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Controlling sound transmission through concrete block walls
Warnock, A. C. C.
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https://nrc-publications.canada.ca/eng/view/object/?id=8fe95aff-adf1-4a91-bc2e-f150870a5aee https://publications-cnrc.canada.ca/fra/voir/objet/?id=8fe95aff-adf1-4a91-bc2e-f150870a5aeeb y A .C.C. Wa rnock
This Update discusses the various factors that affect sound transmission
through different types of concrete block w alls, including single-leaf w alls,
double-leaf w alls and w alls w ith gypsum board attached. Know ledge of
these factors w ill assist construction practitioners to design and build w alls
w ith high levels of acoustic performance economically.
C o n s t r u c t i o n T e c h n o l o g y U p d a t e N o . 1 3
Con crete block w alls are com m on ly u sed to sep arate d w ellin g u n its from each oth er an d to en close m ech an ical equ ip m en t room s in both resid en tial an d office bu ild -in gs becau se of th eir -in h eren t m ass an d stiffn ess. Th e ben d in g stiffn ess of th e m ate-rial m akin g u p th e w all is also im p ortan t. Neith er of th ese fu n d am en tal p rop erties (m ass an d stiffn ess) can be altered by u sers.
How ever, th ere are ad d ition al factors th at n eed to be con sid ered in bu ild in g h igh -qu ality w alls.
Single -Le a f Conc re t e Bloc k Wa lls
Mass per Unit Area
For sin gle-leaf w alls, th e m ost im p ortan t d eterm in an t of sou n d tran sm ission class (STC) is th e m ass p er u n it area: th e h igh er it is, th e better. A sin gle-leaf con crete block w all is h eavy en ou gh to p rovid e STC ratin gs of abou t 45 to 55 w h en th e block su rface is sealed w ith p ain t or p laster.
Figu re 1 sh ow s m easu red STC ratin gs for sin gle-leaf con crete block w alls from a n u m ber of p u blish ed sou rces. Th e con sid -erable scatter d em on strates th at, w h ile im p ortan t, block w eigh t is n ot th e on ly factor th at d eterm in es th e STC for th is typ e of w all. In th e absen ce of m easu red d ata, th e regression lin e in Figu re 1 can be u sed to estim ate th e STC from th e block w eigh t. Altern atively, Table 1 p rovid es rep resen ta-tive STC valu es for 50% solid blocks th at h ave been sealed on at least on e sid e. It sh ow s th e relatively m od est effects of sign ifican t in creases in w all th ickn ess on STC.
Ad d in g m aterials su ch as san d or grou t to th e cores of th e blocks sim p ly in creases th e w eigh t; th e in crease in STC can be
Controlling Sound
Transmission through
Concrete Block Walls
Som e Ba sic Conc e pt s — Tra nsm ission Loss (T L) a nd Sound Tra nsm ission Cla ss (ST C)
For sign ifican t n oise red u ction betw een tw o room s, th e w all (or floor) sep aratin g th em m u st tran sm it on ly a sm all fraction of th e sou n d en ergy th at strikes it. Th e ratio of th e sou n d en ergy strikin g th e w all to th e tran sm itted sou n d en ergy, exp ressed in d ecibels (d B), is called th e tran sm ission loss (TL). Th e less sou n d en ergy tran s-m itted , th e h igh er th e tran ss-m ission loss. In other words, the greater the TL, the better th e w all is at red u cin g n oise.
Sou n d tran sm ission class (STC) is a sin gle-n u m ber ratigle-n g th at su m m arizes tragle-n sm is-sion loss d ata. It is obtain ed by fittin g a stan d ard referen ce con tou r to th e d ata (see Figu res 2 an d 3).
2
estim ated from Figu re 1. Ad d in g sou n d -absorbin g m aterials to th e cores is n ot effective becau se th e sou n d byp asses th e in su lation , takin g th e m ore d irect p ath th rou gh th e block.
Figu re 1 also sh ow s th at u sin g h eavier block to get an STC ratin g of m u ch m ore th an 50 leads to impracticably heavy constructions. High STC ratin gs are m ore easily obtain ed by ad d in g layers of gyp su m board m ou n ted on stu d s or resilien t (flexible) fu rrin g th at d o n ot h ave a solid con n ection to th e con crete block.
