I have some queries regarding the ultrasonic resolution. i did some study on ultrasonic transducer. i placed a transmitter & at the other end a receiver probe on the verticle column surface (non-invasive, with couplant) consist of water. my reason is to study/measure the air bubbles velocity released from the bottom of the column.
if my freq is 40kHz, and taking v= 1500m/s (water), from v = f*lamda, the wavelength of the sound is 37.5mm.
so, what is the minimum bubble size could be detected by the receiver probe? is it equivalent to the wavelength? because i did try smaller than the wavelength (25mm) and the receiver probe still gives the response.
The detection of single bubbles has nothing to do with resolution. This would be the case, if you would be interested in differentiation between two bubbles situated close to each other. In you case the question will be: how strong is the signal reflected by the bubble and what is the sensitivity of your probe and electronics. The wave length is not significant in this case. It can be much larger than the bubble size.