ASTM F2504-05

IMEHDs are alternatives to air conduction hearing aids. They are similar to air conduction hearing aids in that they process incoming sound by applying frequency shaping and compression to create an analog, vibratory audio frequency output. IMEHDs differ from hearing aids in that they do not create an airborne acoustical output signal with an electroacoustical output transducer in the external ear canal, but rather a mechanical stimulation that results in the vibration of the cochlear fluid. Therefore, the IMEHD output signal is not readily accessible after implantation in the way hearing aid output is accessible with real-ear probe microphone measurements. Different devices will use different methods of coupling to the ossicular chain or cochlea. This makes it difficult to design a uniform model of the middle ear in the way the 2-cm³ coupler is used as a model of the external ear canal with conventional hearing aids.

This practice provides uniformity of data collection practices, thus allowing IMEHD in vitro performances to be evaluated and readily compared. Once clinical data are available, the performance specifications can be augmented with corresponding transfer functions or results from measurements in patients.

The temporal bone is a well-accepted model that relates closely to the biomechanics of the living middle ear, which is readily relatable to hearing level. Laser Doppler vibrometry provides accurate velocity measurements in the ranges required for human hearing.

1.1 This practice defines means for describing system performance (ex vivo) and, in particular, system output of an implantable middle ear hearing device (IMEHD) by measuring a physical quantity that is relevant to the insertion gain and output level of the IMEHD when implanted in the patient.

1.2 This practice is similar to headphone calibration on an artificial ear in which the sound pressure level (in decibel sound pressure level (SPL)) measured in the artificial ear can be converted to patient hearing level (in decibel hearing level (HL)) using a known transfer function, as defined by ANSI 3.7. These measurements can then be used to predict system parameters relevant for patient benefit such as functional gain, maximum output, and variability. Measurements defined in this practice should be useful for patients, clinicians, manufacturers, investigators, and regulatory agencies in making comparative evaluations of IMEHDs.

1.3 The values given in SI units are to be considered the standard.

This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.