Effect of Porosity
Wh en th e con crete block is p orou s, sealin g th e su rface w ith p laster or block sealer sign ifican tly im p roves th e sou n d in su la-tion ; th e m ore p orou s th e block, th e greater th e im p rovem en t. Im p rovem en ts of 5 to 10 STC p oin ts, or even m ore, are n ot u n com m on for som e ligh tw eigh t block w alls after sealin g. Con versely, n orm al-w eigh t blocks u su ally sh oal-w little or n o im p rovem en t after sealin g. Th is im p rove-m en t in STC in ligh tw eigh t blocks is related to th e in creased airflow resistivity of th ese blocks. Th e leakage of sou n d th rou gh th e m aterial of th e block is d ifferen t on ly in d egree from leakage of sou n d th rou gh in ad equ ate m ortar join ts. To en su re good sou n d in su lation , m ortar join ts m u st alw ays be p rop erly fin ish ed — th at is, free from obviou s p en etration s.
Gyp su m board attach ed d irectly to blocks by m ean s of glu e or screw s d oes n ot seal th e su rface as w ell as p laster or p ain t sin ce th e gyp su m board d oes n ot bon d com p letely to th e blocks an d beh aves as a sep arate layer. Sou n d rad iated from th e gyp su m board con tin u es to leak th rou gh th e blocks. Table 2 p rovid es exam p les of tw o very d ifferen t typ es of block an d th eir rela-tive STC ratin gs. In each case, th e p laster allow s th e sou n d in su lation of th e w all to reach its fu ll p oten tial; th e gyp su m board d oes n ot. Th e effect of th e p laster on th e very p orou s (w ood -aggregate) blocks, w h ich h ave been in clu d ed for p u rp oses of com p ar-ison , is p articu larly d ram atic. Block sealer w ou ld yield th e sam e resu lt as p laster.
Concrete 90-mm w ood- 190-mm block aggregate lightw eight treatment blocks concrete blocks
Un sealed 14 42 Gyp su m 29 46 board
on on e face
Plastered 43 49
Table 2.STC for very porous wood-aggregate blocks and lightweight concrete blocks sealed with gypsum board or plaster
Wall Lightw eight Normal Thickness, block w eight
mm block STC STC 90 43 44 140 44 46 190 46 48 240 47 49 290 49 51
Table 1.STC ratings for 50% solid normal-weight and lightweight block walls sealed on at least one side
Figure 1.Effect of block weight on STC for single-layer concrete block walls
65 60 55 50 45 40 35 0 10 20 30 40 Block weight, kg STC
In oth er tests carried ou t at IRC, a n u m -ber of d ou ble-leaf cavity block w alls, in w h ich th e layers w ere stru ctu rally isolated from each oth er, ach ieved STC ratin gs in th e ran ge of 70 to 80. Th e con stru ction s p rovid ed th e least am ou n t of stru ctu ral vibration tran sm ission th at can be ach ieved in th e laboratory, w h ich is better th an w h at can be ach ieved in n orm al p ractice. In p ractical situ ation s, carefu l d esign , m eticu -lou s w orkm an sh ip an d skilled su p ervision are n eed ed for su ch w alls to ach ieve th eir fu ll p oten tial.
Conc re t e Bloc k Wa lls w it h Gypsum Boa rd Adde d
Ad d in g gyp su m board su p p orted on fu rrin g or stu d s at a d istan ce from th e su rface of th e block w all can greatly im p rove th e sou n d in su lation of th e w all assem bly. Th ree factors th at govern th e d egree of im p rovem en t are:
1. Th e m eth od of su p p ort — id eally, th e gyp su m board sh ou ld n ot be solid ly con n ected to th e block;
2. Th e d ep th of th e cavity (th e d istan ce betw een facin g su rfaces of th e block an d th e gyp su m board );
3. Th e u se of sou n d -absorbin g m aterial in th e cavity betw een th e gyp su m board an d th e su rface of th e block.
If th e fu rrin g su p p ortin g th e gyp su m board is rigid , sou n d can travel d irectly th rou gh it from th e gyp su m board to th e blocks. How ever, if th e fu rrin g is su fficien tly flexible, th e sou n d w ill be atten u ated . Bu t th e best m eth od of su p p ort for th e gyp su m board is th e u se of in d ep en d en t stu d s th at h ave n o d irect con n ection to th e block. Resilien t m etal fu rrin g m ay be u sed on its ow n or in com bin ation w ith w ood fu rrin g.
