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A R E S E V E R A L WAYS TO C A P T U R E

ur position. However, that'sf surveillan ce, or when

Another op tion is to use a sensitive,mil ar to

icrophones typically have par-l de-ent appro ach,

The major criteria that determine

(bandwidth). Just as frequencyse and directional sensitivity in

na type, norm ally used in

cle is designed u singples which cou ld, in-be applied w ith equal

A very helpful concept in eitheric or electron~ agne tic esign is

r of wave-propagated energy that

Consider for a moment; don't boththe sa me pheno men a of reflec-

DALE B. BLACKWELL

A novelsuper-directionalmicrophone thatoutperforms manycostly commercialdesigns.

like fashion, acoustic energy also ex-hibits the same phenomena. Just asantennas are elecflw?zugtlc~ticenses,so too are microphones and loud-speakers acoustic lenses.Not only are microphones andloudspeakers acoustic transducers orlenses, b ut also acoustic filters. Just asall filters have frequency and phaseresponse, so too do microphones andloudspeakers. However, here, as withantenn as, two types of filtering occur:directional and frequency.Another term for directional sen-sitivity is directivity, often a de sirabletrait, since it prevents spuriou s soundfrom entering from undesired direc-tions. A microphone with uniform di-rectivity is termed om nidirection al;however, f lat direct ional responsedoesn ' t imply f la t f requency re-sponse. A microphone can either havea flat response over the audio spec-trum (20 Hz-20 kH z), or be tailoredfor greater sensitivity over specific au-dio bands. The acoustic horn pre-sented here has very high directivityover the entire audio spectrum.The last property m icrophones andspeakers have in common i s re-

ciprocity, which lets a microphonework equally well as a loud speak er ofidentical d esig n, both directionallyand in frequency response; this prop-erty also holds true for antennas.Different microphone typesMost microphones are omnidirec-tional. as shown in Fig. 1. Figure 1-0shows the basic shape of an om-nidirectional microphone with themain axis, while Fig. 1-0 shows alinear polar plot of relative sensitivityP(8) (dyneslcm" as a function of an-gle 8 about the main axis; all curvesare normalized to 1 at the peak of themain beam. The main beam can be atany angle, although it's normally de-picted at 0'. If several people sitaround a table, an omnidirectionalmicrophone at the center will pickthem all up equally well. Any planethat passes through the main axis willexhibit this sensitivity response.The second most comrnon micro-phone type is the cardioid, shown inFig. 2- a, wh ich has greater directivitytoward the front over most of the audio r;range. The sensitivity pattern shown $in Fig. 2-0 looks like the mathemati- $?

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from the audience. The power f i n -tion is of the form:

phone, shown in (a), has uniform direc-tional sensitivity to sound pressure P(O);the main axis i s out the indicated. In (b) isshown a polar plot of directional sen-sitivity; the response is identical in anyplane through the main axis.

FIG. 2-A CARDlOlD MICROPHONE,shown in (a), has greater sensitivity fromthe front'than the rear. The 0" and 180"directions are along its main axis, point-ing through the main face. The soundpressure sensi tiv ity P(H) shown in (b) wastaken through a plane normal to the mainaxis.cal curve called a cardioid (heart-sha ped ), hence the nam e. An or-hestra in a night club niight use acardioid microphone so that onlytheir music is picked up, not 5our.d~

At 0 = 0 , the sensitivity is max-imized . The sensitivity goes to zero (anull) at H = 180'.The r ibbon element ~nicrophoneshown in Fig. 3-LI s the industry stan-dard, well-known from all the photosof radio stars in front of them. It'ssensitive from both front and rear,producing the figure-8 pattern shownin Fig. 3-b. A microphone that picksup equally well in opposite directionsis advantageous in a talk show wherethe guest sits opposite the host.

shown in (a), is uniformly sensitive tosound from front and rear, but less sen-sitive from the side; the main axis is thesame as that for a cardioid microphone.Note, however, that P(H) in (b) has twolobes, not one, with two maxima and twominims (zeros, or nulls).Increasing d irectivityExperimenting with basic micro-phone directivity patterns yield morespecialized designs that are muchInore sensitive from the front. Figure4-a shows a parabolic reflector m icro-

200 Hz 1 kH z 8 kHz

reflector microphone increases with fre-quency. In (a), the incident parallel raysconverge to the microphone at the focalpoint. In (b ) re shown linear polar plo ts ofacoustic power at four frequencies.

pho ne; all parallel rays, w herever theystrike the curve, are reflected to thefocal point, where the microphone islocated. Parabol ic m icrophones arealso especially directive at higher au-dio frequencies, as shown in the sen-sitivity patterns of Fig. 4 4 .As shown in Fig. 5 - ~ 1 , he line(shotg un) microphone i s anothercommercial directive version, albeitnot quite as focused as a parabolicreflector. he line microphone has ei-ther a single long tube with spacedopen ings, or several tubes of increas-ing length, in front of the microphoneelement. The sensitivity patterns inFig. 5-0 a r m ' t for differing frequen-cies, but for different tube lengths,being integral multiples of hl2, or halfa wavelength.

becomes more directive as the length ofits tubes increase. It's not as directive as aparabolic reflector, and either has onelong tube with spaced openings, or sev-eral tubes of increasing length each withone opening, right i n front of the di-aphragm.

