Dear colleague,

I think this link will be useful:

https://web.archive.org/web/20110720050202/http://www-kyg.stanford.edu/khuriyakub/opencms/en/research/cmuts/general/index.html

Dear Ms. Rada Savkina,

Thanks for the link.  Still, how cMUT overcomes the issues of air coupling compared to piezo transducers is not clear ...

You may find the paper linked below interesting. Piezoelectric materials are used to manufacture air-coupled ultrasonic transducers as they can generate ultrasonic waves in air.

http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1535957

Dear Mr. Benjamin,

Thanks for the link...paper discussed about air coupling of 450 kHz. it also says, very little can be done about the acoustic impedance mismatch.

I found one more literature (see the link) which says,

For harmonic waves, the mean value of sound intensity (J) is given by,

J = 1/2 * Z * (w*dmax)2

Where,

Z - Acoustic impedance (product of sound velocity and specific density of the medium)

w- angular frequency (2*pi*f)

dmax - Max deflection of the oscillating material particles in the propagation medium.

For a given dmax, the sound intensity in any gaseous medium is very low compared with liquids or solids. Therefore, in air a high dmax is needed. This requires large mechanical deflections of the radiating surface of an ultrasonic transducer

operating in air.   Does it mean, cMUT produces more deflection compared to PZT at a given frequency?  

http://iopscience.iop.org/article/10.1088/0957-0233/3/3/001/pdf

What frequencies are you interested in?

Take a gander at the paper below. It explains how cMUTS couple well with air due to their increased bandwidth compared to piezo and an impedance mismatch

Dear Mr. Benjamin,

I am interested in cMUTs for 1 MHz in air.

There is no paper attached in your reply...

Hi, 

You can use matching layer for this purpose.