The requirements in terms of acoustics of measurement rooms and test benches often makes necessary the use of absorbing linings allowing to perform measurements according to standard NF EN SO 3744 Acoustics - Determination of sound power levels and sound energy levels of noise sources using sound pressure - Engineering methods for an essentially free field over a reflecting plane or NF EN ISO 3746 Acoustics - Determination of sound power levels and sound energy levels of noise sources using sound pressure - Survey method using an enveloping measurement surface over a reflecting plane.
For such absorbing linings, innovative linings are offered by ITS, being in the matter - due to their excellent technical features and their small depth - the nec plus ultra.
Those technologies are especially appropriate in case of semi-anechoic or anechoic rooms as well as for aero-acoustic wind tunnels in all branches of industry and for acoustic laboratories for universities or engineering schools as well as for organisms of R&D in acoustics.
In particular, realting to acoustics of measurement rooms and test benches, a special attention should be paid to those technologies in the case of rehabilitation projects because they allow to obtain a better acoustic performance (namely: for low frequency) while increasing the volume available for acoustic measurements i.e. the test of sound sources of larger sizes is allowed (keeping the existing building envelope).
ITS intervenes in relation to the acoustics of measurement rooms (e.g. anechoic chambers) and test rigs (test benches) for research & development (R&D): industry and universities.
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Retrofit of acoustic measurement rooms - text of the article of Faist (whose representative in France is ITS) -
The higher demands of consumers and car manufacturers concerning the reduction of vehicle sound emissions has placed increased demands on the quality of the results from sound-level measurements. This trend can be seen across the automotive industry and has also affected the design of mesurement rooms. Here, broadband compact absorbers (BCA) – which were developped by Faist Anlagenbau, an expert in soundproffing technology – are increasingly used.
These absorbers require a greatly reduced installation depth (250 mm and 350mm are standard) in contrast to conventional wedges absorbers and at the same time reach a very high rate of absorption over the entire frequency range. It is also possible to cover cut-off frequencies in the same space and with minimum effort.
The surface of the BCA modules is smooth and powder-coated. Media interfaces can be easily integrated, and in the case of a retrofit of existing rooms, more space is available.
One current example of the retrofitting of acoustic measurement rooms with BCA absorber systems was at the facility of a German car maker, where the wedge absorbers in a measurement room were replaced by a BCA system. Faist maintained an identical cut-off frequency of 80 Hz and measurements showed that 50 Hz has now been achieved.
Meanwhile, at the premises of an international engine builder, Faist redesigned the acoustic measurement room and converted it from an anechoic chamber to a semi-anechoic chamber. BCA modules replaced wedge absorbers, the cut-off frequency of 100 Hz was reduced to 63 Hz and the measurement range was extended.
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DNW case study - text of the article of Faist (whose representative in France is ITS) -
German sound proofing specialist Faist Anlagenbau recently received an order from the Foundation of German Dutch Wind Tunnels (DNW) to update its wind tunnel facility, currently being used to test scale models of aircraft and components such as landing gear and turbines for their aero-acoustic properties. Based in Braunschweig, Germany, DNW recently converted its aerodynamic wind tunnel into an aero-acoustic wind tunnel, with the goal of reducing wind noises of components or complete aircraft. Per Schneider, sales engineer at Faist Anlagenbau, picks up the story: “One of our tasks was the fabrication and assembly of the more than 7m-high turning vanes at the four corners of the tunnel. They guide the airflow, which has a speed of up to 90m/s, in such a way that no turbulence and only low pressure losses arise and the noise level is effectively reduced.” The result of this design work, which was done in cooperation with experts for aerodynamics and acoustics at DNW and DLR, are individually designed aerodynamic and acoustically optimized profiles that precisely guide the airflow while absorbing the sound. This task is undertaken by turning vane silencers consisting of an external cladding made of perforated sheet metal and an internal absorber. The complete construction is held by an inner steel skeleton comparable to the inner construction of an airplane wing.
