Designing, Building, And Testing Your Own Speaker System

*3

Speaker-Box Construction AND@NsrRucrloN Hou) DES¡GN ENcLosuF.E Trs pRlNcFlEsoF c'ooDSPB^KER prinfor any speakerproiecl regardlessof box tyPe By heeding üose procioles, váu can üuild with confidence and preveDtlong debugging furniture' ""trr"J. ryo., "t" io"tined toward novelty in your choice of vou can exprcss it iD your sPeakor eoclosures by such superncrsl ásoectsofdá"ign as extemal appearance.For l¡ose desiSnleaturesüat aff;ct üe sound, stick to basics

CHAMCTERISTICS ENCI.OSURE DESIRABLE The sinele most deshablecbaBct€ stic ofa good enclosureis that it does not ieúouslv alter ths sou¡d of the sPeakersinstallsd in it A sDe6kersvst€mis different hortr a musical instrument in on€ important thl speakershould produceno sound of its o$'n An instrument a^soect: ,.lau."" toio¿, " "p"akei reproducesit lf üe speal'eraddsan¡'rtring walls il is producingdistortion lf üe eDclosure io the sienalit receives. bass tlue üe weaken "it""t" i"aitfv, they will cotor the sound and shápe enclosüe A bad *"0"""" ¡v aúsorbinglovl-f."quencysnergy can spoil ieproduction by lumping the inevilable air r€so¡ances a¡ounáa nat¡owbandof ftequsncies,Producinga boomysystem SHAPE ENCLOSURE Elclo$res with nonpa¡allel wslls, such as triangular boxes' have wiü internalreflectionsMost spealerbuilders as well fewe¡oroblears üey are because ," orftét"u¿lo f"o", "hooseorünary box-lüe" enclosures _look most of üese the lüe spealers To male """u ro ¡oil¿ and üev no intemal male "nciosr¡es,uuoide*tremsshapesAs a rule of thumb sn enclosüre Such dimension more than thee times that of any other'

24

Speaker-EoxConlruction

can act like a resonantpipe. You can violate üat rule if you follow üe suspestions sivenlat€rin Lhfuchapter'Át the otheraxbeme.if all dimenofall ure eorlal, siñ. vou havea cube.which is oneofüe worstshaPes used Thk ratio' 0 6:1:1.62. prefened is r€Üo of dimensions One by golden ratio used properties, is üe ssme acoustical bere for lts mason to you have a sPecial p¡'ramids. Unless Egyptian artists since th€ the cor¡ect 3_1 lists Table a choice. ratio fu ratio, this useanothe¡ Sood intemal dimensions for spea.ke¡boxes wiü net intemal volumes ftom 400 cubic inches (0.23€ubic foot) to 10,000cubic inches {5.8 cubic feet). Table 3-2 lists a range of cubic volümes measued tn liters with inteúal dim€nsions in cántimete¡s. If you use üe data ftom th€se tabl€6,you¡ enclosu¡ewill have about 10 percent overvolum€to allow soace for clrive¡s, cleats, ajtd oüe¡ internal Psrts. The cohrmns are úbebd D1,D2,andD3. In timespsst,üe smsllestdime¡sionwasussd for enclosuredepth. Now, to obtaio better iúagin8, a Ía¡¡ower box with qeater depti ii often desi¡ed,so D1 becomesthe enclosurehontal üdth. Foi li.t"n"r" wbo preferbassresPonseto imsglnS or bsve a vt¡oofertoo l6rse for the nanower box, m€l(€D2 the \ ¡idth Intem¡l D¡mensions Table3-1. Colden-Raüo for SpeakerEnclosur€r,Entlish units. Des¡red NetVol¡¡me (Cu.In.) 10000 8000 6000 5000 4000 3000 2500 2000 1800 1600 1400 r200 1000 900 800 700 600 500 400

Approx. Tot¡l (Cu.In.)

r r000 8800 6600 5500 4400 t300 2750 2200 1980 1760 1540 1320 1100 990 880 770 660 550 440

lnternal Dim€ris¡ons (ln) D3 D2 D1 13'/." 121f 11" 10,/;' 9l{ 8'/i 8"

6'h' 6fi 6'

22'/: 20'A" 1at 17.[ 16'/r" 14't{ 14" 13" 12'h" 1211'h" 11" 10f," 10" 93t¿"

36' 331'' 3O'/"" 2a'h' 26'4" 24" 22'/i 21" 20v," 19'4" 185/i 17r: 16sK 16li 15\/,'

8'/," ayi 7'{

14\li 133/r" 12'/í

sháPe 25 Enclosurc

'"?"r;il".u1',of l;i,iJi'^.'Jfi#P¿Ti,T"* Desired NetVolume (Liters)

Iotal

lnternalDimensions

(Liters)

D1

D2

D3

34.0 32.0 29.7 28.7 27.O

54-8 51.7 4t.9 46.2 43.6 40.4 38.0 15.3 32.1 30.2 28.0 27.O 25.4 23.6

88.8 83.7 77.6 74.9 70.6 65.4 61.6

150 100 90 75 60 50 40 30 25 20 18 15 '12 10 8 6 5

I10 99 83 66 55 33 27.5 22 19.8 13.2 l1 8.8 6.6 5.5

(cm)

23.6 21.9 19.9 18.7 16.8 15-8 13.8 12.E 10.9

20.6 18.8

51.9 48.9 45.4 41.8 41.2 38.3 36.0 33.4 30.4 2a.b

*fi*ru-e'*$,r,

il;:ñ] s#*i,so;ffiiu$$

**,;**nt*ui**

in EnclosureDesign SomeRecenlTrends

27

The thúd rule isr if the vertical line of drivers is placed off cente¡, make the secondspeakerboffd of your stercopair a mirro¡ image ofthe ñrst (Fig. 3-2). This allows you to plac€ the spealels so that in both enclosuies the drivers will be ofl centered inward, or outward, rather than having one enclosure with the drive¡s inwsid a¡d the oüer with the divels outward from the spacebetween you¡ speakers.

