Redefining the ear
The nature of human hearing is heavily dependent on the shape of the head and torso, their interaction with sound reaching the ears allows for the perception of location within a 3D sound field.
Head And Torso Simulators (HATS) are designed to model this behaviour, they provide an artificial head and torso with which measurements and recordings can be made to replicate the Head Related Transfer Function (HRTF).
HATS are mannequins with built-in mouth simulators and calibrated ear simulators that provide realistic reproduction of the acoustic properties of an average adult human head and torso. They are ideal for performing in-situ electroacoustic tests on, telephone handsets (including mobile and cordless), headsets, audio conference devices, microphones, headphones, hearing aids and hearing protectors.
Critically the shape of the pinna, the outer part of the ear, has a large effect on the behaviour, and as a result it is defined for a HATS by its own standard (IEC TR 60959:1990) to provide consistency across measurements. However, this standard defines the shape of the pinna through a series of 2D cross-sectional profiles. This standards and definition has on occasion proven to be an inadequate guide for manufacturing processes.
As part of a revision of this standard, the Acoustics Team at NPL teamed up with the National Freeform Centre in a novel move to redefine the standard through an on-line 3D CAD specification. A model ear was measured using a CMM with laser scanner to produce a 3D scan of the ear, which can then be used to provide manufacturers with a more practical specification for reproduction and a standard that is easily comparable with similar non-contact freeform measurement techniques.