VRIJE UNIVERSITEIT BRUSSEL Acoustics - the Dept. of ...

4.2. REALIZATION OF ACOUSTIC ABSORPTION 69Thenarrowertheporesare,thestrongerwillbetheairfriction, thusthestronger the damping, i.e. absorption. In order to measure the acousticabsorption of a porous material an experiment can be conducted tomeasure the airflow through the material as shown in Figure 4.6. Thespecific air resistivity R s is a measure for this, defined by R s = −1vIt seems obvious that R s must be sufficiently large to achieve dampingand thus absorption. On the other hand R s should not be too large,becausethentheporeswillbecometoonarrowandonlyasmallfractionof the incident intensity will penetrate, while the main part is reflected.The layer thickness is also an important parameter. Indeed, in orderto get a damped wave, it must penetrate a sufficient distance into theporous material. Moreover, the damping is proportional to the wavevelocity (and not the pressure) which has a maximum at a quarterwavelength away from the wall. One thus comes to the conclusionthat relatively low R s and relatively thick material should give rise tofavorable absorption properties. One should especially not think thathigh R s and small thickness give rise to good absorption. This cheapsolution leads to bad results.Frequency also plays a major role here, since the particle velocity dependson it. The friction losses, which are dependent on the viscosity,increase with increasing speed, thus increasing frequency of sound.Hence porous layers especially absorb high frequencies. Further, onemayalso imaginethatalargelayer thickness isrequired toabsorblowerfrequency (large wavelength).The pore structure also plays a role (granular or fibrous material ...).The structure can be described by means of the so-called tortuosity(this is the randomness with which the fibers are arranged in the material).The method of fixation in front of the wall. If fixed on profiles, a fewcentimeters infrontofahardwall, thenoneobtainsanabsorbenteffect,approximately equivalent to that of a layer absorbent material, equallyto the sum of the thickness of the air layer and the thickness of theabsorbent material.Caution: paint covers the fine pores and annihilates the sound-absorbingeffect of porous materials (there is no danger with large pores).Some numerical values of the specific flow resistivity : R s = 10 4 for glasswooland rockwool, R s = 10 5 for compact glasswool and compact rockwoolR s = 10 6 for compact fiberboard and R s = 10 7 for compact stony materials.∂p. ∂x