Electret Sheet Sensor Technology

Electret Sheet Sensor

In its quest for better sound, Yamaha has long been engaged in the research and development of sensing devices that detect the sound of musical instruments and the vibrations that are the source of that sound. As part of this effort, we have teamed up with the Kobayasi Institute of Physical Research and YUPO CORPORATION to develop the BeatSticker™ sensor, which is made from Yupo synthetic paper. BeatSticker is a sensing device that traps an electric charge in Yupo synthetic paper, and generates electricity when the thickness of the paper changes.

Yupo Synthetic Paper

How the Sheet Sensor Works

Yupo synthetic paper, which is used for election posters and election ballots, has excellent durability and flexibility. Microscopically, it is a porous sheet that looks like a pie crust, with a multi-layered structure and many spaces between each layer. When Yupo synthetic paper is irradiated with a corona discharge to trap an electric charge, a polarized electret sheet is formed.

Element Generating Voltage Proportional to Change in Thickness

As the thickness of the electret sheet changes, the size of the polarization also changes. By extracting this change as an electrical signal, the sheet functions as a sensing device.

The sheet itself is like a very soft spring, and the electrical signal generated is proportional to the change in thickness of the sheet. The polarity of the electrical signal can be switched by switching the connection between the sheet and the electrical signal detector. If there is no change in sheet thickness, no signal is output, so no static load is detected.


How can we use such a sheet to create a convenient device? First, let’s consider what elements (forces) can change the thickness of the sheet.

1. Sound Pressure

Sound pressure is the oscillation of air pressure. A sheet that is free to move on both sides can be used to detect sound pressure, as it collapses and swells according to the density of the air pressure passing through it.

2. Direct Load

A sheet that is fixed on one side will collapse or expand with the push and pull of a force applied to the side that is free to move. This makes it possible to detect the magnitude of the force and the amount of displacement.

3. Acceleration Load

A sheet with a mass on one side will have a mass-spring mechanism. When a sheet in this state oscillates, the sheet collapses or expands due to the force applied to the mass in response to acceleration, which allows acceleration to be detected.

Porous sheets such as Yupo synthetic paper are extremely light and have low elasticity, so they respond to even a slight load or pressure. Additionally, the durability of Yupo synthetic paper allows it to be combined with a strong spring or mass. The detection mechanism, shape, sensitivity, and bandwidth can be freely designed according to the sound and oscillation to be detected, as well as the location where it is to be used.

Use with Atmosfeel™ Guitar Pickup System

The Atmosfeel pickup system that is available in the FG/FS Red Label series of electric-acoustic guitars uses different sensors to pick up the vibration of the strings, the soundboard, and the body, allowing the player to enjoy creating a variety of sounds.

Atmosfeel Pickup System

The BeatSticker is used in the Atmosfeel pickup system as a contact sensor that picks up the vibration of the soundboard.

– Microphone (resonance in the body of the guitar)
– UnderSaddle PickUp (vibration of the strings under the saddle)
– Contact Sensor (vibration of the soundboard: BeatSticker)

Although contact sensors have been used before to pick up the vibration of the soundboard, they have not been able to achieve stable characteristics in the high frequency range. Contact sensors that use BeatSticker are packaged as a mass-spring mechanism that detects vibration acceleration. The layering of soundboard, element, and mass allows the sheet to act like a spring.

By taking advantage of the characteristics of the porous sheet (lightweight, low elasticity) and setting the mass appropriately, the vibration characteristics of the soundboard are not compromised. As a result, BeatSticker, a contact sensor with good high frequency characteristics, contributes to the unique sound quality of the Atmosfeel pickup system.

Future Development

We have introduced the Atmosfeel pickup system as an example of the use of electret sheet sensors, but guitars are not the only musical instruments from Yamaha. We offer many different types of stringed instruments, as well as keyboard instruments, percussion instruments, wind instruments, and even the famous Yamaha Silent Series.

The vibrations that are picked up by the sensor will vary depending on where it is mounted on the instrument. We are continuing our research and development to select the most suitable sensors for sound pressure, load, and acceleration, in order to bring out the best sound from each instrument.

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