Caution calibrating earphones using an IEC reference coupler

When calibrating an audiometer or audiometric earphones, a reference coupler is used.  The reference coupler is an artificial ear which has an acoustical output impedance that is similar to that of an adult human ear.    

Audiologist need to be aware that acoustic couplers represent normative data of real ear impedance values.  Thus if a coupler is substituted for a real human ear, even a typical or average real ear, there will be, in general, differences in the system response.  In addition, a coupler that adequately represents the real ear over a wide frequency range, which is useable with many types of earphones, and is stable and reproducible has not yet been built (ANSI S3.7 - 1973).  Currently available couplers are able to approximate the average human ear only over a limited frequency range and with a restricted class of earphones.       

According to ANSI S3.7, the purpose of coupler calibration of earphones is to provide a simple, convenient, and reproducible means of determining their acoustical output.  ANSI S3.7 describes several different acoustic couplers, all of which have been designed for use with a specific class of earphone.  In this section the IEC 303 reference coupler will be discussed along with a common caveat related to it's use.    

The IEC 303 reference coupler uses a microphone cartridge fitted with a protective grid.  During calibration the microphone grid must be removed according to standard calibration procedures.  The microphone diaphragm is tremendously sensitive and removing the grid leaves the microphone diaphragm susceptible to damage.  To prevent damage, some may elect to leave the grid on during calibration.  Using the protective grid affects the overall volume of coupler and thus the overall acoustic impedance of the system.  During high frequency calibration (6 kHz) it has been found that calibration values could be altered by several decibels. 

The diagram below illustrates the B&K microphone model 4144 and the different adapters used.  The grid (model DD 0015) acts to decrease the overall volume of the cavity, which will cause an increase in SPL.  The grid (model DD 0011) has the opposite effect; it increases the cavity volume causing decreased SPL. 

Figure from Sherwood et al. 1986

The graph below describes the effect of the grid model DD 0015 during calibration.  The grid results in measurement variation  greatest around 6 KHz.  

Graph from Sherwood et al. 1986

Audiologists need to be aware of the effects of the microphone grid on the IEC 303 reference coupler during earphone calibration.  Whether or not the Audiologist will be performing the calibration or not, knowing details such as this will provide the Audiologist with a greater appreciate of the sensitivity and importance of proper calibration techniques.  

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