محصولات صوتی آلارم، بیزر و آژیر

Product Attributes

Introduction

Basic Structure of a Piezoelectric Electronic Alarm

The cross-section above illustrates the basic components used in a piezoelectric sound alarm. The area in front of the transducer element, including the front port opening, forms an acoustic cavity that allows sound to radiate outward at maximum efficiency (i.e., maximum sound level). If the alarm is an indicator containing a circuit board, the board is connected to the piezoelectric sound element via soldered leads. The illustration above can be understood as representing a board-mounted package with pin terminals, but the same concept applies when constructing sound 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 "internal components" of the alarm (including the circuit board and components) are protected against liquid ingress. However, liquid inside the front cavity may impede device operation and cause a significant reduction in sound level. If the alarm requires cleaning after soldering operations, it is strongly recommended to use alarms labeled as washable to prevent cleaning fluid from entering the front cavity.

How Piezoelectric Sound Alarms Work

A piezoelectric electronic sound alarm operates by converting the user's input voltage into an appropriate oscillating signal, which is then applied to the sound element mounted inside the housing. The piezoelectric sound element consists of a metal disc and a special ceramic material that physically bends when voltage is applied. The image above shows a bare piezoelectric sound element. By applying a sinusoidal waveform at an appropriate frequency, the transducer will physically deflect in one direction and then in the opposite direction, following the shape of the input waveform. If this oscillation occurs within the audible frequency range (1 Hz to 20 kHz), pressure waves are generated that the human ear interprets as audible sound. The greater the voltage of the applied waveform, the larger the amplitude of the pressure wave, resulting in a higher sound level. However, the ceramic portion of the transducer can only flex to a certain degree before catastrophic failure occurs. This maximum voltage is approximately 40 to 50 volts. In practice, such high voltages are rarely applied to the transducer, as diminishing returns occur when the voltage significantly exceeds 32 volts. On its own, the transducer element produces very little sound level. To increase the magnitude of the pressure wave (and thus the sound level), the transducer element must be mounted in an acoustic cavity optimized for the transducer's size and resonant frequency. Each transducer has a frequency at which it flexes more efficiently, producing a higher sound level. This frequency at which the transducer performs best is called the resonant frequency. Self-drive devices provide a third terminal connected to an isolated section of the piezoelectric transducer. This third terminal provides a feedback signal that is 180 degrees out of phase with the drive signal. This signal can be fed back into the circuit, enabling the sound element to self-adjust to the transducer's resonant frequency.

Decibel Sound Level Scale

The decibel sound level scale is an arbitrary scale ranging from 0 dB (threshold of hearing) to 130 dB (threshold of pain). The chart below shows the positions of some common sounds on this dB scale. Sound alarm sound levels range from 55 dB at 2 feet to 110 dB at 2 feet.

Reference Sound Levels (Assuming you are standing 2 feet from the sound source)

Fundamental Frequency and Harmonics

Below is a frequency sweep of a piezoelectric sound alarm with a resonant frequency of 2800 Hz. As shown, there is a strong frequency peak at 2.8 kHz, followed by several smaller peaks known as harmonic frequencies. The table below the chart shows that for this particular alarm unit, the harmonic frequencies are significantly smaller in magnitude than the fundamental frequency. Because the fundamental frequency of this alarm is dominant and the harmonics are much smaller, the sound quality of this component is very good. When this alarm is activated, listeners will hear a clear, single frequency (also referred to as pitch) from the alarm. Other electronic alarm technologies, such as electromagnetic or electromechanical alarms, typically exhibit much larger harmonic frequency components, resulting in a less distinct tone.

Circuit for Increasing Turn-On Voltage

Below is a circuit that can be used to prevent the alarm from sounding until a certain voltage is reached. Due to the use of a 10-volt Zener diode, the turn-on voltage for this particular circuit is approximately 10 Vdc, but you can substitute a Zener diode of a different value to achieve the desired turn-on voltage for your circuit.

Controlling Sound Level — Electronic Methods

For piezoelectric sound alarms, the greater the voltage signal applied to the piezoelectric transducer, the higher the sound level. This characteristic can be used to electronically control the sound level of these devices. For sound alarm model MSR320R, the sound level will vary from 65 dB at 2 feet at 3 Vdc to 80 dB at 2 feet at 20 Vdc. By varying the voltage from 3 Vdc to 20 Vdc, the sound level can be varied by 15 dB. A 10 dB reduction in sound level will make the alarm sound approximately half as loud. Several methods for electronically controlling voltage include:

1. Using a manual or digital potentiometer.
2. Using a selection circuit with different (text incomplete in source)