New anechoic chamber for aeroacoustic research: development, qualification tests, use in experiments
V.F. Kopiev1,2, V.V. Palchikovskiy1, I.V. Belyaev,1,2, Yu.V. Bersenev1,3, S.Yu. Makashov2, M.Yu. Zaitsev1,2, I.V. Khramtsov1, O.Yu. Kustov1,
T.A. Viskova1,3, I.A. Korin1, E.V. Sorokin1
1Perm National Research Polytechnic University, Perm, Russia
2Central Aerohydrodynamic Institute, Moscow, Russia
3JSC Aviadvigatel, Perm, Russia
The environmental noise of aircrafts is a key issue in their competitiveness. To satisfy future noise standards it is necessary to create brand-new solutions based on deep scientific research, development of efficient methods and techniques of noise source identification and suppressing. Experimental studies are powerful means for reaching mentioned tasks. Laboratory conditions allow performing experimental studies in the controlled environment, in addition, there is not required high financial, time and human resources as with full-scale tests. However, to ensure reliable experimental data in laboratory research it is necessary to provide the free-field acoustic conditions, i.e. sound propagation as if there are no reflections from the walls. Such a facility is called an anechoic chamber.
In 2014-2015, the anechoic chamber has been designed and built in Perm National Research Polytechnic University (PNRPU) for small-scale experimental studies on jet noise, vortex ring noise and other problems associated with aeronautical acoustics. It should be noted that at present the only other analogous operating facility in Russia is anechoic chamber AC-2 of Central Aerohydrodynamic Institute (TsAGI) built in 1970s, which also enables experiments with small-scale models.
The PNRPU anechoic chamber with concrete walls of 40 cm thickness is a well-isolated room from external noise and vibrations. The walls, ceiling and floor inside the chamber are lined with sound-absorbing wedges from basalt superfine fibers in acoustically transparent glass cloth . The materials used for the acoustic treatment are inflammable, thus ensuring fire safety of the chamber. The chamber also ensures requirements on ventilation and lighting. To make the chamber easy-to-use the following additional rooms were designed and built: apparatus room; room for preparing equipment to experiments; room for fan units of the jet rig; special room for vortex ring generator, which installed into the wall of the anechoic chamber.
The sound-absorbing wedges have width 20 cm, length 100 cm, and total height 80 cm. The optimal density of basalt fibers has been chosen in a series of experiments with the wedges in reverberation chambers of TsAGI  and it is 30 kg/m3. The wedges are assembled in blocks of five wedges inserted into a thin metal wireframe (diameter of the wire is 2 mm) to preserve the geometrical parameters of the structure.
To determine the frequency lower limit of the wedges the impedance tube has been built. It has a cross section of 0.4 m x 0.4 m and a length of 5.2 m. The transfer function method  used in the PNRPU impedance tube determined that the absorption coefficient of the wedges is equal to 0.99 and above starting with 130 Hz. However, final data on the sound-absorbing ability of the lining could be found in qualification tests of the anechoic chamber.
The qualification tests were carried out according to ISO 3745 . They consisted in determination of the maximum allowable radius between a test source and a measurement location where inverse square law spreading holds, within some tolerance. This radius determines the region in the anechoic chamber where quantitative acoustic measurements can be performed without suffering from the reflection of sound from the walls of the chamber, i.e. in the free-field conditions.
To study the acoustic quality of the anechoic chamber in PNRPU, three sound sources were used:
- low-frequency 15”-loudspeaker (100 ‑ 2000 Hz);
- omnidirectional source Bruel & Kjaer 4295 (low and medium frequencies 125 ‑ 5000 Hz);
- high-frequency sound source based on the compressor driver JBL Selenium D408 Ti (5000 - 20000 Hz).
With these sound sources, two types of experiments were performed. In the first case, the source was placed near a wall, in the region of the expected noise source of a turbulent jet, which will be present in the chamber. In the second case, the source was placed in the center of the anechoic chamber, which corresponds to the typical tests of acoustic characteristics for pieces of machinery.
The measurements in both cases were performed for three radial directions 0о, 45о and 90о from the source. For this purpose the traverse microphone system was built in the chamber. The acoustic data were obtained by moving microphones in discrete steps along a radial, with the microphones motionless during data acquisition (30 s) at each microphone location. Then the source was replaced and the measurements were repeated.
The performed measurements have shown that the anechoic chamber in PNRPU realizes the free-field conditions. The radius of the region where the inverse square law spreading is observed equals to 3 m in the frequency range 125 Hz – 20 kHz, provided that microphones are at least at the distance of 1 m from the wedge tips.
To carry out in the chamber research involving noise generation by aerodynamic sources the jet rig and vortex ring generator were designed and built. The jet rig reaches velocity equal to 200 m/s with nozzle exit diameter of 8 cm. Two fans connected in series supply air jet into the chamber and pullout the air through collector. Each fan has frequency converter and jet velocity can smoothly be controlled with PC software from apparatus room. Fans stand outside the chamber in isolated room. To prevent pass of fan self-noise into the anechoic chamber there are silencers in the air ducts. The measurements in the PNRPU anechoic chamber have shown that spectra of turbulent jet noise are the same trends as those of known jet rigs .
For research of vortex ring, which is a simplest aeroacoustic object, the vortex ring generator was built. The experiments carried out in the anechoic chamber have shown that vortex ring noise can be determined on initial path of vortex ring motion even at background noise of the generator . In spectrum there is a narrow frequency range where acoustical pressure is maximum, that is typical of vortex ring. Subsequent motion of the vortex ring leads to shift this pressure maximum in lower frequencies. Observed effects are in good according to those obtained earlier in anechoic chamber AC-2 TsAGI . Thus, obtained results show that PNRPU anechoic chamber can be used in fine aeroacoustic experiments.
The built chamber was also used in experiments on spinning acoustic mode identification . This problem associated with development of effective liners for suppressing aircraft fan noise. Experimental determination of the modal structure can be made with a microphone array mounted inside or outside the duct. In performed research, a planar microphone array placed outside the inlet duct has been used for measuring spinning acoustic modes, and the obtained data were processed with the beamforming method. Spinning modes were produced by a special generator based on the inlet of PS-90 turbofan engine. Sound was generated by 34 acoustic drivers JBL 2451H placed around the circumference under the test facility. Tests were carried out without flow.
The measurement results show that spinning mode is located at a point, which position depends on the mode number (this phenomenon is similar to the results obtained with planar beamforming method for propeller or open rotor noise ). There was also not detected any reflections distorting the noise locations. It indicates that the anechoic chamber can be used in experiments on localization of the noise sources.
In conclusion one can say that designing and building PNRPU anechoic chamber led to restore of domestic capacities in the development and production of wedges for large anechoic chambers. Performed tests demonstrate that the anechoic chamber allows the aeroacoustic measurements to be performed to obtain quantitative results.
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