Index of content:
Volume 104, Issue 4, October 1998
- SELECTED RESEARCH ARTICLES 
104(1998); http://dx.doi.org/10.1121/1.423761View Description Hide Description
Electrostrictive driver materials with large strain capability hold great promise for the advancement of sonar projector technology. However, the nonlinear induced strain of these materials can create acoustic distortion in transducers through higher-order harmonics. Electrostrictors also possess complicated prestress and temperature dependencies, and an elastic modulus that depends strongly on electric field. This investigation examined these issues with a nonlinear, frequency domain model for a flextensional transducer powered by an electrostrictive stacked actuator. A simple, linear lumped-parameter model of a flextensional shell and its surrounding acoustic medium were combined with a nonlinear model of the electrostrictive driver. This model accounted for the material’s nonlinear dependencies and behavior. Predictions of the device’s acoustic/electric response during operation compared favorably with experiments performed on a single element flextensional transducer. The model’s results showed that proper adjustment of the power supply’s parameters minimizes the level of distortion without completely sacrificing the transducer’s improved source level.
Vortex sound in bass-reflex ports of loudspeakers. Part I. Observation of response to harmonic excitation and remedial measures104(1998); http://dx.doi.org/10.1121/1.423760View Description Hide Description
At high sound pressure levels a bass-reflex port produces blowing sounds, especially in the case of small loudspeaker boxes with narrow bass-reflex ports. The blowing sounds are caused by vortex shedding of the acoustic flow at the end of the port at high flow velocities. It has been found that acoustic standing waves in the longitudinal direction of the port are excited in a pulsatile manner by the periodically generated vortices. This is demonstrated by time history measurements of the blowing sounds of a loudspeaker system with a bass-reflex port driven by a harmonic signal. Broadband turbulencesound appears to be weaker than these deterministic sounds. It has been found that, near the 1-kHz port resonance frequency, the power level of the blowing sounds can be reduced by 8 dB by using a port cross section that diverges gradually toward both port ends with a slope angle at the port ends of about 6°, and rounding the edges at both port ends.
Vortex sound in bass-reflex ports of loudspeakers. Part II. A method to estimate the point of separation104(1998); http://dx.doi.org/10.1121/1.423762View Description Hide Description
In part I of this paper, the vortex shedding that may occur in a bass-reflex port of a loudspeaker system was discussed. At the Helmholtz frequency of the bass-reflex port, air is pumped in and out at rather high velocities,vortex shedding occurs at the end of the port, and blowing sounds are generated. It was explained that the key in the design of a port with a minimum of blowing sounds is the point of flow separation from the wall at which vortices are formed. This paper presents a method for estimating the point of separation for an unsteady flow like the flow through a bass-reflex port. Assuming that the flow can be described by a potential flow up to the point where flow separation occurs, it was found that the point of separation can be estimated on the basis of measurement of the sound pressure inside the loudspeaker box and measurement of the sound pressure at a distance of 1 m from the port exit. Application of the proposed technique to a cylindrical port with rounded edges at both port ends revealed that the point of separation is determined by the particle displacement rather than by the particle velocity. It was also found that a good indicator of the onset of severe vortex shedding is the Strouhal number based on the radius of curvature of the port edges.