Figu re 2 p rovid es tw o sets of tran sm ission loss (TL) d ata for w alls w ith u n filled cavities of tw o d ifferen t d ep th s, as w ell as d ata for an u n fin ish ed block w all. Th e effect of th e added gypsum board on acoustic performance is striking — at high frequencies, the perform-an ce of th e w alls w ith th e gyp su m board is better th an th at of th e bare block w all; at low frequ en cies, it is w orse. Th ere is a d ecrease in TL at low frequ en cies an d an in crease in TL at h igh frequ en cies in both w alls relative to th e u n fin ish ed block w all.
Double -Le a f Conc re t e Bloc k Wa lls
In p rin cip le, d ou ble-leaf m ason ry w alls can p rovid e excellen t sou n d in su lation . Th ey ap p ear to m eet th e p rescrip tion for an id eal d ou ble w all: tw o in d ep en d en t, h eavy layers sep arated by an air sp ace. In p ractice, con stru ctin g tw o block w alls th at are n ot solid ly con n ected som ew h ere is very d ifficu lt. Th ere is alw ays som e tran sm is-sion of sou n d en ergy alon g th e w ire ties, th e floor, th e ceilin g an d th e w alls abu ttin g th e p erip h ery of th e d ou ble-leaf w all, an d th rou gh oth er p arts of th e stru ctu re. Th is tran sm itted en ergy, kn ow n as flan kin g tran sm ission , seriou sly im p airs th e effec-tiven ess of th e sou n d in su lation . Flexible ties an d p h ysical breaks in th e floor, ceilin g an d abu ttin g w alls are n eed ed to red u ce it. Even if su ch m easu res are con sid ered in th e d esign , m ortar d rop p in gs or oth er d ebris can brid ge th e gap betw een th e layers an d in crease sou n d tran sm ission . Su ch errors are u su ally con cealed an d im p ossible to rectify after th e w all h as been bu ilt. In on e test con d u cted by IRC research ers, a d ou -ble-leaf w all th at w as exp ected to attain an STC of m ore th an 70 p rovid ed an STC of on ly 60 becau se of m ortar d rop p in gs th at con n ected th e tw o leaves of th e w all.
Figure 2.Sound Transmission Loss (STC) through a 190-mm concrete block wall with 15.9-mm gypsum board attached to one side using a) 13-mm resilient metal channels, and
80 70 60 50 40 30 20 63 125 250 500 Frequency, Hz T ransmission loss, dB 1000 2000 4000 13-mm resilient channels Mass-air-mass resonance Bare blocks 75-mm resilient Z-bars STC 50 STC 51 STC 57
In gen eral, th e greater th e cavity d ep th an d th e greater th e m ass, th e low er th e m ass-air-m ass reson an ce frequ en cy, w h ich u su ally m ean s an in crease in STC. Th ere is an excep tion to th is tren d , h ow ever, w h ich occu rs at frequ en cies arou n d 100 Hz, w h ere th e STC of th e cavity w alls d ecreases relative to th at of th e u n fin ish ed block w all. Wh ile voices are n ot a sou rce of low -frequ en cy n oise, m od ern stereos can be, as can m ech an ical equ ip m en t, m ean in g th at w alls arou n d m ach in e room s sh ou ld be d esign ed w ith great care if th ese room s are ad jacen t or close to livin g or w orkin g sp aces.
Sou n d -absorbin g m aterial is com m on ly ad d ed to th e cavity in a w all or floor to im p rove th e sou n d in su lation . Th e ad d i-tion of th is m aterial:
• low ers th e m ass-air-m ass frequ en cy an d u su ally im p roves th e STC, an d
• red u ces th e d etrim en tal effects of oth er cavity reson an ces at h igh er frequ en cies. Fibrou s m aterials, su ch as cellu lose fibre, glass fibre or rock w ool in su lation , are good m aterials for th is p u rp ose; closed -cell m aterials, su ch as exp an d ed p olystyren e, are n ot, as th ey d o n ot sign ifican tly absorb sou n d .
Figu re 3 sh ow s th e effect of ad d in g glass fibre to th e cavity of th e w all w ith 13-m m resilien t m etal ch an n els. Th e m ass-air-m ass reson an ce frequ en cy d rop s to arou n d 100 Hz an d th e STC in creases to 54.
