New Controllable Sound Absorbers Made of Vacuum Packed Particles - New Contributions in Information Systems and Technologies - page 287

Table 2. Valuesofabsorptioncoecientforselectedfrequenciesofsoundandunder-
pressurelevels(POLandABS)

Frequency [ Hz ]
500 1000 2000 4000 6400

Material Underpressure [ MPa ]

Poly(methyl
methacrylate)

0 0,4570,5740,5810,571 0,58

(POL) 0,01 0,031 0,030,0340,043 0,05

0,02 0,0290,0280,0260,0380,047

0,03 0,0290,0280,0260,0380,048

0,04 0,0290,0270,025 0,040,047

0,05 0,0290,0280,0230,0380,046

0,06 0,0290,0270,0220,0380,046

0,07 0,0290,0270,022 0,040,046

0,08 0,0290,0270,0210,0420,046

0,09 0,0290,0270,022 0,040,046

0 0,2670,4480,4290,5470,464

0,01 0,0310,0340,0470,0580,088

0,02 0,0250,0280,0250,0420,042

0,03 0,0250,0270,0250,0450,038

0,04 0,0240,0260,0250,0440,039

0,05 0,0260,0270,0240,0480,038

0,06 0,0250,0270,0230,0470,045

Acrylonitrile/ 0,07 0,0250,0270,0230,0450,046

Butadiene/

0,08 0,0250,0260,0220,0430,037

Styrene (ABS) 0,09 0,0250,0280,0220,0470,037

4.2 Comparisonof Acoustic Characteristics of Different Materials

The characteristics of the different granular materials under research show a
similarresponseinthefrequencydomain.Infig.7thereareplotsofabsorption
coecient for four different types of granular media with no underpressure in
sample. Additionally, in table 3 there are numerical values. It is easy to ob-
servethattherearesomenoticeabledisagreementsintheshapeoftheiracoustic
absorption characteristicswhen inside the structure there is only atmospheric
pressure. From about 500 to 1000Hz there is a rapid increase in value of ab-
sorptioncoecientforalltestedstructures.Next,fromabout1500to2000Hzit
stabilizesatacertainlevelthatismaintaineduntillabout6000Hz.Abovethat
levelthereisanotherincreaseofabsorptioncoecient.FormaterialsPOL,PS,
andPPthe valueofthetested coecientina broadrangeoffrequenciesfixes
atabout 0,6. A slightly different performance is observedfor ABS (fig. 6). At
thisstageofresearchitisdiculttofindatrustworthyansweraboutthecause
ofsuchbehavior.Onepossibleexplanationfor thiscouldbesomevariancesin
thegrainmix(sizeor/andshape).