Titanium You will find the figures mentioned in this article in the German issue of ATZ 4/2003 beginningMATERIALS on page 372. Das neue Volkswagen-Akustikzentrum in Wolfsburg Teil 2: Reflexionsarme Raumauskleidungen

The New Volkswagen Acoustics Centre in Wolfsburg Part 2: Anechoic Room Linings

By Helmut V. Fuchs, Xueqin Zha, Gerhard Babuke and Peter Friederich

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The first part of this article in the last ATZ issue described the concept of a test bed complex that is tailor-made to meet the requirements of Volkswagen AG. Extremely low ambient noise levels – even when the powerful air conditioning system is switched on – allow acoustic examinations to be performed without any disturbance. This second part of the article deals with the anechoic room lining, which was developed by Fraunhofer IBP, and the quality of the free-field properties that can be achieved by this.

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DEVELOPMENT

Measuring Techniques

1 Exterior Noise Test Stand

The measuring room with a sound-reflecting floor is decoupled from the rest of the building. Due to the problem of condensation, the interior surfaces of the outer walls have a wall heating system. This in turn is covered with trapezoidal sheeting to ensure ventilation. On the room side, the sheeting is finished with a smooth sheet metal cover that forms the surface for attaching the sound-absorbing wall lining [1, Figure 12]. In order to ensure that the doors also have the required noise reduction values, they are fitted from outside to inside with layers of fire protection, sound insulation and absorber materials. In the area of the ventilation ducts, the linings protrude 2 m wide and 0.60 m thick out of the roof lining. The outer wall at one end has a single door measuring 1 x 2 m as an emergency exit, while the wall at the opposite end has a double door measuring 3.5 x 4 m that forms an access opening for the test vehicles as well as a connecting door to the measuring stations. Figure 1 shows a longitudinal section of the semi-anechoic room as well as the ground plan with the rectangular parallelepiped measuring surface and the measuring paths. 1.1 Free-Field Properties of the Roller Test Stand on a Rectangular Parallelepiped Measuring Surface

In consultation with the user, a rectangular parallelepiped measuring surface with the dimensions 13 x 8 x 5 m was defined around the roller test stand, within which the quality of the free-field properties was to be verified. In order to test the sound level drop in accordance with [3] from a central measuring point on the floor, four measuring paths were determined diagonally through the upper corners and one path through the centre of one of the upper edges of the measuring surface shown in Figure 1, and measurements were taken in 0.5 m steps beginning 1 m from the sound source. With up to 9 m of path length, the room meets the requirements of the standard from a lower cut-off frequency of 40 Hz. The stricter VW requirements are met according to [1, Table 2] with ± 1 dB for 100 Hz to 16 kHz and with ± 2.5 dB for 40 Hz to 80 Hz for third-octave bands. In the kHz range, the VW requirements are met on paths 1 to 4 beyond the measuring surface, with distances of more than 12 m instead of up to 9 m. In the area close to the vehicle, measurements in accordance with [3] are even possible up to 8 m at 31.5 Hz and up to 5.5 m at 25 Hz. This is particularly important for measurements taken in close proximity to the vehicles as

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well as for interior noise measurements. It represents a considerable extension of the measuring possibilities compared to the current standard. 1.2 Free-Field Properties at the Measuring Points for Pass-By Simulation

For measuring simulated pass-by, the microphones are located on two measuring paths on the right and left-hand sides of the test stand [1, Figure 2]. To verify the freefield properties at the microphone positions, the sound level drop on 11 paths was determined in accordance with [4] in a quarter of the room, i.e. for 10 m of the room length at intervals of 1 m on the 7.5 m paths. The paths led radially from the test sound source through the 7.5 m reference points at a height of 1.2 m. Measurements were taken in a bandwidth of one-third octaves in 0.5 m steps up to 400 Hz and in 0.25 m steps above 500 Hz. The first measuring point of each path was located at a distance of 3.5 m (up to 400 Hz) and 1.5 m (above 500 Hz) from the 7.5 m reference points. The last measuring point in all paths and frequencies led up to 0.5 m beyond the 7.5 m reference point in the direction of the wall lining. The deviations over the whole particularly interesting frequency range between 100 kHz and 16 kHz remained within the narrow specified tolerance of only ± 1 dB for one-third octave measurements up to the 10 m point of the 7.5 m reference line. The ± 2.5 dB deviation as specified in the standard was met for 50 Hz only up to 9 m and for 40 Hz only up to 8 m in the direction of the end wall along the 7.5 m path. This should come as no surprise because, in this case, the limit distance of λ/4 as defined in [3] from the wall lining to the end wall is exceeded. 2 Roller Test Stands

