PRODUCT SPOTLIGHT:
PIEZO AND MAGNETIC BUZZERS

At the heart of all piezo-type buzzers is the piezoelectric element. The piezoelectric element is composed of a piezoelectric ceramic and a metal plate held together with adhesive. Both sides of the piezoelectric ceramic plate contain an electrode for electrical conduction. Piezo materials exhibit a specific phenomenon known as the piezoelectric effect and the reverse piezoelectric effect. Exposure to mechanical strain will cause the material to develop an electric field, and vice versa.

Working Principle of Piezo Buzzers

When an alternating voltage is applied to the piezoceramic element, the element extends and shrinks diametrically. This characteristic of piezoelectric material is utilized to make the ceramic plate vibrate rapidly to generate sound waves.

Piezo Buzzer Structure

There are two types of piezo buzzers - transducers and indicators. Transducers consist of a casing, a piezoceramic element and a terminal. In order to operate a transducer, the user must send a square wave signal to the buzzer. Indicators consist of a casing, a piezoceramic element, a circuit board and a terminal. In order to operate an indicator, the user must send the buzzer a specified dc voltage.

Piezo Buzzer Feedback

Some CUI Devices piezo buzzers include a feedback line. Driving circuits for buzzers with feedback tend to be simpler than those circuits without. Feedback is accomplished by dividing the piezo element into two, electrically isolated pieces. When the main piezo element is actuated it squeezes the feedback portion, creating a voltage on the feedback line. A simple way to use feedback is to have the feedback line connected to the base of a transistor. As the piezo element oscillates, the feedback signal will oscillate and the transistor will alternately block or allow current to flow.

• The feedback line provides a voltage that is proportional to the strain on the main piezo element
• This voltage can be used to create a simple, self-oscillating circuit

As mentioned earlier in the presentation, piezo and magnetic indicators have the driving circuitry built into the design, creating a "plug and play" solution. Because of this, engineers do not need to worry about building a complex circuit to drive the buzzer. The disadvantage, however, is that indicators operate on a fixed frequency, reducing the flexibility offered to achieve an alternate frequency as application requirements change. Transducers, on the other hand, do not have the driving circuit built-in, so engineers are offered a greater range of flexibility when designing their circuit. The downside comes in the fact that transducers do require an external driving signal to operate properly, potentially adding complexity and time to the design cycle.

Indicator Characteristics

• Built-in driving circuit (frequency generator)
• Simple to design-in
• Fixed frequency (function)

Transducer Characteristics

• External driving circuit required
• Complex to design-in
• User-selected frequencies or multiple frequencies

Key Buzzer Specifications

Frequency Response

How efficiently a buzzer produces sound at a given frequency.

Sound Pressure Level (Unit: dB Pa)

Sound pressure level, SPL, is the deviation from atmospheric pressure caused by the soundwave expressed in decibel Pascals. It is generally proportional to input voltage and decays by 6 dB's when doubling the distance from the buzzer.

Resonant Frequency (Unit: F0 Hz)

All things have a specific frequency at which they tend to vibrate. This frequency is called the resonant frequency. For buzzers, the resonant frequency is the frequency at which they will be the loudest.

Impedance (Unit: ohm)

Electrical impedance is the ratio of applied voltage to current. The electrical impedance varies with frequency.

dB's

A decibel is the scaled logarithm of the ratio of a measured value with respect to a reference value. Decibels are useful because they can show a huge range of values in a small space. For instance a sound pressure scale going from 0-120 dB can represent sound pressures from 20 µPa (micro-pascals) to 20,000,000,000,000 µPa. This roughly represents the lowest SPL a human can hear all the way up to uncomfortably loud sounds. Note: The generally accepted value for "Pref" in the formula above is 20 µPa.

• dB stands for decibel
• It is not a unit, but rather a ratio
• Values increase exponentially, instead of linearly as in counting numbers
• Expressed in "normal" numbers, 20 dB is ten times the power of 10 dB
• Allows for a huge range of values to be expressed in relatively little space

Frequency Response

In a perfect world all devices would recreate every frequency at the exact same amplitude. In real life every device will have frequencies which it may amplify and frequencies which it will tend to reduce or attenuate. Frequency response curves show how a particular device responds to each frequency. SPL is plotted against frequency to indicate how the device will handle certain frequencies. Note: frequency is plotted on an exponential basis, similar to dB's, it allows the full range of human hearing to be fit in a compact space.

The Human Ear and A-Weighting

20 Hz to 20 kHz tends to be the general range for human ears. This range is reduced with age, especially in males. In older males 13 kHz tends to be the upper end of the audible range. The human ear does not have a flat frequency response over the audible range. Certain frequencies tend to be attenuated while others are magnified. A-weighting attempts to compensate for this by discounting frequencies which the human ear is less sensitive to. It places priority on sounds between 1 kHz and 7 kHz.

• Generally, most humans can perceive frequencies from 20 Hz ~ 20,000 Hz
• However, the human ear is more sensitive to some frequencies than others
• A-weighting places more value on frequencies which the human ear is more sensitive to
• Some CUI Devices buzzers specify SPL using the A-weight system, i.e. dBA

Resonant Frequency

Every system has a particular frequency that it tends to vibrate at. For instance, if you pluck a string on a guitar that string will vibrate very near, or at, its resonant frequency. By driving a system at its resonant frequency, very large displacements, relative to the input signal strength can be achieved. Driving a buzzer with an input signal which has the same frequency as the buzzer's resonant frequency, will create the greatest SPL with the least input power.

• Resonant frequency is the natural frequency a system tends to oscillate at
• Driving a system at its resonant frequency will create the largest amplitudes with the smallest input
• Buzzers are loudest when driven at their resonant frequency

SPL Calculator

CUI Devices has developed an SPL calculator to allow users to convert a buzzer's specified SPL on the datasheet to different real-world conditions, or to compare SPLs between two devices with different specified parameters. This tool makes it quick and easy for designers to specify the proper buzzer for their application.

Try the SPL calculator now ▶

Buzzers are typically used for identification and alarm purposes across many major industries. The major application categories that utilize buzzers for indication or alert purposes include: home appliances, automotive electronics, medical, safety and security, industrial, and office automation.

• Home Appliances
• Automotive
• Medical
• Security
• Industrial
• Office Automation