The first major step in noise control is to determine the source of the noise be­fore designing the sound absorber. To give an automotive-related example where biocomposites find applications; the engine, the tire-road interaction and the wind are the three major factors that affect the acoustics of the passenger department of a vehicle. Hence, the floor coverings, the headliner, and the hood insulation are the critical contributions to the acoustic performance of vehicles.2 The other most important characteristic to be sought in the design of absorption materials are the noise absorption capacities in the audible frequency range of interest. For example, medium range (1200-4000 Hz) is of interest for the interior of passenger vehicles.45 Also, cost effectiveness and other conditions should be considered including dura­bility in hostile environments such as high temperature, contamination, and high speed turbulent flow, etc.17 Recyclability, lightweight, thermal comfort and contri­bution to passive safety systems are also desirable characteristics of noise absorbers in vehicles.2

Different researchers give importance to different material parameters as influ — encers of sound absorption. Among the material parameters, which are applicable to porous absorbers, Bies and Hansen29 cite only the flow resistance/resistivity, Cox and D’antonio14 consider flow resistivity and porosity, whereas Fahy,13 and Atten­borough and Ver17 refer to porosity, and structure factor (tortuosity) in addition to flow resistance/resistivity as the primary parameters that affect the sound absorption properties.

Several authors give various parameters as factors affecting the sound absorp­tion of fibrous structures. Cox and D’antonio14 report that sound absorption effi­ciency of fibrous structures can be achieved by manipulating:

• Material density

• Fiber composition

• Fiber orientation

• Fiber dimensions.

Banks-Lee et al.36 reported that the mass per unit area, thickness and porosity of the material and fiber fineness to be of significant importance to the airflow resis­tance and sound absorption of needled fibrous materials.

The factors affecting sound absorption behavior of fibrous materials, biocom­posites in particular, are classified as fiber parameters, macroscopic physical pa­rameters, process parameters of the porous material production and treatments as shown in Table 5.1 and explained below. It is important to note, beforehand, that the parameters to be described are not independent from each other.3

TABLE 5.1 (Continued)

Source: Yilmaz (2009).3

Normal specific sound absorption coefficient of the materials has been determined by using an impedance tube, two microphones, dual channel frequency analyzer and the Indian Institute of Technology Kharagpur developed MATPRO software36. Test has been done as per ASTM E-1050 standard19. The impedance tube fabricated at the Indian Institute of Technology Khaargpur is used to measure the normal specific sound absorption coefficient of the biocomposite materials is shown in Fig. 6.6. Noise reduction coefficient (NRC), a simple quantification of absorption of sound by an acoustical material was calculated by averaging the four values of acoustical normal specific absorption coefficient at specified octave band levels. The NRC val­ue lies anywhere between 0 and 1. Higher the NRC value better is its sound absorb­ing property. Figure 1.7 gives a comparison of the normal specific sound absorption coefficient of some of the sound absorbing materials used for noise control. Each of the material in Fig. 6.7 is of 25 mm thickness with a rigid backing. The normal specific sound absorption coefficient of all the materials increases with frequency. Fiberglass and wool have high sound absorption coefficients. Gypsum board has a low sound absorption coefficients since it is denser with less porosity. The natural materials like jute, cotton and coir also have relatively high sound absorption coef­ficient. The polyurethane open cell has a sound absorption coefficient higher than that of gypsum though less than the porous natural materials and fiberglass.

Table 6.9 gives the values of the normal specific impedance with both the real and imaginary part of a 50 mm jute felt with a density of 117.2 kg/m3 as a function of frequency from 100 Hz to 1000 Hz. The jute felt was provided with a rigid backing. The data in the Table 6.9 can be used in numerical simulations20.

Injection molding process is the most extensively used molding method in the in­dustry used for the production of polymer composites due to its simplicity and fast processing cycle. Injection molding machine mixes and injects a measured amount of matrix and fiber mixture into the mold resulting in the desired product. It consists of three major sections: the injection unit, mold, and ejection and clamping unit (see Fig. 8.4).

