General Transducer Precautions

The most likely earphone types encountered with clinical audiometers are the Telephonics TDH types and ER-3A insert phones.  There are three common TDH style earphones, all of which physically look the same.  The TDH 39, 49 and 50 along with their audiocups which may be type 51 or type MX-41/AR all look identical, but each have different frequency response characteristics.  

A useful difference to know regarding the different TDH style earphones is their impedance values.  The TDH 49 only comes with a 10 ohm impedance, whereas the TDH 39 and 50 come with a variety of different impedance values (10, 60, 300 ohm etc).  In general, transducers with a higher ohm rating are more resistant to noise than low impedance devices.  Noise can occur if the metal contacts of the transducers become oxidized.  When this happens, any turning or twisting at the point of contact may cause noise in the form of static or crackling.  Oxidation on the metal contacts can have up to a 15 dB effect on measured threshold values as found by Rick Cameron of EMI Canada (personal communication), who was kind enough to allow me to observe a day with him during the calibration of the these devices. The problem of oxidation can easily be solved by using an oil-based solvent cleaner on the transducer contacts.    

In clinical testing, the only advantage of using lower resistance transducers is that they do not require as much voltage to drive the device.  However, when doing tests such as ABR's, lower resistance transducers are often useful.  The reason for this is that the high voltage needed to drive the high resistance transducers also tend to introduce more noise into the testing situation.  In threshold testing, this increased system noise is negligible but in ABR testing, it is critical that as much system noise is eliminated in order to get the most accurate measurements, thus the use of lower impedance transducers.     

Some audiometer manufacturers require a transducer of a specified resistance.  For example, GSI instruments use 50 ohm transducers while Madsen and Interacoustics use 10 ohm.  It is important to label transducers with their impedance values so that the correct earphones get used on the correct audiometer.  In busy hospital settings, there may be two or three audiometers present in the clinic, and it is not uncommon for clinicians to switch transducers from one machine to the other if one of them happens to break or get damaged.  Switching transducers among different audiometers is strongly discouraged because each transducer is calibrated specifically to one audiometer.  A study by Valente et al. (1994) captured the error and variability that can occur by arbitrarily plugging in an insert earphone to an audiometer which it was not originally calibrated.  For a summary of this study, click here. 

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