The two test stands are very similar both geometrically and in their acoustic properties, Figure 2, but are used for different measuring tasks. The sectional drawing in Figure 3 shows how the “ventilation ceiling” is fitted with the new absorber materials. The sound level drop measurements in accordance with [3] were once again taken on diagonal paths through the 10 x 5 x 3 m large rectangular parallelepiped measuring surface, but in this case in 0.25 m steps beginning 1 m from the test sound source. Within the virtual measuring surface shown in Figure 2, the room meets the requirements of the standard from a lower cut-off frequency of 50 Hz up to 4.5 m on the five measuring paths. Once again, however, the λ/4 distance of the edge to the lining becomes apparent, with maximum per-

missible measuring distances of 5 m at 63 Hz and 5.5 m at 80 Hz., On some paths, measuring distances of a maximum of 5.75 m (to the corner of the measuring surface) are nevertheless possible below 100 Hz. On paths 1 and 2, one-third octave measurements are even possible at 25 Hz up to this distance. For the frequency range of 100 Hz to 16 kHz, the stricter VW requirements of ± 1 dB are also fulfilled for third octaves. 3 Engine Test Stands and Drive Train Test Stands

The relatively small measuring rooms have numerous installations that are required by the test procedures. Compared to the larger roller test stands, these test stands had to meet the acoustic requirements in terms of the free-field properties according to [2] from 63 Hz and, if possible, below. These rooms also have a two-shell design. The use of BCA modules was especially beneficial in the smaller testing rooms, not only because they are subjected to relatively high loads in terms of wear, damage and dirt but also because the drive shafts to the electric motors in the adjacent rooms are located as close as possible to the walls. Benefits in terms of space are also provided by the ventilation ducts that are completely integrated into the suspended ceiling with outlet vents on the room side. These ducts are at the same time lined with BCA modules to damp the low-frequency contents of the sound level on the duct side. On the room side, the suspended ceilings are lined with sound absorbing ASA, into which the air vents and sound-absorbing lamps are integrated, Figure 4. In the drive train test bed, [1, Figure 9], the engines are mounted at an average height of 1.40 m above the reflective floor. During the sound level drop test with a test sound source, the suitability of the entire measuring room was examined with regard to the original engine as a typical sound source. The size of the room, the dimensions and position of the test object, the room installations and the absorbing lining of the room-limiting surfaces all have an influence on this suitability. The test room should ideally have no reflective installations. In the drive train test bed, however, it was not possible to carry out a test in accordance with [3, Appendix A] along diagonal measuring paths due to the various special installations and equipment stands in the room. Instead, the test was performed according to the “enveloping surface” method described in Appendix B at measuring positions that were coordinated with the users. The measurements according to [3,5] were taken on an inner

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and outer hemisphere corresponding to the location of the subsequent measuring positions. The outer hemisphere was chosen so as to be geometrically similar to the inner one. “Freak” values in the sound level differences between the two hemispheres at individual frequencies indicate reflections from the installations or from unlined engine supports, or from drive shaft bearings, the frame for the microphone stand or pipes and cables. If these surfaces are additionally covered with sound-absorbing material and measuring positions close to the installations are avoided, it is also possible to carry out one-third octave measurements down to 50 Hz according to Precision Class 1 in this room. The semi-anechoic room fulfils the condition of δ ≤ 0.5 dB according to [3] for f ≥ 100 Hz. Due to the installations required by the test object, deviations of up to 2 dB occur between 50 and 80 Hz. The test room and the original measuring surface are classed as being suitable for their purposes in accordance with this international standard. In the engine test beds, [1, Figure 8], the engine is mounted on four supports at a height of 1.20 m (centre axis of the drive shaft) above the reflective floor. The drop in the sound level was measured by establishing five paths that extend radially from the position of the sound source into the upper corners of the room. The sound level was measured at a distance of 1 m from the source in steps of 0.25 m. During this process, deviations were recorded which can clearly be assigned to reflections and interference from the installations described, which influence the free field. Nevertheless, in these small rooms the values remained within the permissible tolerances specified in [3] for Precision Class 1 at 63 Hz up to a measuring radius of approximately 2 m, at 100 Hz up to 3 m and above 100 Hz even up to 3.5 m. Due to the special properties of the BCA lining, it was therefore possible to classify engine test beds 1 and 2 as “semi-anechoic rooms in accordance with ISO 3745” that are optimally adapted to all practical specifications for frequencies down to 50 Hz. 4 Window Test Bed

