hc-sr04 measures distance from what point?

Hi,

I know this sensor is not the most accurate, but I was wondering if anyone knows from what point the HC-SR04 ultrasonic sensor measures from? Is the distance measured from the board, or from where the signal is actually sent out, or is it from the top of the plastic casing for the transmitter/receiver?

Thanks

From the vibrating diaphragm of the transducer.