The injection unit consists of a heated screw barrel having a compression screw, which can rotate as well as reciprocate. The function of the heated barrel is to pro­vide heat to the polymer matrix to melt before injection. The function of the recip­rocating screw is to carry and compress the pellets from the hopper into the heated barrel, mix the polymer matrix and fiber, provide heat to the matrix by viscous shearing and inject the mixture into the closed mold by acting as a piston. In other words injection unit consists of an extruder with an added function of reciprocating screw that injects the mixture into the mold. The cavities in the mold are the replica of the desired geometry of the product. The molds consist of cooling and/or heat­ing coils to regulate the mold temperature. The mold temperature determines the cooling rate of the product. The clamping unit clamps the mold tightly against the injection pressure to prevent burr formation and the ejector unit actuates the ejectors in the mold to eject the part when the cycle completes.

A typical injection molding process cycle is shown in Fig. 8.5. Generally the injection molding cycle is assumed to start from mold close position. After the mold closes it is tightly clamped against the injection pressure by the clamping unit. In the mean time the screw is retracted to its back position and then it injects the molten mixture with the desired injection pressure and speed into the mold cavity. The in­jected mixture undergoes shrinkage during solidification and to compensate that the screw is kept forward by the desired holding pressure for some time. After this point the screw starts to retract and plasticize the mixture while the part is being cooled in the mold. The part is allowed to cool sufficiently to be able to bear the ejector force and meet the desired dimensions. In the mean time the screw is being pushed backward as it is rotating and accumulating the mixture in the front. The part is then ejected and the cycle repeats itself.

FIGURE 8.5 Injection molding cycle.

Among all the natural fiber reinforced biocomposites, the flax based composite shows the best properties when compared to other natural composites, including glass-reinforced traditional composites. Flax fibers offer higher reinforcing prop­erties than hemp and kenaf natural fibers. Namely, comparison of the mechanical properties of the natural fiber reinforced composites has shown that composites based on flax fibers exhibited higher tensile strength relative to those based on hemp or kenaf fibers. Flax fibers exhibit a higher fineness and more unique distribution compared to hemp or kenaf. According to current theories, a higher fiber fineness should results in better fiber embedment during compression molding and conse­quently higher mechanical properties. Generally, mechanical properties of natural fibers are determined by the cellulose content and microfibrillar angle. The cells of the flax fibers consist mostly of pure cellulose cemented by means of noncellulosic incrusting such as lignin, hemicellulose, pectin or mineral substances, resins, tan­nins and small amount of waxes and fats. Flax cell wall consists of about 70-75% cellulose, 15% hemicellulose and pectin materials. The Young’s modulus of the nat­ural fibers decreases with the increase of diameter. The mechanical properties of the natural fibers are also closely related to the degree of polymerization of the cellulose in the fiber21. Basic physicochemical properties and cellulose content for flax fibers versus other natural reinforcing fibers are shown in Table 10.3.

Oksman et al.22 have studied the mechanical properties of PLA/Flax composites versus PP/Flax. The addition of flax fibers increase the modulus, but the higher fiber content has not improved the modulus in PLA composites as it has been ob­served for PP composites due to the fiber orientation in the polymer matrix. The test composites were compression molded and the fibers could be orientated differently from one sample to another. Because of the brittle nature of PLA, triacetin was used to plasticize the pure PLA and for the PLA/Flax composites. The addition of triac­etin has shown a positive effect on the elongation to break for pure PLA and PLA/ Flax composites, which was expected because of the softening effect. The highest triacetin addition (15%) clearly shows a negative effect for PLA/Flax composites, both the stress and stiffness were strongly decreased. As expected, it was shown that the addition of triacetin did not affect the impact properties of the PLA/Flax com­posites. The addition of 5% triacetin in PLA has shown the best results on impact strength. The authors also reported that thermal properties of PLA were increased with the incorporation of flax fibers. The softening temperature was increased from about 50°C for pure PLA to 60°C with flax fibers, and it is further increased if the composite is crystallized.

