How to Squeeze Piezo Acoustic Transducers into Shrinking Devices

What you’ll learn:

• The wide application of piezoelectric technology.
• Why trends to miniaturize devices while retaining precision present challenges for design engineers.
• How multiphysics software tools can address the inherent multiphysics challenges of designing piezoelectric acoustic transducers.

The increasing miniaturization and sophistication of electronic products, ranging from consumer media devices to medical diagnostic tools to defense-related sonar applications, presents a bounty of utility and ease for consumers—and an ongoing challenge for design engineers. These seemingly disparate products (audio/mobile device speakers, certain non-invasive medical devices, and sonar arrays) share in common a reliance on piezoelectric transducers to both generate and receive acoustic signals.

Piezoelectric materials have been valued since the first half of the 20th century for their ability to convert mechanical energy into electrical energy and vice versa. However, 21st century technology demands that these same materials produce more sound or more precise frequencies within smaller and smaller packages, all while utilizing as little energy as possible.

The challenge of designing piezoelectric-containing devices is inherently multiphysics in nature due to the confluence of electricity, vibration, and acoustics. Thus, designers must have tools that can calculate the multiple physics within their products.

Piezoelectric Material Overview

Piezoelectric materials are materials that can produce electricity due to mechanical stress, such as compression. These materials also can deform when voltage (electricity) is applied. Typical piezoceramic materials, whether non-conductive ceramic or crystal, are placed between two metal plates.

To generate piezoelectricity, the material must be compressed or squeezed. Mechanical stress applied to piezoelectric ceramic material generates electricity. The piezoelectric effect can be reversed, which is referred to as the inverse piezoelectric effect. This is created by applying electrical voltage to make a piezoelectric crystal shrink or expand. The inverse piezoelectric effect converts electrical energy to mechanical energy.

Piezoelectric materials are found in a surprising array of everyday products. The flame that leaps to life when you press the button of a “click-and-flame” lighter was aided into existence by the compression of piezoelectric material, which produces a spark.

Now, let’s look at some other products that present more of a challenge for design engineers due to the need for increased output within smaller devices.

Mics and Speakers

Piezoelectric materials are used extensively in acoustics. Microphones contain piezoelectric crystals that convert the incoming sound waves into signals that are then processed to create outgoing amplified sound. Small speakers, such as those within cell phones and other mobile devices, also are driven by piezoelectric crystals. The device’s battery vibrates the crystal at a frequency that produces sound.

The challenge here is in designing piezoelectric transducers that can produce very-high-quality sound within a small package, and without draining too much of the device’s battery.

Medical Devices

Non-invasive medical devices such as hearing aids also rely on piezoelectrics for a portion of their operation. So, too, does ultrasound technology, which is a major application of piezoelectric material.

In ultrasonics, piezoelectric materials are electrified to create high-frequency sound waves (between 1.5 and 8 MHz) which are able to penetrate bodily tissues. As the waves bounce back, piezoelectric crystals convert the received mechanical energy into electrical energy, sending it back to the ultrasound machine for conversion into an image.

Other medical devices such as harmonic scalpels utilize piezoelectric materials’ vibrational properties to cut and cauterize tissue during surgery. The piezoelectric crystals within the device generate both the kinetic energy and heat energy needed to simultaneously cut and cauterize.

Ultrasonic design challenges focus on the need to determine the correct shape and material composition of the piezoelectric components to create the very precise frequencies used in ultrasound. And, in the example of harmonic scalpels, the design must account for the effects of heating on the device’s vibrational response.

Sonar

Perhaps the broadest and most long-standing use of piezoelectric technology can be found within sonar applications. During World War I, sonar was the first commercial application of piezoelectricity, and its use skyrocketed in the period between the two world wars. 

Today, all sonar-based systems, including those used by the military, commercial fishermen, and in numerous other marine applications, utilize a piezo-containing transducer to both generate and receive sound waves.

It seems simple, but designing transducers for the propagation of sound through water rather than air can present its own set of complex engineering challenges. These applications often require the piezoelectric device to generate high-power signals to propagate long distances without attenuating below detectable levels.

New Uses

An emerging application of piezoelectric materials is within energy-harvesting technology. Because of the unique properties of piezo materials, they can be successfully used in any application that requires or produces vibration.

In energy harvesting, exogenous vibration produces a mechanical strain to the piezoelectric material that’s converted to electrical energy. That piezo-created energy can then be used to power other components of the device or system.

Battery-independent tire-pressure-monitoring systems (TPMS) represent one such example. As a vehicle’s tires rotate, mechanical energy is produced. A piezo-containing sensor harvests that energy, stores it, and sends a signal to the driver’s display panel. TPMSs have historically been battery-powered, but increasing interest in environmentally friendly battery alternatives has led to a new focus on the energy-harvesting potential of piezoelectric materials.

Old Discovery, Modern Challenges

Although piezoelectric materials have been utilized for over a century, the current need for their application within smaller and more complex products presents a challenge for design engineers. Choosing the correct materials and designing the right crystal shape are critically important to the functionality of a prototype.

Piezos have very complex material properties that are highly intertwined, and material composition matters. Similarly, if the shape of a piezoelectric crystal doesn’t produce the correct resonant frequency, the device won’t work. And, in elegant lockstep with the “Observer Effect,” the very electrification of a piezoelectric crystal deforms its shape while also producing more electricity.

It’s an incredibly complicated feedback loop crying out for a design solution that eliminates the guesswork involved in lengthy build-test prototype processes. 

Why Simulation Matters

Simulation is always helpful when dealing with nonlinearities. It prevents designers from the thankless (and often budgetarily unfeasible) task of building and testing amid too many unknowns. When considering electroacoustic transducers, the unique combination of electrical energy, mechanical energy, and acoustics is decidedly nonlinear, and inherently multiphysics in nature.

Multiphysics simulation can provide design engineers with the tools to develop products more effectively by enabling them to simulate their device designs within operating conditions. In addition, these simulations may include the entire ecosystem from control circuit to piezoelectric transducer to surrounding acoustic environment. Multiphysics simulations will take into account factors such as:

• The constitutive equations of mechanical and electrical response
• Poling direction of piezoelectric material properties
• Boundary conditions
• Structural mechanics/vibrational heating

As piezoelectric-dependent devices become smaller and more complex to meet the demands of sophisticated consumers (be those individuals or industries), design engineers must have tools that calculate the multiple physics within their products. Multiphysics simulation tools can provide clarity and direction to complicated design challenges.

You can find out more about piezoelectric technology by watching the Designing Piezoelectric Acoustic Transducers with Simulations webinar.