it is called piezoelectric ceramic products, ultrasonic transducer. it is ultrasonic product by piezoelectric effect.

Middle-higher frequency series (33-60 KHz Type             Frequency(KHz) Capacitance(pF) Impedance(Ω) DimensionD*H(mm) Power(W) Isolation Resistance (2500VDC) Notes HNC-8SS-2540 40±0.8 3100±10% ≤30Ω 25*58 20w ≥100MΩ   HNC-8SS-3045 46±1.0 2400±10% ≤25Ω 45*43 35w ≥100MΩ   HNC-8SH-3833 33±0.5 3800±10% ≤20Ω 48*61 60w ≥100MΩ   HNC-8SH-3840 40±0.5 3800±10% ≤20Ω 48*51 60w ≥100MΩ   HNC-4AS-1550 51.0±2.0 1700±10% ≤50Ω 15*58 10W ≥100MΩ   HNC-4AS-1540 40±2.0 3400±10% ≤30Ω 15*58 15W ≥100MΩ   HNC-4SS-2540 40±0.8 2400±10% ≤30Ω 25*58 20w ≥100MΩ   HNC-4SH-2540 40±0.8 2400±10% ≤30Ω 30*50 20w ≥100MΩ   HNC-4AH-2560 60±1.5 2300±10% ≤35Ω 30*35 20w ≥100MΩ   HNC-4AS-2565 65±1.5 2300±10% ≤35Ω 25*31 20w ≥100MΩ   HNC-4AS-3055 55±1.0 3100±10% ≤35Ω 30*45 35w ≥100MΩ   HNC-4AH-3050 52±2.0 3100±10% ≤35Ω 38*40 35w ≥100MΩ   HNC-4AH-3540 40 ±1.0 4000±10% ≤20Ω 45*55 50w ≥100MΩ   HNC-4SS-3833 33±0.5 5100±10% ≤20Ω 45*61 60w ≥100MΩ   HNC-4SS-3835 35±0.5 5100±10% ≤20Ω 45*55 60w ≥100MΩ   HNC-4SH-3840 40±0.5 5100±10% ≤20Ω 48*51 60w ≥100MΩ