New earplug can protect hearing while preserving sound quality

Hearing loss is irreversible and the most common work-related injury worldwide. Prevention is the only way to tackle this issue. But who has not been annoyed by how foam earplugs make a rock concert sound muffled and bass-heavy? or struggled to hear a colleague when wearing bulky ear protectors in a noisy workplace?

Whether it’s to improve the music experience or to hear your colleagues say, it’s easy to give in to the temptation of lifting or even completely removing your hearing protection in these situations. This exposes us to dangerous noise levels and has made permanent hearing loss a global public health issue.

Developing a technology that helps prevent this was the driving motivation behind the SINTEF and Minuendo collaborative project, “Picovatt.” The result? A piezoelectric microelectromechanical system (piezoMEMS) with volume control and no sound distortion, developed at SINTEF MiNaLab, (part of the NorFab infrastructure), enabling volume control without sound distortion. As a bonus, the chip also functions as both a microphone and speaker.

Passive and active hearing protection

Today, passive and active hearing protectors exist. Foam earplugs and ear muffs mentioned earlier are passive, meaning they lack integrated electronics or advanced functionality. Active hearing protection uses built-in microphones, signal processors, and speakers to adjust and control sound levels based on the environment.

But these headphones can be bulky, uncomfortable, and produce an unnatural sound. Despite the availability of active hearing protection, passive options still dominate the market.

Our hybrid earplug combines the best of both worlds: it’s small, adjusts the volume without distorting the sound, and is far less power-consuming than conventional active earplugs.

Rock concert with volume control

Minuendo, spun off from SINTEF in 2018, sells critically acclaimed hearing protection worldwide. In their current version of the earplug, shown on the left in Figure 1, the sound volume is adjusted by opening and closing a small slit using a tiny manual lever. The sound passes through naturally, with a manually adjustable damping of 9 to 25 dB.

The new piezoMEMS chip, developed in the Picovatt project, electronically opens and closes multiple slits by bending small “beam structures,” or tiny “lids” situated on top of each slit.

This allows volume control ranging from nearly 0 dB (open) to 40 dB (closed). The active part, the actuator, consists of thin silicon coated with micron-thin piezoelectric layers of lead zirconate titanate (PZT). PZT contracts in-plane when a voltage is applied, “pulling” the surface and bending the beam upwards.

At 30 volts, the tip of the beam lifts about 100 micrometers, allowing sound to pass without damping. When the voltage drops, the slit closes.

By varying the voltage, you thus have an earplug that lets you stand at the front row of a rock concert and adjust the volume to a safe and comfortable level, with sound quality unchanged. No more muffled, bass-heavy, distorted sound. Just pure and clear rock n’ roll at the perfect safe volume.

Attenuator, speaker, and microphone

The hybrid earplug does more than volume control; it can also play sounds or other audio signals and work as a microphone. In many cases, monitoring the outside noise levels is desirable so that the system can adjust its attenuation to the situation. It is, therefore, quite convenient that piezoelectric materials work both ways: small mechanical vibrations from sound waves generate electrical signals that can be readily picked up.

Therefore, the same chip functions as both a speaker and a microphone. This was also demonstrated in the Picovatt project. In fact, it works surprisingly well as a speaker and has very useful microphone properties. This way, the same component can serve as a “three-in-one” solution.

Facts about piezoMEMS and acoustic filtering:

• Sensors convert physical input into an electrical signal. Actuators convert an electrical signal into mechanical motion. Both allow microprocessors to interact with their surroundings.
• Piezoelectric materials convert electricity into motion and vice versa, a process known as electromechanical transduction.
• Thin piezoelectric layers can bend small structures, such as microbeams in the earplug, with less than one microwatt of power.
• Piezoelectric microelectromechanical systems, or piezoMEMS, are micron-sized piezoelectric structures used as sensors and actuators.
• The hybrid earplug is an example of how piezoMEMS can miniaturize and enhance acoustic “circuits” made up of membranes, tubes, or slits that form a filter.
• PiezoMEMS can be integrated into large silicon wafers, allowing for cost-effective mass production.

Smaller, cheaper, better

Like most technology today, it is also a desire that hearing protection and headphones become smaller, more functional, and cheaper, with increased battery life. Thin-film piezoMEMS make this possible.

As shown on the right in Figure 1, the prototype is about one-ninth the size of a coin. Even though it already works well in an earplug, future models could shrink 10 times more. The energy consumption of just a few hundred nanowatts would be even lower, allowing for smaller batteries and longer-lasting earplugs. At the same time, more chips can fit on each silicon wafer, reducing the price further. All in all, chips that are smaller, cheaper, and better.