Audio transducers and buzzers produce a continuous or intermittent tone by converting electrical energy into acoustic energy. These devices are found in smoke detectors and other products that produce sound. Some audio transducers, like the one in the video below, are programmable and are capable of producing a series of multiple tones.

Video credit: aquaticusrov / CC BY-SA 4.0

Technology

Audio transducers and buzzers may use electromagnetic or piezoelectric technology to produce a tone.

Electromagnetic

Electromagnetic transducers use a magnet and an electric coil to produce acoustic energy. When current is applied to the coil from a power source, it creates a magnetic field that switches the transducer or buzzer on, producing a tone or buzzing sound. Compared to piezoelectric products, electromagnetic transducers produce a relatively raw, low power signal, and are typically larger devices.

A PCB-mounted electromagnetic buzzer. Image credit: Advanced Acoustic Technology Corp.

Piezoelectric

Piezoelectricity is the electrical charge that accumulates in crystals and certain ceramics in response to stress. Piezoelectric audio transducers use a metal plate in conjunction with the piezo ceramic element. The metal plate vibrates when the element expands and contracts, producing an audible sound.

A PCB-mounted piezo buzzer. Image credit: Advanced Acoustic Technology Corp.

Audio Specifications

Frequency

Frequency refers to a number of sound wave cycles, expressed in Hertz (Hz), which determines an audio signal's pitch. The image below shows several waves of various frequencies; the waves at the top of the image are vibrating at lower frequency than the bottom ones.

Image credit: Kieff

The human ear is capable of hearing pitches ranging from 20 Hz to 20,000 Hz (or 20 kHz). Common frequency ranges, and the audible sounds they produce, are:

• 16 to 32 Hz: the lowest tones audible to a human. These pitches of low vibration are sometimes described as being "felt" rather than heard.
• 32 to 512 Hz: the lower half of a piano keyboard, or "bass" notes.
• 1024 to 2048 Hz: the upper half of a piano keyboard. These pitches give a horn-like or tinny sound quality.
• 4096 to 8192 Hz: labial and fricative sounds of human speech.
• 16384 to 32768 Hz: tones in this range become less audible. For reference, the tone emitted by a cathode ray tube (CRT) television measures approximately 16700 Hz.

Audio transducers may be manufactured to emit a fixed tone or may specify a range of programmable tones.

Sound Pressure

Sound pressure refers to the local pressure deviation from ambient pressure caused by a sound wave. When dealing with audio transducers, sound pressure level (SPL) is a more useful specification. SPL logarithmically expresses a wave's sound pressure relative to a reference value, usually the threshold of human hearing at 1 kHz. The reference value is expressed as

p_ref = 20 μPa (rms)

where rms = root mean square.

This value can be roughly translated as the sound of a mosquito flying 3 meters away from a human ear.

Sound pressure level is expressed in decibels (dB). Some common sounds, along with their sound pressure values (in Pa RMS) and sound pressure level values include:

Table credit: Wikipedia

As shown in the table, sound pressure level is calculated to express an enormous range of sounds using a relatively small scale. Because decibels are logarithmic units, a change in power ratio by a factor of 10 is represented as a 10 dB change.

Physical Specifications

Terminations

A transducer's terminations determine how it connects to an input source.

• Surface mount (SMT) devices do not include terminations and are connected to a printed circuit board (PCB) using wave solder.
• Some transducers may include pre-wired leads or pigtails to make a connection.
• Multi-pin connectors (PC pins) are used primarily for external interfacing.
• Quick connects are plugs with sensor probes.

• Screw terminals provide a secure physical connection by screwing a connecting wire onto an electrical contact.

• Solder tabs are flat metal pieces to which wires can be soldered, and are used for connecting to circuit boards.

RoHS Certification

The Restriction of Hazardous Substances directive (RoHS) is a European Union order which restricts the use of six hazardous materials in the manufacture of electronic and electrical devices. Although they are often simply referred to as "lead-free", RoHS products contain less than 0.1% of:

• Lead
• Mercury
• Cadmium
• Hexavalent chromium (Cr⁶⁺)
• Polybrominated biphenyls (PBB)
• Polybrominated diphenyl ether (PBDE)

The RoHS directive protects the health of electronics industry workers and those in close proximity to areas where high-tech waste is stored. Despite initial concerns about the reliability of lead-free solder, RoHS products have been favorably tested to withstand harsh environmental conditions and severe vibration and shock requirements.

While RoHS compliant products do not carry a standard mark or seal, the image below presents a typical example.

Image credit: Advanced Digital Cable Inc.

References

Electronics Tutorials - Sound Transducers

Image credit: Pfannenberg | Americor Electronics | RDI, Inc.