Bone conduction calibration:  The Artificial Mastoid

  Beltone artificial mastoid 5A - Beltone artificial mastoid 5B  

Bruel & Kjaer 4930 - Larson Davis AMC493

The artificial mastoid (sometimes referred to as a mechanical coupler) is the tool used most commonly to calibrate bone transducers.  

When using an artificial mastoid, an electrical input can be applied to the bone transducer which is then measured as being equivalent to a certain sound pressure level.

The Beltone artificial mastoid model 5B is a redesign of the 5A model.  For clinical purposes, it is not necessary to detail the specifications of the artificial mastoid.  I will however highlight what I feel are to be important aspects of this artificial mastoid. 

The Beltone model 5B is intended to replace the 5A model because the impedance of the 5A was found to be higher than what was specified for accurate bone conduction calibration.  The specification for bone conduction calibration requires that the surface area which contacts the mastoid, be 1.75 cm² with a contact force of 5.4 Newtons.  Audiologists who have the Beltone model 5A should be aware that if they have returned their model 5A to Beltone for calibration, then it is more than likely their 5A would have been converted to 5B specifications. 

The Beltone 5A was used for the HAIC (Hearing Aid Industry Council) interim norms and was found to be quite accurate.  Despite this, the impedance of the 5A was found to be higher at all frequencies compared to the recommended ANSI calibration figures.  The Beltone 5B which has a lower characteristic impedance is also a good device for interim use, but is not as widely used.       

A study by Frank and Richards (1980) compared the performance of both the Beltone 5A and 5B artificial mastoids.  Below is a simplified summary of their findings compared to other studies.

                                                   Frequency (Hz)

BC threshold values are shown in dB

Frank and Richards (1982) found that the output from their Beltone 5B was 6-7 dB less than the output of a 5A at 2000, 3000, and 4000 Hz for the same input voltage to a B-70-A vibrator.   

Beltone artificial mastoid 5A - Beltone artificial mastoid 5B  

The Beltone artificial mastoid 5A and 5B have been taken off the market by the manufacturer.  It has been obsolete for the past 12-13 years.  Audiologists who may have either one of these devices should be aware that they are longer provide valid means of calibration.     

Bruel and Kjaer Artifical Mastoid 4930

The Bruel and Kjaer Artificial mastoid 4930 was designed for the calibration of bone conduction hearing aids and bone vibrators used in audiometry.  It consists of a mechanical simulation of the human head, incorporating a built-in force transducer to monitor the output of the device to be calibrated.  The 4930 consists of an inertial mass, simulating the human head, mounted on a baseplate by resilient plastic spacers which surround the securing-bolts.  The baseplate is carried on a suspension composed of three conical springs filed with foam rubber to provide damping. 

The 4930 is designed to meet both North American and international standards.  This includes International Electrotechnical Commission IEC R373 (1971) including revisions, British Standards BS 4009 (1975) and American National Standard ANSI S3.26 (1981).

The Bruel and Kjaer Artifical mastoid model 4930 has been problematic in the past in terms of measurement reliability.  Dirks et al (1976) studied the input-output characteristics of the model 4930.  What they found was that when identical input signals were used, the force level outputs on the bone conduction vibrators were significantly different.  What this means is that the model 4930 does not provide a stable bone conduction calibration level.  Since this study, the 4930 artificial mastoid has been revised from the 4930 original pad to 4930 new pad. 

Bruel and Kjaer Artifical Mastoid 4930

Audiologist may be interested in knowing that the 4930 new pad delivers a more consistent and reliable measure compared to the 4930 original pad, but it does so at a higher impedance than the original pad.  Audiologists who are curious to know whether the 4930 used to calibrate their equipment is acceptable for calibration purposes need only to check the serial number of the artificial mastoid.  If the artificial mastoid contains a serial numbers above 526226, then it is considered acceptable for calibration purposes (Robinson et al. 1982).           

The B&K 4930 is a very robust piece of equipment and service and maintenance is often not necessary.  However, if something goes wrong and service is needed, it must be returned to the Bruel and Kjaer manufacturer in Denmark.  No service is available in North America.   

Larkson Davis AMC493

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