## 3 Results and Discussion  
### Transmission Loss  
Figure 5 show the measured TL values for the studied fluted PVC panels. For studied panels, the transmission loss is influenced by four factors: size, stiffness, mass, and damping. In Figure 5, similar TL spectrums are observed. In the low and medium frequencies, the three visible peaks are observed: around 100 Hz, 160 Hz, and 315 Hz, and dip at 125 Hz and 200 Hz. These dips can be interpreted as a specific vibrational mode of the panel or the lowest acoustic coincidence of the panels. At low frequencies, the transmission depends heavily on stiffness, as can be observed for GW700 panels under 63 Hz. With greater bending stiffness and a shorter span, the transmission loss is the highest. However, the measured transmission loss of all fluted panels indicates no distinct lower critical frequency shift with the change of panel shape. The low-frequency response of panels of different depths is similar. Figure 4: Reverberation room with the studied panel. Figure 5: Measured transmission loss (TL) of studied PVC panels. However, at the frequency of the incident, the wave increases and the panels will resonate at a series of frequencies, such as resonance or eigenfrequencies. This is because the panels have a finite boundary and edge fixing. The resonance frequencies comprise a fundamental frequency and integer multiples of this called harmonics. At the fundamental resonance, \(f_{1,1}\), the sound transmission through the panel is enhanced, and the transmission loss drops significantly. Damping, as an isolation material, provides the only means to control resonance. Presently, damped materials (such as rubber, fibrous substances, plastic sheets, mastic solutions, and adhesive films) specifically planned to minimize resonance problems are widely used. Highly damped materials also excel at controlling or preventing rebound, thus making damped isolation mounts ideal for controlling shock problems. The fundamental resonance frequencies, \(f_{1,1}\), of a supported rectangular orthotropic panel of width a and length b were calculated according to:  
\[f_{i,n}=\frac{\pi}{2m^{1/2}}\sqrt{\left(\frac{B_{x}i^{4}}{a^{4}}+\frac{B_{y}u^ {4}}{b^{4}}+\frac{B_{xy}i^{2}n^{2}}{a^{2}b^{2}}\right)} \tag{22}\]  
\[B_{xy}=\frac{1}{2}\left(B_{x}\
u+B_{y}\
u+Gh^{3}/3\right) \tag{23}\]  
where \(G=E/[2(1+\
u)]\) is material modulus rigidity, \(E\) is [PERSON]'s modulus, \(\
u\) is [PERSON]'s ratio, \(a\) and \(b\) are panel dimensions, and \(h\) is the panel thickness (Table 1). Above 800 Hz, the sound transmission depends on the mass inertia of panels and is independent of its stiffness. In this frequency region, part of the acoustic energy is transmitted through the panel, and the rest is reflected. A direct relationship exists between panels' weight and the resulting transmission loss, except for the GW300 panel. Generally, an increase in transmission loss is expected with increasing mass because the heavier the panel, the less it vibrates in response to the sound waves, and hence the less sound energy it radiates on the other side. The mass law predicts that the transmission loss will increase by approximately 6 dB for each: doubling of the surface mass or doubling of the frequency. The mass can be increased using thicker material or denser material. Theoretically, all noise control problems could be solved by increasing the partition mass. However, practical considerations limit the usefulness of this option. In our case, the way of joining the panels and their corrugated construction causes the sound insulation to not be improved only by an increase in mass. One of the most critical factors affecting the sound insulation across building elements is sound transmission through narrow or joining gaps that occur in the joining of construction elements (panels, windows, doors, floor panels, etc.). The sound leakage through such gaps deteriorates the sound insulation performance of the elements.