Hi Mathan Sambu,

Based on your results, the SAC drop in the middle frequency range, which mean the sample reflected the sound at those frequencies. It is might due to the physical properties and the effect of binder/adhesive. Thus, you try to relate your SAC results with the physical properties; fiber diameter, density, porosity, tortuosity, flow resistivity and other properties which influencing to the SAC performance. The more open pores in the sample, the less will be the effect on the mid-high frequencies range (refer: architectural acoustics: principles and practices, sub topic 1.2 basic concepts page 12). The fibre orientation distribution during sample preparation also might effect on the SAC. 

Also refer to this paper :

(https://www.ncsu.edu/bioresources/BioRes_07/BioRes_07_4_5700_Karlinasari_HMMNH_Acoustic_Prop_Particleboard_Bamboo_2681.pdf)

Hi Mathan:

For porous absorbers place on a rigid backing (Kundt tube set up) , you could have a resonance frequency where the absorption drops. This resonance depends on the thickness of the sample (the thicker, the lower the resonance). You might have a look  to the book "Acoustics Absorbers and Diffusers", by Trevor Cox and Peter D'Antonio (I am using the second edition). Chapter 5 is about porous absorption. Figure 5.7 shows a drop of the absorption at 1000 Hz of a inorganic foam. Figure 5.20 compares two configurations of porous absorbers (different thickness) calculated using the matrix transfer method. The book I recommended also includes some matlab code, so you can study this effect (check the appendix B, Matlab Scripts, Absorption of a rigid backed porouse absorbers). I recommend you to check this in order to get a good insight of the behavior of the absortion coefficient of porous absorbers.

The deepness of the drop of the absortion will depend on the physical properties of the material, as Hanif Latif mentioned in his answer.

Regards

Thanks alot to Mr.Hanif Latif and Mr.Manuel A.S.S for your reply.

In my view, it depends on type of testing (Normal incident or random incident waves), instrument (two microphone impedance for reflected waves or four microphone impedance for transfer waves) and of-course accuracy of samples like uniformity of fiber and resin distribution and exact fitting of sample in holder. May I can explain you more detail...if you want...normally amplitude of waves depends on frequency and attenuation of waves depends on material parameters especially thickness, density and surface characteristics decides the resistivity. if surface is completely laminated with latex during processing of nonwoven composite, it wont give any absorption value in two microphone impedance test method, consider as everything is reflected, it is contradict to open window..every thing transferred...lot of factors affects absorption of sound waves(meant noise)..can be discussed in personnel mail..

Hi, Mathan,

As additional resources for Trevor J Cox book, you may find an interesting detailed explanation in J.F. Allard and N. Atalla’s book Propagation of Sound in Porous Media (Willey, 2009). You may also find another brief explanation in Hoda S. Seddeq’s paper published in Australian Journal of Basic and Applied Sciences, 3(4): 4610-4617, 2009: Factors Influencing Acoustic Performance of Sound Absorptive Materials.  Almost similar topic provided by Tiuc et al. in their paper entitled The analysis of factors that influence the sound absorption coefficient of porous materials which are available at RG.

According to my own experience in working with felt, it is possible to increase the sound absorption of the fibrous material based sound absorber in the mid-frequency range. The simplest way is by using thin elastic membrane backed fibrous layer. When the fibrous layer coated with a thin elastic membrane, it changes the reactance of the structure and brings the possibility for absorbing the sound waves energy through combined resonant and viscous damping mechanism simultaneously. Should you need further discussion, please feel free to send an email privately.

Best regards,

Iwan.