DESICN IN ENCLOSURE SOMERECENTTRENDS A gene¡ationago therc were as many horizontal enclosuresas vertical d¿signs,but that is no longe¡ so. In fact, some curent speakercab' inets are io skinny and tall that they oÍce would have been considered extreme. Figures 3_3and 3_4show some wavs that you can use a tall' na¡row enclosure while miniñizin8 the dsk of piPe resonance lnse¡ting a partition conve¡ts üe pipe to an ordinary box Placing the d¡iver at a point that is one-fifth the distance from the end of the pipe supprcssesthe fi{lh harmonic, a major causeof resonancei¡ closed piP€s. Ifyou add stuffing to the enclosure,results can be quite satisláctory. Columnar speakers. such as those shown in Figs. 3-3 and 3'4, have seveml practical advantages.The nanow front usually produces better Ete¡eo imaging than the wider f¡ont of conventional boxes AIso, the height ofthe cabinetsputs the sound at ear level without the need to place ihe speakersatop other Pieces of fumiture or on separat€ sta¡ds.

s¡e¿rP,'ne' ---r--...-

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Fígüe 3-2. when dt¡ve' arc nounted off cente, the t||o speakersshould be ñiíol ir¡,agesof each other The namal anngenent ¡s ta ¡nstall the drivers so that they arc offset towad the niddle arca of the s¡ereospace.

?a

Spe¿leÉ8o\Conetruction

,:y:;:,i:;, ,?;i.-*

i;3í:,;í;,::,::

" tuitd a to||e, speake,fheto||e, pa,tof ke cotunn serves

instailedat a f¡fthof thetotatpipetenlthion thetop, iif#1 ,¡s,

:fffi'+il:;il"',T:';"',1".1i.,ff :$;"*:l::,:ll";l*nltn** have,alr-eadynoticed the proliferationof speaker -.- lou nrobablV

asme oes,gnrn | ¡gs.3-5 and 3_6.Suchslandssre designed to r¿iseancl

Som€RecenrTrends in Enclosur€ Desien 29 tip back spealels of intermediate size. Most medium-size speslers sound much more open and natuml when placed on such a stand. The secondkind of Btandyou might have seenis the type shown in Fig. 3-7. Thesestsnds are useful for ulhacompact to compact-sizespe€lers. As speaker designers have become more álert to problems of difhaction causedby some kinds ofcabinet const¡uction, they have made chsn8osin the extemal shape of speakerenclosures.Figure 3-8 show6 some colrlmon ways speakerswere once installed in poorly designed boxes. In th€ cabinet shown in Fig. 3-9, the worst features of Fig. 3-8 have been corected. Figues 3-10 and 3-11 show somo ways to solve the problem ol placing a grille fiame over üe front of the enclosu¡e. When üe grille is installed on standoffs, as in Fig. 3-11, sound can move horizontally wiüout hitting any interfering projections. If yoüi drive¡s are installed ñom the hont ofüe enclosurcand you plan to use an ordinary halne to hold the g¡ille cloth, you can cover th€ speaker board area betwe€n üe d ve¡s and the fra¡re wiü thick felt or

FiSue 3-5, A useful stand fór speake\ oÍ intermediate size. The doping platforn l¡tu and til$ rhe spe.aker,nak¡nq the sound mote open and natural.

30

SP€aker-BoxConsructioñ

rigt¡e 3-6, *

Íigtte 3'7

iñ FiS t'5 * *^O t 'hePt¿nsshown

bv plac¡ngúen on inproveddranatically be can speakers Many srnall

SomeRecentTrc¡d,in E¡closureDesign

-11

Fig"rc 3-8. A bad speakerenclosurc. Ihe drivet installation behind the boad and the shary pro¡ecting edqes inteíerc with sound wave propagat¡on

F - - - - - - - i

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Figu¡e3-9. A good speaketenclosure.lherc are ro obsttuctionsto inteíere with

foam (Fig. 3-12). You can buy sheets of foam at carpet deale$ Such foam is designed fo¡ carpet u¡derlaymenti it comes in va¡ious colors and thicknesses.The %-inchfoa¡r shouldbe about riSht for most speakers. Cut tho fosrn to fit the space and glue it to the speaker boa¡d, the smooth surface against the board. Such material will absorb sound that might be rcradiated hom üe spealer board, causinSinte¡ference wiü sound hom the d ver. You can hear the impmvement wiü some spealels easilf but it is Dot as obvious wiü oüels. To test the usefuInessof thiÁ beatment, tempo¡arily tack or stapl€ the foam to the speakerboard on one of your stereo sp€alers and compare the sound wiü the untlsat€d speater. If you cs¡ hea¡ a difference, it is wo¡th doin8.