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Both the reflector and line micro-are directive. but neither com -6. Figure 6-crs the geometry of the basic horn

6-6 shows the

All microp hone s, of whatever type,The narrow beam

6-HORN MICROPHONESARE VERYdiaphragm to open air. In (a) are the

phasing and pressurethan sides or rear. In (b)areots for 1 , 4 , and 7 kHz.

Th e high directivity of all horn mi-

r& iv it One reason for the high

X = Cif, where AC is speed of(34 0 rn/s), and is frequency inz.Since 1 ft = 30.48 cm , then from 20

relative to wavelength, directivity in-creases, since audio wavelength i s com-parable to mouth size. Shown aredirectional patterns of decreasing beam-width, for four horn diameters relative toA.

Hz to a few hundred Hz, the wave-l e n g t h s a r e o v e r a f o o t . A t,f= 1.115483 kH z, then A = 1 ft , so the1-foot diameter horn presented hereshould be quite directive at that fre-quency. Figure 7 shows additional di-rectivity patterns, but not fo r explicitfrequencies. Note that those patternsare for various mouth sizes relative towavelength. As the ratio of mouthsize to wavelength increases, so doesdirectivity. Another way to achievehigher directivity is to increase hornlength for a given mouth si2.e. Asshown in Fig. 8, to achieve this, thehorn angle a must be reduced.

rectivity in a horn microphone is to in-crease length versus mouth size, requir-ing that horn angle u be reduced.

Horns of different shapes are com-monly used as loudspeakers, with theexponential, hyperbolic, and conicalversions the most common, in thatorder. Horns are uniquely able totransform and match acoustic imped-ances. The horn loudspeaker is anacoustic transformer, changing largepressures and small volun~e urrentsin the throat to small pressures andlarge volume currents in its mouth;horn microphones do the reverse.As shown in Fig. 9 , the conicalhorn has a gradual impedance-trans-

formation curve as cutoff frequency isauuroached. with a smo oth transition1from a high-directivity pattern to oneof lower directivity. Such smoothtransitions are m ore desirable than theabrupt low-frequency cutoff of bothexponential and hyperbolic horns.In the horn of Fig. 6-0, the transi-tion from square horn to receptor issmoothed into a cone using modelingclay. At the higher audio freq uencie s,the conical walls reflect the shortwavelengths (a few inches or less)down to the microphone diaphragm,helping to optimize high-end audiodirectivity for a narrower beamw idth.

FIG. 9-RELATIVE ACOUSTIC resistancefor several horn microphones of size andbandwidth similar to Fig. 6. Each worksjust as well as a loudspeaker by re-ciprocity, with the exponential, hyper-bolic, and conical the most common.ConstructionThe horn presented here can bemade using low-cost materials and alittle time. Because sound pressurewaves exert low force, light-weightmaterials can be used. Figure 10shows the prototype, made from cor-OPFN MOUTH BASIC

FIG. 10-THE PROTOTYPE HORN WASmade from corrugated cardboard; a re-movable extension with larger mouth anda carrying handle was added. At high au-dio frequencies, the walls reflect shortv~avelengths f a few inches or less to thediaphragm, to optimize directivity.

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side of the basic horn; note the directionof the corrugated ribs. The edges haveslight curvature so the sides have addedstrength, and don't resonate easily. Theedges were taped, and paper glue wasused on the inner and outer corners. Thesmall end was cu tto al -inch diameter, andthe microphone slides in and is held bythe four sides. A metal washer slippedinto the throat face acts as a stop, yet letsthe sound reach the diaphragm.

FIG. 12-WHEN MOUNTING THE micro-phone in the horn, the washer apertureshould be at least 75% of the diaphragmdiameter. The modeling clay smoothedthe transition from the square horn to thewasher opening, so that the s ~ u n dasn'trestricted from reaching the diaphragm.

rugated cardboard, cut to the correctsize and glued together, with a carry-ing handle added. The horn was con-structed, asse mbled , and tested; then ,a removable extension was added togauge the benefits of a larger mouth.The basic horn was built with foursides from the pattern in Fig. 11. Theedges have slight curvature for addi-tional strength, so they won't resonateeasily. The edges were taped enoughto hold them in place, and simplewhite paper glue was applied to boththe outside and inside corners. Thesmall end was cut to a 1-inch diameter,letting the m icrophone slide in and beheld by the four cardboard sides. Ametal washer slipped into the throatagainst the microphone face acts as aposition stop, while letting the soundeach the diaphragm.As Fig. 12 shows, modeling claysmoothed the t rans i t ion f rom the

