Basically, a skull simulator consists of an inertial mass with a coupling surface and a measurement device or means. The coupling surface serves as a receptacle on which a vibration element of a bone-conduction hearing device or a bone-conduction vibrator being part of such a bone-conduction hearing device may be mounted for testing, and the measurement device or means serves to determine the vibration force applied by the bone-conduction vibrator to the inertial mass. The inertial mass is ideally designed to provide an acoustic impedance towards the bone-conduction vibrator equal to that provided by the skull bone or the head of an average hearing-device user at the position on the skull bone or the head where the bone-conduction vibrator is to be arranged during normal use of the hearing device. The skull simulator may thus be used to measure the output force of bone-conduction vibrators under realistic operating conditions, e.g. for testing or calibration purposes.
In known skull simulators, the inertial mass typically comprises an elastically suspended, rigid body, such as a metal cylinder, with an accelerometer rigidly attached at a rear end. The opposite front end of the rigid body may serve directly as coupling surface, or a suitable fixture with a coupling surface may be rigidly attached thereto. For measuring of transcutaneous bone-conduction vibrators, the coupling surface may be covered by one or more layers of materials, such as rubber, designed to simulate the acoustic impedance of skin and tissue covering the skull bone. Due to the known correlation between the acceleration of a body and the vibration force applied to it, the output of the accelerometer may be used as a measure for the force applied to the inertial mass and thus for the output force of the bone-conduction vibrator.
Some known skull simulators comprise a protective casing shaped substantially as a rectangular cuboid, i.e. with top and bottom walls, two side walls, a front wall and a rear wall. The top, bottom and side walls are typically integral with each other and form a sleeve within which the inertial mass and the elastic suspension is mounted. The front and rear walls are secured to the sleeve with bolts or screws, either directly or indirectly via parts of the suspension. The coupling surface is either accessible through a central through hole in the front wall, or the corresponding portion of the rigid body or the fixture extends through such a through hole such that the coupling surface is external to the casing. In use, the skull simulator is arranged to stand on the bottom wall, preferably acoustically decoupled from the supporting surface by means of elastic and/or damping feet or pads. The coupling surface is arranged such that the bone-conduction vibrator applies its vibration force horizontally, which allows for using relatively simple, vertically oriented planar springs to suspend the inertial mass. In other known skull simulators, the casing is cylindrical and stands on one end of the cylinder, and the coupling surface is arranged such that the bone-conduction vibrator applies its vibration force vertically.
In the prior art skull simulators known to the inventors of the present invention, the front wall of the casing—and/or exposed parts of the suspension—have one or more planar surfaces on the front, and these planar surfaces are thus oriented towards the bone-conduction vibrator when it is mounted on the skull simulator.
The inventors of the present invention have now surprisingly established that this particular feature poses a cause for irregularities in the measurement results. The planar surfaces may reflect airborne sound emitted from the housing of the bone-conduction vibrator or emit sounds themselves when vibrating, and the reflected or emitted sound contributes to the build-up of resonances which are not present when the bone-conduction vibrator is mounted on a human head.