32

Sp€aker-Boxconstrucr|on

lin¡n8

I

Íigtú.3-lO. to ñÁo,*"o^ inteiot eoF.

dE a g¡tte taÍE' slue leh ot foan atons

(frooi !ic*) F4.Jfie3-tl. one'ay¡o¡nn¿ll¿Stillefnne$ eing acrots the enckxure rront

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nol hpde

dtesoundtrav'

Materials 3J

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Íoan, canbe glued to thespeakel ÍEür¿3-t2. A soundabsoóent,suchas fettor the dr/'f]rsto rcducediffnctton'

MATERIALS

matedal is %-inch (19 cmj Tbe most commooly used speal(er-box

{medium-densitv p*i"leuo-áorMD.F pry"i.¿. iffi*lilirul ihick¡ess ihould be choser to oatch encl(

:li}:xt#,ltr'#"i*:':" rrt;iy"tii"tl",'gi,i'fi -same to.pan6lsize A Sood üickness so you shouldadiust lhickness tor r" tó use'¡-inch ir'g cml mate¡ial in. soc.losures ñ" "f ü"-l

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lfvouus€

p€nelsintite;l,S,",h, larse ""-tl"ahonsBrace ""." g¡a¡ good a pl)'wood choose

:*",3ii T":x¿'-lilH"IH;""'*:xl:Xi;:,:lT *"*""

-lf you usefir'ask for tattlás cause " p-"i. *¿ f";se layersca-n

otiot"¡-qtvY:.91 üuTnÁ:''c,"¿" lr.vr'i.fr ill :::f.:I"il,[tJ; enclosur€ by 1 ¡ncn {z : tle enla¡ge wood.

iii"-t¡"',"i"".i-*"ii" *i t h iocb(r 27cmlPaflic',i"11$,**o*,*

1i,ilil*,r"u"-'r"'p*'ra" r""i;:"":'1""1'"J':,F;H"T:ffi 'l;¿;t""iff;'"'l'Jrffi:lJH

j"ff :rS":i."J# wellpaper.

Ext.aWallTrealment 35 Particle board is a good choice becauseof its hiSh density' lack of voi¿lso¡ loose layers, ;d low cost some builders pÉfe¡ high-density the oarticle bo6rd -iile oüets choose medjum-density fiberboa-rdin pafl¡cre Lelief lhat it is betler dsñped You can identi¡' hiSh-density Uo"rJ¡v "". in¡C l¡"particlesize althe sdgesoftheboard-ThehiShha"-smali particles and smoot¡ edges which mates il ¿"*irv'¡o"t¿ *"i". io ¿,r". co""tt""tion adhesiveis a good, low-cost choice of material to us; for bonding parti.le bodrd or €ny kind ol fiberboard lt ioinls ror dont matcb wel[, epoxy is better'CPoxyc€n be qurte expenslve place doioqan enlirecabi;et bul itworks well for Eluingbrscesinlo b"iw?un oppo.;t" punels Cul lhe brac" Io fil ss snugly as possible then the atr"ctrit *iifr epoxy,u"ing nails to hold the bracein place until contact close it requires glue but is useful wood eooxv\els.Carp;nter's oithé parts for ma-ximumstrenSü. ó; disadvanlageol hjgh'densityps-rücleboad is that il dulls ordinary saw bladesi After o-nly one pass tlrough üe- board, ^üe ¡lade needst; beresharpefled,Fo¡ fastercutting ard longerblade life' choose a tlade with carbiáe teeth to cut paficle board Such bladescan be danger;us if.,sed lncorr""tly lfyou get a blade with carbide-tipPedte€th' follow these precautions: . Use a blade guard on your saw. . Don't use tñe blade in a saw that exceeds the úaximum recommended rpm for üe blade. The maximum rate probably is printed somewhereon the blade Package. . Savethc orieinalpack€gefor blade'toragc\ahennot in use . Use careat áU tirn"s;n handlingthe blade lo ensu|pthát you nevec qurtace bump lhe Ldrbjdeleelb a8ainslsny

WALLTREAIMENT EXTRA If vou are a perfeLlionist. thete are some e\ba touches üal can improíe almosrary speakerenclosure.parlicularlythosemadeof ply' -;d. one kind oibr;.e úral occupiésvcry litlle spaceis on€ üat ties anv opposing walls togeüer' Such a brace need not be thick becaúse That brace the compress sbetchor to movemust tJriaut "| to. ttt"rtváu" movement is in üe düection ofgreatest stiffness' Almost any enclosure wall ia¡ be made less resonant by making üe wail into a composite of more üan one kind of material lt's a parricularlygood ideaü add ¿n] mdl€rialthal can damp üe walls suchas ""on"f,'.É"n"g material You can often 8"1 scraps of asphalt roofing hJm constructlon silos. If ¡ ou glue pieces of il lo the cent¡al arcaof üe wall Danels,the walls will be relatively dead to sound You can use asphjlt cementfor elue but it takcsso long lo setthal ) ou m¡ghtprefer adhesive.one all-purposeadhesjvemade by Macco a a iaste¡-actinp

3ó

SPeakerBoxConsruction

g$g¡3#1******li*

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ConstructionRules

JZ

boad and back'suchasthesg Fiqrrc 3-14. Close-fittingcleatsbehindthe speaker hélpprcventa¡ teaks. üe speaker hol€s to ioitrt. then install the back. You can r€ach throügh taulk the ioints atound the back

iHi:i:Üli:Jl"iHH;"$ -.tll:'*;ttlm:iT"1"f hsr:oi;¡" r,fa'1ir;?il*i'i,*::.":",1i;; o",nq"ls rorq"1 3,o"o.t "#:*,iil::::"f *á i:ffi ;,"- "i,¡"*'"..*" i,j:1.."Tll

'**:'J;"-*-i"';Hi' .:T:,"J,i,,ffi $it"#T:":'iPf

* t",:T::fi""i"'J,,"J".:il. 1$i!1r"11;1,, 5,'""*::.,i",:lül ,"''ü11 inte' onrhe mareriar ;iliií"::tli"':iiliilil;'"l''",iio"-'ins l;l[ilJffi"::; "*""iy;l"'",'ffi ;1"'#;ff ffi:*fi',::Tfl

38

Speaker'BoxCo¡struction

tiEue 3-15. Add cornetglue blocks,scrcwedot nailedin, for strength.

