Transducers- Self Drive- Piezo

Table of Contents

General Alarm Technology

An electronic audible alarm produces an audible warning sound using electronic means. This is in contrast to electro-mechanical alarms that produce sound by mechanical means. Examples of electro-mechanical alarms include the old clapper type alarm clocks, school bells, and car horns. Examples of applications that use electronic audible alarms include smoke detectors and microwave ovens.

Buzzers, beepers, audible signals, piezo’s, sounders, alerts, audio alarms, indicators, transducers, and various combinations of these terms (audio alerts, piezo indicators, etc.).

Audible alarms work by using electronic components to convert the user’s input voltage into an appropriate oscillating signal that drives a metal sounder diaphragm.  This metal sounder diaphragm then physically flexes up and down producing air pressure waves that the human ear interprets as sound.  For a more detailed description, please read the Article titled, “Audible Alarm Basics” and see Technical Application Guide, "Piezoelectric Alarm Operation".

Piezoelectric type alarms utilize a piezoelectric transducer which consists of a metal disc that has a ceramic material bonded to it.  When voltage is applied to the ceramic material, it causes the metal disc to physically flex.  If the piezoelectric transducer is physically flexed at an appropriate frequency, the air pressure waves are produced that are heard as an audible sound.

Electro-magnetic type alarms utilize an electro-magnet and a nearby bare metal disc that is mounted to the housing.  When the electro-magnet is energized, the resulting magnetic field physically deflects the bare metal disc.  If the bare metal disc is flexed at an appropriate frequency, an audible sound is produced.

Alarms that use piezoelectric technology draw less current, are capable of louder sound levels, and do not generate magnetic fields (possible EMI/EMC concerns).  Alarms that use electro-magnetic technology excel at producing low frequency pitch sounds in small packages.  This is why many miniature board mount or surface mount audible alarms use electro-magnetic technology.

Electronic audible alarms are considered components by equipment designers, but in actuality, they are a complex electromechanical assembly.  See Technical Application Guide, "Piezoelectric Alarm Construction".

An indicator is an electronic alarm that has internal circuitry.  The user only needs to apply an input voltage, and the alarm will automatically sound.

A transducer does not contain any internal circuitry.  The user has to supply the complex AC signal that will make the sounder diaphragm flex at the appropriate rate and amplitude.

Indicators are always appropriate to use.  Mallory’s design engineering (which holds over a dozen active patents) has already designed the most efficient circuit needed to produce the required sound and has tested that circuit against a wide variety of environmental conditions.

Transducers may be justified to use when there is sufficient volume to the application to justify the time and expense required to design, de-bug, test, re-design, and validate the circuit design needed to drive the transducer under the environmental extremes that will be seen in the application.  While the operation of the transducer may seem simple from the outside, there are many potential application problems that can arise unexpectedly.

Electrical Application Issues

To adjust the volume level, change the voltage going to the alarm. The sound level of the alarm is directly related to the voltage applied across the sounder element. For a fixed supply voltage, you can use resistors or a potentiometer (analog or digital) to adjust the voltage being applied to the alarm. It does take a fairly wide voltage swing to get a significant change in sound level, so there will be little sound level change with electromagnetic type buzzers because their voltage ranges are already too small to begin with. See Technical Application Guide, “Controlling Sound Level Electronically".  

A piezoelectric or electromagnetic alarm generates sound by physically deflecting a metal disc. It does take a small amount of time once the voltage signal is applied to the part before the metal disc is flexing to its fullest potential. Our recommended minimum pulse duration is 50 msec. but some customers have reported being able to use even shorter durations without affecting the sound level.  At some point as the pulse duration continues to decrease, the "beep" sound will become a "click" sound and the sound level will decrease.

You can use a simple zener diode & transistor circuit. See Technical Application Guide, "Circuit to Increase Turn-On Voltage".  

The input impedance of a piezoelectric transducer looks like pure capacitance. The current and voltage waveforms across the transducer can be predicted from the source voltage impedance and the transducer capacitance.  

For Mallory Sonalert piezoelectric transducer type alarms, the sound level curves are generated with a square-wave with a 50-Ohm source impedance. You can increase the source impedance to a few hundred Ohms with negligible effect on the resulting sound output.  

For Mallory Alarms and buzzers that use piezoelectric transducers, we have never had a report of these devices being affected by EMC/EMI.  However, there have been a few reports of electromagnetic type buzzers being effected.  If you are worried about the alarm being affected by EMC/EMI, you can consider using an EMC protective product such as 3M EMC tape or fabric that could be wrapped around the alarm.  Google "3M EMC Products" to find 3M's website with information on these products.

