Audible Alarm BasicsEverything you wanted to know, but were afraid to askby Dan O’Brien, Sales Engineer, Mallory Sonalert Products, Inc.Fromsmoke detectors to automobiles,audible alarms (also known to be calledbuzzers) have become a part of oureveryday life. Some of the uses of thesealarms are innocuous such as in amicrowave oven. However, in someapplications such as in a smoke detectoror medical equipment, a person’s lifemay depend upon the audible warningsound. In all cases, the equipmentdesigner should consider the desiredcharacteristics of the audible alarm at theinitial design planning phase to obtainsatisfactory performance and avoidcostly redesign. The first characteristicfor a designer to consider is the type ofsound such as a continuous, intermittent,or specialty sound. Other critical criteriainclude sound level, frequency, currentdraw, quality, mounting configuration,cost, and availability.Electromagnetic vs PiezoelectricTwo main types of audible alarms arethe electromagnetic type and thepiezoelectric type. Figure I shows asummary of each type along with thedifferent packages s of a plastic housing with a metaldiaphragm that is flexed by an electromagnet. The main benefit of this designis the ability to obtain low frequencies ina small package. Drawbacks includemechanical wear, electrical noise, lowersound output, and large powerconsumption.The piezoelectric type avoids electromechanical contacts by using apiezoelectric transducer. The transducerconsists of a polarized ceramic materialbonded to a metal disc. A voltageapplied to the transducer will cause it toflex. A voltage that is applied andreleased at an appropriate rate orfrequency will produce a continuousaudible sound. Piezoelectric audiblealarms offer no mechanical wear, anabsence of electrical noise, low powerconsumption, and a clear penetratingsound. Advances in transducer designhave allowed these devices to shrink insize and lower the sound frequencyenabling them to move into applicationstraditionally serviced by electromagneticdevices.
Indicator vs. TransducerPiezoelectricaudiblealarmsareavailable in two packages: with a drivingcircuit (indicator) and without a drivingcircuit (transducer). Transducer devicesdo not have a circuit included, so theuser must supply the drive signal to thedevice to generate the audible sound.The drive signal is typically a sine orsquare wave applied at the ratedfrequency of the particular transducermodel. A transducer device has twopackage options: self-drive (withfeedback) and external-drive (nofeedback). An external-drive packagewill have only two external terminals onthe device. The user must provide a fullsquare or sine wave to the device togenerate an audible sound. A self-drivepackage option has a third terminal thatprovides a feedback signal that is 180degrees out of phase with the drivesignal. A simple oscillation circuit canbe used with the self-drive package thatgreatly reduces the component count(and cost). Schematics of electroniccircuits that will provide drive signalsfor both the external-drive and self-drivetypes of transducer devices are availablefrom the manufacturer of these devices.Piezoelectricaudiblealarms that include adriving circuit are calledindicators.For thesedevices, the user needsonly to supply anappropriate voltage toproducetheaudiblesound. Indicator modelsare available that operateover various voltageranges from 1 to 250 voltsAC or DC. Advantagesto the designer ofindicator audible alarms include ease ofuse and the confidence that the designhas been thoroughly tested over theentire operating temperature range andenvironmental conditions.While a transducer type audible alarmwill have a lower part cost since noelectronic circuitry is included, the userwill incur significant circuit design timeand testing costs to ensure the operatingreliability of their circuit design over theentiretemperaturerangeandenvironmental conditions.If theapplication using the audible alarm hassignificant sales volume and the designand testing resources are available, thenthe cost savings of using a transducertype audible alarm may be justified.Most designers will find, however, thatthe value added by an indicator modelwill more than justify its higher cost.Sound LevelAfter deciding upon the type of soundneeded such as a continuous tone,intermittent tone, or specialty sound, thedesigner must choose the sound leveldesired. The sound level is the size oramplitude of the air pressure fluctuationsproduced by the audibleDecibelsalarm. A useful scale for this130Threshold of Painpurpose is the decibel or dB120Rock Concertscale.This relative scale110Jet Take-off100ranges from the threshold of90Heavy Truckhearing (0 dB) to the80threshold of pain (130 dB).70Loud SingingFor an audible alarm to be60Conversationuseful, it must be at least 105040dB above the ambient30Librarybackground noise in order to20Quiet Whisperbe easily heard. As Figure II10shows,theambient0Hearing Thresholdbackground noise can varyFigure IIwidely from 30 dB at a library