dimension will absorb rea¡ ¡eflections. h üe real world, you should aim for a ma¡gin of safety srld put damping mate¡ial on every interior wall except üe speaker board (Fig. 3-16). For ñnal adjustment, use your ear. Add more Eat€dal to any system üat sounds loud at mod€st volume levels. Ma.kesu¡e the wslls n€ar Oreütoofe¡ a¡e heÁvily cove¡ed snd, if necessary, ha¡rg a cuftain of mate¡ial b€hind the woofer o¡ frll the spac€ b€hind it. when ñtüng the area nea¡ üe woofer, don't üse ñberSlass. It can get into üe voice coil region and cause problems. For such areas,use polyester batting or Acousta-stuf Il there is a single_note boom, ty shetching a layer of úate¡ial over ü€ back of the woofer, stapling it to the speaker boa¡d. Make sule this dsmping pad is shetched üghdy. In additioü to absorbing reflections, you can use dsmpinS matedal to incroase the effective cubic volune of the box. It does this by chsnging the operation of üe box air ftom adiabaüc (constart heatl to isotherms.l (constant tempe¡atur€). Wiü isothemal operation, the stuffia8 absorbg and gives up heat, maintaining a constant box tsmpemtule ánd ¡educing souad velocity. This shofiens the wavelengths. In theory you can add about 40% to box vólume by stuffing; in pmctice, about 20% is the limit. To stuff a box, cut the damping matedal into súall pieces and loosely ñll üe box. Don't forc€ etrough mateúal into üe box to comprcss it.

ConstructioñRules

39

what kind of damping material should you use?Fibe¡8]assis prob' ably üe most used becauseit is widely available and its cha¡acteristics a¡e well known. The acousticalglad€ comes in packagesof one square yard of material 1 inch [2.54 cm) üick. Acoustical g¡ade is p¡efe¡¡ed becauseit stays in place better than looser kinds. Ask for Radio Shack CaL No. 42-1082.You ca¡ substitute other materials such as polyester batting, rug und€rla]'rnent,or old rags;however,beware ofdense mateals üat would significantly changebox volu]n€. As a rule of üumb, small closed-boxspeakerenclosu¡esshould be filled with damping matelial. For this purpose, as p¡eviously mentioned, polyester fibe¡ or Acousta-stuf is the prcferred choice. Fiberglass,when used too close to üe driver. can get into the voice coil and . Rul€ 4: Install Drive$ from Outside the Box. ln earlier days, the twical high-fidelity speakerwas bolted to the rear of üe speakerboard (Fig. 3-171.In some €nclosures,that is the only practical way to mount a drive¡; but, if üe panel is very thick, such mounting produces a cavity at the hont of the spesler that can color the sound. The shary edge

Fieurc3-16- Doni lorye¡d¿nptnqn¿redal

40

Sp€akeFEoxConstructron

JI

F a¡leña Fígur 5-12. fheorct¡calconeexcursionol a wooferin thrcekindsof enclosutes:a closedfux, a sinple reflex,anda double-chanbetreflex.

HOW TO DESICNA DOUEI"E{HAMBERREFIEX You can apply Thiele data to ü€ double-chamber¡eflex a6wsll as to siDtle-chamberboxes.He¡s is how to do it in a four"stepprocedr¡re: . Chooseyour woofer. Use its specificationsto fitrd the optimum volume. Maks thb volume the total volumg of üe two chambe¡s.Or, you ca¡ reve$s üis procedure by setting th6 total volume, as desi¡ed,and choosüs a woofer to match rL.

74

Ported-BoxSpeakerSystems

..,j:rit:i:"{j,"".' h[?*LiH:i Rid';'"l'ff ffiil'i;j'li*

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advantagesover singledoubt"-chamberreflex has .some

,l*'r,'ffi;rl**¡**l:;-t ü, }tü', Ji'fi *+*iÍ""#:'tr, li,*ü*'FI':'"t,""i$.il+ii:*'#:f ji"i,j,#T",:"*l,H#i*"'"*¡i' 'J":l'i'':"'$ ;l{}.;+.

liüHr$iffi',ilft*$iliffi ffRftq'i*"$j:.3r.TLlr":'"jT:: SYSfEM5 BAND-PASS

lir;H:"i;::llr*ff :i,:5:t:'ff i:{i"1,[{ii¡ ,n",u;:1ff

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Band,hssSystems Z5 b€¡d-pssssystemcan be employedteiú s sjng¡ewooferor wirh .A mur|lprewooters,tl two ¿re used.one ca¡ bc wired lo eachchannelof a stcreo_ system.as shown in Fjg. 5- 13.Tbe ba¡d-passbo\ LUts otf l¡c hrBnerhBquencyoutput oIthe woote¡s.allowingyou to usethF syslpm as a subwoofer.It might be desirableto use a crossoverfilter to fufher block higher frequency sound ftom the band-pass svstem. Trial a¡d efror can $ride you with that. By_choosing different alignments, you catr alter üe Íiequency ran8e,flatnessof r€sponse.and efficiency of tbe system.The range of á band-pa5sspeal.er is deñned as the band of frequencies betwc;n the lwo points whe¡e üe responseis down 3 dB bom the level ar mid_ band. This r€nge is dets¡mi[ed by the location of the cent¡a] ftequency, set b-yfc ofthe closed box chamber,and the damping factor.The damp_ irg factor, S, cán range ftom 0.s (or even lower), to ó.2 in a twical sys_ tem. The lower values fo¡ S allow a wide¡ ¡ange, but wiü increasing rippleinüe responsecurve. At án S of0.Z, therásponseis üeoreticalti flat. Above 0.7 the ¡ange nanows mpidly. The voiume of the cornpart'm€nt in Iront of the woofer is detemined by the value of S and the chaEcteristics oI the driver. Jn addition to choosinga woofer and S, lhe remain¡nBdps¡sn . cho¡Lc, is thc system Q lQ¡) for tie d¡iver in the closed-boxiompalment.You cancalculatetie properclosedvolume justasyou wouli for 8nY Oürerc¡oseo-boxsvstem,