Mechanical Application Issues

Yes, but the alarm will be attenuated 15-20 decibels. This means that the sound level will be about ½ to ¼ as loud as it would be if it was mounted externally or if there were openings made in the enclosure so that the sound could radiate out. For a full discussion on mounting alarms inside of equipment, read the article titled, “Audible Alarm Use and Equipment Integrity Issues.”  

Yes! Visit our contact page on the website or email or call 317-612-1000

Soldering & Washing Issues

330ºC for 1.5 seconds or 270ºC for 4 seconds. 

270ºC for 3 seconds. 

Unlike resistors, capacitors, IC’s, or other components, electronic alarms are actually a complex electro-mechanical assembly. Unless you are using a transducer or buzzer unit that utilizes spring clip connections, there are usually a multitude of solder joints internal to the audible alarm. If you put too much heat for too long of a time on the audible alarm terminals, you can cause a variety of problems including:

  1. The solder joint connecting the audible alarm terminal to the PC board may re-flow causing a cold solder joint.
  2. Components on the circuit board near to the terminal may heat up and fail due to either thermal shock of the component or cold solder joints.
  3. The solder plating on the terminals may fail.

It is only recommended to wash those audible alarms that are sealed in the back and have a wash label over the sound opening. Not all board mount audible alarms are available in this configuration, so it is recommended to view the part data sheet or visit our contact page on the website or email or call 317-612-1000

If the part does not have a wash label, the cleaning solution will get into the front of the part through the front hole acoustic opening. This will not likely damage the part, but an audible alarm will likely not sound (or barely sound) if there is liquid solution in this front chamber. The cleaning solution may dry out over time on its own, or it can be removed more quickly by using an air circulating oven. However, the time it takes for the part to dry out depends on many different factors.

If the part is not sealed in back, several things may happen. The cleaning solution may get trapped inside the part and may seep out over time or at an inappropriate time. If the part has a circuit board, the cleaning solution may get up on the circuit board and cause electrical shorting and possibly permanent damage to the part or the circuit board the part is mounted on. Finally, the cleaning solution may cause corrosion internally to the audible alarm which could result in the part failing catastrophically or slowly over time.  

Sound Issues

Sound level is measured in decibels (abbreviated dB).  The dB scale is an arbitrary scale that reflects the loudness of the sound that is being measured.  It ranges from 0 dB (threshold of hearing) to 130 dB (threshold of pain).  For a better understanding of the decibel sound level scale, see Technical Application Guide, "Decibel Sound Level Scale".

The audible alarm should be at least 10 dB louder than the ambient back ground noise so that it can be easily heard. You can estimate the ambient background noise by using the chart found in the Technical Application Guide, “Decibel Sound Level Scale" or you can use a sound level meter to measure the actual ambient noise level.  

Every time the sound level increases by 10 dB, it will sound twice as loud to the human ear. For example, an alarm specified as 90 dB at 2 feet will sound half as loud as one specified as 100 dB at 2 feet.  

Sound level falls off over distance. We intuitively know this because we have to talk louder (or even shout) when people are farther away. The rule of thumb is that every time the distance doubles, the sound level drops off by 6 dB. For example, if an audible alarm measures 60 dB at 2 feet, by the time it reaches 4 feet, it will only be 54 dB. By the time it reaches 8 feet, it will only be 48 dB, and so on. 

Unfortunately, there is no one standard distance for specifying the sound level for audible alarms. However, there are some common distances such as 2 feet (60 cm), 1 foot (30 cm), and 10 cm (4 in). An excel spreadsheet has been developed to convert among the most common distances used. The link for the spreadsheet is in our TECHINCAL RESOURCES webpage.

For example, if you want to compare an alarm that is specified as 100 dB at 10 cm and one specified as 88 dB at 2 feet, you must choose one distance that you want to use to compare the parts. Using the distance conversion spreadsheet, you would find that 88 dB at 2 feet equates to 103 dB at 10 cm, so the alarm specified as 88 dB at 2 feet is actually louder than the other one when they are compared apples to apples.  

Most people can only distinguish a sound level change only when it increases or decreases by 3 decibels. For example if a person was listening to an audible alarm that changed from 90 to 92 dB, that person would most likely say that the alarm did not get louder. If the sound level changed from 90 dB to 93 dB, the person would say that the sound level is slightly louder. If the sound level changed from 90 to 96 dB, the person would say that the sound level is significantly louder. If the sound level changed from 90 to 100 dB, the person would say that the sound level is twice as loud as before. 

Pulsing tones are more easily distinguished than constant tones. Also, pulsing tones convey typically convey more urgency to a person than a constant tone. On the other hand, it takes more electronic circuitry to make a tone pulse, so pulsing audible alarms are usually more expensive than constant tone alarms. If a more pleasant sounding tone is needed, a chime sound may be preferred.

