## 3 Measurements and Methodology  
### Standards and Regulations  
1.1 Methodology in Line with ISO 17497: 2004+A1:2014, Part 1--Measurement of Random-Incidence Scattering Coefficient in a Reverberant Room  
Part 1 of the ISO 17497 deals with the method for measuring a random-incident scattering coefficient by describing the number of reflections deviating specularly from a surface. The frequency range of the measurements is comprised between 100 Hz and 5 kHz, in third-octave bands. The test should be undertaken in a reverberant room (as fully described in ISO 354:2003 [21]) such that the absorption (A) inside the testing room should not exceed A \(\leq\) 0.3 \(\times\) V\({}^{2/3}\). The test sample fixed to a rigid base/plate is rotated by a turntable mechanized system. The plate where the sample is fixed should be symmetrical with respect to the axis of rotation and should have a size corresponding to the maximum dimension of the test sample. The test sample should be circular or flush-mounted, having a surface area as large as possible in order to obtain a good measurement accuracy. It is recommended that the sample structural depth should be small compared to the size of the surface to be tested, it should be \(h\leq d/16\), where \(d\) is the diameter of the turntable. Another condition to be considered for the test sample is that the random-incidence absorption coefficient should not exceed \(\alpha_{\rm s}=0.50\). Procedures for the measurement methodology should follow the ISO 18233:2006 [22]. The room impulse response (RIRs) should be measured without and with the test sample accordingly with the ISO 354, with two source positions and three microphone positions at least. The duration of the measurement should be equal to the time that the turntable employs to make a complete revolution. Measurement should be taken with and without the test sample, in static and rotating turntable conditions, for a total of four different settings. Figure 1: Quadratic residue diffuser (QRD) used during the experimental measurements: (**a**) elevation; (**b**) horizontal section; (**c**) vertical section. All the numbers are in mm. 1.2 Methodology in Line with ISO 17497:2012, Part 2--Measurement of the Directional Diffusion Coefficient in a Free Field  
Part 2 of the ISO 17497 deals with the method for measuring the directional diffusion coefficient by describing the uniformity of the reflected polar distribution, best known as the quality of the diffusing surface. The diffusion coefficient describes the energy reflected by a surface, which is spatially distributed by the description of a polar response. For this methodology, the microphones are positioned radially in front of the test sample. The frequency range is the same used for the turntable method, comprised between 100 Hz and 5 kHz in third-octave bands. The environment of the measurements should be an anechoic chamber or a large semi-reverberant room simulating the reflection-free environment. It is recommended that the RIR should be taken in the far-field for monitoring the achievement of the amount of diffusion and use the near field for any aberration or focusing effect. In relation to the measurement procedures, at least 80% of the receiving positions should be outside of the specular zone in order to meet the far-field conditions and the distance between the source and the test sample should be 10 m, while the receivers' radius should be 5 m. The receiver angular resolution should be of a maximum of 5\({}^{\circ}\). Regarding the equipment, the sound source should be omnidirectional, emitting the sound signal as described in ISO 18233:2006, while the microphones should have the same sensitivity to all the conceivable reflection paths. The measurements should be undertaken without and with the test sample. The polar response processing is based on the isolation of the reflections given by the test sample from other types of reflections (e.g., room boundaries). The influence of background reflections shall be removed through windowing the RIR.