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Sound fields
Published in Carl Hopkins, Sound Insulation, 2020
For engineering calculations it is convenient to relate the reverberation time directly to the absorption coefficient. Normal mode theory uses the specific acoustic admittance, which is the reciprocal of the specific acoustic impedance; hence it is linked to the absorption coefficient. From Eq. 1.76 we see that the absorption coefficient is dependent upon the angle of incidence and the specific acoustic impedance. Depending on the mode, the waves will be incident upon the room surfaces at different angles. For axial modes the waves always impinge upon two opposite surfaces at an angle of incidence that is normal to these surfaces. For oblique and tangential modes, the angle of incidence varies depending upon the mode and the room boundary upon which the waves are impinging. For simplicity, the angle dependence is ignored in the following examples and a single value for the specific acoustic admittance is used for all of the room surfaces. This still allows us to see the general effect of the modes on the decay curves; we simply acknowledge that the situation is more complex in reality.
Computational modeling in architectural acoustics
Published in Jan L.M. Hensen, Roberto Lamberts, Building Performance Simulation for Design and Operation, 2019
The oldest and still broadly applied numeric indicator of room acoustic is the reverberation time T. It is defined as the time that it takes for the mean sound pressure level in a room to drop 60 dB after a stationary source of a sound in a room is turned off. Reverberation time in a room can be estimated, among others, via Equation 9.1 (Sabine 1927) and Equations 9.2 and 9.3 (Eyring 1930). Given the fact that absorption coefficients and equivalent absorption areas are frequency dependent, the resulting reverberation times are obviously frequency dependent as well. Sabine equation is applicable only to rooms with smaller values of mean absorption coefficient (α < 0.3). Eyrings formula applies in principle also to rooms with higher sound absorption levels (Fasold and Veres 2003).
Acoustics of Enclosures
Published in Malcolm J. Crocker, A. John Price, Noise and Noise Control, 2018
Malcolm J. Crocker, A. John Price
The original experiments conducted by Sabine1,8 showed that the effective absorption coefficient of open windows increased as the total window area was decreased. Since then, various experimenters9,10 have shown quite clearly that the effective absorption per unit area changes with the area of material present in the enclosure during the test measurements. For example, in one set of measurements made by Kuttruff,'11 an absorption coefficient of 0.18 was obtained when the test material covered one entire wall of the reverberant room used for the experiment, yet when the same material was put on adjacent perpendicular walls the effective absorption coefficient increased to 0.59! Undoubtedly, some of this "area effect" can be attributed to sound diffraction at the edges of the material. However, this appears to be very difficult to predict. In any case this assumption does not account for many of the observed large differences in the reverberation time of a room with a given amount of surface absorption material placed in a number of different locations within the room. This fact is not accounted for in the previous equations for the reverberation time of a room (Equations 1.64, 4.12, 4.13, 4.20, 4.21, 4.24, and 4.25).
Thermal-Hydraulic System-Level Analysis of a Molten Salt Natural Circulation Loop
Published in Nuclear Science and Engineering, 2023
Sheng Zhang, Hsun-Chia Lin, Xiaodong Sun
The radiative heat transfer is predicted by Eq. (9) for the nitrate salt in both the heating and cooling sections. To estimate radiative heat transfer in molten salts, the absorption coefficient needs to be known. The absorption coefficient depends on a few factors, such as the salt species, impurity species, impurity level (concentration), wavelength, and temperature. It is difficult to quantitatively identify the relationship between the impurity level and absorption coefficient, especially considering that the impurity level changes after running a facility for a long period of time. Future experimental research is needed to establish the relationship between the impurity level and absorption coefficient for the molten salts of interest. In the current analysis, the Planck mean absorption coefficient, 873 m−1, is used for NaNO3-KNO3 (60–40 wt%) as a starting point.23,25
Mechanical and acoustic absorption characteristics of UHMWPE weft-knitted structures of flexible porous laminated composites
Published in The Journal of The Textile Institute, 2022
Ruosi Yan, Xingteng Zhang, Mengjin Wu, Zhengkun Zhang, Tuo Liu, Lixia Jia
The acoustic absorption capacity of the UHMWPE composites was evaluated using a SZZB standing wave tube testing system (Beijing Acoustic Vibration Research Institute, China) according to the ASTM E1050-2019 standards. The porosity of composite materials was controlled by adjusting the sinking depth, yarn fineness, and composite material thickness. Currently, the acoustic absorption coefficient is the most common parameter used to characterize acoustic absorption performance of materials. The higher the value of acoustic absorption coefficient, the greater the acoustic absorption performance. Acoustic absorbing materials have an average acoustic absorption coefficient value that is greater than 0.2. The formula for determining the acoustic absorption coefficient was as follows: where α is the acoustic absorption coefficient. ΔL is the difference between the maximum and minimum sound pressure in dB. In the present study, the acoustic absorption effect of the fabric was expressed using the noise reduction coefficient (NRC), which was the average acoustic absorption coefficient of measurements taken at frequencies of 250, 500, 1000, 2000 Hz.
Low-Noise pavement technologies and evaluation techniques: a literature review
Published in International Journal of Pavement Engineering, 2022
Peter Mikhailenko, Zhengyin Piao, Muhammad Rafiq Kakar, Moises Bueno, Sahand Athari, Reto Pieren, Kurt Heutschi, Lily Poulikakos
The sound absorption coefficient is defined as the ratio of energy absorbed by a material to the energy incident upon its surface. The measurement process for normal sound incidence has been specified in ISO 10534 parts 1 and -2. Part 1 specifies the measurement of absorption coefficient using the standing wave ratio method. Part 2 specifies the use of the transfer function method. As shown in Figure 9, a rigid tube is used for testing, in which one end of the tube has a sample holder and onthe other end, a loudspeaker is mounted facing the sample. Two microphones are inserted on either side of loudspeaker in the tube. The incident sinusoidal sound wave is generated by the loudspeaker and is reflected from the test sample. In the standing wave method (ISO 10534-1), an incident sinusoidal wave is generated by the loudspeaker at the favourable frequency and is reflected from the test sample. The superposition of incident and reflected waves, forms a partially standing wave. A probe microphone is then moved along the tube to measure the maximum and minimum sound pressure amplitudes of the standing wave. From these values, the standing wave ratio hence the sound absorption coefficient of the material is calculated. In the transfer function method (ISO 10534-2), the signal is a random noise and two microphones are mounted in the tube. Using the spectra of the sound signal received by these two microphones, the transfer function between the them can be calculated. Using this transfer function, the complex reflection factor, hence the absorption coefficient is determined.