Figure 5-t3. Foutth-odet band-pass enctosures.when usedas a subwoofet, the Ieft and dght channel bass can be n¡xed in one enctoswe by using two woofe' ol with a double vo¡ce co¡l woofet

76

Ported-BoxSpeak€.syst€m, For the vohme of üe ported chambe¡ {VT}, uss this forEulal

vr = (z XsXersD,0x) a¡d for f¡ f" = {ac"} (fs/Qls) This system is easily designedwiü a computer program süch as LDP.With it you can quickly exploro how cbangesin S or systemQ will alter üe frequencyrangeof your syst€m.Afte¡ you ente¡ t¡e pa¡aBetels for your woofe! arld the desiied values for Q and S, LDP lists üe low and high frequencycut-offpoiats, the po¡t dimeDsions,and the rclative efñciency of üo system,comparedto ths output level of the woofer in a conventional box.

*10

Projects A MAIoR coA! IN sErEcrl,\c coNfoNENTs AND tN DlslGN cHolcEs FoR THES!

proiects was valüe. Any spealer system is a compromise between cost and difficulty of construction ve$us level ofpe¡formance. Alúost any spealer can be improved by specifying more exotic components' but vou can reach a point oI diminishirg ¡etuns. As in pasle¿itionsofthis bool. simple crosso!ernelworksare t¡e of the design"boires for crossor"rPans might sPcm ¡ule here.-Some appear to violate the ¡ules, it is becausethe final deciIf they unusual. was made by a listening test. case sion in each ofevery constructronstep here' descriptions find detaiied You won't expected that you follow the but it is discussed, arc Soecial o¡oblems 3 The photographsshould in Chapter outlined ptocedures cónst oction you questions have abo¡¡tp¡ocedüe' any to answer show enough detail asPectof any pmi is an impofant volume cabinet that Remember space,as long fit any special to dimensions change can usually ect. You be mainshould volume piPe, but cubic or a you a cube don't ms](e as fiBure. at the sDecified tained Varioussuistitr¡tjoni ca¡ be made,bul don t Pxpe(tro substitute another wooler fo¡ one of the po ed-box proiects and 8et optimüm perfoünance. Such substitution can ruin arry ported-boxproject unless the new woofer has similar specifications for Q and compliance The speake$ that specify L_pads on the tw€eters require carefirl adiuslmentof üose conÚols Start l¡e ddiuslmentproccssby luming th€ t\¡eetersll lhe wav do$rn,l¡an advdncathe conbol unlil the souDd ofthe tweeter is blenáed with üat ofthe woofer' Ifthe tweeter appea¡s to srandoul. you haveSoneloo far' Of it eveDonesiogingor speal_ ine sounds¿s il lhev ha! " a falserPethwh i"t le. you hdve8on" way too fa.riVoicerecording!are usuallyb€sl Iof t$Feteradiuslmentberauseit is easv to notice when t¡e human voice sounds natural o¡ unnatural' N;te that üe description of each project includes tbe values for maximum and minimum imP€dance These can be useful, pa¡ticularly

the value for low impedancs. Some speale¡ systemshave impedance lows clown to 3 o¡ 4 ohms. a¡d sometimeseven lower. Some amplifiers don't work well with such low impedance.Speakerswith highei mini_ mum impedance,and with a low ratio of high to low' are easie¡to drive a¡drork well with low-priced receiversoiamplifiers. N€t box cubic volume is also listed. tfyou don,t like the shape of thebox used in the p¡oject, you can use on; of the tables tn Chapie¡ :, oI t¡e computer program for box dimensions in Chapter 7 or in LDp to-design an enclosure of anoth€r shape with the specified cubic volum€.

PROIECT 1:A MtNt-SPEAKER . Low impeda¡ce: 8.8 ohms @ g0o Hz . High impedance: 2Z.Sohms @ 5.S kHz . Box volume: 0.07 cu. ft. (2 liters) The small tull-range Radio Shack speaker(Table10_1)used i¡lthis Droi_ ecl has b€en around for many years. A spealer in such a tiny box iar:t Deexpectedlo reproduce Iow bassor fill á¡ auditorium with sound. Fo¡

- *y rimitedspace €nvironment, it canoffervery

li::11::i?á:.

A cssuálgldncear rhe phorograpba lFigs.g_l and r0_1)mighrload vo\rlo concludetiat the spealeris instslledin a cube_shaped box. Not so. An inspection of Fig. 1O-2wiII show that üe intemai dim€nsions are in the ¡atio of 4:5:6, so no dimensiotr is a multiple of either of üe others. Becausethe box is so small, %-inch (1.3_c;) pl],.wood is ade_ quate.lbrthe sicrewdlls. For the speal€rboard and bacl, t-inch (1.9. cm, plywood or paf icle bodrdis a bettercboice.One unusualleatureis the way üe spealer board is set in the box. The cleats that hold the spaale"rboard a-reset ba_ckonly x inch lo.g5 cm), le ing the board pro_ lrude hom tle hont of the bor. you can usa any k_inch f0.95-;m) rounded molding,as shown in tbe phorographs. tó cover r}e spea_kei board-sraw edge.Nole thal lhe back is setin flush.