Due to its special measuring tasks, the receiving room [1, Figure 7] is designed as a semi-anechoic room. However, if sound power or window tests in accordance with [6,7] need to be carried out, a reverberation booth can be installed in the receiving room. The two adjacent source reverberation rooms are each linked with the receiving room via a closable test opening. The source room, which is located next to the

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receiving room, is connected on the wall side via the above-mentioned test opening with a maximum of 10.2 m2. Depending on the test object, the test opening can be closed off using small segments. The test opening in the second source room with an access door to the basement area is required, for example, for measuring the sound damping of a vehicle floor assembly. For this purpose, a movable crane is installed in a lengthwise direction in the centre of the ceiling. In order to avoid secondary sound transmission from the source room to the receiving room, the two flanking walls and the intermediate wall with the test opening were fitted with insulating shells underneath the actual sound-absorbing lining of the receiving room. The entire ventilation ducting is installed over the ceiling area and between the ceiling joists. The ducts are lined inside with a sound-absorbing material. The cavity between the ventilation ducts and the craneway is also soundinsulated. The ceiling is then finished on the room side with a covering of 2 mm thick steel sheeting which also serves as the backing for the BCA lining with the standard dimensions of 1.4 x 1 m and a thickness of 250 mm. To provide mechanical protection, the absorbers are attached to the walls and the ceiling with perforated sheet baskets. The area of the test opening that is not occupied by the test object is also lined. Due to the sub-division of the opening into 24 single elements, the BCA modules have been replaced by soft foam elements. The craneway was also lined with 250 mm thick foam absorbers, which can be folded back if required. Figure 5 shows the test opening from the receiving and transmitting room side. The sound level drop measurements were taken at 0.25 m steps, beginning 1 m from the sound source on diagonal paths 1 to 4 into the upper corners of the room as well as on path 5 into the centre of the upper edge of the room above the test opening. The rectangular parallelepiped measuring surface was defined in this case by the measurements directly in front of the test objects installed in the test openings. For this reason, the ventilation vents on the ceiling do not begin until a distance of 2 m from the wall with the test opening. If the test sound sources are arranged on the reflective sealed floor, the free-field conditions according to [3] for third-octave measurements from 125 Hz are fulfilled without reservation up to at least 3.25 m. In spite of the relatively large surfaces which could not be lined with BCA modules, it was nevertheless possible to meet the free-field requirements of the standard up to 2.75 m at

MATERIALS

63 Hz, up to 1.75 m at 80 Hz and up to 2.25 m at 100 Hz. The upper source reverberation space was damped, as described in [8], using 6 CPR modules, which were permanently installed in two upper corners. The lower reverberation room was also provided with basic low-frequency damping by plasterboard shells with a large surface area. The reverberation times of the empty reverberation spaces shown in Figure 6 create the conditions for performing reproducible and comparable measurements of the sound damping of test objects in the test openings down to 63 Hz. 5 Listening Studio

