The Acoustics Team at NPL and its National Freeform Centre created a 3D model of the ear to help redefine the standard for pinna simulators. A pinna is the outer part of the human ear that critically affects how one perceives sound and its location within a 3D field.
Models of the pinna are a crucial component in Head and Torso Simulators (HATS), which are mannequins with built-in calibrated ear simulators that replicate the acoustic properties of an average adult human head and torso. These simulators are used by manufacturers of a range of acoustic products, such as telephone handsets, headphones and hearing aids.
The pinna’s importance in these simulations means that it is defined for HATS by its own standard (IEC TR 60959:1990) to provide consistency across measurements.
Ian Butterworth from NPL said the problem is that the current standard defines the pinna through a series of 2D cross-sectional profiles and this has led to the manufacture of inconsistent models.
‘In reality [the standard] is multiple pages of separated cross sections and the thing is you have to compare that to this three-dimensional object you are trying to recreate,’ he said. ‘It’s reasonably pointless turning something 3D into a series of two-dimensional cross sections and then working it back into 3D.’
The NPL set out to change this. At its freeform centre, a model ear was measured using a coordinate measuring machine with a laser scanner that was passed over the ear from multiple directions.
Andy Robinson, from the National Freeform Centre, said this built up close to a million data points that effectively produced a digital representation of the ear.
‘There’s a real advantage now the scan is digitised,’ he said. ‘There is a real advantage in being able to share that data with other measurement institutes and organisations because we can effectively send that file and scan data where previously you would just have a table of measurement points. In principle anyone can get hold of the file we produced now and compare model ears back to that reference. So there is a real improvement in the way that data can be shared and accessed.’
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