Figu re 3 also sh ow s th at w h en gyp su m board is ap p lied to both sid es of th e blocks, th e TL in creases to a greater d egree at h igh er frequ en cies, bu t d ecreases d ram ati-cally arou n d th e m ass-air-m ass reson an ce. Th e STC for th e w all w ith gyp su m board on both sid es is on e p oin t less th an th at for th e w all th at d oes n ot h ave gyp su m board ad d ed . More im p ortan tly, in th e case of th e w all w ith gyp su m board ad d ed to both sid es, th e effectiven ess of th e sou n d in su la-tion arou n d 100 Hz is sign ifican tly red u ced by th e ad d ed m aterial. Th is d im in ish ed p erform an ce occu rs becau se th e cavity d ep th is too sm all, w h ich lead s to a m ass-air-m ass reson an ce th at low ers th e STC. To m axim ize th e im p rovem en t in sou n d in su lation , th e cavity d ep th sh ou ld be as large as is p ractical. As a gen eral gu id e-lin e, th e p rod u ct of th e m ass p er u n it area of th e gyp su m board (in kg/ m2) an d th e cavity d ep th (in m m ) sh ou ld exceed 425 for a cavity filled w ith sou n d -absorbin g m aterial, an d 720 for an u n filled cavity.
Est im a t ing ST C for Conc re t e Bloc k Wa lls w it h Gypsum Boa rd Adde d
Normal-w eight, non-porous block
Data from an exten sive series of m easu re-m en ts on n orre-m al-w eigh t block w alls w ere u sed to d evelop an em p irical m eth od for p red ictin g STC for block w alls w ith gyp su m
4
Figure 3.Sound Transmission Loss (STC) for a 190-mm concrete block wall with glass fibre batts in the cavity and with 15.9-mm gypsum board attached on 13-mm resilient metal channels to a) one side and b) both sides of the wall
90 80 70 60 50 40 30 20 63 125 250 500 Frequency, Hz T ransmission loss, dB 10 1000 2000 4000 Bare blocks
One side only Both sides
Mass-air-mass resonance
STC 50 STC 54
STC 49
M a ss-a ir-m a ss re sona nc e fre que nc y
Th is is th e sp ecific frequ en cy at w h ich tw o layers of m aterial sep arated by an air sp ace reson ate, som ew h at like th e skin of a d ru m . In th e case of a block w all w ith gyp su m board ad d ed , vibration al en ergy tran sfers from th e gyp su m board th rou gh th e air in th e cavity to th e block w all m ore effectively th an it d oes th rou gh th e bare block w all.
board su p p orted on resilien t fu rrin g or in d ep en d en t stu d s. Th e sim p lified version of th is m eth od , p rovid ed in Table 3, u ses on ly STC ratin gs bu t still p rovid es reason -able accu racy. Th e equ ation s give th e STC in crem en t (∆STC) th at can be ach ieved , w ith or w ith ou t sou n d -absorbin g m aterial, w h en gyp su m board is ad d ed to on e or both sid es of a bare block w all.
For exam p le, if 15.9-m m gyp su m board is ad d ed to both sid es of a block w all filled w ith sou n d -absorbin g m aterial, u sin g 65-m 65-m steel stu d s, ∆STC is 0.44x65 - 7.37 = 21.2. Th u s th e STC ratin g of th e bare w all im p roves from 46 to abou t 46 + 21 = 67.
Note th at for sm all valu es of d (i.e., sm all cavities), ∆STC can be n egative — in oth er w ord s, th e effectiven ess of th e w all in con -trollin g sou n d is red u ced . Pred iction s m ad e u sin g th is p roced u re h ave an u n cer-tain ty of abou t ±2d B, bu t can be u sed w ith som e d egree of reliability for 12.7- or 15.9-m m th ick gyp su m board .
Porous block w alls
Th e leakage of sou n d th rou gh a p orou s con crete block w all can be u sed to ad van -tage w h en gyp su m board is ad d ed to fin ish th e w all. Becau se sou n d p en etrates th e block, th e effective d ep th of th e cavity is greater th an th e actu al d istan ce betw een th e in n er faces of th e gyp su m board an d th e block. Th e effective d ep th d ep en d s on th e p orosity of th e block. Th e best w ay to fin ish a leaky con crete block w all is as follow s:
• Plaster or p ain t on e sid e on ly. If a gyp su m board fin ish is requ ired on th is sid e, it sh ou ld be glu ed or screw ed on d irectly as th is h as th e least d etrim en tal effect on STC.
