Keywords

1 Introduction

The sound absorption coefficient of a material is defined as the fraction of the incident sound energy absorbed by it. It depends upon the nature of the material, frequency of the sound waves and the angle at which the sound waves strike the surface of the material and is measured at frequencies of 100–5000 Hz. There are numerous sound absorbing materials that can be used for acoustical treatment of buildings, dwellings, offices, theatres, halls and auditoriums. The choice of a suitable absorbent material depends, however, upon the requirements of the low or middle frequency acoustical treatment, durability, maintainability and availability of the material. The majority of the sound absorbing materials are either fibrous or porous in nature, Fibrous materials are in the form of boards or tiles while porous materials are in the form of tiles, blankets and resin-bonded slabs. Acoustic tiles are available in various textures and resin-bonded mineral or glass wool is available in the form of semi-rigid slabs, mats, etc. The cavity of Helmholtz resonators is also typical sound absorber used in auditoria or noisy rooms in a narrow region of the low frequency band. The use of wedge-shaped fibrous materials is extensively used for constructing anechoic chambers [1]. There had been numerous studies in the past on reverberation chambers method and sound impedance tube method that had analyzed the parametric analysis of the factors affecting the sound absorption characteristics of the various acoustical materials. However, the reverberation chamber method has proved to be the best method having lesser uncertainty and employing a larger specimen size for evaluating the random incidence sound absorption coefficient. A proper understanding of the factors affecting sound absorption characteristics and mounting considerations shall be indispensable in the design and development of better acoustical materials for noise control.

2 Mounting Considerations

The mountings test specimens during sound absorption tests as recommended by ASTM E795-16 standard as follows [2]:

  • Type A Mounting-Test specimen laid directly against the test surface

  • Type B Mounting-Test specimen cemented to gypsum board and laid directly against the test surface

  • Type C Mounting-Test specimen comprising sound absorptive material behind a perforated, expanded, open facing or other porous material

  • Type D Mounting-Test specimen mounted on wood furring stripes

  • Type E Mounting-Test specimen mounted with an air space behind it

  • Type F Mounting-Test specimen mounted with an air space behind it

  • Type G Mounting-Test specimen is a drapery, window shade, or blind hung parallel to the test surface

  • Type H Mounting-Test specimen is a drapery suspended away from any vertical surface Type I Mounting-The specimen is a spray- or trowel-applied material on an acoustically hard substrate

  • Type J Mounting- The specimen is a sound absorbing unit or set of sound absorbing units

  • Type K Mounting-Test specimen is an office screen

  • Type L Mounting-This mounting is for use with concrete blocks or block-like specimen that are normally assembled using mortar

  • Type M Mounting-Test specimen is theatre seats

Type C, D, E, and G Mountings are further designated by a numerical suffix which indicates the distance (in millimeters from the specimen to the test surface rounded to the nearest integral multiple of 5 mm. For example, a Type E-400 mounting is a plenum mounting in which the face of the test specimen is 400 mm away from the test surface. In case of G mounting, the test specimen shall be drapery, window shade, or window blind hung parallel to the test surface. The suffix of the mounting designation shall be the distance from the test surface to the centreline of the hangers rounded to the nearest integral multiple of 5 mm. The preferred distance between the centreline of the hangers and the test surface is 75 mm. In the case of Type J Mounting, the test specimen shall be a sound absorbing unit or set of sound absorbing units that are directly attached to or hanging from a ceiling, wall or other room surface. If the units are suspended flat panels (baffles), and an installation pattern is not specified, it is recommended that the following panel size and arrangement be tested. The total absorptive area (all exposed surfaces) of the sound absorbing units shall be at least 10 m2. The distance between any sound absorbing unit and any reflective surface (other than the test surface), rotating vane or diffuser panel shall be consistent with the requirements of Test Method C423. The measured sound absorption is in square meters per unit or Sabins per unit [2]. The analysis of the observations made for different mounting considerations is shown in Figs. 1 and 2. The following observations associated with various mounting considerations as presented by Sharma and Singal [1] are as follows [1]:

Fig. 1
4 multi-line graphs plot absorption coefficient versus frequency for 4 mounting considerations. Graph 1 has 3 lines that follow an increasing trend between 0.2 and 1. Graph 2 has 2 distribution curves with some fluctuation between 0.2 and 1. Graphs 3 and 4 have 3 increasing trends between 0 and 0.8.

