IEC 61828:2001 pdf download

IEC 61828:2001 pdf download.Ultrasonics – Focusing transducers – Definitions and measurement methods for the transmitted fields.
IEC 61157:1992, Requirements for the declaration of the acoustic output of medical diagnostic ultrasonic equipment
IEC 61689:1996, Ultrasonics — Physiotherapy systems — Performance requirements and methods of measurement in the frequency range 0,5 MHz to 5 MHz
3 General
The information contained in this clause is an introduction to the definitions given in clause 4
and the measurement methods given in clause 6.
3.1 Focusing transducers
The term •focu.ing transducer”2 is commonly used for a device which has a smaller beamwidth in some regions of the field than a device which is non-focusing. A nonfocusing transducer can still have a natural focus, so it is necessary to distinguish a focusing transducer as having a greater concentration of pressure amplitude (for a given power output) than a non-focusing transducer at its natural focus For example, a non- focusing transducer made of a simple disc of uniformly poled piezoelectric material has a beam whose intensity at its natural focus can be as much as four times the average intensity at the source, and whose —6 dB beamwldth can be approximately half of that at the source A definition of a focusing transducer is given in 4.2.33 to make a quantitative distinction between focusing and non-focusing transducers.
3.1.1 Focusing methods
The simplest means of intentionally focusing an utrasonle transducer, borrowed from analogous optical principles, Is that of shaping the ultrasonic transducer Into a concave form or adding to it a physical lens as illustrated in figure 1. In the top part of this figure, a transducer curved with a radius R is shown focusing to the centre of curvature, where R Is positive by convention. By the geometrical-optics approximation, the focal length F is equal to R and hence Is also positive. In the middle of figure 1 Is shown a transducer with a piano- concave lens made of a material with longitudinal velocity. c1, which Is curved on one side with a radius. RLENS. and radiates into a medium In which the velocity Is Cw, In acoustics. Cw Is typically less than c1, I.e., the index of refraction n (equal to cwlcij Is less than 1. When this is true, the radius is considered to be negative and the local length, given by the geometric-acoustics approximation as LENS divided by (n — 1), Is positive. At the bottom of the figure. for comparison, the typical situation for a convex lens in optics is shown: n Is greater than 1 and the radius is considered to be positive, so the focal length is positive.
3.1.2 Known and unknown focusing transducers
For ultrasonic transducers currently used in medical ultrasound applications, it is difficult to determine from physical observation if an ultrasonic transducer is focusing, because additionally many other focusing methods such as geometric shaping and arrangement, reflectors, arrays with electronic phasing and delay. Fresnel lenses, shading, etc. may be used singly or in combination. Because of inherent natural focusing and the potential complexity of additional focusing means used, any generally useful definition of a focusing transducer must be in terms of its field raiher than its construction. If a focusing source were to be defined in terms of its pressure field, then this would be relatively easy to apply in practice, since the pressure can be measured directly with a hydrophone.
A distinction is also made between ultrasonic transducers whose construction is known and
transducers about which very ittle information is available. For the first category of ultrasonic transducers, certain theoretical definitions, such as geometric focal length, are useful for describing and modelling focusing characteristics. ultrasonic transducers falling in the second category function as an unknown “black box” and only the field may be accessible.