You can listen to the various sounds that Mallory audible alarms make on our SOUNDS webpage.

dB is the abbreviation for decibels which is how the sound level of audible alarms is measured. The “a” in dBa means that the sound level was measured on an A-Weighting scale. The A-Weighting scale was developed to compensate for the fact that the human ear is not a perfect microphone. By applying the A-Weighting scale to sound level measurements, you put the different frequencies (pitches) that the audible alarms produce on an even basis (i.e. comparing apples to apples). Mallory always uses A-Weighting for their sound level measurements, but not all audible alarm manufacturers are this diligent.

Yes. See Technical Application Guide, "Controlling Sound Level Electronically".

Mallory Sonalert has worked with Professors at Rose Hulman University in an attempt to model the sound chamber using Helmholtz equations, but these equations do not work well in predicting the resulting sound characteristics of the alarm. When Mallory Sonalert engineering designs new audible alarms, we rely on past designs and experience to give guidance on a starting point. However, the final design of the sound chamber is based on careful process of building prototype after prototype in order to find that sweet spot in sound performance."

The acoustic sound chamber of audible alarms includes the area inside the housing that is in front of the sounder element and includes the front hole opening.

The sound chamber does not work like organ pipes. In organ pipes, there are standing waves of different size depending on the frequency generated. This is why the organ pipes are different lengths. If the standing wave principle was used for electronic audible alarms, the alarms would have to be many inches or feet in length.

Perhaps the best way to explain how the acoustic sound chamber works is to think of it using a more visceral medium. If you think of the air sound waves being replaced by water, the sound chamber would work by providing an efficient shape for the water to move out of the housing without being obstructed by eddies, reverse currents, and dead spots. Essentially, the acoustic sound chamber provides a low impedance path for the air pressure wave to escape the housing with maximum intensity.

Yes! Visit our CONTACT US webpage or email or call 317-612-1000

Environmental Issues

The alarms, buzzers, transducers, speakers, and other products & accessories sold by Mallory Sonalert Products, Inc. are individual components that must be incorporated into final equipment in order to be useful.  Since their safety and use depends to a very large extent on how they are incorporated, they are not covered by the various European Directives, and need not be CE marked.  In fact, per the Low Voltage Directive, components must not be CE Marked.

While a vast majority of Mallory's alarms are used in industrial and non-aerospace applications, Mallory's alarms have been used by the aerospace industry for over 30 years, and end customers include nearly all major and minor jet, airplane, and helicopter manufacturers. All the various alarm models used in these applications have been certified with the FAA by the alarm user.  While Mallory has not been directly involved with the FAA during the PMA (Parts Manufacturer Approval) process, Mallory has (and will) supply all needed information for any certification and/or approvals that are required by the application to the alarm user.  It is up to the alarm user to work with the FAA to gain approval.

Mallory is not aware of anyone who has ever had a shelf life issue with our alarms. That being said, some alarm models contain aluminum electrolytic capacitors. The recommended shelf life for these capacitors is 5 to 10 years depending on how they are used. Our application of these capacitors is not especially sensitive to the shelf life issues of these components, so we would expect that they would last 8-10 years or longer in our alarms just sitting on the shelf (no voltage applied during that time).

MTBF data has only been generated for the SC, SNP, and SBM Series. Historical life test data at maximum temperature and voltage has resulted in the following failure rates for a majority of the models in these series that we sell when calculated per Mil-Handbook-217:

F.R. = 0.08% per 1000 hrs @ 60% confidence level

MTBF = 1,250,000 hrs @ 60% confidence level. 

Mallory Sonalert Products alarms, buzzers, and speakers do not require an ECCN Number.  However, if you absolutely need to assign an ECCN Number, use EAR99 (which means that our product is not regulated).

Customer returns of Mallory audible alarms for failure to operate are very rare.  Of the few parts returned each year, the vast majority of the root cause of failure is an over-voltage or voltage spike condition caused by the customer’s application.  For more details, see Technical Application Guide, "Typical Failure Modes". 

For our Rugged/Military panel mount models, these alarms are already at the limits of the technology (-55 C). However, it is likely that our other alarms will work at colder temperatures.  Visit our CONTACT US webpage, email or call 317-612-1000

Design Engineering uses a variety of tests during the verification and validation design phases.  These tests can include: surge voltage, reverse voltage, hot & cold life tests, room temperature life test, humidity, vibration, shock, salt spray, and terminal strength.  The Environmental Tests for each alarm are listed in that alarm’s Environmental Durability PDF available on the website.

MSL 1 (Unlimited)

Yes! Use our CONTACT US webpage, email or call 317-612-1000


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