Product Application Guide- Industrial Alarms
Part Numbering System
SC, LSC, SBM, SNP, & VSB Series
Part Number Structure
Series
SC = Screw Neck Panel Mount
SBM = Large Board Mount
VSB= Specialty Sound
Voltage Range & Type
307 = 3 to 7 Vdc
628A = 6 to 28 Vac/dc
110 = 30 to 120 Vac/dc
Suffixes
CP = Chime
H = 4500 Hz
M = Military
Q = Chirp
LSC = LED Alarm
SNP = Snap-in Mount
SC
648A
NP
R
616 = 6 to 16 Vdc
648 = 10 to 48 Vdc
250 = 60 to 250 Vac/dc
628 = 6 to 28 Vdc
648A = 10 to 48 Vac/dc
D = 1900 Hz
J = Slow Pulse
N = Loud
S = Siren
E = Soft / 1900 Hz
K = Short Pulse
P = Fast Pulse
W = Warble
F = Soft
L = Lead Wires
PU = Dual Mode
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 3
rd
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.
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