Table10-1. Components Usedin proiectt. Componenl R C L S

Descriplion 50 Ohm,1OWatt 814 N.P.tl€c. 0.1mH,20 gá. 4"full range,RadioShackt40-¡ 197

Project2: A Low-CostTwo WavSpeaker

to limsht¿Á fírru¡el0-I PrcFc¡| bo\ in 'on9fuctrcnNotPtheuacol ñoldin+ eáEe, ot plyvood -de. ¿ndprcie.tiñ] 9e¿retbo¿¡d The sDeaker'sperformanceis enhancedby a filter that contols.an jevel in the re8ion n"ar 5 kHz The raw spea-ker excessiveenergy vou "l'"iá¿ "l¡' ñárd until the filtei was added To male the filrer' corc' a 1-inch can either buY a 0.1-mH coil or wind your own lf you use magnel wrre ;-incb wide, try using 20 leet t6 l matets)of 2O-gauge jour homemaoe Even ifvou have no lesl equipment lou ca¡ parallel thP max'ng coil witl ar 8- oi lo-tlF Lapácilorand a 50 ohm resislor' final choice ofcapacitor bY ea¡. tests' The value ofihe resisior, 50 ohms' was dictated by listening in filte$ fl" "ái"nt"t"¿ u"lu" *as low€r, but the discussion of notch ChaDter6 explainswhy such a valuecan be righl can 8do" ih""" tpiul"tt *"." used without a Srille Il desircd lou a n vk i n do t l h i n$ i l l e L l o t h u s j n g a t h i n S r i l l e b o a r d s u c h a s a p ¡ e c e o l %-inch(0.3-cm)ha¡dboard

fwO-wAY SPEAKER PRolEcT2: A LOW-COST . Low imDedance:7.3 ohms @ 200 Hz . Hish im;edance: 35 ohms @ 2 kHz . Boi volume: 0.¡s cu. ft (10 litersl

(1.9)

1 " I i .9)

t* - ri ..1 i--' i . i l l 6',(15.2)

7'117.a)

r

i i i r i i il i!-.1 1 t 4 - U O . 2 |t !

]L

(0.e5)

s

*40-1197

Fígure tO-Z. Const uction plansfor prcject 1.

l':*fi i#iil'*+*t***i:t;ii*dylüi

r*Hfifil*#rflTlilt¡* iütri**!]iii''#:#il**m#l*+

Prciect2; A Low'CostTwo-WavSpeak€r 149 Tablel0-2.

Component

cr LI R1 L2 R2 C2 S1

s2

componenrsUsedin Proiect2.

Descr¡ptior 2.2UF,mylar 0.5 mH,20 ga. L-pad,I-Ohm 2.0 mH, 16 ga.oI better 10Ohm,l0 Watt 6.8pt N.PEl€c. ,/¡"domeMeetet PEl,from MCM, t53-440 61¿" woofer,MCM,551 170

After adding qsphalt lining to the inner wall6, the volume was further - _r€duced. the enclosure(Fig. 10-3)occupiesvery little spaces¡d canbe built tom any Una otz-io'ch (l s cmi material,suchas pl''wood or Pafiicle ¡o"ta. i¡" iote*at constructronis show¡ in tle phologmPhof Fi8 ióa. Aftertiat photowasmade l addeda layerofasphsllroofingmate' ri¡l to the iDlemalwalls gluing il in with construclionadhestve l s .eduiesthe ¡et voLtmeaiit, but filling the box wiü polyesterbatting can make up for any volume loss. After iostalliüg th€ back wiü coü_ stmction adiesive. i caulked üe joints sround the back by working through the woofel cut-out in üe hont pansl' M"oretime v/as sp€nt on crossove¡ tinkering than on any other sugaspedof the designof this proiect.Teslsand lonS-lermIistening-.ror' a two Problems se;t€dthat l¡e id€alcrossovershouldconecl tbr it'*á; *r"4. twical of all small speakersand some peaksin üe r"rooo"". I vi,otii¿ on t¡e pests ñrsl: tryioBvsriouspeákfilters Afr€r a n;mber of testsit becameevidentthat the addsdfilters would make fo. " "o.ol"* ..o".o*'er' Retumingto the goalsof simplicity s¡d low cost, a much easierpath seemedpossible That was lo incrBasethe valueofthe coll in üe low-passñlier to üe woofe¡ With an L-padon tbe üeeler, it pe¡mitteda Soodaudio balanc€ The exha largecboke coil doesdoubieduty: it shápesüe responseof tbe woofer'conecting ior the rising """pott"" ",,-i, ánd alÁo dspressing the peaks at üe uDper -"itt" end of ths woofer'srange. i.o".ou". network iJ a hybrid, part fhst-o¡der (woofer) and second-orde¡(twe€ter) I used a Erylar capacito¡ in üe tweeter leg' but an N.P.electrol]'tic will probably not show any obvjou6 degradationin souDd.For the ¿o acrosstl¡e tweeter circuit, note that 2o-gaugewi¡e is hoaw enough.Tfyou wiDd your own' smallerwire suchas 22 Sauge' l"n ú" u""a]¡o. r¡" toofer óoil.on üe othsrhand.theheavierüe wi re

150

Projects

s¿"(21.6) 7'117.81

-____+1 101"(26)

r_

If--

':1'o-*l

I ..","

' ). -'.-;-l ). r)- ' - ' " . 1;,,';) (

:,

,'-: É

t lo*"ry íu"¿ -N '

- , . _ '@ ¡ ¡l vi t€u8 tn€gr ' ,

3*'18.e)

,l

.-

Box

, ; , , , ' . ' itl , ; 1 'l ",..'--í;-¡:,-,1','.' s¡'1s.e)

