Viscoelasticity sound absorption

Dynamic specific modulus E /y, the ratio of dynamic Young s modulus to specific gravity, and loss tangent tan 8 can be used to study the viscoelastic nature of wood. E /y is related to sound velocity and tan 8 to sound absorption or damping within the wood. A large E /y and small tan 8 characterize the acoustic quality of soundboard wood [3]. 

Absorption, definition, 168 Absorption of sound in viscoelastic materials. See Sound absorption in viscoelastic materials Acoustic, definition, 25-26... 

Due to the high energy-absorbing abilities of viscoelastic foams, the foams are sometimes called energy-absorbing foams or sound-damping foams. However, die sound-absorption and impact-absorption properties are somewhat different. Therefore, optimum formulations for making them are not exactly the same. 

Because of the time-temperature (or frequency-temperature) relation for the viscoelastic properties of polymers there is of course also a corresponding frequency dependence of the acoustic properties of polymers. In Table 14.2 frequency derivatives of sound speeds and absorptions are listed. 

The stress-strain relations for viscoelastic materials are reviewed. The simplest case of intrinsic absorption in polymers is a molecular relaxation mechanism with a single relaxation time. However, the relaxation mechanisms which lead to absorption of sound are usually more complicated, and are characterized by a distribution of relaxation times. Under causal linear response conditions the attenuation and dispersion of sound in a... 

The most widely used materials for absorption of sound are viscoelastic polymers, and these material are discussed in a separate section. 

In the specific area of development of viscoelastic polymers for sound attenuation and vibration damping, the need is to identify materials with high intrinsic absorption which are also relatively temperature and pressure independent. These conditions can be optimized within the constraints of the general causal relations between the real and imaginary parts of the moduli. 

The ultrasonic relaxation observed in CTAB/Pentanol/water-systems has previously been interpreted in terms of a perturbation of the equilibrium between alcohol molecules in the water phase and micelle phase respectively. Generally speaking the sound wave detects the alcohol molecules jumping in and out of the CTAB-micel-les. The interesting question is whether or not this process is affected by additives that make the system viscoelastic. It is important to notice that the ultrasonic absorption data at different frequencies for CTAB in pure water are pretty close to those obtained from CTAB made viscoelastic by adding Sodium Salicylate. Thus the ultrasonic wave hardly detects the change that causes the viscoelasticity. 

---