Product Application Guide- Industrial Alarms
Part Numbering System
SC, LSC, SBM, SNP, & VSB Series
Part Number Structure SC 648A NP R
Series
SC = Screw Neck Panel Mount LSC = LED Alarm
SBM = Large Board Mount SNP = Snap-in Mount
VSB= Specialty Sound
Voltage Range & Type
307 = 3 to 7 Vdc 616 = 6 to 16 Vdc 628 = 6 to 28 Vdc
628A = 6 to 28 Vac/dc 648 = 10 to 48 Vdc 648A = 10 to 48 Vac/dc
110 = 30 to 120 Vac/dc 250 = 60 to 250 Vac/dc
Suffixes
CP = Chime D = 1900 Hz E = Soft / 1900 Hz F = Soft
H = 4500 Hz J = Slow Pulse K = Short Pulse L = Lead Wires
M = Military N = Loud P = Fast Pulse PU = Dual Mode
Q = Chirp S = Siren W = Warble WY/WXY = Multi Sound
RoHS: R = RoHS Compliant; -BTR = Flat Blades & RoHS Compliant
Page 2
Piezoelectric Electronic Alarm Construction
The above cross section picture shows the basic elements used in a piezoelectric audible
alarm. The area in front of the transducer element including the front hole opening forms
an acoustic cavity that lets the sound radiate out with the most efficiency (i.e. loudest
sound level). If the alarm is an indicator that contains a circuit board, the circuit board is
attached to the piezoelectric sounder element via soldered wires.
The above picture can be interpreted to represent a board mount package with pc pin
terminations, but the same concept is used when building audible alarms in other
mounting configurations such as SMT, Flange Mount, and Panel Mount alarms.
If the back of the alarm is sealed with epoxy or other material, the “guts” of the alarm (including
the circuit board and components) are protected against fluid intrusion. However, fluid sitting
inside the front cavity can obstruct the operation of the device causing the sound level to
decrease significantly. If you need to wash the alarms after a soldering operation, it is strongly
recommended to use an alarm that comes with a wash label that keeps the washing fluid from
getting inside of the front cavity.
Page 3
Operation of Piezoelectric Audible Alarms
Piezoelectric electronic audible alarms work by converting the user input voltage to an
appropriate oscillating signal that is applied to a sounder element that is mounted in a housing.
The piezoelectric sounder element consists of a metal disc that has a special ceramic material
bonded to it that physically bends when voltage is applied to it.
The above picture shows a bare piezoelectric sounder element. By applying a sinusoidal wave-
form at an appropriate frequency, the transducer will physically deflect in one direction and then
in the opposite direction following the shape of the input wave-form. If this oscillation occurs in
the audible frequency range (1 Hz to 20 kHz), then air pressure waves are produced that the
human ear interprets as an audible sound.
The larger the voltage of the applied wave-form, the larger the amplitude of the air pressure
waves resulting in a louder sound level. However, the ceramic portion of the transducer can only
bend so far before there is a risk of a catastrophic failure. This maximum voltage is somewhere
around 40 to 50 volts. However, it is rare to apply this much voltage to a transducer as you reach
a point of diminishing returns for voltages much greater than 32 volts.
By itself, the sound level produced by a transducer element is insignificant. To increase the size
of the air pressure waves (and thus the sound level), the transducer element must be mounted
inside an acoustic chamber that is optimized for the transducer size and resonant frequency.
Every transducer has one frequency where it flexes more efficiently producing the louder sound
levels. This frequency where the transducer performs the best is called the resonant frequency.
Self-Drive type devices provide a 3rd terminal that connects to an isolated portion of the
piezoelectric transducer. This third terminal provides a feed-back signal that is 180 degrees out
of phase with the drive signal. This signal can be fed back into the circuit to allow the sounder
element to self-tune itself to the transducer’s resonant frequency.
Page 4
Decibel Sound Level Scale
The decibel sound level scale is an arbitrary scale that ranges from 0 dB (threshold of
hearing) to 130 dB (threshold of pain). The chart below shows where some common
sounds fall on this dB scale. Audible alarms are available that have sound levels as soft
as 55 dB at 2 feet and as loud as 110 dB at 2 feet.
Page 5
Fundamental Frequency & Harmonics
Below is a frequency scan of a piezoelectric audible alarm that has a resonant frequency
of 2,800 Hz. As you can see, there is a strong frequency peak at 2.8 kHz and several
smaller frequency peaks that follow called harmonic frequencies. The table below the
chart shows that the size of the harmonic frequencies are significantly smaller than the
fundamental frequency for this particular alarm unit. Because this alarm has a large
fundamental frequency and much smaller harmonic frequencies, the sound quality of this
part will be very good. When this alarm is activated, the listener will hear one clear
frequency (also called sound pitch) from the alarm. Other electronic alarm technologies
such as electro-magnetic or electro-mechanical type alarms often have much larger
harmonic frequency components resulting in less clear tone.
Page 6
Circuit to Increase Turn-On Voltage
Below is a circuit that can be used to prevent the alarm from sounding until a certain
voltage is reached. This particular circuit has a turn-on voltage around 10 Vdc due to the
10 volt Zener Diode, but you can just substitute other values of Zener Diodes to get the
needed turn-on voltage for your circuit.
Page 7
Controlling Sound Level- Electronic Method
For piezoelectric type audible alarms, the larger the voltage signal applied to the piezoelectric
transducer, the louder the sound level. This property can be used to electronically control the
sound level of these devices.