Water absorption is due to the hydrogen bonding of water molecules to the hydroxyl groups on the cells walls of the wood or lignocellulosic fibers.4870 The long-term water absorption as a function of time for the various LPCs at room temperature is shown in Fig. 13.5. All composites tested absorbed water during the incubation peri­od and no distinct saturation levels were achieved after 872 h of soaking (Fig. 13.5). The HDPE and HDPE-MAPE samples exhibited inconsequential weight gains (i. e., less than a 1% increase) after the immersion incubation time (872 h) compared to the biocomposites (Table 13.4; Fig. 13.5). Absorption of water by composites is a crucial factor in evaluating the ability of biocomposite to be commercially used.87172 To improve the resistance to water absorption inclusion of MAPE into the compos­ite formulation is routinely conducted.7172 However, we did not confirm this situa­tion for the composites used in this study. In fact, in some cases inclusion of MAPE in the formulation actually resulted in greater water absorption than that from com­posites without MAPE. For example, HDPE-25DDGS and HDPE-25DDGS-MAPE exhibited weight gains of 1.1 and 1.8%, respectively. This trend was observed for several STDDGS, PW and STPW formulations (Table 13.4; Fig. 13.5). These re­sults are somewhat surprising, since several other investigators have reported that inclusion of maleate olefins with the composite blend considerably reduces wa­ter absorption when using bio-fillers such as with Paulownia wood, loblolly wood, pine wood, sisal fiber, and wheat.437173 Practically no difference in water absorp­tion rates occurred between the HDPE-PW and HDPE-PW-MAPE formulations or the HDPE-STPW and HDPE-STPW-MAPE formulations (Table 13.4; Fig. 13.5). This observation seems counterintuitive and contrary to prior observations yet it is clearly demonstrated in this study with the PW formulations employed.437173 One explanation for the discrepancy between this study and others could be the thick­ness of the tensile bar formulations, method of processing and/or injection molding procedures employed in various studies.

The various composites exhibited distinctly different tensile bar thickness. Gen­erally, the gate section of the tensile bar is slightly thicker than the neck, which in turn is thicker than the end section (Table 13.5). This is due to the method the plastic resin and composite blend is injected into the mold; the gate portion is subjected to more injection time than the end portion and therefore contains more plastic resin than the other portions of the tensile bar. For example, the HDPE-25DDGS com­posite tensile bar exhibits an initial gate, neck and end section thickness of 3.18, 3.13 and 3.11 mm, respectively (Table 13.5). No significant increase in thickness for the tensile bar sections (gate, neck or end) of the neat HDPE and HDPE-MAPE were observed. However, increases in thickness could occur in the various com­posite formulations tested when given a soaking treatment (Table 13.5). Signifi­cant increases in thickness for the tensile bar gate, neck and end sections occurred in the HDPE-DDGS composites (e. g., HDPE-25DDGS and HDPE-25STDDGS, HDPE-25DDGS-MAPE and HDPE-25STDDGS-MAPE (Table 13.5). In contrast, the HDPE-PW composites (HDPE-25PW, HDPE-25PW-MAPE, HDPE-25STPW, and HDPE-25STPW-MAPE) only exhibited significant increases in the end section of the tensile bar. We attribute this occurrence to the presence of more PWF and less HDPE in the end portion of the tensile bar compared to that occurring in the gate and neck sections. Bio-fillers are hydrophilic in nature due to the presence of the abundant hydroxyl groups on the cellulose, lignin and hemicellulose which readily interacts with water molecules by hydrogen bonding.43 DDGS composites were ini­tially thicker than PW composites and when soaked for 872 h exhibited higher per­centages in thickness increases than PW composites (Table 13.5). We can conclude that DDGS composites are less dimensionally stable than PW composites. Inclusion of MAPE into the DDGS or PW composites did not notably alter the thickness measurements of the initial tensile bars compared to formulation without MAPE. Soaked composites containing MAPE exhibited slightly less thickness increases in terms of percent thickness increases than composites without MAPE (Table 13.5). Similarly this trend was also observed for “combination” composite mixtures (HDPE-12.5STDDGS/12.5PINEW and HDPE-12.5STDDGS/12.5PINEW-MAPE) where the inclusion of MAPE in the formulation reduced the thickness increase compared to the formulation without MAPE. This observation conforms to previous observations where inclusion of a maleate polyolefin in the formulation increases dimensional stability of the resulting composites through the binding of the cou­pling agent with the hydroxyl groups of the filler thereby preventing fillers binding to water molecules.43

