This is a two-part story on me scoring a working clinical tympanometer for $25… And why it’s such a valuable tool for both detecting Auditory Neuropathy Spectrum Disorder, and also for adjusting & troubleshooting hearing aids.¹
One of the big mistakes I made when I closed my hearing aid dispensing lab in 1995 was letting go of my Rexton I-28 tympanometer (impedance bridge), as it was providing valuable auditory neuropathy/dys-synchrony diagnostic information — But I didn’t recognize it at the time… Nor in fact, did almost anyone else in the hearing care profession. More on this in an upcoming article on auditory neuropathy spectrum disorder.
So, recognizing the need to add a clinical tympanometer back into my arsenal against hearing loss, I had been combing the pages of eBay for another I-28, or maybe something a bit newer (and nicer), to find something that I could properly test stapedial reflex thresholds again. (These are also called middle ear muscle reflexes or acoustic reflexes.) Besides searching for tympanometer I was also combing through impedance looking for a mis-labeled bargain, when this gem popped up:
Up for your bid is a Madsen Impedance Audiometer ZS 76 1B. It is in overall good shape as shown in the pictures. There is some wear here and there as expected. The case has some writing on it and the top looks like something dribbled down it leaving a light stain. The unit and case will need a light cleaning by the new owner. When the unit is plugged in the light came on as shown. The knobs seemed to turn easily. The one red light to the left of the db meter did not come on but it might be because of the settings of the unit at the time. When I put the headphone on my ear I did hear some noise coming out of it but not alot. There again it may be the settings of the unit. Please note: I know nothing about these so you are buying it “as is”. This is why I listed it as for parts and repair. It may work fine but I don’t know. My loss is your gain. Please feel free to ask any questions you have before bidding and I will try to answer them. Thank you for looking and your bid.
Well, I’m an Electrical Engineer, and I figured that if the seller couldn’t figure out how to use it (there was no manual), there was a either a chance that it indeed worked, or that if it wasn’t working, at least I could use my voltmeter & ‘scope to take a shot at fixing it. Well, guess what folks? It works! So, all that is left to do is get a 1.0 cm³ calibrator cavity (or have my machine shop make one), get some eartips, and I’m off to the races. There is one modification I’ll most likely add: Although I don’t test infants, the 226 Hz probe tone has too long a wavelength to use for tiny ears, so I’ll splice in a 1000 Hz oscillator, to give me that capacity.
Not too shabby for a total of $39.52, including shipping to my door.
1) Now, about my statement on using a tympanometer for adjusting & troubleshooting hearing aids: As it turns out, one trick taught to me many years ago (I think it was by the late Sam Lybarger) is to measure the acoustic reflex thresholds, and use those figures to set the instruments’ SSPL-90 (now called MPO), on the theory that you don’t want to have the hearing aid output constantly triggering the stapedial reflex, as it is tiring to the patient. The fact that this is now being done to set C/M levels on CI’s for infants & others who can’t report back, as described here by my friend Ed Overstreet PhD, appears to validate this tried-and-true concept~