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Acoustics as important as ergonomics

12-08-2016
Chairs exert a great influence on the acoustics of the hall. How does Forum Seating test its armchairs?
Joanna Machaj, Forum Seating Director for National Sales: Sound-absorbing properties of armchairs and chairs, as well as the audience are very important factors that have a huge influence on the acoustic conditions of concert, theatre and congress halls. In rooms with properly designed acoustics (those where the role of the space is closely linked with the sound reaching the listeners) reverberation time is always key; therefore it is necessary to determine exactly what this is. It is highly important to precisely define the sound absorption coefficients for armchairs and chairs. In order to achieve optimal parameters for a given space, we work with the best acoustic specialists and test our armchairs in Warsaw at the ITB Acoustic Laboratory, registered by the Polish Centre for Accreditation, as well as in Cracow at the AGH Laboratory of Technical Acoustics. For individual projects, we also cooperate with foreign acoustic institutes.
 
The way we test our chairs is strictly defined by the standard PN-EN ISO 354:2005 Acoustics. Measurement of sound absorption in a reverberation room, according to which chairs and armchairs have to be tested in the same arrangement as they are used in practice or in a particular space. The test is carried out in a reverberation room with a capacity of 200 m³. Armchairs are placed on plywood boards and the whole tested sample, which can consist of up to several dozen armchairs, is surrounded with a plank fence. Before measuring sound absorption, the measurement chain has to be calibrated. Then, the tests of empty chairs or chairs with an audience are carried out using the interrupted noise method or the total impulse response method.
 
What features ensure an armchair has high acoustic properties? Are they sound-absorbing foams, high quality upholstery fabrics or holes at the bottom of the seat?
In terms of armchairs designed for philharmonics and theatres, everything is important since all components affect the acoustics. The type of material we use, the fact whether chair elements are upholstered or not, the density of the foam used to fill the seat and backrest, the type of upholstery fabric we apply – all these aspects matter a lot. Other important factors include the shape, structure and even the technique of cutting the armrest. Each completed project enables us to expand our knowledge in the field of chair acoustics. That is why we care so much about maintaining setpoints for acoustic parameters, whilst meeting the designer's stylistic and qualitative requirements, as well as all issues related to ergonomics and durability that are so crucial for the users.
 
Does an armchair always have to be tested in a particular context? Is it always necessary to take into consideration a particular space, or can a product itself have high acoustic properties?
Chairs are always examined with respect to a particular area. In order to avoid side absorption, wooden panels adjusted to the chair height are put around armchairs during tests. This allows us to obtain representative test results for a particular number of seats or an auditorium hall. 
High acoustic properties of a chair cannot be easily defined. Some qualities are desirable for a congress hall, where high speech intelligibility is important, and different ones are important for concert halls. Depending on the purpose of the room and the prevailing conditions, the chair should primarily absorb sound or provide appropriate "reverberation."
 
How many acoustic tests of one chair have to be conducted in order to achieve the desired result?
In our experience, for large, acoustically demanding concert halls it is necessary to conduct a series of at least two tests to achieve the best match of components to be used for the production of the chairs. In exceptional and complex projects, the work must be repeated many times to achieve perfect results.
 
The MIA armchair assembled in the National Forum of Music has a unique leg construction with ventilation holes that ensure the flow of air – what kind of influence does this exert on the hall acoustics?
The project of the armchairs designed for the National Forum of Music is unique in terms of the way the chairs are assembled to the floor. The designers' guidelines were very strict. The chair bases had to be placed just above air conditioning holes; they couldn't have any sharp edges, but they had to be equipped with a specially adjusted perforation that would make the sound of airflow imperceptible to the human ear. Therefore, we needed some specialist analyses that Polish laboratories refused to conduct. Finally, we carried out the acoustic tests in Berlin. Their results for our specially designed chair bases showed that they were able to ensure air conditioning and prevent its sound from being heard by the users and disturbing a concert.
 