I

PEI MCU *5$4

s2 rg gaor beder

10o< R2 f cz

*5s-1170 (17)

plansfot Ptoj*t 2. figüe 10-3. Cons¡ruction

üe better.Id€ally it shouldbe 16 gauge,or better,on a4 air-cor€coil, but 18gaugsis probsblygoodenough. MlNI-SPEAKER PROIICT3: A FORTED . LoveimDedance:6.2 ohms@330Hz . Hkb impeda¡ce:21 obms@88 Hz . Box volume:0.2 cu. ft. [5.7liters)

Project3:A PortedMini-Sf€aker

figurc l0-4. Boxe of P¡oject2 duing @nstructioñ.walls weredanped trygtuine p¡ecesof asphaltroolingon thm with consttuctionadhesive. This prciect uses ¿lrivers of higher cost than those specified for the first two projects (Tablo 10-3). These mini-speakers offer good imaging, probably due to t¡eir narrow boxes, as well as gleat clarity. Several young musicians aided in the final development of this Foject by participatinS in critical listening tests. Lat€r, when some listerers heard Erya's "Watemark" recording on these speak€$, they werc amazed at the full sound coming from such small boxas. The only caveat fu that they can't fill an auditorirm with the sound of a ¡ock band and no one should expect that from 4-inch (1o-cm) v,¡oofe¡s.

Table10-3. Components Usedin Proiccl3. Componenf

Descripl¡on

c1

4 pf, Myla¡ 4 Ohm, l0 Watt LI 0.4mH, 168a. R2 7.5 Ohm, 10 Watt c2 10 pF,N.P.Elec. S1 %" dome tweet€r,Vfa Dl9TD 05 52 4" Kevlarwoofe.,Audax HTI OoKO Driversavailabl€lrcm Meniscusor Parbtxpr6s

R1

:

t

752

Projectj

Small spsake¡ssre best usod on stands€¡d, ifyou wadt optimum imaging and depth of ñeld, thoy should be r^'ell away fiom üe rear wall. However üey can tre used on shelvesif you don't shovethem too close to üe wall and block the po¡t actio¡. A ¡€a¡ port hs! c€Itain advantages(Fig. 10-5). One vi¡tue is that the additio¡ of a cosmetic grille has ao effect on po¡t action. Thi6 catr be a p¡oblem fo¡ pofis installed on the speale¡ boa¡d. Even minor port blockegecan altor üe Q of the port, li¡¡itin8 its dampiüg on th6 woof$, Anoüer advsntag€of a rear po¡t is that a¡y midrange sound üat oscapesis lo6s likely to bs noticedby listene¡s.

f-7"(f7.8)-1

Noich

r- 8¡,,er.o)_--¡ l l ( 0 . 6 4 x 2 . s 4 )I r-/'(r7.8)--l I

I r- 5¡"(1a)-t| 1""t" ll

j-

,.=,=.=i I

iil

+ t ' 1 1 2 4i.36" (3.4)

__-T

.J

ii'i i.i.,i

f-

II

-1 7" 117.8'

ct

F1

i

*i"

I

tu*r.*3*¡ ¡'1r'8)

J

10pl =

Grill€llam€ dn trcrh plywood figue taLs,

Constuction plans fo¡ tuojed 3.

-

P r o l e clt: A P o n e dM i ¡ i S p e a k e r

',,ouo,o '', i':;';"',:;';,,,ili:ifri:.::'":,í::':'tr:,::; :::1;:;tr"i!,,!!;*,

'!:::ii::i::;::;'Jiír:':"::;,::!:?::;.!:f;f::::,::l: Figúe t9-7.

frcnt iteft) an(t rcar rte||s oi an attenúte rrpe af t)on íat p.ajeú 3 The

154 P.ojects Th€ port tr¡b€susedin üis p¡ojectweremadeto be üsedaselectical co¡duit tubjog.Sucbtubjngis lesssxpensivethanmostothert,?€s. bul bewareof dimensionslvariations.Althougbüe tubeswereplain)y marked "1%," when measued by a vemier caliper, üe diame¿r was 34.4mm, or about1.35inches.ü you build this p¡ojectand use t¡üe r.2s-inchlubing,try cuttin8r¡e tonSthbackto abour4.2Sinches. Tbe $ossovornetworkis a simple frrst-orderoetwork.The capac¡to¡ i¡ the tweeter leg is üe value you Eight exp€ctto use for a 36O¡-Hz crossover,but üe coil in the woofer ñlter night be chosenfor a lower hequency.Witb the impeda-nce equaüzerin the circuit, ths woofer.s 'mpedánceisapproximately6.5 ohms.At üat valu€.üe coi¡ suggested herc is, by fo|mula, riSht fo¡ about 2600Hz. A8aid, listening tesi;often dictate a value diffe¡eút from üat suggestedby formulas. A mylar capacitor is ¡ecomr¡ended fo¡ the tweeto¡ ñlter, and 16-gaugewire for the woofer coil. The resistor in the tweete¡ ci¡cuit can be replácedbv an L-padif you wa¡t a more flexiblecontrol on t¡e twester.I¡ that iase yoü might ñnd it necessaryto increasetho value of the capacitor in the tweelercircuil Fom 4 to 4.7 uF. - Noticein the photoofthe frnishedproject(Fig.10-6)that the g¡ille board is iDstalled by dúlling holes in the ñont edéeof the top anJbottom paneiand installingshortpiecesof dowel at üe top andbottomof the gdlle ftame to ñt üe holes. To makesu¡e the location of the dowels is ¡ight to match that of the holss, tempora¡ily nail the uncoveredgille frsme to the ftont of th€ enclosure and drill holes top and bottom throuShthe grille ftame ¡nro üe pa¡€l ed8es.piacesof t-in f0.64cm) I¡ble 10-4.

