The Electromagnetic Drive Circuit shown is the typical circuit used for electromagnetic transducers. The positive pin of the transducer is connected to a DC voltage source while the negative pin is connected to the collector of the npn transistor. The emitter of the transistor is connected to ground. The base of the transistor is connected to the AC signal source.

The DC source determines the amplitude of the signal while the AC source determines the frequency. Once both are supplied, the transistor will sound at the given frequency.

AC Signal Drive Circuit

The AC Signal Drive Circuit is the simplest drive circuit for a piezoelectric sound transducer. The positive pin of the transducer is connected to one end of the AC source while the negative pin is connected to the other. An AC signal is then applied across the transducer. While this can be a sine signal, a square signal is recommended instead as the sharp change from alternating voltage peaks and vice versa causes the piezoelectric element to bend quicker and more forcibly which produces higher SPL.

The limitation of this type of circuit is that the voltage range is determined by the limitations of the AC generator which are typically lower than a DC power supply.

To extend the range, the user could use the same Electromagnetic Drive Circuit shown before with some slight variations to account for the transducer being a piezoelectric device. Here, the limit will be the DC power supply and the AC power supply only needs to be high enough to supply a discernable high and low input signal to enable the transistor.

Piezoelectric Drive Circuit

Rp is a discharge resister to ensure there is a path for stored charge in the piezoelectric disk to discharge safely and can be between 600 ohm to 1,000 ohm. Rs can be used optionally to adjust the loudness of the piezoelectric transducer.

Resonant Drive Circuit

In either of the above cases, the circuit requires a high voltage source to achieve high SPL which can be impractical especially with portable devices. To achieve higher SPL with a lower voltage supply, the Resonant Drive Circuit above can be used.

Here, an inductor is introduced into the previous circuit. An inductor resists instantaneous changes in current. When the transistor switches from on to off, the flow of current should ideally be cut. The inductor, however, wants to maintain whatever current is flowing through it and so will compensate by introducing a sharp temporary voltage spike that is greater than the supply voltage. The piezoelectric element will bend in response to the voltage spike and produce an SPL higher than what it would without it.

To maximize the gain with this addition, the chosen inductor needs to be selected based on the capacitance of the piezoelectric element such that the combination of both matches the part’s resonant frequency as per the formula below. The part should then be driven at the resonant frequency as that is where the SPL gain is particularly effective and the benefits at other frequencies are noticeably lower.

Resonant Frequency Formula

Full-Bridge Drive Circuit

This uses a single DC power supply and AC signal source to drive the piezoelectric transducer with a Vpp signal. Both positive and negative pins of the transducers are each connected to a set of transistor switches with an AC signal, or its inverse connected to the bases as shown. This configuration creates 2 independent paths for the current to flow with no overlap.

Positive to Negative Path

When the AC signal is high, the transistors create the above path for the current to flow.

Negative to Positive Path

When the AC signal is low, the opposite happens and lets the current flow through the piezoelectric transducer in the other direction.

The benefit to this drive circuit is that you can take advantage of the full range of motion of a piezoelectric element without using a high voltage AC source. The DC source can also effectively provide twice the amplitude in this configuration.

The percentage of open space to closed space for a grille covering the output of a speaker should be set between 60-65%. Hole diameter will be dependent on the size of the speaker though a typical size of 3mm is recommended. The grille should be designed so as to not have a parallel plane to the speaker face.

Any speaker assembly has the potential to achieve an IP rating if an IP rated mesh is incorporated on the grille. Many speaker diaphragm and surround materials will of themselves achieve an IP rating though water drainage should be considered.

A designer has the option of many chamber types which will offer unique speaker performance. The design can be very complex and involved and only a basic approach will be provided here. The most common and straight forward design types are sealed and aperiodic. For a sealed chamber design, thrice the volume of the speaker should be included into the chamber. Thiele Small parameters can be provided for all speakers at customer request if a more advanced design methodology is desired.

Two mounting environments need to be considered, the speaker to the overall assembly and the chamber to the speaker.

A. Two methodologies can be used to mount the speaker to eliminate vibration either a gasket stack-up or a glue joint. This guide will only discuss the gasket design as this is the preferred methodology for customer application and provides enhanced reliability for a manufacturing environment not designed to produce speaker assemblies.

• A typical gasket stack-up for the front face of the speaker uses a compressible gasket with a double-sided adhesive and pull tab mounted to the speaker front face. A typical gasket stack-up for the rear face of the speaker uses a compressible gasket mounted either to the speaker magnet or outer rear perimeter.
• Speaker selection can be made to include screw bosses for mounting to create the needed compression.

B. The same two design concepts applied to the speaker can be applied to mounting the acoustic chamber. The main intent when mounting the chamber is create an air-tight seal along the mounting joint.

A Piezoelectric Element, or Disk (often called a ’Bender’) is the most basic type of electronic component used to produce sound.

1. Low power consumption.
2. Inexpensive.
3. Used in many electronic products such as consumer, fire, & safety and medical applications.
4. Typically are round using metallic disk with a layer of ceramic that is ’sandwiched’ by an electrode.

When voltage is applied, the disk vibrates at a given frequency and produces sound. More voltage means a louder disk. The sound can be:

1. A steady ’Buzz’ or a ’Beep’.
2. A series of several notes, often called ’Sing-Tones’.

For a piezoelectric to make sound, it needs to sit atop a housing called a ’Chamber’. If placed on a flat surface, it will not be able to properly flex to produce sound. The chamber (a ’Helmholtz Resonator’) is typically designed to match the resonant frequency of the piezoelectric disk.

1. Metallic Disk
2. Ceramic Disk
3. Chamber

The best example of a piezoelectric application is a smoke detector.

Bi-Morph

A ’Bi-Morph’ Piezoelectric disk has the electrode and ceramic layer on both sides of the metal disk. When pulsed, a ’Push-Pull’ effect is created making the piezoelectric slightly louder.

A piezoelectric disk can also be used as a low-cost switch. When ’stressed’, a piezoelectric disk creates a small amount of voltage that can trigger a microprocessor or microcontroller.

Most Piezoelectric Transducers are IP capable on the front diaphragm, some even up to IP68. IP67 means the transducer can be submerged into a body of water up to a meter deep for 30 minutes and still operate. IP68 guarantees protection in water up to 3 meters deep also for 30 minutes and still operate. Both IP ratings are dust proof.

Electromagnetic Transducers are used when a design has tight width constraints or low frquency requirements. They consist of eight components that are quite small and are made with highly automated coil winding and assembly equipment.

1. Upper Housing
2. Magnetic Plate
3. Diaphragm
4. Permanent Magnet
5. Coil
6. Yolk Plate
7. Lower Housing
8. Terminals