to 90 dB near a heavy truck. If adesigner is not able to determine theambient background noise where theaudible alarm will be used, then thedesigner must rely on the experience ofthe audible alarm supplier to determinethe needed sound level for the audiblewarning device. As a general rule ofthumb, the following suggestions can bea starting point:Application: Outside/Heavy IndustrialUse: Loud/Extra Loud Audible AlarmApplication: Light IndustrialUse: Medium Level Audible AlarmApplication: Office/ElevatorUse: Soft Level Audible AlarmWhen comparing audible alarm soundlevelspecificationsfromonemanufacture to another, it is importantfor the designer to note whether or notthe sound level specifications are listedas minimums or typical readings. Somebuzzer manufacturers will strictly ratetheir parts to a guaranteed minimum dBlevel while others will be much looser intheir promises and show typical values.Another key to comparing sound levelreadings is to note the distance the soundlevel reading is specified at. Sound levelwill fall off 6 dB every time the distancetraveled doubles, so the distance atwhich the measurement is made iscritical. Unfortunately, there are no setdistance standards for the industry, butan informal standard for the NorthAmerican industry is a 2 footmeasurement distance. Other commondistances for sound level readings are10cm, 30cm, and 100cm. A chart inAppendix I is attached to help convertsound level readings from differentdistances.Finally, it must be noted that audiblealarm providers must associate the rateddB level to an input voltage. Forexample, an indicator could have thefollowing sound level specifications:70 dB min. @ 2 ft. @ 6 Vdc.85 dB min. @ 2 ft. @ 16 Vdc.The first line above reads 70 decibelsminimum at 2 feet at 6 Volts DC. Asthis example shows, the sound levelincreases from 70 dB to 85 dB as thevoltage varies from 6 Vdc to 16 Vdc.This voltage versus sound level effect isrelated to the operation of thepiezoelectric transducer.A largervoltage across the transducer willproduce a greater physical deflectionthat will be heard as a louder sound.A word must also be said about themethod used by manufacturers to ratethe dB level of their audible alarms. Toavoid sound reflections and outsidenoise, and anechoic sound chambershould be used. This anechoic chamberis typically a box or a room lined withsound dampening material to ensure thatthe sound level readings are untainted.Not all manufacturers use anechoicchambers when rating their buzzers, socare must be taken when comparing requency RatingThe next important characteristic toconsider for an audible alarm is the ratedfrequency (also known musically aspitch).The number of pressurevariations per second is the frequency ofthe sound, and it is measured in Hertz(Hz). The rumble of distant thunder has
a low frequency while a whistle has arelatively high frequency. The normalhearing rage for a healthy young personis 20 Hz to 20,000 Hz (20 kHz), but as aperson ages, they tend to lose their highfrequency hearing capability.A typical range of frequencies foraudible alarms is as follows:Low Frequency: 1 to 2 kHzMedium Frequency: 2 to 3.5 kHzHigh Frequency: 3.5 to 4.5 kHzWhile a piano will range from 27.5 Hzto 4,186 Hz, the human ear is mostsensitive to sounds between 2 kHz and 5kHz. Experiments have been run tounderstand how the sound level heard bythe human ear is affected by frequency.It turns out that the human ear is not aperfect microphone. Low frequencysounds must have a higher sound level inorder to “sound” as equally loud to thehuman ear as higher frequency sounds.To compensate for this facet of humanhearing, the measurement of the soundlevel should be adjusted so that itreflects how the ear hears the sound. Arecognized standard for this adjustmentis the A-Weighting curve shown inFigure III. When this curve is used tomeasure the sound level of an audiblesound, the readings will be given in unitsof dBA showing that an A-Weightingcurve was applied. An audible alarmuser, therefore, should expect soundlevel readings specified in units of dBAsuch as:90 dBA @ 10cm @ 6 Vdc.The above statement reads: 90 decibelsA-weighted at 10 centimeters at 6 VoltsDC.Power ConsumptionAs product design requirements movetoward lower voltage levels and troniccomponents has become an importantissue. Audible alarms require anywherefrom tens of milliamps (mA) to 80 mAto operate.Audible alarm circuitdesigns using inductors or transformerswill typically require greater current(40 mA) while designs utilizingdiscrete components or IC’s willtypically use less than20mA.Electromagnetictype audible alarms requiregreater current becausetheyutilizeanelectromagnetic in theirconstruction.If powerconsumption or electronicnoise is an omagneticdevicesand audible indicators thatutilizeinductorsortransformers should beavoided.
