Deep vs. Shallow Insertion Depth for ER-3A Earphones

This article deals with the advantages of using a deep versus shallow insertion depth for ER-3A styple earphone and it's misunderstood advantage when applied to masking click stimuli used in ABR testing.  

Audiologists are taught that deep insertion of earphones (ER-3A style) will provide greater interaural attenuation value compared to shallow insertions for specific stimuli.  An interaural attenuation of 100 dB at 500 Hz with deeply inserted eartips can drop to 75 dB or less with shallow insertion (Killion et al., 1985).  In addition, it has been proven that using ER-3A style earphones will provide a greater interaural attenuation for pure-tone stimuli compared to using supra-aural earphones (Killion et al, 1985).

Using ER-3A earphones can eliminate the use of masking for certain testing (i.e. ABR) because of the increase in interaural attenuation.  According to Beauchaine et a. (1997), insert earphones should be used in ABR evaluations “when concerns exist for problems associated with masking” (p. 296). 

Audiologists need to know however that using ER-3A style earphones is only advantageous for masking for certain types of stimuli.  Studies revealed that the average interaural attenuation using behavioural methods with pure tone stimuli for supra-aural thresholds is 60 dB, compared to 72 dB for ER-3A with shallow insertion depth.  A shallow insertion depth can be defined as a depth where the outer part of the earphone is flush with the opening of the ear canal. 

The obvious next step would be to see if a deeper insertion depth would be more advantageous.  This logic stems from the fact that deeper insertion depths have been proven to provide greater interaural attenuation for pure tone stimuli (Killion 1985).  A study by Laws et al. 1993 revealed that deep insertion interaural attenuation does not differ significantly from shallow insertion interaural attenuation for click stimulus.  Also, Van Campen et al. 1990 found that using ER-3A over supra-aural earphones with a click stimulus only produced a small advantage when using behavioural and ABR thresholds. 

For click stimuli, a deep insertion depth will produce (on average) an advantage of 2 dB interaural attenuation (Laws et al., 1993).  The depth of the insertion as conducted by Laws et al., (1993) ranged from 2 to 8 mm.  Audiologists are reminded that click stimuli often used in ABR testing is related to the 1000-4000 Hz range of the cochlea.  Van Campen et. al. (1990) think that the high frequency response nature of the ABR might explain the lack of increased interaural attenuation when they compared shallow insertion with supra-aural earphones.      

Wilber et al., 1987 recommend the use of deeply inserted foam eartips for critical testing.  That is, when a 2 dB difference will be important and/or accurate thresholds at 125 Hz are required.  125 Hz is the frequency in which the greatest range of variability across testing has been observed (Wilber et al., 1987).  With shallow earphone insertion testing at 125 Hz octave band, physiological noise can cause elevated thresholds due to masking (i.e. the noise is masking the tone).     

The take home message here is, when using click stimuli, it is not necessary to us a deep insertion since the advantage of doing so is not clinically significant to alleviate problems associated with masking the click stimulus.  One thing Audiologists need to be aware of is the inherent danger of deep insertions of earphones.  Anytime you use a deep insertion, you run the risk of hitting the tympanic membrane or pushing cerumen further down the ear canal.  Another reason not to use deep insertion when it is not necessary is that deep insertions often negatively affect patient comfort (especially for narrow canals).  From my clinical experience, I found that using deep insertions often caused my patients to cough.  According to my clinical supervisor, this was due to the foam plug touching a nerve which runs along the canal.  No further explanation of this was given however.

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