Calculation of acoustic impedance of multi-layer absorbers

Optimising flat-walled multi-layered anechoic linings using evolutionary algorithms

The results of measuring the sound absorption by acoustic wedges, which were performed in AC-3 and AC-11 reverberation chambers at the Central Aerohydrodynamic Institute (TsAGI), are presented. Wedges of different densities manufactured from superfine basaltic and thin mineral fibers were investigated. The results of tests of these wedges were compared to the sound absorption of wedges of the operating AC-2 anechoic facility at TsAGI. It is shown that basaltic-fiber wedges have better sound-absorption characteristics than the investigated analogs and can be recommended for facing anechoic facilities under construction.

Experimental investigation of sound absorption of acoustic wedges for anechoic chambers

Vertical axis wind turbines (VAWT) are a source of renewable energy and are used for both industrial and domestic purposes. The study of noise characteristics of a VAWT is an important performance parameter for the turbine. This study focuses on the development of a linear microphone array and measuring acoustic signals on a cambered five-bladed 45 W VAWT in an anechoic chamber at different tip speed ratios. The sound pressure level spectrum of VAWT shows that tonal noises such as blade passing frequencies dominate at lower frequencies whereas broadband noise corresponds to all audible ranges of frequencies. This study shows that the major portion of noise from the source is dominated by aerodynamic noises generated due to vortex generation and trailing edge serrations. The research also predicts that dynamic stall is evident in the lower Tip speed ratio (TSR) region making smaller TSR values unsuitable for a quiet VAWT. This paper compares the results of linear aeroacoustic array with a 128-MEMS acoustic camera with higher resolution. The study depicts a 3 dB margin between two systems at lower TSR values. The research approves the usage of the 8 mic linear array for small radius rotary machinery considering the results comparison with a NORSONIC camera and its resolution. These observations serve as a basis for noise reduction and blade optimization techniques.

Development of a Linear Acoustic Array for Aero-Acoustic Quantification of Camber-Bladed Vertical Axis Wind Turbine

In this paper, the design, construction and verification of a low-cost and easy-to-install anechoic chamber are described. The chamber is constructed with affordable, local materials. The quality, according to the deviation from the inverse square law, complies with ISO 3745. The working dimensions of the chamber are 1.94 m long x 1.91 m wide x 1.84 m high. The nominal cutoff frequency is 400 Hz. The cost per square meter of the effective finished volume is 200 USD. With this design, small- and medium-sized enterprises are able to build anechoic chambers with a small investment, adding value to their products through characterization and subsequent mitigation, conditioning or amplification, as necessary, of emitted sound or noise. This study also contributes to the development and appropriation of knowledge in acoustics, a branch of physics of emerging importance in developing countries.

/pdf/low-cost-and-easily-implemented-anechoic-acoustic-chambers-5987879gbd.pdf

W.A. Davern

Papers

Polyurethane ether foam wedges for anechoic chamber