Ear simulators and hearing aid testing

ISVR Consulting and the National Physical Laboratory (NPL) recently collaborated in research on ear simulators and hearing aid testing.  The project was part of the UK Department of Trade and Industry's National Measurement System Programme for Acoustical Metrology and covers acoustic couplers, artificial ears, ear simulators and head and torso simulators (HATS). 

An acoustic coupler is a simple cavity of specified shape and volume which is used for the calibration of an earphone. It contains a calibrated microphone to measure the sound pressure developed within the cavity when the earphone is placed over the cavity. A coupler gives only a rough approximation to the acoustic properties of the human ear but has the advantage of simple design and construction. 

An artificial ear

An artificial ear

An artificial ear or ear simulator is a far more sophisticated device for the calibration of an earphone.  Like the acoustic coupler, it contains a calibrated microphone for the measurement of the sound pressure developed within a cavity by the earphone .  Unlike the coupler it presents to the earphone an acoustic load impedance equivalent to that presented by the average human ear.  The appropriate impedance is obtained by providing acoustic leakage paths to small volumes or side branches off the main cavity.  These side branches are damped resonators tuned to different sound frequencies.

 

A group of HATS

Head and Torso Simulators (HATS)

A head and torso simulator (HATS) is a device simulating the geometry and acoustical properties of the average human head, including the external ears, and the upper torso. It is equipped with ear simulators representing the ear canals, with microphones at the positions corresponding to the eardrums.  The sound levels generated at the eardrum microphones approximate the sound levels which would be generated at the eardrums of a median human adult exposed to the same sound source.  The generic term manikin is also used for head and torso simulator.

Couplers, artificial ears, ear simulators, and head and torso simulators are used in audiology for the calibration of audiometers and measurements of hearing aid response, in the audio and telecommunications industries for measuring the frequency responses and sound output levels of earphones and telephones, and in research for studies of hearing and sound perception.

Over the years the audiology, telecommunications and audio industries have adapted the basic artificial ears and ear simulators, and independently developed their own ear simulators and HATS to meet their specific needs.  The devices used by the different groups have diverged, as have the national and international standards which describe and specify them. 

An important objective of this project was to assess the potential for the harmonisation of  the various different but related standards for ear simulators and HATS which were in use in the different fields.

The project also addressed the use of ear simulators for the measurements of short duration or transient signals which are used to assess patients' ears and hearing.  A further topic, largely unrelated to the rest of the project, was to assess which methods and test signals would be appropriate for testing of digital hearing aids which incorporate signal processing and non-linear circuitry.

The project overall had three main themes:

  1. The development and harmonisation of specification standards for ear simulators and hearing thresholds
  2. Research and development to encourage consistent use of ear simulators and common calibration practices.
  3. Methods and signals for the testing of digital signal processing (DSP) hearing aids

The issues raised by these themes was addressed by the following activities:

Both ISVR Consulting and the National Physical Laboratory disseminated their findings in reports and published papers.  Two reports are currently available for downloading from ISVR Consulting in Acrobat/pdf format.  These are:

The partnership of ISVR and NPL drew upon two of the largest and best equipped research teams in acoustics in the UK. The partnership was well placed to take proper account of the needs of ear simulator users, and to use its contacts with these users to add value to the work.