Repri nt ed f rom THE J OURNAL SOCI ETY OF AMERI CA,  Vol .  31,  No. 5,  628- 629,Copyri ght ,  1959 by t he Acoust i cal  Soci et y of  Ameri ca.
Pri nt ed i n U. S. A.
Dependence of  Sound Absorpt i on Coef f i ci entDependence of  Sound Absorpt i on Coef f i ci entDependence of  Sound Absorpt i on Coef f i ci entDependence of  Sound Absorpt i on Coef f i ci entDependence of  Sound Absorpt i on Coef f i ci entupon Area of  Acoust i c Mat eri al supon Area of  Acoust i c Mat eri al supon Area of  Acoust i c Mat eri al supon Area of  Acoust i c Mat eri al supon Area of  Acoust i c Mat eri al s
KOZI  SATO AND MASARU KOYASUKobayashi  I nst i t ut e of  Physi cal  Research,  Kokubunj i ,  Tokyo,  J apan( Recei ved December 23,  1958)
The dependence of  t he absorpt i on coef f i ci enton t he area of  t he sampl e mat eri al swhen t he sound absorpt i on i s measured byt he reverberat i on chamber was al readyreport ed i n 1934 by V. L. Chri sl er.   We i nves-t i gat ed t hi s rel at i on by usi ng our new
reverberat i on chamber and al so obt ai ned amore ext ensi ve range of  absorpt i on coef f i -ci ent s t han t hat  of  Chri sl er.  The reverberat i on chamber used f or t hi sresearch has a vol ume of  about        , sur-rounded bynonparal l el  wal l s.   The empt ychamber absorpt i on ext ends f rom 2. 7 t o 8. 2f or t he f requency range f rom 250t o 2000cps.  Sampl e mat eri al s were several  ki nds ofmi neral  wool  bl anket s wi t h di f f erent  t hi ck-nesses,  pl aced on t he cent er of  t he f l oor oft hi s chamber.  The di mensi ons of  sampl e mat e-ri al s are shown i nTabl e I .  I n order t o obt ai n t he absorpt i on coef f i -ci ent   f or t he sampl e area l ess t han      , wef ol l owed t he met hod t hat  was adopt ed by
Table I.  Dimensions of sample materials.
Lengthwidth          (m)×Area(m2)Perimeter / area         ratio
0910 601210 911820911821822 73 1823641825461825462 42546303
..................
×××××××××
054110165331497661996132165
.........
5603853312 20183165146119103
.........
FIG.1.  Variation in measured absorption coefficient with distancebetween each area when a number of equal small aeas were placedseparately.  Unit size is about 0.91m0.60m.×
Chri sl er.  That  i s,  a number of  equal  smal lareas were pl aced i n t he reverberat i on cham-ber f ar enough apart  f rom one anot her.  Ast he pr el i mi nar y st udy of  t hi s met hod,  wei nvest i gat ed ef f ect  of  t he di st ance bet weeneach area on t he absorpt i on coef f i ci ent .  Thesi des of  sampl e mat eri al s were covered wi t hmat eri al s whi ch have a f ar l ower absorpt i oncoef f i ci ent  compared wi t h t he sampl e sur-f ace.  Fi gur e 1 shows an exampl e of  t heseresul t s.  For t he sampl e area l ess t han about1    ,  t he measured absorpt i on coef f i ci enti ncreases as t he di st ance i ncreases.  But ,when t he di st ance becomes more t han 1 m,t he absorpt i on coef f i ci ent  approaches t hel i mi t i ng val ue.  Thus,  we t ook t hi s val ueas t he absorpt i on coef f i ci ent  correspondi ngt o t hat  area.
FIG.2.  variation in measured absorption coefficient with area at250 cps.  Dotted lines show Chrisler's experiments.
FIG.3.  Variation in measured absorption coefficient witharea at 500 cps.
LETTERS TO THE EDI TOR
sampl e area varyi ng f rom 0. 5 t o 15    areshown i n Fi gs. 2 t hrough 5.  I n t hese f i gures,t he resul t s report ed i n Chri sl er' spaper are al so shown.  Wi t hi n t he same rangeof  absorpt i on coef f i ci ent s,  t hese t wo mea-surement s coi nci de f ai rl y wel l  wi t h eachot her.  The resul t s of  measurement s of  t heabsorpt i on coef f i ci ent  wi t h
  The det ai l s of  t hi s research wi l l  be pub-l i shed l at er i n t he Bul l et i n of  t he KobayashiI nst i t ut e of  Physi cal  Research.1 V. L. Chri sl er,  J . Research Nat l .  Bur.  St andards 1313131313,169-  187 ( 1934) .2 K. Sat o and M. Koyasu,  J . Acoust . Soc. J apan 1313131313,  231-255  ( 1957) .
FIG.4.  variation in measured absorption coefficient witharea at 1000 cps.
FIG.5.  Variation in measured absorption coefficient witharea at 2000 cps.