Reference Equivalent Threshold Force Levels For bone Vibrators

Bone vibrators used during audiometric evaluation are calibrated using an artificial mastoid, as specified by ANSI 1987 guidelines.  During calibration, the vibrator is coupled to the mechanical coupler with a static force of 5.4 +/- 0.5 N and the artificial mastoid is equipped with an electromechanical transducer that measures the force level at the point where the vibrator is in contact with the surface of the artificial mastoid.  A Bruel & Kjaer artificial mastoid type 4930 is generally used for the calibration of bone vibrators.  The typical setup is diagramed below.  

An artificial mastoid needs to exert an impedance onto the tip of the bone vibrator that will have an impedance which is similar to an average human head-bone situation over the range of test frequencies (normally 250 – 4000 Hz).  Artificial mastoids that meet ANSI S3.26 1981 standards must not differ from the manufacturer’s values by more than +/- 25% over the range of 250 –4000 Hz.  If this condition is not met, the artificial mastoid must be re-calibrated as indicated in the manufacturer’s manual.  Calibration should be done under an ambient room temperature of 23 +/- 2^oC.  Every artificial mastoid, regardless of who manufactured it, needs to include a manual with a curve or table indicating the mechanical impedance vs. frequency characteristics of the device. 

Reference equivalent threshold force level calibration values for the B&K type 4930 (with serial numbers higher than 526226) used with a Radioear B-71 attached to a P3333 headband are indicated below.

 Values taken from ANSI S3.26 1981

If another artificial mastoid and bone vibrator combination is to be used, calibration values can be determined using subjective techniques.

Audiologists need to know that when the bone vibrator is placed across the front of the head, rather than on the mastoid, more force needs to be added to obtain thresholds comparable to the mastoid placement.  With a mastoid placement, the recommended level of force against the mastoid is 5.4 +/- 0.5 N.  The following values are the correction factors that must be added to the reference equivalent threshold force level calibration figures above for forehead placement of the bone vibrator. 

Note:  This site is best viewed using IE 4.X or higher under 800x600 resolution