Influence of ultrasound on air pressure levels I am wondering if Ultrasonic waves can influence air pressure. Would there be change in air pressure in the order of Pascals where ultrasound waves pass?
 A: Ultrasound (like regular sound) IS a change in air pressure. Sound is defined as a pressure wave in a fluid medium and for sound to happen, pressure must change. This pressure change is typically a small modulation of the steady state pressure of the medium. 
A: Like Hilmar mentioned, ultrasound refers to the frequency of the excitation. In the linear treatment of sound, this does not change anything with respect to the solution. Sound is indeed pressure variations (or differentials in the language of calculus). Most often, pressure deviates from a constant ("DC") value, which for sound in air is the atmospheric pressure.
Nevertheless, due to the relatively high frequency, which corresponds to shorter wavelengths for constant speed (a reasonable approximation for low to mid pressure amplitudes) some phenomena such as diffraction are not so prominent. While on the other hand, attenuation is a major issue here and this is why many applications of ultrasounds are applied in solids (such as non-invasive testing). For an introduction on the treatment, you can have a look at the book Acoustics: An Introduction by Heinrich Kuttruff.
Other applications of ultrasounds include room scale model excitation (see Modelling in Auditorium Acoustics - From Ripple Tank and Scale Models to Computer Simulations article by Jens Rindel), medical applications (see the The Medical Applications of Ultrasonics article by Wells) and sensing (see the Precision Ultrasound Sensing on a Chip article by Esfahani et al. and some Ultrasonic Sensors by Banner). For more general information one can have a look at the Ultrasound - Wikipedia link.
Now, regarding the pressure wall, I haven't found anything similar so far. From my small experience on spatial audio and sound fields, I believe you would have to create a spatial "discontinuity" (or at least an abrupt change) in pressure, to create a big difference in the impedance at this point. This is definitely not a trivial task to achieve though.
For more information on manipulating wavefields, you can have a look at the Local Control of Audio Environment: A Review of Methods and Applications paper by Kuutti et al. (although this is not a pure ultrasound treatment). In addition, you may want to have a look at active noise control techniques (Active Noise Control: A Tutorial Review by Kuo and Morgan and references therein can serve as a general introduction and Generation of Zones of Quiet Using a Virtual Microphone Arrangement article by Garcia-Bonito et al. can prove to be a good starting point for a possible application).
In addition, you could also have a look at sound field synthesis techniques. A quite instructive starting point could be the Analytic Methods of Sound Field Synthesis book by Jens Ahrens.
I know I couldn't provide a direct solution to your "problem" but I hope you'll manage to find something useful in the references.
A: Thank you for the replies. I want to create localized pressure wall in air of a height of about 40 mm. Is it possible to have a pressure wall with ultrasound waves? It would be wonderful if someone can point me to any publication or an equation for the same.
Thanks again