• On th e secon d sid e of th e w all, u se resilien t m etal fu rrin g or in d ep en d en t stu d s to su p p ort a layer of gyp su m board . As w ith n on -p orou s block, th e larger th e cavity d ep th , th e better. • Ad d sou n d -absorbin g m aterial to th e
cavity.
Porou s block w alls fin ish ed in th is m an n er can p rovid e STC ratin gs as good as th ose p rovid ed by n on -p orou s, h eavier blocks. Cu rren tly, it is n ot p ossible to p red ict th e STC for com p osite w alls su ch as th ese. Ap p roxim ate estim ates m ay be obtained using the procedures for non-porous blocks. In critical situ ation s, m easu rem en t of th e sou n d tran sm ission loss (TL) valu es m ay be n eed ed .
Sum m a ry
Becau se of th eir w eigh t, w ith little effort on th e p art of th e d esign er, con crete block w alls p rovid e good sou n d in su lation . In gen eral, th e greater th e
• m ass of th e w all • th e d ep th of th e cavity
• th e am ou n t of sou n d -absorbin g m aterial th e better th e p erform an ce of th e block w all, alth ou gh th ere are som e excep tion s to th is ap p roach . How ever, by ap p lyin g th e p rin cip les d escribed in th is Up d ate, it is p ossible to con stru ct h igh -qu ality w alls th at can m eet th e m ost acou stically d em an d in g situ ation s.
Re fe re nc e s
1. A.C.C. Warn ock an d D.W. Mon k. Sou n d tran sm ission loss of m ason ry w alls: tests on 90, 140, 190, 240 an d 290 m m con -crete block w alls w ith variou s su rface fin ish es. Division of Bu ild in g Research , Nation al Research Cou n cil, Bu ild in g Research Note 217, Ju n e 1984.
2. T.D. North w ood an d D.W. Mon k. Sou n d transmission loss of masonry walls: twelve-in ch ligh tw eigh t con crete blocks — com p arison of latex an d p laster sealers. Division of Bu ild in g Research , Nation al Research Cou n cil, Bu ild in g Research Note 93. Sep tem ber 1974.
∆STC = 0.11xd - 1.22 on e sid e on ly ∆STC = 0.14xd - 2.78 both sid es ∆STC = 0.12xd + 1.87 on e sid e on ly ∆STC = 0.44xd - 7.37 both sid es
Table 3.Equations for estimating increments in STC using the cavity depth d (in mm) and assuming a single layer of gypsum board
No sou n d -absorbin g m aterial in th e cavity Cavity filled w ith sou n d -absorbin g m aterial
3. T.D. North w ood an d D.W. Mon k. Sou n d tran sm ission loss of m ason ry w alls: tw elve-in ch ligh tw eigh t con crete blocks w ith variou s su rface fin ish es. Division of Bu ild in g Research , Nation al Research Cou n cil, Bu ild in g Research Note 90. Ap ril 1974.
4. A.C.C. Warn ock. Factors affectin g sou n d tran sm ission loss. In stitu te for Research in Con stru ction , Nation al Research Cou n cil, Can ad ian Bu ild in g Digest 239, Ottaw a, 1985.
5. A.C.C. Warn ock. Sou n d tran sm ission loss m easu rem en ts th rou gh 190 m m an d 140 m m blocks w ith ad d ed d ryw all an d th rou gh cavity block w alls. In stitu te for Research in Con stru ction , Nation al Research Cou n cil, In tern al Rep ort 586, 1990.
Dr. A .C.C. Wa rnockis a sen ior research officer in th e In d oor En viron m en t Program of th e N ation al Research Cou n cil’s In stitu te for Research in Con stru ction .
“Construction Te chnology Up d a te s” is a se rie s of te chnica l a rticle s conta ining p ra ctica l inform a tion d istille d from re ce nt construction re se a rch.
For more information, contact Institute for Research in Construction, National Research Council of Canada, Ottaw a K1A 0R6
Telephone: (613) 993-2607; Facsimile: (613) 952-7673; Internet: http://irc.nrc-cnrc.gc.ca
© 1998
Nation al Research Cou n cil of Can ad a May 1998
