tailieunhanh - 2001 Fundamentals Part 3

Tham khảo tài liệu '2001 fundamentals part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 2001 ASHRAE Fundamentals Handbook SI concrete. The transmission loss of a single-leaf partition depends mainly on its surface mass mass per unit area because the heavier the partition the less it vibrates in response to sound waves and the less sound it radiates on the side opposite the sound source. Increased surface mass can be achieved either by an increase in the partition s thickness or its density. The mass law is a semi-empirical expression that may be used to predict transmission loss for randomly incident sound for thin homogeneous single-leaf panels. It is written as TL 20log Ws f - 42 13 where ws surface mass of panel kg m2 f frequency Hz The mass law predicts that transmission loss increases by 6 dB for each doubling of surface mass or frequency. If sound is incident only perpendicularly on the panel the TL is about 5 dB greater than that predicted by the mass law. Transmission loss also depends on material properties such as stiffness and internal damping. The transmission losses of three single-leaf walls are illustrated in Figure 2. For the 16 mm gypsum board TL depends mainly on the surface mass of the wall at frequencies below about 1 kHz agreement with the mass law is good. At higher frequencies there is a dip in the TL curve called the coincidence dip because it occurs at the frequency where the wavelength of flexural vibrations in the wall coincides with the wavelength of sound in the air. The frequency where the minimum value of TL occurs in the coincidence dip is called the critical frequency which depends on material stiffness and thickness. The stiffer or thicker the layer of material the lower the critical frequency. For example the 150 mm concrete slab has a surface mass of about 370 kg m2 and a coincidence frequency at 125 Hz. Thus over most of the frequency range shown in Figure 2 the transmission loss for the 150 mm concrete slab is well below that predicted by mass law. The coincidence dip for the 25 gage mm thick steel sheet