Directly adjacent to the seven acoustic test beds, a listening room with studio-like properties was planned on the upper floor. In designing the listening studio, the main aim was to ensure the faithful reproduction and presentation of test results that are in no way acoustically influenced by the room. At the same time, however, emphasis was also placed on achieving a visually pleasing atmosphere. The colour and structure of the wall lining was deliberately chosen to take up the perforate design of the engine test beds [1, Figure 10]. The room, which has a volume of 120 m3 and a height of approximately 3 m, is located directly above the tyre noise test bed. Figure 7 shows a longitudinal section and a ground plan of the room. With its massive construction, highly insulated doors, elevated double floor and carefully sound-insulated ventilation and air conditioning system, a background sound level of below 25 dB(A) was achieved when the tyre noise test bed is in operation. The acoustic design uses foam, CPA and BCA modules behind perforated sheet cassettes with a fleece backing with a maximum depth of 150 mm. In order to ensure that sounds recorded in the test rooms and elsewhere can be correctly assessed both objectively and subjectively even down to the lowest frequencies, a neutral sound field has to be produced at all seats. This is possible only if: ■ the room’s natural resonances (“modes”) are strongly damped ■ early reflections (within 15 ms) are at least 10 dB below the direct sound ■ the reverberation period is as short as possible and increases only slightly above all at low frequencies. The reverberation period, averaged over eight measuring points, of approximately 0.15 s, as shown in Figure 8, which hardly increases above 0.2 s even at 63 and 50 Hz, is well within internationally applicable re-

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DEVELOPMENT

Measuring Techniques

7 Summary Table: Main acoustic data of the seven acoustic test beds in the VW Acoustics Centre

quirements for professional sound studios [9]. 6 Experience with the New Standard

Designing and equipping acoustic test beds demands great flexibility on the part of the project planner in adapting to the wishes and ideas of the user. Finding out these wishes and ideas in careful communication with IBP and combining them to form a verifiable requirements profile was the job of the acoustics engineer at Volkswagen who was responsible for the whole project. The actual planning and construction was supervised by Volkswagen’s buildings department. Continuous consultation between all those responsible made it possible to master the coordination problems that inevitably occur in such a complex building project, and to achieve all the ambitious aims concerning noise and vibration protection in accordance with the recognised rules of engineering and technology. With regard to the new standard for room acoustic requirements with low tolerances for third-octave measurements as well as compliance with the standards [3] also for sine measurements, it was necessary – and tolerated by the customer – to optimise some of the detail designs concerning the room lining and solve some of the measuring problems for the complex and difficult sound drop measurements on

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site while the test beds were being built. Our thanks go to those responsible at Volkswagen for making additional time and resources available in order to achieve the ambitious aims in absolutely every respect and without compromises. Determining the thickness of the vibration sheeting for the CPR in the various wall and ceiling areas, all of which are available only once for absorption at the lowest frequencies, requires a great deal of experience, which can only be gained during the construction of anechoic rooms of various dimensions. If corrections were necessary in the design of the measuring rooms in order to meet the requirements of the standard at low frequencies [9] (by increasing the proportion of the lowest tuned vibration sheeting), they immediately fulfilled even the stricter requirements according to [1, Table 2] from 100 Hz upwards. However, at a tolerance of only ± 1 dB, some measuring problems already become clearly apparent during the sound drop measurement. Since these problems are of general importance, also with regard to the subsequent measurements in the new test beds, they are discussed in the following. According to [3], the test sound source should be either smaller than 0.1 λ (i.e. only 4.3 cm at 800 Hz, only 4.3 mm at 8 kHz) or be completely sunk into the hard floor in order to avoid reflections from the latter and the ensuing interference with the direct field of the source. Such small sources

cannot, however, be produced with a sufficiently high sound power and omnidirectional directivity. Even with the very ambitious and cooperative procedure in this pilot project, it was not possible to sink a suitable sound source into a hole in the floor. Instead, the loudspeaker in each case had to be arranged approximately 0.2 m above the floor, as shown in Figure 9. This gives a qualitative explanation of the curve in Figure 9, top. In fact, the floor interference is less strongly noticeable in measurements with third-octave noise. However, it nevertheless seems justified to eliminate this effect – which has nothing to do with the quality of the anechoic lining – by placing a foam panel with the dimensions of only 1 x 0.4 m directly next to the sound source. The measuring curve then follows the theoretically expected one very well. In the case of an optimally adapted straight line, the measured values would not even touch the ± 1 dB tolerance range (cf. Figure 9, lower curve). Similar interference effects also occurred at higher frequencies during the sound drop measurements, but once again these had nothing to do with the lining. Because the sound source itself can never be an absolute point and in the case in question, Figure 10, for example, a small collar surrounds the opening of the pressure chamber, the sound level drop at 6.3 kHz showed a similar deviation to that in Figure 9, although it was possible to correct this

