James J. Jerome, Senior Occupational Audiologist

I recently calibrated a Monitor MI-5000B and noticed that Telephonics TDH-39 earphones were being used. I found this strange, since, in my experience, all Monitor audiometers exclusively use Telephonics TDH-49 earphones.

I contacted Van Anderson, President of Monitor Instruments, and asked him if they had dropped using TDH-49 earphones. His answer was no. At times TDH-49 earphones can be in short supply. If they cannot find other supply sources for TDH-49 earphones, Monitor will use TDH-39 earphones until TDH-49 earphones become available again.

Does this alternate earphone have an impact on the performance of Monitor audiometers? The answer is no, since the earphones are calibrated to ANSI audiometer standards. All things equal, they will yield the same hearing threshold levels as TDH-49 earphones.

Can either TDH-39 or TDH-49 earphones be calibrated to Monitor audiometers? The answer is yes and no. Mr. Anderson stated that Monitor audiometer firmware version 9.23 has the best output latitude to accommodate both models of earphones. For lesser firmware versions, TDH-39 earphone may be used, but it’s best to contact Monitor Instruments to discuss feasibility before proceeding.

How does this impact the customer? This does not impact the customer in any way. They can be assured that Monitor audiometers will perform as specified.

How does this impact service providers who distribute and calibrate audiometric equipment? If the technician is calibrating the same Monitor audiometer year after year, there should be no problem in knowing what model earphone is being used. However, if a new Monitor audiometer is ordered for a customer and that audiometer is calibrated to TDH-39 earphones, distributors will be given the choice of taking possession or deferring possession until the TDH-49 earphone becomes available again. If a customer states that they just acquired a new Monitor audiometer, technicians will need to pay attention to the model earphone being used and calibrate accordingly. Mr. Anderson stated that TDH-49 earphones will always be the preferred earphone for their Monitor audiometers. Thus, few audiometers will ever be using the TDH-39 earphones.  Regardless, anyone who calibrates Monitor audiometers will need to be vigilant.