Acoustics

Ultrasonic transducers comprise a piezo-electric element that deforms when an electric voltage is applied to it. It deforms in the opposite way if the polarity is reversed. By rapidly changing the polarity the transducer can be made to vibrate and emit sound. Ultrasonic sound ranges in frequency from about 25 kHz to about 80 kHz in air. Lower frequencies become audible sound, whereas higher frequencies are absorbed too much by air to be of practical use.
[ Transducer ] Depicted here is the core of a -so called- open type transducer. It comprises a base and the piezo electric element onto which an impedance matching cone is mounted. The unit is less than 10mm in diameter. In the simulation the transducer is excited with a number of pulses at its resonant frequency, making the cone vibrate at increasing amplitude. This creates alternating wave of low and high pressure in the column of air above the cone, which is what creates the ultra-sound. When the drive pulse is halted the cone continues to vibrate for a while in a number of eigen modes. These cause complex wave patterns that need to be thoroughly understood.
Sound waves travel through air as longitudinal pressure variations. Because they use air as their transportation medium, they are affected by changes in that medium due to temperature, humidity, or flow. Because they are waves, they are diffracted, and attenuated and can form complex interference patterns. Understanding those diffraction patterns and their relationship with the characteristics of the transducer is subject of study.
[ SoundWaves ] On the left is an example of the study of sound waves, using the Finite Element Analysis program Dyna3D. In this case a sound wave is traveling through a straight channel until it reaches an opening at the end. There it forms the expected defracting pattern. What is of interest here though is that the corners of the channel act as sound wave sources and send pressure waves right back into the channel. Although in magnitude much smaller than the outgoing sound wave, such waves can obscure echoes from nearby objects and are therefore undesirable. This study is of importance to the development of the ATI/Autoliv Occupant Spatial Sensor System.

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