Measurement of Sound Absorption by Impedance Tube Method
The impedance tube method, also known as double microphone impedance tube method, is a popular method for measuring the sound absorbing properties of materials. It is stipulated in the ISO article, ISO 10534-2. The absorption coefficient is determined by measuring the pressure difference inside a tube by the use of microphones and a sound generator (Özdil and Kayseri, 2019). This is illustrated below:
The performance of the test material is determined by a software to obtain the sound absorption coefficient. This is done through the following formula (Özdil and Kayseri, 2019): 
Where:
r is a reflection factor of normal incidence
x1 the distance between the sample and the farther microphone
k = 2π f/c
Фr the phase angle of the normal incidence reflection
H12 the transfer function from microphone one to two and
HR and HI the real and imaginary parts of H12
The sound absorption coefficient(α) is calculated as:
Results
The sound performance of the acoustic curtain was determined by running an impedance tube analysis as described above. Through the works of (Gabor and Vasile, 2016) and (Özdil and Kayseri, 2019), the sound absorption coefficients of the polypropylene fabric and the polyurethane composite were determined and the results illustrated below: 
References
Gabor, T. and Vasile, O. (2016) ‘Improved sound absorption properties of polyurethane foam mixed with textile waste’, 85(November 2015), pp. 559–565. doi: 10.1016/j.egypro.2015.12.245.
Özdil, N. and Kayseri, G. Ö. (2019) ‘Investigation of Sound Absorption Characteristics of Textile Materials Produced from Recycled Fibers’, pp. 1–20.