Accuracy and flexibility
The manufacturing of these elements was a challenge. Vlado Lazic, head of project management for acoustic measurement rooms and test cells at Faist, explains: “Angle of attack, height and cross-sections of a total of 27 turning vanes were individually defined. The turning vane contour had to be shaped in an aerodynamic way.” Values calculated by a CFD program by DLR were applied to determine the perfect contour. These data sets were the basis for implementing the design planning. Complex details of the design were made by CAD and then imported into the CAM production system. Faist produced various prototypes that were checked by the customer and then released for series production. The installation of the modules was at least as demanding as the manufacturing. Vlado Lazic notes, “The challenge was the fact that the floor only had a limited load capacity and the turning vanes could neither be fixed at the top nor to each other. So the force had to be equally distributed into the floor and the turning vanes had to guarantee a very high stability because wind loads of 13kN act on the individual turning vanes under operation.”
Fan discharge silencer
Together with the company TLT, Faist also manufactured a fan tailcone designed as a silencer downstream of the fan, the tailcone has a diameter of almost 5m. Per Schneider says, “Its task is to absorb the noise emission of the fan already at its source. This especially applies to low frequency ranges, which set specific demands on noise reduction. At the same time it is the task of the tailcone to act aerodynamically and to ensure the high efficiency of the fan. The flow-optimized contours create a significantly reduced pressure loss enabling a reduced energy consumption at the fan drive, and so positively influences the operating costs of the facility.” Dr Andreas Bergmann, head of DNW-NWB in Braunschweig, comments, “In order to meet the noise requirements on future airplanes, it is necessary to be able also to detect noise sources of the lowest intensity which make up the total noise emission in their sum. At the moment this is only possible in the ultra-quiet wind tunnel DNWNWB.”
Sound absorption
The first wind tunnel has an acoustic performance with a sound pressure level of 55 dB(A) at a maximum air flow of 717m3/s generated by a 2MW fan. At this wind tunnel, and also at others, especially in the plenum and the tunnel, the technology of the broadband compact absorbers (BCA) is applied. This technology reaches a sound absorption of as=0.99 acc. This acoustic cladding consists of modules in a sandwich structure with a smooth surface that does not reveal its sound-absorbing function when you look at it. Its intelligence is hidden in the sandwich structure of the modules. Layers of open-cell absorption material are built in an acoustically transparent perforated sheet-metal basket. The layer next to the wall consists of a sheet-metal resonator fixed on an additional acoustic layer. These two components in combination build the compound panel absorber (CPA), a mass–spring system. The unique combination of the CPA for low frequencies and the open-cell absorber for medium and high frequencies is called the broadband compact absorber (BCA). The BCA provides sound absorption efficiency over a wide frequency range. Depending on the calculated mode field of the unclad test room, which mainly depends on position and frequency of the noise source, the free field conditions, especially in the range of low excitation frequencies, are ensured by variable resonators and their selective positioning in the test room. These assessments guarantee free field conditions down to the required cut-off frequency.
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Absorption Technology for Acoustic Test Rooms - text of the article of Faist whose representative in France is ITS)-
When given the order to design and build an acoustic test room, Faist’s acoustic experts precisely analyze the needs of the customer. Then, in most projects, they do not use the conventional wedge absorbers with their pyramid structure that reaches far into the room and demands a lot of space. Instead, the company applies its so-called broadband compact absorber technology.
Reactive Sound Absorption
This acoustic cladding consists of modules in a sandwich structure with a smooth surface that does not reveal its sound-absorbing function when you look at it. Its intelligence is hidden in the sandwich structure of the modules. Layers of open-cell absorption material are built in an acoustically transparent perforated sheet-metal basket. The layer next to the wall consists of a sheet-metal resonator fixed on an additional acoustic layer. These two components in combination build the compound panel absorber (CPA), a mass–spring system.
The unique combination of the CPA for low frequencies and the open-cell absorber for medium and high frequencies is called the Broadband Compact Absorber (BCA). The BCA provides great sound absorption efficiency over a wide frequency range.
Depending on the calculated mode field of the unclad test room, the free field conditions, especially in the range of low excitation frequencies are ensured by variable resonators and their selective positioning in the test room. The necessary tools for calculation and simulation were developed at the Fraunhofer Institute for Building Physics in Stuttgart, Germany. They are used in order to meet the free field conditions at the required cut-off frequency.