Component

cl LI

R1 R2

c2 L2

c3 R3 c4

pcofipoJlents

Used in

Descr¡pt¡or 4.0It Mylar 1.0mH,20ga. L-pad,I Ohm l0 ohm,5 wan lpF 1.0mH,16ga. 4.7yF I Ohm,10Wan 10pt N.PE¡ec. VifaD26fC-Os

52 wooleravailable fromMeniscus, olhe. €omponents fromMeniscus or hrts Express.

P¡oject3rA PorredM¡ni-Speakef r5j

cl_owel ffea suitable siz€ for üis job. Use dowel lengt-hsof ,1i¡ f1.9 cmJ ano dnrr the eclges.oftlle pa¡els to a depth ofjust over , in (t 27 cmJ. . As a nnaj touch, cove¡ üe bont of üe speal.erboard \a,irhfe¡r. ti.s also a _Boodidea to cove¡ tho intemal edge;f the grille ftan; ;iü-; stlip oIopen-Lell foam weat}lerstripping mateci¿llo prevenl rsflections oac1.r: rrr: remove rhp g¡it¡F framp thite list€ning. Írilers.put 9r.just rr you plan to tbesesmallspealerson a shell closeto a ¡ei sur, . race.you mr8ht .onside¡moving tle port fiom thp redr to the front.To oo úat. you can mále a ¡eclangular port at the bottom of the froot panet. You cán uso LDp to design "uch a porl tuned to ábout S5 Hz. One way rs ro use üre construction meüod shown in Fig. 10-7. Tty using '-in.11.9,cm) rails undF¡th€ pa_flition. a duct abourz inchesls.bBcmi maxrn8tne port arc€ . inch by 2 inches,or L9 by 5.08 cenrimet"r".it urar ts loo crowded lor (he drivers. you can male the slot oven nar-

¡--t+.l'ieoz1---

¡ |.-rzi"oz.rr1

-r

I I

I

9 É

a t

ei

at'ú al --l rd' (25'4) l'-

I é

1 tt gt

plyrcod 3"(1cm)rhick 2"

8n 10¡t

Íigurc 1O-8. Ptansíat proFc¡ 4

-

case'the rower. such as I inch (1.27cm) by 2%inches (7 3 cm) ln €ithe¡ at i""niii "iift" ¿""t "t'"tld be about 5 inches Note that' theo¡etically slightly to i"."? "t tf" slot gets narrower, the area must be ¡edüced of duct' length the same with fiequency -ui"t"i" ttt" *-Jt""ing PROIECT4: ANOTHTR PORTEDTWO'WAY SPEAKER . Low impedance:6 4 ohms @ 230 Hz . HiPh impedance:20 ohms @ 74 Hz . Boi volume: 0.6a cu ft. (18lite¡s) wooferand 3 Yearsago,mosl 'ompd' t \péaker'I onsisledof dn 8-inch Now the mosl populararrangPmenJ.seems tweeter'ina two-nav_.ystPm wiih a small dome tweeter' Using a (ri woofer ál ur-i""}' io L" " protects ,.ult"t ,"oof". p"t-its a higher crossoverfrequency' which ranSe the lwpeierfro; perlormingal d freouenL)belo\"/ils oplimum ol lhe srze This sv"lemrpod; an P\r eplion¿ll)robustbdcsre\pon\aior the wóofer. üe For this oioi"cl, Ihe ' hoi.e was a $ell knoltn Vila woofet j viia rw".r.t rhc D¿61G-05{TableI0-a) The specificaPr7i.v;;;; o'35' and a tions Íár the woofer show bassresonanceat 37 Hz' a Qrs of v . . " i r . z t " " . h . ( J 4 7 l i t e r s lA r L ¡ n l h f o u g h L D P s h o t { e d á n o p l i m u m u o i u m e o { O g , r , h ( 2 55 l i l p r \ ) .r u n e dt o 4 0 H z T h i s s y s l e ms h o u l o

\h¿pP ¡ hc. / 1/FOY ÍiEurc lT-c. P'oi t a 'PFJkar' (ub| is nat ¿tLcretl eatúne tt be u"ed si¿pes Gn

Etia oí LDP.Altenate

152 Prciect4:Another Po¡tedTwo'WavSpeaker

i'x.: ift::l:i:';;T1;'' ilTtri", :1il"":l;:ixil:i::"".11í:í il,r,,:l$rii*',:lill:t"i"l;m;x""T:"^::; '"i;i'il -rhis\ asdonebur'aherlinit,;;:.;; ;"t cu rr'rFis ro-8)

ii*:,'*;:t*n:*:1""t*:t'ru:1il:i;:;', " tiilXlt;.".t".

network'verv "",-ork forP¡oiect4 is a secon¿l-order ibook'íde"ignto¡ a-ohmdriversat 2400Hz' "rrni*"," " iiJ*tirorey """ii üe -'ata''je reflectionsto üe woofercone'I filled th€ "ir-*"* of Makesüreth€ toP "o"* ¡"fti"J t¡" .."fer with Acousta-stuf

wlil,;:1',:1"ffi^":""i;; ú,epoñopFnins áá-pr'e;"L¡¡" u"row

"r ""4 i"'i"i"¡¿" i*"""t1"J"""1t'l'ffJ:l""iffiH:,1'*:s;;:i ü;*ln*;"'JH":l$::i:l';1:J#:J""xll"'e^

y;"*il{ii*,'*,i:','"'ilm::'H'il^:;::

,il";n:i:'i''J,::1"H# í":::.1r*iÍ',:*liT:ixTffi hoiect 3.