The problem of the rigid-piston radiator mounted in an infinite baffle has been studied widely for tutorial as well as for practical reasons. The resulting theory is commonly applied to model a loudspeaker in the audio-frequency range. A special function, the Struve function H1(z), occurs in the expressions for the rigid-piston radiator. This Struve function is not readily available in programs such as Matlab or Mathcad, nor in computer languages such as FORTRAN and C. Therefore a simple and effective approximation of H1(z) which is valid for all z is developed. Some examples of the application of the Struve function in acoustics are presented. © 2003 Acoustical Society of America.