Cutting costs
On the one hand the appeal of this wall cladding lies in the highly efficient sound absorption even at low frequencies down to 40 Hz. On the other hand the space requirement is considerably lower than for conventional absorbers, as a comparison shows. If a test room with a cut-off frequency of 100 Hz is designed with conventional acoustic treatment, a wedge absorber with a total construction length of 0.85 m is used. If a cut-off frequency of 50 Hz must be reached, the wedges must have a length of 1.70 m. The BCA technology, however, gets along with a construction depth of 35 cm – irrespective of the lower cut-off frequency.
So the user of the test room can save space and costs, also because doors and windows can be realized in a more cost-effective way when using the BCA technology. Moreover, the construction time is reduced because the pre-assembled BCA panels are easy to handle and can be quickly installed on-site.
Acceptance measurement by an independent institute
The BCA technology was developed by the well-known Fraunhofer Institute for Building Physics and is proving itself in high-quality anechoic and hemi-anechoic rooms and test rigs all over the world. This has been confirmed also by third party experts because FAIST generally offers the option to have an acceptance measurement carried out by external experts during the commissioning.
Worldwide references
Skepticism is absolutely appropriate if you consider the smooth surface of the BCA panel: Can such a system, which looks anything but absorptive not only to laymen, really meet the high demands of NVH test engineers? As nothing is more convincing than practical examples, you will find here a short description of some projects which have been realized by FAIST.
BMW Dingolfing: lower 25 Hz cut-off frequency
FAIST developed two acoustic test rooms with room volumes of 1040 m³ and 273 m³ for the plant in Dingolfing, Germany, where BMW manufactures the 5, 6 and 7 Series models. The requirements of the customer were very demanding. Among other things, a cut-off frequency of 25 Hz was desired. In order to balance cost and benefit, BMW allowed an additional 1.5 dB tolerance for the frequencies below 80 Hz. The remaining frequency range was in strict accordance with ISO 3745. If using a conventional technology, wedge absorbers with a length of 3.40 m would have been necessary. Using the BCA technology, however, a construction depth of 0.35 m was sufficient, providing considerable space, material and therefore cost savings. Proof that the requirements of BMW were met was provided by the acceptance measurements that were carried out by acoustic specialists Müller BBM.
The larger test room is mainly used for interior measurements. It is equipped with a four-wheel drive dynamometer and a cooling air system that simulates the air flow up to 110 km/h.
FAW Changchun: chamber for simulated pass-by testing
First Automobile Works (FAW), one of the biggest car manufacturers in China and production partner of Volkswagen since 1990, uses a 4-wheel drive roller test stand in its development center in Changchun, which was mainly designed for acoustic measurements of accelerated pass-by testing according to ISO 362. The dimensions of the test room are 25.4 x 20.8 x 6.5m. A second anechoic room is used for power train NVH testing. Both rooms were designed by the acoustic experts of FAIST according to the wishes of the user, and realized with the BCA technology. Before that, measurement technicians and purchasers informed themselves about this sound absorption technology at their partner Volkswagen AG: Faist also planned and built the acoustic measurement rooms for the VW acoustic test center in Wolfsburg which are based on the CPA principle as well.
Faurecia: sound test room for exhaust system
The French company Faurecia S.A. belongs to the top ten of the international automotive suppliers. The company develops and manufactures, among other things, exhaust systems and catalytic converters. In order to generate a low noise level (or, in case of sports cars, the right “sound”) the Emissions Control Technologies business division runs an acoustic center with a test room that was planned by Faist and is equipped with BCA cladding.
Perfect for wind tunnels
The broadband compact absorbers are also ideal for application in aero-acoustic wind tunnels used for the measurement of wind noise. They require only a minimum of space and ensure that the space available can be optimally used for both aero-acoustic and aero-dynamic testing with high flow speeds. Faist has experience in this field of testing technology - both in project management and in the installation of suitable sound insulation measures in wind tunnels, as for example fan discharge silencers, turning vane silencers, sound absorbing treatment of the air-line and BCA cladding in the plenum.