Interview with architect Petr Hájek and acoustics expert Martin Vondrášek

What would you say about the multifunctional hall in the Imperial Spa at the beginning?
Petr Hájek: The Imperial Spa is a pinnacle building from the late 19th century, built with a unique spa technology. It featured a machine for transporting peat from the peat pavilion to individual baths, which were arranged around an inner courtyard in the shape of a horseshoe. The inner engine room was the metaphorical heart of the building, and we replaced that machine with a new one, integrating contemporary concert hall technology instead of the spa technology. However, the hall can easily be adapted to different productions through variable acoustics and spatial variability, transforming it into a cinema, conference forum, or dance floor.

How did you manage to solve the acoustics in the hall?
Martin Vondrášek (AVT Group a.s.): The hall is very unique and variable from an acoustic perspective. For classical music, the ideal reverberation time for the given volume of the hall is around two seconds; for cinema and spoken word, a shorter reverberation time of less than one second is preferable, and for organ music, a three-second duration is optimal for the given volume of the hall. Our hall is unique in its variable acoustic parameters, which allows performers to adjust them according to their needs and requirements. I am not aware of any other hall with such properties and acoustic and spatial variability.

Rotating elements on the ceiling are a great help in this regard; they consist of three parts. One part is reflective acoustically, the second part absorbs sound, and the third part is diffusive, meaning that sound is scattered in all directions and is specifically tuned for the needs of a symphony orchestra. Additionally, the hall will be equipped with a curtain that will allow us to reduce the reverberation time to less than one second, so it can also serve as a cinema and officially become the festival cinema next year. This curtain will also be variable, and we will achieve this variability by installing tracks with automated control around the hall. Motors will control individual segments of the curtain, allowing for the closure of the entire space or the use of individual segments. Thanks to all this, the organizer will be able to further adjust the acoustics according to their needs.

But it wasn't easy. At the beginning, we found that the basic proportions, such as the length, width, and height of the hall, deviated from the norm. Honestly, when Petr first brought this up, I wanted to reject him because acoustically it is not a standard space, but in the end, we embarked on the project, viewing it as a professional challenge.

What exactly constitutes the multifunctionality of the hall?
Petr Hájek: More or less everything necessary for an effective transformation is movable and adjustable. There are theater tables that can be adjusted in height and arranged according to the operator's needs. The mobile audience changes the ratio of seating capacity to floor area, part of the rear wall of the orchestra pit retracts down under the structure, while a screen emerges from above. The rotating acoustic panels on the ceiling modify the sound parameters towards the audience. And we must not forget that the hall is on six legs, and there is a universal space beneath it that can be used for exhibitions, receptions, or as an additional foyer.

Martin Vondrášek: What I am proud of and what we managed to achieve is to place all systems under one control. The liftable stage tables, rotating ceiling acoustic elements, the rear wall, and the mobile audience can all be controlled together via a touchscreen. Users can set all configurations and change them with the push of a button in an instant. It is practically self-service, so one person can set everything up, save all user configurations, and recall them from memory.

You mentioned a film festival; will the hall be used this year?
Petr Hájek: Unfortunately, it won't be the official festival hall yet this year. Certain requirements must be met first; for example, we are missing a curtain for darkening. However, this curtain is also a key element that affects the reverberation time for the cinema. In a film hall, a so-called dry sound is desirable, meaning without reverberation, so that the reproduced sound can be perfectly heard.

When will the hall open to the public?
Petr Hájek: The official opening to the public is June 17th. However, in the first half year to year, trial operations occur in such halls, during which all issues related to acoustics, controls, and other aspects are addressed. The staff also learns how to operate the technologies and handle all conceivable situations. You can imagine this hall as a metaphorical musical instrument that needs to be tuned beforehand. We must first try out a number of configurations and test them, and then we will save these settings to memory. After that, it is basically just a matter of pressing one button.

How did you construct the hall? Do you know how many parts it consists of?
Petr Hájek: I don't know the exact number, but I do know that it was 150 tons of material, and the way in was through the roof. All construction components were manufactured as a kit and were subsequently assembled on-site using screw connections. This is not common for such large structures, as it significantly complicates the work and requires precision.

How long did it take to build the hall?
Petr Hájek: Initial discussions started a year ago, and the project went into production at the end of November. The first truck arrived in mid-December before Christmas. I think there were a total of 12 or 14 trucks. Frankly, the time for realization was extremely short.

Why did you choose the color red?
Petr Hájek: The main reason is the emotional effect that the color red evokes. We needed people entering the hall to reset their senses, awaken emotions, and be prepared for a strong experience. The second reason is scenographic. Our eyes perceive the color red as very dark even in dim light, and in some cases even darker than black. The illuminated parts stand out significantly, while the slightly illuminated parts dip into darkness. Therefore, only the important things happening on stage are displayed.

Could it be said that this is unique or does the hall have competition somewhere in the world?
Petr Hájek: It depends on how one assesses uniqueness. Of course, from an artistic perspective, the author’s handwriting and architectural concept are always unique by their nature. The technological aspect is more complex. Yes, there are halls with mobile audiences and variable acoustics, but there are very few with such a complexity of elements and technologies. However, I do not know of any comparable hall with such a wide range of modifiable reverberation times. After the installation of the curtain, we expect a range from 0.8 to 2.6 to 2.8 seconds.

What was the most difficult aspect for you in the hall?
Petr Hájek: The difficulty was that we wanted to achieve top parameters in all categories of acoustics and stage variability for all requested productions. Primarily for classical music concerts, lectures, conferences, and film projections. That was the biggest challenge of the entire project, and achieving the goal was truly difficult for us. Experienced experts from all related fields participated in its realization. I cannot fail to mention the main ones. Through acoustics, AVT Group, and through the construction of the hall, Gradior Tech. Together with the general contractor of the construction, a consortium of companies Geosan and Metrostav, they literally threaded the whole structure through a hole in the roof and assembled it. It would not have been possible without them. And of course, nothing would have come to fruition without the persistence of the clients, the Karlovy Vary Region and the city of Karlovy Vary, who confirmed their joint will for a concert hall in a joint memorandum. So it is a collective work in every sense.