sounds a little bit darker – but you have to really listen for it to find it. It works very well as an alternative and you cannot hear the switchover.’
The analogue network is based around a 1U 19-inch AM MX81 analogue mixer from Australian Monitor. ‘We wanted the technical things like delay and comp limiters done in the digital domain, so we opted for a simple analogue network,’ says Mr Setterberg. The extra network is something that Mr Setterberg is keen to have to ensure any issues on the digital side will not affect the routine of the mosque.
One of the major benefits that the network provided was that it enabled the designers to use Renkus-Heinz’s Rhaon software to control the speakers for filtering. ‘Mainly I used low-pass high-pass filters as well as shelving filters and in onlytwo places I used band-pass filtering. I was very careful with the equalisation, and avoiding high Q filters.’ The use of high-pass filtering was an important factor to aid the acoustic treatment of the room. The limited thickness of the Fellert dampening material meant that there was a limit to the dampening in the low-end frequencies, but the filtering from the speakers was able to compensate for this.
The various zones of the network are controlled via a touch screen computer from SDL Systems in the main rack room adjacent to the courtyard. ‘It has very simple settings, you can adjust the volume and mute various areas where you don’t want the speakers on,’ explains Mr Setterberg. ‘It also has three settings depending on the number of visitors to the mosque – empty hall, half full and full hall – because the number of people affect the acoustics in the room,’ he continues. ‘They are very small differences but we didn’t want the staff to have to adjust anything in the system.’
One of the major positive factors for the project at the State Mosque was the architect in charge. Mr Wael A Zaher A Tavab from the Arab Engineering Bureau not only cared about how the speakers in the hall looked, but how they sounded too. ‘He really cared about the acoustics. At our first meeting there were 12 people there discussing sound and acoustics – I was a little bit shocked!’ This was a refreshing change for Mr Setterberg, and something he would like to experience more often.
Also of importance has been the relationship with AV-Tech, and Mr Setterberg offers particular praise for the project to MD Hadi Arzouni as well as project manager Harish Kunju and his team. ‘We taught some of the team a lot about Rhaon and programming systems, and I think they learnt very quickly.’
While the building project itself has been ongoing for a number of years – as any building with so much hand crafted work would – it has been an 18 month job for Soliflex. ‘I’m very pleased with the quality,’ says Mr Setterberg. ‘I visited the site in July to see the final cabling and I was really surprised, it’s a very good quality of workmanship. It’s been a fun job and a very different job for me.’
Projects like this show that while reverberant spaces can pose a major challenge for acoustics, they can also be tackled. Selecting the right equipment is key to getting intelligible sound, and the work done here by Soliflex and AV-Tech has ensured Qatar’s new State Mosque will get the best out of its system.