Measurement Variability:  TDH 39 vs. ER-3A Insert phones

This article looks at the Telephonics TDH-39 supra-aural earphone with MX41-AR cushions versus the Etymotic Research ER-3A insert phone.  There are numerous advantages to using the ER-3A insert phone, some of which have been discussed in the article comparing the TDH49 to ER-3A.  Some notable advantages of using insert phones as opposed to supra-aural are that it reduces the occurrence of collapsing ear canals and also increased the interaural attenuation (IA).

It is advantageous to increase the IA because a greater IA reduces the risk of cross-hearing as well as the need for masking of the non-test ear.  Compared to the TDH-39, the ER-3A insert phone offers greater IA, especially at the low frequencies.

The above graph is taken from Munro et al (1999).  The solid circles represent the IA value for the TDH-39 earphones.  The solid squares represent the IA values of a ER-3A deep insertion and the open squares represent the shallow insertion.  As the graph indicates, the IA decreases with a shallow insertion depth.  What isn’t illustrated on the graph is that IA increases as the area of the head exposed to the stimulus is decreased.  This makes sense because as the area decrease, there is less of a medium for sound to travel.   

According to Skare et al. (1987), cross hearing should be suspect if a pure tone stimulus transduced by an ER-3A insert phone exceeds the bone conduction threshold of the opposite ear by 75 dB HL for low frequencies (i.e. 250-1000 Hz) and 50 dB HL for higher frequencies.  With a deeper insertion depth, the IA values were typically 15-20 dB greater than the TDH-39 earphones. 

According to the British Society of Audiology, transmission loss is typically 60 dB when using supra aural earphones, but can occur at values as low as 40 dB.  According to Munro et al. (1999), cross hearing using the ER-3A insert phones will occur when the difference between the test stimulus and the threshold in the non-test ear exceeds 55-70 dB, depending on the frequency.  If a shallow insertion depth is present, then the IA can be expected to be approximately 5 dB less.   

Knowing when to use masking and knowing the limitations of the transducer can save an Audiologist time when conducting hearing tests.  In addition, it is important to know when masking is needed because inaccurate information about the IA values of a transducer may result in inaccurate threshold measurement of patients due to cross hearing.       

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