B A S E OF MICROPHONE ELECTRICAL TAPEFIG. 13-A CLOSE-UP VIEW OF THE EXTERIOR of the neck. The carboard is tapered,producing an opening of proper size for the microphone, and the microphone i s nserted.Note the silvery r ing at the base of the horn, just behind the base of the horn. The bottomof the microphone protrudes from the base of the horn, and was sealed mechanically andacoustically with duct tape, while the base of the horn was stiffened with e lectrical tape.square horn to the round washer open-ing, so the sound wasn't preventedfrom reaching the diaphragm. Thewasher needs an ope ning at least 75%of the microphone diameter. Figure 13shows a close-u p view of the exteriorof the neck of the horn. You can seehow the cardboard is tapered to pro-duce an op ening of the proper size forthe microphone, and how the micro-phone is inserted.Note the silvery ring at the base ofthe horn in Fig. 13, just in front ofwhere the microphone apparent lyends. The base of the microphoneprotrudes from the base of the horn,and is sealed mechanically and acous-tically with duct tape. The extensionin Figs. 14 and 15 slips over the frontof the basic ho rn, to extend the lengthand expand the mouth, and two 114-20screws with washers hold both sec-tions together.By adding th e extension, the mouthwas increased in size from 1x 1 ft to2 x 2 ft, quadrupling the area. Also,the new size is one wavelength acrossat f=557.742 Hz, matching wave-lengths dow n to lower audio frequen-cies and increasing directivity beyondthat of the basic horn alone. Thelarger diame ter and greater total areaimproves pick-up, raising the the-oretical pressure level by 3 dB. inpractice, the horn picks up more atlower frequencies because the impe d-ance m atching at those frequencies isimproved.

ribs stiffen the cardboard. The extensionslips over the horn, extending its lengthand expanding its mouth, and two 114-20screws with washers hold both together.The mouth is now 2 x 2 ft, one wavelengthacross at f=557.742 Hz, matching wave-lengths down t o lower audio frequencies,improving directivity, raising the pressurelevel by 3 dB, and providing better low-frequency pick-up, since impedancematching is improved.TestingThe preliminary tests were con-ducted at a large parking lot at a localbeach. in actual use, aim the horn inthe direction of the desired sou nd, andplug the microphone into a tape re-corder, allowing playback later on . Inevaluating the prototype, all testswere recorded to allow detailed soundpressure evaluation of an individual(Continued on page 5 2 )

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FIG. 6-INTERFACING THE HD44780-BASEDLCD module to a microprocessor requiressome additional logic.

temperatures throughout your house with your own custom messages.pability of the port pins of the micro-controller, the LCD is powered di-r e c t l y f r o m a p o r t p i n o f t h emicrocontroller. That allows the co n-venient feature of letting the micro-controller power down the displaywhen it's not needed in order to con-serve battery power. It should benoted that the entire design u ses fewerthan thirty interconnecting wires.

As a su ggestion for your own proj-ect using an LCD module, why don'tyou try to build a multi-zone ther-mometer that displays temperaturesthroughout your house with simple,non-cryptic messages. For example,you could display "THE TEM PERA-TURE IN STEVE 'S ROOM IS 72"."A block diagram of such a project isshown in Fig. 7 . R-E

speaking, in a normal voice, 100 ftfrom the mouth. Th e resultant record-ing was quite intelligible even aboveseagu l ls squaw ki ng o ve rhead , t hesurf, and car noises 500 ft away.The higher audio frequencies sonecessary for speech intelligibilitytend to be very directive. Noticeableroll-off occurred 5" away from themain axis of the horn; in fact, speechwasn't understandable when the hornmicrophone wasn't pointed directly atsomeone. Bevond 10-15" off-axis. avoice vanished com pletely into back-ground noise. However, seagulls andbirds 75-100 ft away sounded likethey were 2 ft in front of a regularmicrophone

BASICHORN MICROPHONE HANDLE

EXTENSION

FIG. 15-HERE'S THE COMPL ETEDHORN MICROPHONE. At the top is thereceptor microphone, then comes thebasic horn, and lastly, the horn extensionis shown with its support ribs.

Surprisingly, the extension didn'treally improve directivity, and appar-ently wasn't worth the effort, giventhe time and effort neede d, as well asits size. Frequency res pon se tests withpolar pattern measurements would beneeded for verification of this, and tooptimize the extension performance.However, recording bird calls and ani-mal sounds is a perfect application forthis horn, since both the horn andextension are small enough for fielduse, and give excellent performanceover the full audio range. R-E