Construction QualityBecause of the critical role audiblewarning devices can play in anapplication, the quality of the buzzershould be an important consideration.Circuit designers often have themisconception that an audible alarm is asingle component, whereas in reality, anaudible alarm is a complicated assembly.While two different manufacturer’saudible alarm devices may look andsound the same on the outside, theinternal construction quality may be attwo very different levels.Whenevaluating a particular audible alarmmodel, a tear-down of the unit should bestandard practice. Peeling down the caseof the audible alarm with side cutterswill expose the circuit board and thetransducer.Questions that can beanswered by a tear-down include: Is the housing of sufficientstrength?Is the circuit board relativelyclean of excess flux?Do the solder joints have a goodappearance?Is the circuit board well laid outwithout the use of jumpers?If the transducer is mounted tothe housing using a siliconeadhesive, is the adhesive beadclean, and does it have adequateadhesion strength?One application question that issometimes asked is whether thetransducer metal type is important.Piezoelectric transducers are typicallymade of either brass or stainless steel.The advantage of brass is that it ischeaper and it is easier to process andtherefore more reliable in the field. Theonly advantage of using stainless steel isthat it does not need to be coated with aprotective coating for harsh salt waterapplications.For 99.9% of theapplications (i.e. not subjected to harshsalt water), brass is the preferredtransducer choice. For those other 0.1%,either a stainless steel or a brasstransducer with a protective coating canbe used.Environmental TestingFor most applications, it is alsoimportant to note the environmental andelectrical testing methods the audiblealarm manufacturer uses to ensure theirproducts are qualified. Testing shouldinclude: reverse voltage, surge voltage,vibration, shock, salt spray, thermalshock, hot and cold life & storage tests,and intermittent voltage life tests. Thiskind of data should be able to beprovided upon demand.Someproductionprocessesorapplication environments may alsodictate whether the audible alarm shouldbe completely sealed or open to theoutside air. Manufacturers of audiblealarms typically offer both sealed andunsealed units. Sealed units will oftenbe back-filled in the housing with anepoxy compound. Unsealed units willoften have a snap-in type cover. Sealedunits may cost slightly more thanunsealed units, and they may also be alittle taller in height to provide additionalroom for the epoxy back-fill.A similar processing issue is whether awash label is needed to cover the soundemission hole during the circuit boardcleaning process. If water penetratesinside the audible alarm during cleaning,the audible alarm may be eithertemporarily or permanently damaged by
the water intrusion. After the audiblealarm makes it through the circuit boardcleaning process, the wash label must beremoved by hand for the audible alarmto function correctly. If the audiblealarm will be seeing a wash typeprocess, then a wash label is needed.Market Trendswarning sounds have become morecommonplace, using a specialty soundhas emerged as a new way to catchsomeone’s attention. Specialty soundsinclude chimes, chirps, warbles, andsirens. Advances in technology hasenabled siren sounds to be placed on pcboard mountable audible alarmsmeasuring only 23 mm diameter by 10.5mm high.Several trends have emerged in theaudible alarm market in the last fewyears. While the market will continue tosteadily grow for large panel mountindustrial units, the demand for smallerand louder pc board mount units hasmaintained it strong upward pace. Theproduct offering of surface mountaudible alarms has also increased to stayup with the ever growing demand forthis type package.Another challenge for audib