Measurement Variability: TDH 49P vs ER-3A  

The performance of the Telephonics TDH-49P and the ER-3A insert earphones are not identical at all frequencies, but can be comparable if the manufacturer correction values are used, or if the audiometer is re-calibrated for the appropriate earphone.  Therefore to gain the advantages of using ER-3A insert earphones, one needs to apply the manufacturer correction values or re-calibrate the audiometer.       

A study conducted by Lindgren (1990), compared the variability in threshold measured with insert and conventional supra-aural earphones.  The earphones used were the TDH-49P with MX-41/AR cushions and the EA-3A inserts.  The study showed that when switching to ER-3A insert phones, the measured threshold values show significant variation above 5 kHz.  To solve this problem, two options are available.  The first is to re-calibrate the audiometer for the ER-3A insert phones.  The second is to apply manufacturer correction values to the obtained threshold values before they are entered on an audiogram.  When either of these two procedures are followed correctly, the TDH-49P and ER-3A insert phones provide similar threshold values.  

One of the problems with the standard TDH earphones with MX-41/AR cushions is that for occupational hearing conservation purposes, TDH earphones are poor at attenuating background noise to the test ear.  For this reason, a sound booth is required to test hearing sensitivity at 0 to –10 dB HL.  Noise reduction muffs attached to TDH earphones are often used to reduce background noise.  Audiologists need to be aware that these Audiocups are capable of reducing the pressure of the MX-41/AR cushions on the outer ear.  This makes it difficult to center the earphones over the ear canal entrance, which can have implications when testing higher frequencies.  Less reliable results and poorer thresholds may result.  Audiologists should also know that proper calibration of the TDH earphones with MX-41/AR cushions cannot be conducted unless the headband portion is removed.  

Further sources of measurement error using standard headphones can occur if the ear canal collapses.  This may be caused from the pressure of the earphones on the outer ear.

If a discrepancy in hearing thresholds exist between the two test ears, this may be due to the result of the test signal crossing over to the contralateral side.  The solution to this problem is masking in the contralateral ear, but this is rarely implemented in occupational Audiology.

An alternative transducer to the TDH earphones is the ER-3A insert phones.  When using ER-3A insert phones, the audiometer must be adjusted and correction factors must be applied when testing frequencies above 4 kHz.  Not doing this can result in differences that equal or exceed 7.9 dB between THD earphones.  ER-3A insert phones can be equally as reliable as THD earphones if the proper correction factors are applied.

Manufacturer's suggested correction factors 

Manufacturer data from Lindgren 1990

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