Chemical modification treatments (A and AM) performed on the HDPE-STD — DGS formulations (HDPE-25STDDGS/A, HDPE-25 STDDGS/A-MAPE, HDPE — 25STDDGS/AM, and HDPE-STDDGS/AM-MAPE) did not improve absorption rates (% weight gain) compared to untreated controls (HDPE-25STDDGS) (Table 13.4). Thickness of DDGS formulations that were chemically modified (A and AM) were less initially than the untreated formulation. However, both formulations in­creased significantly following soaking (Table 13.5). In contrast, chemical modifi­cation treatments (A and A/M) of HDPE-STPW formulations (HDPE-25STPW/A, HDPE-25STPW/A-MAPE, HDPE-25STPW/AM, and HDPE-STPW/AM-MAPE) exhibited less weight gain compared to un-modified controls (HDPE-25STPW and HDPE-25STPW-MAPE) (Table 13.4). Thickness of STPW formulations that were chemically modified (A and AM) were initially comparable to untreated controls (HDPW-25STPW). However, all soaked STPW composites whether chemically modified or not exhibited significant increases in the end section of the tensile bar only but not in the gate or neck sections, which were much less affected (Table 13.5). One explanation for the difference in responses between the two filler for­mulations was the abundance of protein content in the DDGS formulations which contains more hydroxyl groups that can interact with water molecules during the soaking process than present in the PW formulations.

An important attribute of WPC is their ability to retain their original color and this characteristic greatly contributes toward its commercial value.7197475 Weather­ing causes color changes in WPC which is both undesirable and irreversible.71974 Water soaking is an important weathering test and is useful in determining the du­rable nature of a thermoplastic composite.764’7176 Weathering (e. g., water soaking) causes HDPE-composites to undergo chemical reactions such as breakdown of lig­nins into water soluble products which form chromophoric functional groups such as carboxylic acids, quinones, and hydroperoxy radicals.74

Figure 13.6 compares color values (L*, a* and b*) of the original composites to the soaked composites. Almost all of the composites exhibited lightness (L*) following the soaking treatment. This trend has been observed in other immersion studies employing WPC.7 Coupling agents are included in the biocomposites to im­prove bio-filler binding to the thermoplastic resin and they may combat lightness (L* value) changes.197778 Similarly, in this study, composites containing MAPE are darker than the corresponding composites without MAPE and retained their L* val­ues to a greater extent following the soaking process (Table 13.6; Fig. 13.6). For example, following soaking the HDPE-25DDGS formulation exhibited a 16% light­ening response; while HDPE-DDGS-MAPE exhibited a 6% lightening response. Overall, DDGS formulations exhibited much higher color change values (L*, a* and b*) following soaking than PW formulations. This may be attributed to water induced chemical reactions in the DDGS; suggesting that DDGS are less chemically stable than PWF. Chemical modification of DDGS resulted in less change in color values compared to that occurring in the nonchemically modified DDGS formula­tions. Changes in redness (a*) and yellowness (b*) values generally followed the L* trends, but not always (Table 13.6). Changes in the C*ab (chromaticity, color qual­ity), and H*ab (hue) values also occurred when comparing the original and soaked composites. Notable changes in color values even occurred in the neat HDPE and HDPE-MAPE polymers (Table 13.6; Fig. 13.6).

presence of the asterisk “*” indicates significant difference between soaking treatments (p £ 0.05).

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2 4 6 0 5 10 15 20

a* Values b* Values

FIGURE 13.6 Influence of soaking on color analysis of various PW/DDGS/PINEW composites.

Environmental stresses such as water soaking may cause changes in the me­chanical properties to occur which needs to be determined in order to assess the po­tential commercial value of a composite.7,64,70,73,76,79 For example, flexural properties have been reported to decrease when LPC are weathered.7,71,72 The response of the mechanical properties of composites as well as neat HDPE and HDPE-MAPE by water soaking are presented in Table 13.4. The neat HDPE and HDPE-MAPE blends exhibited significant changes in their oU and E values when given a soaking treat­ment (Table 13.4). Soaking caused oU values increased 3 and 3% for neat HDPE and HDPE-MAPE, respectively while %El values decreased 5 and 2% for neat HDPE and HDPE-MAPP, respectively. Soaking caused neat HDPE and HDPE-MAPE E values to change +4 and -5%, respectively. Changes in the mechanical properties for the composites varied considerably depending on the composition of the filler and MAPP concentration employed (Table 13.4). It is difficult to discern trends for the mechanical properties in the soaked and un-soaked formulations. Chemical modification of the PW and DDGS resulted in composites that when soaked main­tained their oU values (Table 13.4). However, several of these chemically modified formulations exhibited significant changes occurred for the E and %El values fol­lowing soaking. Absorption was found to be somewhat related to the variation of the mechanical properties of the composites. Formulations that maintained their me­chanical properties after prolonged soaking generally exhibited low absorption rates (Table 13.4). Even when significant differences occurred, less than a 5% change in values occurred (Table 13.4). Further work needs to be conducted to address how water absorbance affects the long-term mechanical properties of composites.