What was the most difficult acoustic project realised by Forum Seating and why?
The most demanding project is the one we are carrying out at the moment and which is connected with supplying armchairs to the Congress Hall at the Palace of Culture and Science in Warsaw. The Congress Hall is known as an interior with famous, very good acoustic parameters perfect for events requiring high speech intelligibility, such as congresses, and great sound attenuation resulting in short reverberation time, for example during concerts with audio power amplifiers. The previous armchairs, manufactured over 60 years ago, had high sound absorption, yet much bigger dimensions than it is now allowed by current fire regulations. Forum Seating, as the manufacturer of the armchairs for the Congress Hall, was faced with a highly demanding task: to maintain the current look of the armchairs, but to make them smaller and apply the most modern materials. We had to focus on ergonomics in order to provide the users of our chairs with excellent sitting comfort during several hours' conferences, ensure a high durability level, and maintain the acoustic properties of the previous chairs. The team of our specialists has managed to face up to this challenge: the armchair prototypes are comfortable, their look meets the requirements of the Conservator of Monuments, and their durability and flammability parameters have been tested by accredited testing institutes. Moreover, the acoustic test results show that thanks to the use of special foams, an acoustic and solid upholstery fabric, and a well-thought-out perforation in wooden elements of the seats, the armchairs have perfect acoustic parameters despite the fact that their dimensions have been changed.
 
The Forum Seating brand supplied armchairs to the National Forum of Music in Wrocław and the Polish National Radio Symphony Orchestra in Katowice, where famous studios, such as Nagata Acoustics or Arup, were responsible for the acoustic parameters. What did the cooperation look like? 
The armchairs for the Hall in Katowice were manufactured according to strict guidelines received from the Japanese company Nagata Acoustics. The work was supervised by a master in his field – Yasushisa Toyota. The American department of the Nagata company made changes in our armchairs or approved particular project implementation stages through the agency of Konior Studio. In order to achieve a perfect sound quality in the Hall, we carried out acoustic tests in accordance with the tips given by specialists from a few independent acoustic laboratories.
We consulted the British company Arup directly when we were designing the armchairs for the National Forum of Music in Wrocław. All components of the armchairs had to be approved by the company. Arup was also responsible for supervising the acoustic tests concerning the flow of air in diffusers; before that the company defined the size and arrangement of perforations, as well as the thickness of particular parts of our chair bases. The armchair folding mechanisms needed to be approved as well. They had to be silent and meet the defined decibel criterion. 
 
Is the process of supplying armchairs to famous halls more difficult than supplying them to new ones?
Theatre armchairs have the biggest acoustic absorption of all the other hall equipment elements. Their replacement usually leads to changes in the hall acoustics that can be heard to the human ear. This is because new armchairs usually have different construction and different covering than the previous seats that were used for, let's say, a dozen or so years, or even longer. One huge challenge is maintaining the acoustic parameters of a chair in renovated halls, as the dimensions of the chairs need to be changed to adapt the design of existing building steps and meet current fire regulations.
In such cases, it is essential to work closely with acousticians and make investors aware that acoustic measurements should be supplemented by measuring sound absorption in both the old chairs after they have been removed and in the new ones offered by suppliers. Only by undertaking this kind of action can we avoid uncontrolled changes in the hall acoustics. 
 
You sometimes supply armchairs to facilities where albums are recorded without an audience. Is it possible to ensure the same acoustics both in the full and empty hall? 
The arrangement of armchairs, their shape and density of the materials used should ensure the orchestra is always able to hear itself in the same way, regardless of whether the hall is full or not during recording. Perfection is when the level of acoustic absorption with empty seats is close to that with a seated audience, meaning that a full or empty hall always has the same acoustic climate. We always do our best to achieve this.
 
Do you think that people's awareness of the benefits resulting from armchair acoustic tests is growing?
The hall users' awareness of the importance of acoustic aspects is definitively growing and the market has a more professional approach to the implementation of musical, cultural and auditorium projects. People are also becoming more and more aware of the fact that a good hall – one that they are willing to visit – has good acoustics and comfortable seats.
 
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