The product shown is a piezoelectric ceramic element or disc, commonly used as a sensor or transducer. 

Product Description:

• Functionality: These discs utilize the piezoelectric effect to convert mechanical vibrations, pressure, or impact into electrical signals, or vice versa, to generate sound when an electrical voltage is applied. 
• Construction: Typically, they consist of a brass outer plate and a ceramic inner plate, ensuring good conductivity and durability. 
• Dimensions: Common sizes include a 27mm or 35mm brass outer diameter. 
• Features: They are known for their high sensitivity, reliability, and stability. They are also ultra-thin, lightweight, and offer low power consumption for voltage-type applications. 
• Applications: Highly versatile, they are ideal for various uses, including:
  • Acoustic instrument pickups (e.g., guitars, drums, stompboxes). 
  • Contact microphones. 
  • Buzzer or sounder applications when glued to a surface. 
  • Percussive sensors in musical instruments or drum pads. 
• Specifications (for a 27mm disc example):
  • Resonant Frequency: Approximately 4.6KHz. 
  • Resonant Resistance: 300 ohm max. 
  • Capacitance: Around 16,000pF +/- 30%
  • The image displays Piezoelectric Ceramic Elements with Leads, commonly known as Piezo Buzzers or Sensors. These versatile components utilize the piezoelectric effect to convert mechanical energy (like pressure or vibration) into electrical signals, and vice-versa. 
    Key Features:
    • Piezoelectric Effect: Generates an electrical charge when subjected to mechanical stress (e.g., pressure, vibration, sound waves).
    • Reverse Piezoelectric Effect: Deforms mechanically when an electrical voltage is applied, enabling sound generation or precise actuation.
    • Construction: Typically comprises a thin piezoelectric ceramic disc (often Lead Zirconate Titanate, or PZT) adhered to a metal plate, with pre-soldered wires for easy connection.
    • Compact Size: Available in various diameters (e.g., 15mm, 20mm, 35mm), making them suitable for space-constrained applications.
    • Low Power Consumption: Piezoelectric buzzers, especially, require minimal current for operation compared to electromagnetic buzzers. 
    Common Applications:
    • Sound Generation: Used in buzzers, alarms, speakers, and musical instruments (e.g., acoustic guitar pickups).
    • Sensors: Employed as vibration sensors, knock sensors, drum triggers, and even in medical equipment for heart rate monitoring or ultrasound.
    • Actuation: Can be found in applications requiring precise, small-scale mechanical movement, such as micropositioning devices or inkjet printer heads.
    • Energy Harvesting: Explores the potential to convert ambient mechanical energy (like footsteps) into electrical energy. 
    These elements are valued for their simplicity, reliability, and ability to convert between electrical and mechanical energy efficiently across a range of applications.