Real Ear Method for Earphone Calibration

The real ear calibration method involves testing and averaging thresholds of a group of normal hearing people, and checking to see that the average hearing of this group falls at 0 dB HL at each frequency tested on the audiometer.  Theoretically, this procedure will work with a large enough sample size, but clinically it is not a practical procedure.         

A more efficient method of conducting this calibration is to use 10 or more young normal hearing subjects.  ANSI S3.9-1996 recommends using minimally 6 subjects while other authors (Wilber 1994) recommend 10.

The procedure for calibrating the earphone according to ANSI S3.6 - 1996, is as follows:

The first step is to place a reference earphone over the subject's test ear.  Once this is done, the voltage required to reach the subject's threshold needs to be recorded.  This process needs to be conducted for the entire range of test frequencies.

Following the measurements obtained using a reference earphone, a test earphone needs to be applied to the same ear and the process of measuring thresholds needs to be repeated. 

The standard reference earphone is then placed on a standard coupler or artificial ear and the threshold values obtained earlier are increased by 70 dB and applied to the input terminals of the earphone.  This procedure is then repeated using the test earphone and threshold values obtain for it.  

The sound pressure level developed in the appropriate coupler are then used to transfer the reference equivalent threshold values of the standard earphone to the test earphone and specified coupler.        

Another method outlined in ANSI S3.6-1996 is one involving a probe tube microphone.  

In this procedure, a probe tube microphone is positioned so that the tip of the probe is situated at the entrance of the ear canal.  What this does is that it makes the subjects ear act as if it were an acoustic coupler.  

A reference earphone is then positioned on the ear in such as way that the probe tube microphone is not moved.  

An audiometer is then used to present pure tone signals of a specified sound pressure level into the the reference earphone where the sound will then be picked up and measured at the probe tube.  The sound pressure level involved in this procedure should not exceed 80 dB.  

Next the same steps are repeated using a non-standard earphone.  

The standard reference earphone is then needs to be placed on a standard acoustic coupler or artificial ear.  The measurements recorded at the ear canal by the probe tube are then reproduced and the sound pressure level of resulting signal in the coupler are recorded.  This procedure is then repeated with the non standard earphone.  

The sound pressure levels measured in the couplers are then used to transfer the reference equivalent threshold values of the standard reference earphone to the test earphone and specified coupler or artificial ear.

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