The optimized structural parameters were used in the vibrational wavenumber calculation with AM1 method to characterize all stationary points as minima. The structural parameters were calculated select the Constrain bond and length options of Build menu for two method of analysis.

15.2.2 FTIR

The energy of each peak in an absorption spectrum corresponds to the frequency of the vibration of a molecule part, thus allowing qualitative identification of certain bond types in the sample.

The FTIR was obtained by first selecting menu Compute, vibrational, rotational option, once completed this analysis, using the option vibrational spectrum of FTIR spectrum pattern is obtained for two methods of analysis. The analysis of the struc­ture of LDPE, benzophenone and WG husk with AM1 method was show in Tables 15.2-15.4, respectively.

Bionanocomposites are a novel class of nanosized materials in the modern day world. The terminology “bionanocomposites” is introduced several years ago to classify an emerging class of biohybrid materials. Bionanocomposites are the com­bination of biopolymers such as proteins, polysaccharides, nucleic acids, etc. with a reinforcing agent having at least one dimension in the nanometer range. The rein­forcing agent may include plant fibers and by products from lignocellulosic renew­able resources or synthetic inorganic fraction of finely divided solids, spanning from clays to phosphates or carbonates, whose origin can be either natural or synthetic. The most important challenge in bionanocomposites is to achieve materials with improved performance characteristics, by the elusive management of the individual properties of the incorporated components. There are some similarities of bionano­composites with nanocomposites prepared by using commodity polymers but also have fundamental differences in the methods of preparation, functionalities, proper­ties, biodegradability, and applications. Processes and structure of bionanocompos­ites are regulated by water that is added in an amount to only hydrate functional groups in the carbohydrate macromolecule. In particular, the biodegradability and biocompatibility nature of biopolymers, along with the thermal and mechanical properties of the reinforcing counterpart, bridge the gap between functional and structural materials.114117 Engineered biopolymer-layered silicate nanocomposites are reported to have markedly improved physical properties including higher gas barrier properties, tensile strength, and thermal stability.118120 Chemically treated nanoscale silicate plates incorporated with appropriate polymers can provide ef­fective barrier performance against water, gases, and grease.121 These hyper-platy, nanodimensional thickness crystals create a tortuous path structure that inherently resists penetration.

The minor ampullate (MI) gland is morphologically similar to the MA gland, but it is smaller in size (Fig. 1.1). MI silks are often present with MA silks, which form four fibers in dragline silks; two large diameter MA silk fibers and two smaller MI silk threads (Fig. 1.2). MI silk has been proposed to consist of two spidroin family members, Minor Ampullate Spidroin 1 (MiSp1) and Minor Ampullate Spidroin 2 (MiSp2).44 Recently, the complete cDNA sequence for MiSp from the orb-weaver spider, Araneus ventricosus, was deposited in the GenBank database.45 Analysis of the MiSp1 cDNA sequence predicts a 1766 amino acid residue protein organized into conserved nonrepetitive N — and C-terminal domains and internal repetitive re­gions composed of four submodules that are iterated in a nonregular manner. Simi­lar to MA fibroins, the predicted sequence is dominated by poly A and GGX, as well as Gly Gly Gly X (GGGX), Gly X (GX) motifs and spacer segments.