For audible alarm model MSR320R,
the sound level will vary from 65 dB
at 2 ft. at 3 Vdc up to 80 dB at 2 ft. at
20 Vdc. By varying the voltage from
3 to 20 Vdc, the sound level can be
varied by 15 dB. A 10 dB drop in
sound level will make the alarm sound
half as loud.
There are several ways of electronically controlling the voltage including:
1. Using a manual or digital potentiometer.
2. Using a selector circuit and different values of resistors.
3. Using PWM voltage signals from microcontrollers.
In all cases, care must be taken to make sure that the circuitry and voltage signals used do not
interfere with the internal circuitry of the audible alarm. This method of controlling the sound
level will probably not work well in the following cases:
1. The sound level of electro-magnetic type alarms do not vary much over the voltage range of
these devices, so most electro-magnetic type alarms are not suitable for this method.
2. When you only have a narrow operating voltage range to work with. For example, if you are
using the MSR320R at 5 Vdc, the sound level change from 5 Vdc to 3 Vdc is not very much.
3. When the voltage range of the audible alarm is narrow. For example, panel mount model
SC307NR only has a voltage range of 3 to 7 Vdc. Over this range, the sound level will vary
by only 6 dB. This sound level change is significant, but may not be enough to make a
difference in the application.
Page 8
Controlling Sound Level Using a Potentiometer
A potentiometer can be used on many Mallory Sonalert alarm models to control the
sound level. However, there are some alarm models where the use of a potentiometer
actually interferes with the operation of the alarm circuitry. The first step to using a
potentiometer, therefore, is to verify that it will not affect the operation of the audible
alarm.
For panel mount model SC628R, as the left graph below shows, the sound level will
change as much as 10 dB over the rated voltage range. By putting a potentiometer in
series with the SC628R, we can adjust the voltage to the alarm and thus adjust the alarm
sound level.
It was verified that the circuitry of model SC628R is not affected by a potentiometer, so
data was gathered to find the value of a potentiometer that gives the greatest range of
sound change.
Results:
At 12 Vdc, it was found that a 7 kOhm potentiometer in series with the SC628R will give
the widest swing of sound level. At 0 Ohm, the sound level is 84 dB, and with the POT
turned all the way up to 7 kOhm, the sound level drops to 73 dB. This gives a fairly wide
swing of 11 dB.
At 28 Vdc, it was found that a 20 kOhm potentiometer in series with the SC628R works
well. At 0 Ohm, the sound level is 89 dB, and with the POT set at 20 kOhm, the sound
level drops to 73 dB. This gives an even wider swing of 16 dB.
Page 9
It should be noted that with the two potentiometers listed above turned all the way up to
their maximum value, the voltage across the SC628R is only 3 Vdc. While this is outside
the rated voltage range, the alarm will still work at this voltage, so it is okay to drop the
voltage this low in order to get a wider swing of sound level as long as this sound level is
still adequate for the application.
Page 10
Controlling Sound Level- Mechanical Method
There are two ways to control sound level in an electronic audible alarm. One is mechanical
and the other is electrical. The mechanical method involves changing the size of the front hole
opening of the audible alarm. The open area in the front of the audible alarm including the front
hole opening is an acoustically tuned cavity. By partially covering the front hole opening, you
are changing the cavity tuning making it less efficient. The more the front hole opening is
covered, the more the sound level will decrease.
An example of how to do this is shown above. The picture above shows our manual sound
baffle accessory with part number: SCVC being assembled onto the front of a panel mount
alarm housing. The SCVC accessory consists of a screw and two pieces of plastic. After the
sound baffle is attached to the front of the alarm, the sound level can be changed by manually
turning the top piece of plastic. The sound level of the alarm will attenuate anywhere from 10
to 15 dB’s making the alarm sound about half as loud as before.
The main disadvantage of this manual method of controlling the sound level is that it is
controlled by the operator. In some situations, it is dangerous to let the operator have the ability
to decrease the sound level of the alarm. The most obvious potential problem is that the
operator could turn the sound level down too much so that the alarm will not easily be heard the
next time it is activated.
Page 11
Typical Failure Modes of Piezoelectric Audible Alarms
Component/Subsystem Failure Mode End Result Occurrence
Circuit Components
(Resistors, Capacitors,
Diodes, IC’s, etc.)
Over-voltage by customer’s
application
Unit ceases working. Vast
Majority of
Returns
Transducer/Wire Solder
Operation
Not enough wire strands in
solder joint
Wire breaks after
period of time & unit
ceases sounding
Rare
Physical Assembly Transducer wire pinched,
adhesive/epoxy run down
onto transducer, or RTV
adhesive seal failure
Intermittent operation Rare
Soldering Operation Incorrect Solder Temperature
or Time Causing Cold Solder
Joint
Intermittent operation
or unit ceases working
after period of time
Very Rare
Circuit Components Random Component Failure;
Wrong Component Used;
Missing Component
Unit ceases working
under normal
operating conditions
Very Rare
Transducer Wire Defect in Wire;
Wire Strands Damaged in
Production
Wire breaks after
period of time & unit
ceases sounding
Very Rare
Piezo Transducer Incorrect Polarization by
Manufacturer;
Glue Bonding Failure
Sound volume level
decreases over time.
Exceedingly
Rare
Notes:
1. 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.
2. All Mallory alarms are, at a minimum, function tested 100% during production, and a
final audit is performed. Mallory SC/SBM/SBT/SBS/SNP/LSC/VSB/MSR/MSO/ZA
series of alarms are audited 100% at final test by checking that sound level, frequency,
and current are within specification limits from 2 to 4 different voltage levels.
