Can ultrasonic signal be guided through a tube?

Question

I have an industrial application using an ultrasonic sensor to detect whether transparent film is present in a plastic ring. Ring minimum diameter is > 30mm. Sensing distance is 90mm. Sensor transducer looks about 10 - 15mm diameter - it's covered by a membrane so I can't measure easily. I don't know the frequency but let's assume 20kHz giving a wavelength of about 18mm.

Film is nominally at right angles to ultrasonic beam but I'm finding that when it's distorted the sensor doesn't detect. It's as though the sound is reflected off-axis by the angled film. I can't increase the sensitivity because I'll start sensing the background.

+-------------+                           |          |
| Sensor      ) ) ) ) ) ) ) ) ) ) ) ) ) ) |          |
+-------------+                           |Target    |Background

One option I'm considering is making an ultrasonic waveguide as sketched below.

+-------------+ ----------------          |          |
| Sensor      ) ) ) ) ) ) ) ) ) ) ) ) ) ) |          |
+-------------+ ----------------          |Target    |Background
                 waveguide tube

The idea is that I could guide the wave to within 30mm of the target and better capture the echo and guide it back to the sensor.

My question is what tube diameter would form an optimum waveguide given the 18mm diameter wavelenght? Or is there a reason this is a bad idea? I'm also assuming a steel or hard plastic tube with smooth internal bore would improve my chances.

Many thanks.

[Edit 2015-02-27] The manufacturer has replied, "Adding a sonic tube or choke to the sensor may help with the problem. It should have an ID of 12-14mm and no longer than 20mm with the current software configuration in the sensor."

+-------------+ ------                    |          |
| Sensor      ) ) ) ) ) ) ) ) ) ) ) ) ) ) |          |
+-------------+ ------                    |Target    |Background
                20mm waveguide tube

Can anyone suggest what physics principle would be involved with this solution? i.e., Why would this work in preference to me extending the tube as close as possible to my target? Why would he refer to it as a choke?

Answer 1

I suggest that you don't want to pass ultrasonic signals through air.

I worked in an ultrasonic lab once and we used solid crystals and water to pass the signal.... I would suggest that this is the way to go rather than air

send waves down metal rod perhaps and have water to couple to the sample if this is possible... or even a rubber foot if water is not possible - or put the metal straight onto the sample.

***************** edit after comment ************

ok so air works for you...

I would say the key thing is the length of the tube - I guess it would be most efficient to have a standing wave set up with either a whole or integer plus a half number of wavelengths distance in air (or plus quater wavelength) - that would be my first approach to optimize the system, but maybe you tried that already...

youtube video about stading waves in pipes

text about standing waves in pipes...