TECHNICAL NOTES

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

No. 629

A SOUND PRESSUBE-LEVEL I;:ETER WITHOUT AMPLIFICATION

By E . Z. StorTell Langley Memorial Aeronautical Laboratory

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IVa shi n ES t on Dec e rll b e r 1 9 3 7

https://ntrs.nasa.gov/search.jsp?R=19930081445 2020-03-05T08:25:27+00:00Z

NATIONAL ADVISORY CO MM I TTEE FOR AERONAUTICS

TECHN ICAL NOTE NO . 629

A SOUND PRESSURE-LEVE L METER WI THOUT AMPLIFICATION

By E. Z. Stowell

The N . A.C.A . h~s developed a simple pressure-leve l meter for the neasurement of sound- pressure levels above 70 db . The instrum ent emp loys a carbon micro~hone but has no amplification . The source of powe r is f iv e flash­light batter i es . . '1e~surements may be made up to the threshold of foeling wi th an ac curacy of ±2 db; band anal y ­sis of complox spect r~ m~y be made if des ired .

GEr ERAL DESCRIPTIOl

For 3 ye~rs the ~ . A. C.A. has been using a very simple in st ru~ent for me asuring sound pressureg above 70 db . The ci r cuit diagran of the p ressure - level Qetor is shown in f i gu~e 1 . The carbon mic~ophone , be in g of the double ­button type, requires a split reactor L, wh ich i n this cas o consists of the pri r.l ar;:.- of an old 1 icrophone trans­forner uith the secondary left open . The filte r imme d i ate ­ly ~~jacent to the microp hone co mpr ises three series in­ductors of 0 . 0015 henry each and two pa r al lel condens e rs of 0 . 02 mi cro farad each . This filter pre v en ts stick i ng of th e c~rbon g r anu l es to one another uhen the off- on swit c h S is o~erated . The ~ illia~Dete r reads the direct current to both buttons and h~s a range of 100 mill ampe r es . Th e transformer T se rves to ma tch the attenuator i mped­an c e of 200 ohms to the rectif i e r impedance of 3 , 500 ohms . Th e final outyut meter is a v ar y sensitive micro~mme ter .

?igure 2 shows the instrument opened to make observa­t i ons of total sound ; tho n icr ophone is soon hung in tho en clo sure at the end of tho box tOTIard the reader . The cord s in the lid are for ensy connect i on to the filters as shoun i n fi ~ure 3 . The pa_c l contains the s~ itch S, tho mill i ~rnmetor l , tho attonu~to r, and the output moter .

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2 N .A. C.A. Technic~l Noto No . 629

THE MI CROPHONE

The micr op hone is of the douole-outton ' tK type made b y the Universal Microphon e Company with a stretched dia­phragm g ol d- p late d at th~ ' carDon cont~ct . The sensitiv­ity is a b out - 30 do when suppl i ed with 45 milliamperes or more d irec t current . This comparatively large output is p artly re spons iole f o r the aOsence of amplification . The variation of s e nsitivity with current through the out to ns is sho.n in f igure 4 ; in pract ice, the current is simply kept in excess of 50 milliamperes and no corrections are applied . The 7 - 1/2 volt flashlight battery, which is th e only po er sourc e requir ed , is replaced when the cur­rent falls oelow this value .

THE ATTENUATOR

The attonuator is of Ge neral Radio design consisting of a T- secti on g ivi ng 45 do attenuation in steps of 3/ 4 do wi th a p ract ic a l l y constant impe dance of 200 ohms. The insertion loss is 6 db . Eeyond 45 db the attenuat ion in­cre~~ e s very rap i dly to in f inity .

