Electropneumatic transducers as secondary actuators for active noise control, part I: Theoretical analysis

A theoretical analysis is presented of an acoustic source which operates by the release of compressed air through an aperture, the area of which is made to vary with time. A distinction is made between sonic and subsonic compressed air sources (depending on the Mach number at the throat), and in both cases simple equations are derived that describe the acoustical characteristics of the device. The theory is also developed with a view to using the source as a secondary actuator in an active noise control system, and the pneumatic efficiency and linearity of compressed air sources are discussed. Although the sonic source has a high internal acoustic impedance and its output is almost linearly dependant on the aperture opening, it is shown to be very inefficient. The subsonic source can be much more efficient than the sonic one but the output is generally no longer a linear function of aperture opening. A simple method of linearizing such a source is discussed. A numerical comparison between compressed air sources and electrodynamic loudspeakers shows that the former offer a useful alternative for active noise control, especially when the secondary source has to act in an extreme environment. The subsonic compressed air source is particularly useful when efficiency is a major issue. Details and results of the experimental evaluation of a subsonic compressed air source built at the Laboratory of Acoustics of the Faculte Polytechnique de Mons (Belgium) are presented in companion paper (Part II). (C) 1999 Academic Press.