6E-4 Acoustic ID-Tags for Under Water Use

Passive ID-tags that could be read remotely is an area of growing commercial interest. Here we look at a small tag in the low MHz range for under water use, based on acoustic resonances. It could be used in sea water, where RF-signals are heavily damped, and it could easily be made small, as there is no need for bulky antennas and hence will have an advantage over the more common RF-ID-tags based on electromagnetic waves. The tag is intended for an as yet unspecified laboratory use, where we may assume a moderate distance between tag and detector system. As a design example we look at a circular tag with diameter 300 micrometer, and resonances in the 1 to 4 MHz range. The resonators are circular nitride membranes over an evacuated cavity in the tag substrate. We review the analysis of the resonators, which is based on coupling of the acoustic vibration modes of the membranes and the tag substrate through the fluid A detailed response of an ID-tag with on the order of ten to twenty resonators may be found easily and fast from predetermined data for resonators and substrate. Both radiation losses and viscosity losses in water are included. It is found that resonator Q´s are limited to about 20 in water at 4 MHz, and further that the viscosity leads to a small shift in the resonance frequencies. The number of identities for a tag design as discussed here could be in the range 50,000 to 100,000 or more.