Sound absorption characteristics of absorptive material in ASTM mountings A and D [1]

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Fig. 2
4 multi-line graphs plot absorption coefficient versus frequency for 3 mounting considerations. Graph 1 has 3 distribution curves with some variations between 0.2 and 1. Graph 2 has 2 distribution curves between 0.2 and 1. Graphs 3 has 3 increasing trends and graph 4 has 2 increasing trends.

Sound absorption characteristics of absorptive material in ASTM mountings C, D and E [1]

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  • With increasing thickness, the absorption coefficient of porous soft materials at any frequency increases and for a slab of thickness 100 mm of mineral wool, the absorption coefficient attains a maximum value at 500 Hz which is maintained more or less uniformly at higher frequencies also.

  • With air-gap between the porous absorption material and the solid wall backing, the absorbing material and the solid wall backing, the absorption coefficient becomes more or less uniform in the mid frequency range 500–2000 Hz.

  • The addition of the air-gap between the paneled absorbing material and the solid wall increases the maximum absorption coefficient while the increase in the open area in this situation not only shifts the resonance frequency but for a certain percentage of the open area (perforated 30%), absorption coefficient becomes uniformly high over a wider frequency range [1].

3 Edge Effect in Sound Absorption

The diffraction from the edges at low frequencies causes reflected waves to diffract that produces edge effect whereby more sound absorption occurs near the edges of an absorber than its centre. Thus, the wave diffraction at the edges of the specimen causes the sound absorption coefficient greater than unity. The amount by which the absorber's effective area increases is proportional to the ratio of the perimeter of the edges to the area of the absorber. The effect increases with decreasing frequency, decreasing specimen size and increasing aspect ratio [3,4,5].

4 Parametric Study

The parametric sensitivity of the various factors affecting the sound absorption characteristics of acoustical materials are enlisted in Table 1 based on exhaustive literature surveys of various previous studies carried out in this field [6,7,8,9,10,11,12,13]. It may be noted that several factors such as perforation of the material, air-gap behind the absorptive material, thickness and density of the absorptive material, airflow resistivity etc. play a vital role in determining the sound absorption characteristics. The diffusivity characteristics of the reverberation chamber are also an important aspect in the determination of the sound absorption characteristics.

Table 1 Effects of various parameters of acoustical materials on sound absorption characteristics [6,7,8,9,10,11,12,13]
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Table 2 describes the details of some of the sandwich constructions tested and observed to show higher sound absorption characteristics in the entire measurement frequency range. The sandwich constructions in many cases show enhanced sound absorption coefficient as shown in Fig. 3.

Table 2 Description of sound absorptive materials of higher sound absorption characteristics
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Fig. 3
A multi-line graph plots sound absorption coefficient versus one-third octave frequency band. 4 lines for S 1 through S 4 begin between 0.10 and 0.80 and follow an increasing trend. S 2 has the highest value at (4800, 0.85), followed by S 1, S 3, and S 4. Values are estimated.

Sound absorption coefficient of various sandwich acoustical materials of higher sound absorption characteristics tested in Reverberation chambers

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5 Conclusions

The paper reports the parametric sensitivity of the factors affecting the sound absorption characteristics of the acoustical materials while these are tested in the Reverberation chambers. The various factors affecting the sound absorption characteristics such as air-gap, thickness, bulk density, porosity, tortuosity and air flow resistivity etc. have been highlighted in the study that has a significant role in determining the sound absorption characteristics in the entire measurement frequency range. The study shows the application of sandwich absorptive materials showing enhanced sound absorption characteristics that can be used widely for noise control applications. Future studies shall focus on the parametric sensitivity analysis of all the factors affecting the sound absorption characteristics using the analytical and experimental evaluation.