Measuring Insert Earphone RETSPL using Hearing Aid Couplers

There are certainly clear advantages for using insert earphones for obtaining accurate audiological measurements.  Insert style earphones eliminates the need for headbands (thus increasing patient comfort), prevents collapsing ear canals, and allows for hearing tests under higher ambient noise levels.  The use of insert style earphones allows an increase in interaural attenuation thus reducing the need for masking. 

Audiologists may find it interesting to know that there is currently a disagreement and a certain degree of uncertainty regarding the RETSPL for insert phones generated in HA couplers.  More specifically, the RETSPL concerning HA coupler type output SPL variability and correction levels between HA coupler types. 

Variability in RETSPL values for ER-3A insert phones used with the HA-1 coupler exists because of differences in physical dimensions, total volume and sound inlet hole diameter across several models of the HA-1 coupler.  However, Larson & Cooper (1988) found that HA-1 coupler output SPL variability to be clinically acceptable for ER-3A measurements.  Currently, the output SPL variability for HA-2 coupler types using insert style earphones are not known.   

The issue of coupler output SPL variability is directly applicable to Audiologists because these are the couplers used to calibrate the accuracy of the insert earphones.  Using a hearing aid coupler allows measurements of the SPL output response for the insert earphones.   

A study by Frank et al. 1991 examined three hearing aid couplers (HA-1, HA-2 and DB-0138 (HA-2 with entrance through a rigid tube)) with two pairs of insert earphones (ER-3A left and right, EARTONE-3A left and right).  For each coupler types, the mean coupler SPLs were found to be very similar across the four insert earphones, which is not surprising since both the EA-3A and EARTONE insert phones are manufactured to identical specifications.  

It is important to note that it is not uncommon to find output variations of 2 to 3 dB within the same earphone type.  For example, at 250 Hz, the ER-3A (R) and at 8000 Hz ER-3A (L) demonstrated lower than average coupler SPLs compared with other insert phones.  This trend was visible across all couplers types in the Frank et al. 1991 study. 

The HA-1 and HA-2 coupler SPLs are very similar for each insert phone within a range of 125-8000 Hz.  The response characteristic in the range of 125 - 3000 Hz was generally flat.  The DB0138 coupler has a similar response as above except that the flat frequency response occurs between 125 – 4000 Hz.

The input frequency for each insert earphone has a +/- 1 Hz tolerance.  In clinical practice, audiometers have a +/- 1% frequency tolerance for type 1 audiometers, +/- 2% frequency tolerance for type 2 and +/- 3% for type 3, 4 and 5 (ANSI S3.6 – 1996).  What Frank et al. 1991 was concerned with was the steep high frequency response characteristic for each earphone.  Audiologists should know that there is a general rule that measurement repeatability decreases if any transducer that is operated at a discrete a frequency that lies along a steep slope (click here for a related related article)   

The following is a review of the redsults and findings of Frank et al. 1991.  

The SPL variations were less than 1 dB for both the HA-1 and HA-2 coupler between the range of 125 – 3000 Hz and 125 – 4000 Hz for the DB0138. 

The SPL variations were greater than 1 dB for the HA-1 and HA-2 SPL between the range of 4000 – 8000 Hz with a maximum variation of 4.1 dB at 8000 Hz.  For the DB0138 this range is 6000 – 8000 Hz with a maximum variation of 4.6 dB occurring at 8000 Hz. 

Higher frequency output variations can be directly related to the steeply sloping high frequency response for each earphone measured in each coupler. 

For specifying high frequency insert earphone RETSPL, a +/- 3% frequency tolerance should be avoided (especially at 8000 Hz), hence type 3 audiometers and above should be avoided, unless they can be provide frequency accuracy in the +/- 1% range.  Clinically it is recommended that the audiometer frequency should be as close as possible to the nominal center frequency so that insert earphone output variations that could influence calibration and testing are minimized.   

From the Frank et al. 1991 study, all three hearing aid couplers provided stable coupler SPLs.  However, Frank et al 1991 recommends the use of the DB0138 coupler (HA-2 coupler with entrance though a rigid tube) to be used for referencing normative thresholds and for routine clinical calibration of ER-3A and EARTONE 3A insert earphones.  Their reason for selecting the DB0138 is because it is the easiest one to use for calibration.  It is mentioned that Larson and Cooper (1989) recommend the HA-2 coupler.  Frank et al. 1991 found that when using the HA-2 coupler, placement of the eartip tube flush with the earmold substitute sound channel outlet may be difficult to obtain because the eartip termination point cannot be visualized when the HA-2 coupler is intact.  In addition, they found that some eartip tubes do not fit as tightly as others in the earmold substitute sound channel once the eartip has been removed, hence questioning the adequacy of the seal.     

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