Mauri Luukkala

The impulse response and pressure nearfield of a curved ultrasonic radiator

A method of calculating sound pressure with the aid of the impulse response is applied to a circular concave ultrasonic radiator. First the impulse response of a curved radiator is derived and found to be expressible in closed form; then the impulse response function is used to give a closed-form formula for the pressure at the axis of symmetry. An approximate formula is shown to give satisfactory results in the region near the centre of curvature in the focal plane. Finally a complete pressure amplitude pattern of a curved radiator based on numerical computations is given.

Ultrasound detection and identification of foreign bodies in food products

Abstract This paper introduces a concept based on an ultrasound reflection measurement with an echo classifier which has shown ability to detect and classify foreign bodies (FBs) in commercial food samples. The probed products were different kinds of cheese and marmalade. The FBs ranged from bone to steel and their size were 1 to 14 mm in diameter. The frequency was 5 MHz and the probing depths were 25, 50 and 75 mm. The research showed that all of the FBs investigated could be detected and identified in one of the tested food products. The best detection temperature interval was 1–5°C. Discrimination of FBs was more difficult in the inhomogenous samples than in the homogeneous samples investigated.

Metal plate testing using airborne ultrasound

Abstract A method of exciting plate waves in metal plates is described in which the wave is generated from air without touching the sample. By using this method contactless non-destructive ‘on-line’ testing of metal plates is possible within a thickness range from 0.2 mm up to several millimetres. Defect resolution is best in thin plates.

Photoacoustic evaluation of elasticity and integrity of pharmaceutical tablets

A nondestructive method based on pulse photoacoustics was applied for evaluation of elasticity and integrity of pharmaceutical tablets. Variations in porosity, density and sodium chloride content of microcrystalline cellulose tablets were found to be related to parameters extracted from the through-transmitted ultrasonic wave forms. By using the amplitudes and ultrasonic velocities of these wave forms, it was possible to obtain values of a transverse to longitudinal amplitude ratio, and also elastic parameters, such as Youngs and shear moduli, for the tablets. Poissons ratio was calculated from the elastic moduli as well as from the amplitudes. An exponential relationship between tablet porosity and the attenuation of longitudinal wave form was noticed. The transverse to longitudinal amplitude ratio and the amplitudinal Poissons ratio were indicative of structural variations, e.g., changes in the porosity and the sodium chloride content of tablets. Youngs and shear moduli of microcrystalline cellulose tablets were found to follow similar porosity trends to those in previously published beam bending and twisting studies, although the absolute values and the values extrapolated to zero porosity were slightly smaller. The Poissons ratio calculated from the experimental Youngs and shear modulus values was also in agreement with earlier studies, but the values extrapolated to zero porosity differed significantly. The method is a promising tool for evaluating the elastic properties of tableting materials and the structural variations in tablets.

Factors affecting the sensitivity of electrostatic ultrasonic transducers

The sensitivity of an electrostatic transducer determines the range of pulse-echo detection. Maximum sensitivity can be achieved by increasing the bias voltage within certain limits and by using a light membrane. The tension of the membrane was not found to affect the transducers sensitivity. By using a uniformly grooved backplate the acoustic properties of the transducer can be optimized. The different sensitivity factors of the electrostatic ultrasonic transducer were confirmed with experimental results. High sensitivity can be achieved by using well manufactured backplates with shallow grooves and a small ridge angle.

A model for an electrostatic ultrasonic transducer with a grooved backplate

A model has been constructed for an electrostatic ultrasonic transducer with a uniformly grooved backplate. The membrane and the groove pattern of the transducer was divided into individual elements, and each element was treated as a Helmholtz resonator. The resonant frequency of the transducer can be calculated using one single resonator element. The calculated resonant frequencies were compared with experimental results.

Paper roughness measurement using airborne ultrasound

Abstract Paper roughness has been studied using high-frequency airborne ultrasound. The measurement principle is based on the attenuation that occurs when ultrasound is reflected from a rough surface. Paper samples have been tested and results compared with data from conventional air-leak measurements. The proposed method is non-contacting and it allows the possibility of developing fast and rugged applications for the laboratory or for on-location product testing.

Measurement of paper-wetting processes by ultrasound transmission

Understanding of wetting processes for papers of various qualities is important for the analysis and the adjustment of paper-production processes as well as for designing properties of paper for printing. In this investigation, papers of one kind with three different sizing-content values were examined by time-dependent attenuation of ultrasound transmission in mixtures of isopropanol and water. The attenuation depends on the stage of absorption of liquid into the paper sample, which furnishes the signature of the paper type and quality. Papers that have contact angles of 40° (without size), 70° (low size content) and 110° (high size content) were tested and a measurement procedure was developed. The measurement results showed that the development of attenuation with time differs at different frequencies, indicating the possibility of detecting various phases of wetting phenomena.

Sound pressure near the focal area of an ultrasonic lens

A method of computing the pressure distribution of an ultrasonic single-surface lens is derived. The method is based on an impulse response method used in pressure field calculations of spherically curved ultrasonic transducers. By a simple transformation the lens is transformed into a curved transducer having the same focusing properties as the lens. The pressure distribution of this transducer is then computed using the rapid impulse response method. Expressions for focal point size and pressure gains are given. An example of the pressure distribution near the focal point of a lens is given pictorially.

Air-coupled ultrasonic measurement of the change in roughness of paper during wetting

We investigated the change in surface roughness of paper during wetting. Four kinds of paper were examined by time-dependent attenuation of focused 1.2 MHz airborne ultrasound reflected from a paper sample. The samples were wetted with fog from mixtures of water and isopropanol and the transmitted ultrasonic signal was measured at 15 ms intervals for 16 s after the application of the mixture. The results showed that the attenuation of the reflected sound by time differs for various kinds of paper, indicating that there are differences in the development of wetting.