4.2.1 BIO-BASED EPOXY RESINS

Bio-based aliphatic epoxy resins such as glycerol polyglycidyl ether (GPE), poly­glycerol polyglycidyl ether (PGPE), sorbitol polyglycidyl ether (SPE), epoxidized soybean oil (ESO), epoxidized linseed oil (ELO), diglycidyl ester of dimer aid (DGEDA) and limonene diepoxide (LMDE), etc. are industrially available in large volumes at a reasonable cost (Fig. 4.1). Glycerol is an abundant and inexpensive bio-based aliphatic polyols, which can be derived from triglyceride vegetable oil. The biodiesel boom of the recent decade led to a significant increase in biomass — derived glycerol.27 At present, the worldwide glycerol production is around 1.2-1.4 million tons. This amount will further increase to 1.54 million tons in 2015.28 How­ever, the total global demand is less than 1 Mt. Thus, the market for petrochemical derived glycerol that is available via propene, allyl chloride and epichlorohydrin (ECH) no longer exists. The utilization of glycerol for the production of other in­termediate chemicals and final materials will become very important in near future. Indeed, some of these products are close to commercialization or already introduced into the market. One such example is the Epicerol® Process introduces by Solvay in 2007 enabling the ECH synthesis from bio-derived glycerol.29 Also, glycerol is an attractive renewable building block for synthesis of polyglycerols which have several uses in different field. Polyglycerols, especially diglycerol and triglycerol are the main products of glycerol etherification.30 Therefore, GPE and PGPE which are synthesized by the reactions of glycerol and polyglycerol with ECH should be very promising bio-based epoxy resins. Although the industrially available GPE and PGPE have been used in textile and paper processing agents and reactive diluents, etc., their epoxy resins have not yet been applied to matrix resins for fiber-reinforced plastics.

Sorbitol is also abundant and inexpensive bio-based aliphatic polyols, which can be produced by the catalytic hydrogenation of glucose derived from corn starch.3132 It is widely used in the food industry, not only as a sweetener but also as a humec — tant, texturizer, and softener. The SPE which is prepared by the reaction of sorbitol and ECH has been mainly used for tackifier, coatings, and paper and fiber-modifier, etc. Epoxidized vegetable oils such as ESO and ELO are manufactured by the epoxi — dation of the double bonds of vegetable oils with hydrogen peroxide, either in acetic acid or in formic acid,3335 and have mainly been used as plasticizer or stabilizer to modify the properties of plastic resins such as poly(vinyl chloride). Because ELO has a 30% more oxirane content than ESO does, the cured ELO has a higher cross­linking density, which results in a better performance.

Dimer acid-based DGEDA is a flexible epoxy resin produced from dimer acid and ECH. Although dimer acid can be also obtained from animal fats or vegetable oils, most of the dimer acids appearing in the market are synthesized from the crude tall oil provided as a byproduct of Kraft pulp.36 The commercially available dimer acid usually contains monomer (1-5%) and trimer or more (14-16%) in addition to dimer. Limonene-based LMDE (1-methyl-4-(2-Methyl-2-oxiranyl)-7-oxabicyc- lo[4.1.0]heptane) is commercially produced by the reaction of limonene and per­acetic acid, which is used as cationically curable resins and reactive diluents. (+)-d — Limonene is a popular monoterpene which is commercially obtained from citrus fruits.

However, their bio-based aliphatic epoxy resins had not been versatile mate­rials because of inferior mechanical and thermal properties to the bisphenol-A or novolac-based epoxy resins. Therefore, the selection of hardener and the addition of natural fibers are important for the use of the bio-based aliphatic epoxy resins in wide applications. In the following section, we used GPE, PGPE, SPE, and ESO as bio-based aliphatic epoxy resins for the preparation of green composites. Their physical properties and suppliers are summarized in Table 4.1. The average number of epoxy groups per molecule of GPE, PGPE, SPE and ESO are 2.0, 4.1, 3.6, and 4, respectively.

It is a common practice to leave an air gap between the absorber material and the hard backing wall in applications such as suspended acoustical ceilings, although the practical use is relatively limited.68 The highest NAC will be achieved when the distance between the absorber and the wall is odd multiples of a quarter wavelengths for the sound frequency of concern. This phenomenon is due to the fact that the incident waves and the reflected waves will have a phase difference of 180°. Con­versely, when the air gap is a multiple of half wavelengths, the airspace becomes totally ineffective as the incident and the reflected waves will be in phase.17 Air gap behind the material increases NAC substantially in the low frequency range at the cost of high frequencies.68 This effect is seen in Fig. 5.10. Jayaraman61 found that the air gap caused an increase in the absorption in the frequency range between 500 and 4500 Hz. He did not find a significant difference in NAC values for fibrous mats with 5-mm air gap compared to those with 10-mm air gap, whereas the maximum peak is at a lower frequency, that is, a greater quarter wavelength for the greater depth of the gap. There are useful design charts in Attenborough and Ver17 to predict the NAC of absorbers separated from the wall with an air gap with the knowledge of air gap distance, flow resistivity of the absorber and sound frequency.