External Drive piezo element, namely two pole piezo element or piezo disc, it is made of substrate (copper,steel,iron,alloy) and piezoelectric ceramic piece processing, only two contact points, can weld positive and negative two lines. Other buzzer is usually used for passive buzzer, FT-15T-6.0A1, 15T stands for diameter of 15mm, frequency of 6.0khz. The frequency of the acoustic element is determined by the thickness. The thinner the thickness, the higher the frequency. piezo element is a voltage element, that is to say, the greater the voltage applied to the device, the louder the device will be. The sound of the piezo element is extremely clear. FBelec China leading provider of piezo disk of various sizes and frequencies