The Acoustics and Ultrasounds Department carries out research in the field of applied and physical acoustics. Measurement services to users include calibration of transducers and instruments, such as microphones, sound level meters, acoustic calibrators, sound sources. Testing activity concerns the measurement of the acoustic properties of materials.
Secondary calibrations of measurement microphones and of Sound Level Meters need new normalised methods in order to obtain more reliable verification of the instrumentation used in legal metrology. Therefore in 2005 effort was devoted to the improvement of free field and pressure comparison calibrations. The effect of the sound field (an approximation of plane progressive waves) generated by different sound sources in the comparison of sensors of different geometrical dimensions in a free field was studied. Analysis of the uncertainty was extended to the variations of microphone parts like protection grids for free field response and adaptor rings for pressure calibrations. The results will support IEC activity in working groups regarding microphone calibration and sound level meter verifications and are part of the EUROMET project 792. A sound level meter has been type approved according to the new standard IEC 61672 - part 2. The preparation of Draft B for EUROMET.A-K3 key comparison started, and the preliminary comparison of its results with those of the corresponding CCAUV comparison shows good agreement.
Main uncertainty components of pressure to free field correction of Sound Level Meters response.
New application software has been developed for the automated measurement of ultrasonic power emitted by commercial or laboratory standard transducers, that reduces the time necessary for a complete measurement and the drift in force measurements. The uncertainty in the determination of the radiation conductance of the transducer (the main parameter used in laboratory intercomparison) has been lowered with the use of thermal voltage converters for the measurement of the driving voltage in the range 0.5 MHz to 20 MHz. EUROMET project 879 "Bilateral comparison of ultrasonic power (10 mW to 15 W) in the frequency range from 1.8 MHz to 11 MHz" has been approved, and the comparison between PTB and INRIM will take place at the beginning of 2006. Co-operation with a company internationally active in the field of ultrasonic diagnostics has been active in the field of calibration of radiation force balances, and in the study of the field emitted by ultrasonic transducer. A new scanning tank apparatus for the characterisation of the ultrasonic sound field in the space around the transducer, especially useful for highly focused types, has been tested and the measurement of the parameter associated with safety in ultrasonic medicine has begun.
Carbon fiber support of Radiation Force Target.
Two apparatuses, one for the measurement of the compressive properties of the materials and one for compressive creep, have been developed, built and tested. New measurement services of these properties have been made available. The techniques for the measurement of structural damping, based on acceleration measurements and determination of impulse response, have been applied to materials and structures used in buildings, in particular roof covers. An omni-directional sound source has been designed and tested in the semi-anechoic room. It has been used for the validation of rooms for the measurement of sound power according to ISO standards.
Within the framework of the international research effort aiming at a possible new definition of the kelvin, INRIM is developing an experiment for the determination of the molar gas constant R and the Boltzmann constant k with ppm uncertainty. For the achievement of this result several experimental and conceptual improvements of the acoustic thermometry experiment are required. It is particularly necessary to reduce the uncertainty associated with the absolute determination of the volume of the resonator. Two different strategies, involving different perturbations of ideal spherical geometry, have been pursued, namely misalignment of the two hemispheres comprising the resonator, and the realisation of an ellipsoidal cavity. In both cases the geometrical perturbations are intended to increase the precision achievable in measuring multiple degenerate microwave modes, while maintaining the corresponding perturbation of the acoustic field within a few ppm. This work is carried on in cooperation with the Thermal Metrology Department.
As part of an ongoing research program to measure the physical properties of n-alkanes, we performed accurate measurements of speed of sound in n-Nonane along six isotherms for temperatures between 293.15 K and 393.15 K and at pressures from 0.1 MPa up to 100 MPa. The overall estimated uncertainty is less than 0.2%. The results were compared with literature values and the predictions of a dedicated equation of state. Comparisons show deviations within 1%. These results have been useful in the formulation of a new global equation of state for n-Nonane and in the development of improved thermodynamic models for hydrocarbon systems.
Deviations of experimental values of speed of sound in n-Nonane from the values predicted by a dedicated equation of state.
Ultrasonic waves have been used to generate new materials, in relation to their property of inducing the formation of particles of much smaller size and higher surface area than those achieved by other methods. The chemical effects of ultrasound arise from acoustic cavitation, that is the formation, growth and implosive collapse of bubbles in the liquid, which generates transient temperatures of about 7000 K, pressures of 2000 atm and a cooling rate of 109 K/s. These extreme conditions can drive chemical reactions such as oxidation, reduction dissolution and decomposition, and have been explored to generate metals, metal carbides and metal oxide. Recently we have performed the sonochemical preparation of MgAl2O4 that is an important material used in catalytic and optical research. Ultrasound synthesis allowed to obtain an amorphous product with a surface area of 220 m2/g.
FESEM image of a sample of MgAl2O4 calcined at 500 º C.