THE OUTPUT .tfETER

The output meter is the model JS microammet er supplied oy the Sensit i v e Research In s trument Corporation . The re ­sistance of the micr oammoter is 3,500 ohms, which gives an exc e llent Batch for the copper - oxide rectifier . The o rig­inal scale of the instrument has oeen r e moved and a dire~ t­Tead ing dec i oel scale suost itut e d. The calioration to de­ter mine po ints on th e s cale was e ffe cted oy noting readings on the orig inal scale with known frequency- modulated sound p r essu r es impres sod. u-pon th e c[1.,roon microphon e , th e [1.,ttenu­ator ' oe in g kept a t zero . Th e new scale was then suppli ed oy the Da ke rs of th e instrumen t . Th e dat[1., necessary to 00-tain the now s cale nre shown in f igure 5 . A 12 do ran ge , from 70 to 82 do , i s ma r ke d unon the n e~ scale. The actua l soun d p ressure mensured wi th ~ he i ns t~ument is then the sum of th e scale ead ing ~nd the a tt enuator, g iving a total r nnge of from 70 to 1 27 d o .

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r-N.A . C. A. Technical Note No . 629 3

OVER -ALL CHARACTERISTICS OF THE I NSTRUMENT

The over - al l f r equen cy charact er is t ics of the pres­sure - l e v el me t e r are shown i n f i gure 6 . This curv e re p r e ­sents the moan of many checks a ga i ns t a mov in g - c o il micro ­phon e ove r a lo ng ge riod . DisBg r eem~nts with tho labo ra­tory i ntensity- level mete r , 'h~n used in free space , amount to 2 db on th o ave r age . The day-to - day accuracy in r epeat ­ing r eadings is a l so of th i s a rd o r . Mo r e accu racy is ~ot r equired for loudne s s - l e vel co mputat ion s i n ce the uncer­tainti es in tho Fletcho r- Muns6n fo r mula a r e about 2" db .

The over -all range of p r essures cove r e d has b oe n s een to extend fro m sl i ghtly hi g her than conversational l eve l (70 do) to the uppe r t h r eshold (1 2 7 do ), whe re the zer o of th o de cioel s c ~ l e is 0 . 0002 dyne /c m2 •

Th e f ilt e rs used are those r egularly employed in rou­tin e me asur ements of n o ise from p r opel l er s on a test stand , and c o ver the r anges 0 - 1 00 , 1 00 - 500, 500-1, 000 , 1 , 000- 5 , 000, and the r ange above 5 , 000 cycl es pe r second . The ir i mped­ance is 3 , 000 ohms, and the i nsertion l oss is 1 . 5 do .

The we i gh t of the ~ r escur e - l e v o l me t e r is 20 pounds and of the filters , 35 pounds .

I t is essen tial that he i nstr u men t De in a f i xed uo ­s i t i on and leve l to wi t hin aoout 3 0 i n o rde r that the n~ o ­dI e of the output me ter ma y sw i ng f r ee l y and ao out it s nor­mal ze r o . A slant of th i s amount i o r ead ily dete ctao l e oy th e eye so tha t no espec i a l d i ff iculty r esults from this featur e .

The follo~ i ng p roc edur e is car ri e d out i n making a measure~en t : (a) I ns trnmen t i~ leveled ; (0 ) micro phone d oo r opened ; (c) at t enuBto r set at i nf i n it y ; (d) needle of output ~e t e r r e l ease and adjusted t o zero ; ( e) current to mi cr ophone turned on ; (f) at t enuation slowly de cr ea sed until need l e roads on scal e , and attonuato r and s cale read­ing noted ; a nd" (g) at t enuato r r esot at in f inity and cu r­r ent turn ed off .

THE I NSTRU.ErT IN USE

Th e p res sur e -l e vel me t er i s we ll adapted to th e mea s­ur ement of hi g h p r essu r e levels , the portab ility of the in -

4 N . A . C. A . Techn i ~al Note No . 629

stru~ent having proved to be one of the important features . Large changes in le ve l, such as result from maj·or altera­t ion s to the sou rc e of sound over a long per i od of t i me , are readily detectable wi th an accuracy or ± 2 db; this aC ­cu r acy is ' suffic i ent to permit loudnes~ - leve l computations by the Fle tc he r- Mun so n fo r mul a whenever the app ro p riate band analys i s i s 9arr i ed out . Smaller changes in level a r e , of cour se , detectab l e when the changes in the sour c e c an be more quick l y made .

Langley 1emor i a l A e r onaut ic ~l Laboratory, National Ad vi so r y Co mm i ttee fo r Ae ron aut ics,

· Lang ley F i e l d , Va ., Novembe r 11 , 19 37 .

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