On the sound field requirements in the hearing protector standard ISO 4869-1

The sound field requirements in the ISO 4869 1 standard for hearing protector attenuation measurements comprise two parts: 1) a sound level difference requirement for positions around the head of the listener (ie at positions 15 cm from a reference point; up-down, front-back and left-right) and 2) a directivity requirement for the sound incidence at the reference point, measured with a directional microphone, to ensure an approximate diffuse sound field. The level difference requirement (1) is not difficult to fulfil but the directivity requirement (2) may lead to contradicting results if the measurement procedure in the standard is used with different types of microphones. In this paper the directivity requirement is discussed on a theoretical basis. The requirement is evaluated in relation to the widely used anechoic tetrahedron setup (four loudspeakers in the corners of a tetrahedron) and an anechoic square setup (four loudspeakers in the corners of a rectangle). It is shown that the formulation in the standard is misleading and that a better test method is needed for the diffuseness requirement.