A buzzer is a signal element (a device) which announces a certain, pre-defined, expected event through a sound. In German the word is „summer“ which means buzzing. In English the word describes an audio transmitter (an audio alarm). This is the element that generates sound in alarms. We could say that generally, a buzzer is a man-machine interface. It may be electromechanical (magnetic), electronic or piezoelectric. Buzzers which use piezoelectric ceramics are called piezoelectric buzzers. Buzzers are used in different types of sirens and alarms, fire alarms, in home appliances, industry, car manufacture, phones, etc. Their most common usage is as a warning signal of forgotten switched on lights in the car or switched off lights when starting the car, but also as a signaling apparatus of forgotten open door to a room with restricted access. Buzzers are also used to give signal of an unauthorized attempt to press a button or move a computer mouse and in the household, in microwaves and standard ovens when the appliance is preset to a certain time of work so that the hostess knows that the dish is ready *, in door bells, alarms of electronic watches, etc. The sound may be continuous, periodic or single. But what constitutes a sound?
Sound. Characteristics of Sound
Sound is a vibration of air. It is a vibration of matter leading to a periodic change of air pressure, perceived by the hearing-aid of a person. Except through the air, sound propagates in gases, liquids and solids and cannot propagate in a vacuum. A vibration is a type of longitudinal mechanical wave, and as such it has all the characteristic properties of waves: intensity, frequency, amplitude, period, wavelength, speed and direction of propagation.
The main features of buzzers that are significant are volume, frequency and speed of propagation. The speed of sound in the air at a given temperature is constant, and at 20℃ it is 340 m / s.
Sound intensity is measured in bel (B) but the value that is used more widely today is the tenth part of a bel – the decibel (dB). The most frequently selected sound intensity when constructing a buzzer is about 80-90 dB. We will give as a reference point the sound of rustling leaves or the movement of a mouse – 10-20 dB, and over 120 dB sound is painfully loud and can cause damages.
Frequency is the number of oscillations per second. It is measured in hertz (Hz) and is named after the German physicist Heinrich Hertz who demonstrated experimentally the existence of electromagnetic waves. The human ear perceives as sound frequencies ranging from 20 Hz to 20 kHz, although there are individuals who are able to hear outside these limits. The human ear is constructed so that it can enhance the sound pressure. Three different physical principles function over an area of about 4 cm, taken up by the middle and the inner ear. They serve to amplify weak vibrations in the air in such a way as to create acoustic waves in a liquid.
The effect of the ear canal which resembles a resonance of an organ pipe (Helmholtz resonator) amplifies air pressure about 10 times. The lever system formed by the bones of the middle ear, amplifies the signal 3 times further. The difference between the area of the eardrum and the area of the oval window provide additional 30-fold amplification. The result of these three mechanisms can lead to amplification of the sound wave over 800 times before it sets into motion the inner ear fluid. This enhancement allows us to hear faint and distant sounds. Buzzers are designed so that the sound emitted is about the average audible range, because sound is best heard when it is in the middle portion of the frequency spectrum. The usual sound frequencies of buzzers are from 2.8 to 6 kHz.
Buzzers. Principle of operation, structure, types
Piezoelectric buzzers consist of a piezoelectric diaphragm which is a piezoelectric ceramic disk adhered to a metal plate made of brass or nickel alloy. Around the disc there is a resonating box with a hole in the middle whose purpose is to emit the sound. In the rear of the element there are two terminals. One of them is connected to the metal disk and the other – to the piezoelectric disc. When there is voltage supply the piezoelectric crystal expands and contracts carrying along the metal disk that produces sound, generated through its vibrations. The sound is amplified by the acoustic box, in which it is located and goes out through the hole in the middle of the box.
Parameters characterizing the buzzer:
• rated voltage – the piezoelectric buzzer (piezo transducer) works with rectangular pulse waves.
• Supply voltage – the voltage needed for the proper functioning of the device.
• Current consumption – the current, which is needed for the proper functioning of the buzzer. At the beginning of the working process this current is 3 times higher. The piezo buzzer can produce higher SPL (sound pressure level) but it will consume more electricity.
• output sound – its intensity is measured when rated voltage is applied by a decibelmetre located at a distance of 10 cm.
• a resonant frequency – a buzzer can generate sound with different frequencies, but it is assumed that the highest and most stable SPL is emitted at a resonance frequency.
• Operating temperature – buzzers operate normally at temperatures from -30 to +70 ℃
How to choose a buzzer?
There are many different methods for selecting a buzzer, but the most important among them are several parameters such as voltage, current, principle of operation, dimensions, mounting dimensions and of course the most important one – how big is the sound pressure level – SPL, and at what frequency we want it to be. The resonant frequency of emission of the piezo element is directly related to its diameter. The larger the diameter of the element is, the lower the frequency, and vice versa.
Operating voltage: normally the operating voltage for a magnetic buzzer is from 1.5V to 24V, for a piezo buzzer – from 3V to 220V. However, in order to obtain a sufficiently high SPL, we offer at least 9V for managing a piezo buzzer.
Current consumption: according to the voltage, the current consumption of the magnetic buzzer is tens to hundreds of milliamps; vice versa – the piezo buzzer saves much more electricity and needs only a few milliamps, consuming three times more than the operating current when the buzzer starts operating.
Size : a buzzer’s sizes are inversely proportional to the frequency, they have influence on the SPL, the magnitude of the magnetic buzzer is 7 mm to 25 mm, and the piezo buzzer is 12 mm to 50 mm or even larger.
Type of connection: SMD type, by wires, etc.
Determination of the sound intensity (sound pressure level SPL): sound intensity in decibels is measured 10cm from the buzzer and can be of the order of 85dB, and common sirens make the laudest sound.
The main operating element in any buzzer is the piezoelectric plate (diaphragm). According to the way in which the signal is received, we distinguish two types of piezoelectric components, producing sound. One of them has such a connection that the diaphragm actuates through an external signal (a buzzer without a generator) and the other one –self-propelled through resistors and transistors (a buzzer with a generator).