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Measurement of sound absorption in a very near field using active noise control approach [Elektronski vir]
Šteblaj, Peter ...
Sound-absorbing materials are commonly used to reduce noise. It is important to have some information about sound absorption coefficient of already installed and applied materials especially in low ...frequency range. A new in-situ method for measuring the sound absorption in low frequency range was developed. Presented method utilizes a dipole characteristic of the source, which generates sound pressure from the test signal. A dipole source in a perfect free field conditions generates a specific shape of the sound filed. If the dipole source is placed in the vicinity of arbitrary acoustic obstacle, sound field is deformed and it depends only on the acoustic properties of the obstacle. A difference between sound pressure filed of the dipole source in a perfectly free field conditions and the sound pressure field just above the acoustic obstacle gives sufficient information for the calculation of the sound absorption of the obstacle itself. When measuring in-situ, background noise cancelation is of a significant importance to achieve good signal to noise ratio. Therefore an active noise control approach was used to reduce the impact of background noise, and to compensate the asymmetry of the dipole source in free field, consequently leading to better results. Numerical results indicated that the dipole source should be placed as close as possible to the surface of the sample. Based on that, a measuring setup was build. Two pairs of separately driven dipole sources were used and placed just above the acoustic obstacle. Measurement results were later compared with results obtained with the well-known method of standing wave ratio in the impedance tube.
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