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easily by fitting a foam disk just 5 mm thick to the collar. The situation is similar for an interference at 10 kHz, which has its simple cause not directly at the sound source but at the receiver, Figure 10. The phenomenon, which can also occur in practice in every free-field measurement, disappears only when the small remote-controlled microphone that moves on a wire stretched across the room is also lined with a thin absorber. Once these acoustic problems have been carefully eliminated, the quality of the anechoic pass-by hall can also be evaluated from the draw-away curves shown in Figure 11. It must be pointed out, however, that interference effects such as those described above occur almost inevitably in practice in nearly all measurements in a free field with a reflective floor. Finally, the suitability of the pass-by measuring hall for narrow-band (sine) measurements in accordance with Precision Class 1 as described in [3] was verified, even though this sophisticated measuring technology, as in most free-field measurements in industrial practice, did not expressly need to be verified even at Volkswagen. Figure 12 shows that, if all reflections and interferences in the measuring room and on the measuring equipment are avoided, the measuring distance for f ≥ 80 Hz can be more than 9 m, for 63 Hz 7.5 m and for 31.5 Hz once again more than 9 m, corresponding to the target specification of

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the user and customer. In other anechoic rooms built in the meantime using the new absorber technology, their suitability in the frequency range 80 Hz ≤ f ≤ 16 kHz for thirdoctave and sine measurements according to [3] could also be tested in smaller rooms. 7 Summary

The Acoustics Centre presented in this article impressively underlines the high demands that Volkswagen places on development quality with regard to noise and vibration comfort. For all acoustic test beds, the sound level drop test proves their suitability as semi-anechoic rooms in accordance with the international standard ISO 3745 [3]. The Table shows the main acoustic data of the room. It was found that, in the exterior noise measuring hall, free-field conditions are achieved up to a radius of 6 m around the centre of the test bed above a lower cut-off frequency of 25 Hz, thus surpassing the specified requirements. In addition to the existing facilities, the Centre offers the 150 employees working in this field a new standard for automotive test bed and measuring technology that is hard to beat.

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[2]

DIN ISO 362 (1997): Messung des von beschleunigten Straßenfahrzeugen abgestrahlten Geräuschs – Verfahren der Genauigkeitsklasse 2 [3] ISO/DIS 3745.2 (2002): Acoustics – Determination of sound power levels of noise sources using sound pressure – Precision methods for anechoic and hemi-anechoic rooms [4] VDIRichtlinie 2563 (1990): Geräuschanteile von Straßenfahrzeugen - Messtechnische Erfassung und Bewertung [5] DIN 45 635 (1987): Geräuschmessung an Maschinen. Luftschallemission, Hüllflächenverfahren. Teil 11: Verbrennungsmotoren [6] DIN EN 23741 (1991): Ermittlung der Schallleistungspegel von Geräuschquellen. Hallraumverfahren der Genauigkeitsklasse 1 für breitbandige Quellen [7] DIN EN 23742 (1991): Ermittlung der Schallleistungspegel von Geräuschquellen. Hallraumverfahren der Genauigkeitsklasse 1 für tonale und schmalbandige Quellen [8] Brandstätt, P.; Fuchs, H. V.; Roller, M.: Novel silencers and absorbers for wind tunnels and acoustic test facilities. In: Noise Control Engin. J. 50 (2002), No. 2, S. 41–49 [9] Fuchs, H. V.; Zha, X.; Babuke, G.: FreifeldMessräume. Ein neuer Standard für die Automobil-Akustik. Wiesbaden: Teubner/ Vieweg, 2003 (in Vorbereitung) [10] DIN 45 635 (1984): Geräuschmessung an Maschinen. Teil 1: Luftschallemission, Hüllflächen-Verfahren. Rahmenverfahren für 3 Genauigkeitsklassen [11] ISO 37 45 (1991): Acoustics – Determination of sound power levels of noise sources – Precision methods for anechoic and semi-anechoic rooms

References [1]

Dreyer, H.; Hoppe; P.; Friederich, P.; Fuchs, H. V.: Das neue Volkswagen-Akustikzentrum in Wolfsburg, Teil 1: Prüfstandsauslegungen. In: ATZ 105 (2003), Nr. 3, S. 250–260

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