Following on three previous Zenith items:
Zenith didn’t “belong” in the hearing business. Zenith had always built radio receivers and TV receivers. They added phonographs and clocks to their radios, but never branched out into audio systems or clocks. They never made instrumentation or lab equipment of any sort. With this one exception, Zenith focused tightly on receiving and demodulating RF signals.
How did Zenith get into the hearing business? Books and other materials don’t answer the question.
Pure theory: Zenith’s CEO in its glory days was Commodore Eugene McDonald, a serious adventurer and navigator. McDonald organized and funded Arctic expeditions that lasted for months, during which he tested and publicized Zenith equipment. He always owned a fleet of yachts. The Trans-Oceanic was McDonald’s idea, a battery-powered shortwave radio for boaters and campers. Zenith thus always sold portable and battery radios along with its more complex home receivers. Hearing aids absolutely require battery power and efficient use of space, so Zenith’s shipboard talent extended to hearing aids. Maybe.
For whatever reason, Zenith succeeded with hearing aids, first in the era of submini tubes and then with transistors. Their hearing aid salesmen needed test equipment, so Zenith built audiometers for them. These audiometers weren’t complex enough for clinical use.
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Zenith saleswoman Polistra is testing the hearing of Fran, a waitress who works in a noisy diner. A proper test setup always starts with an acoustically isolated room to shut out traffic and building noises.
From Polistra’s view:
The audiometer has three main controls, the frequency selector on top, the intensity dial below, and the tone switch. This Zenith unit has three extra features: Noise intensity at upper left, pulsed tone switch, and the left / right / bone switch at upper right.
A basic test tries to determine the upper and lower limits of audibility at each selected frequency. The tester starts at a comfortable level (if any) and gives brief beeps. The patient indicates whether she’s hearing the beep, by raising her hand or by hitting a button on the panel. The tester gradually reduces the level until the patient can’t hear it, then does a more refined up and down to pin the level.
Noise, also called masking, is typically sent to the opposite ear. It keeps the opposite side of the auditory system occupied with a steady input on all frequencies so the single-frequency response of the test ear is the sole variable.
Tone pulsing can help to make the test tone more distinct when tinnitus is loud.
At Fran’s end, she is hooked up with earphones and a bone conduction transducer. The earphones are just high-fidelity magnetic phones, carefully calibrated with a fake head containing a microphone so the tester can be sure that 60 dB on the intensity dial is actually producing 60 dB into an ear canal.
Bone conduction is really the most important ‘controlled experiment’. Comparing the earphone to the bone response determines the nature of the hearing loss. If both are about the same, the loss is in the cochlea or the nerves beyond the cochlea. If the sound is easier to hear via bone, it means that at least part of the loss is conductive, from a defect in the mechanical parts of the hearing system. It could simply be mucus jamming up the middle ear cavity, or a punctured eardrum, or an arthritic stiffness of the three little ossicles that conduct the movement from the eardrum into the cochlea.
Here’s the normal action of sound pushing the eardrum, then moving the three little ossicles to send vibrations into the cochlea. (All of these animations are wildly exaggerated; the real motions are nearly microscopic.)
The bone conduction transducer is pressing against the temporal bone in front of the ear. It pushes the bone in and out, without moving the air and the eardrum. (Again the real action is just a microscopic distortion of the bone, not a huge slamming back and forth!)
Up close, the relative motion of the ossicles versus the opening of the cochlea is the same. If the bones are frozen up, or the eardrum can’t move, this action will create an audible sound while the airborne input is weak or nonexistent.
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Tech sidenote:
Audiometers of this generation, tubes and analog transistors, had a unique analog trick to prevent clicks. Our hearing system is exquisitely sensitive to every sudden change of amplitude or frequency or phase, because speech depends on sudden changes in all those variables. An abruptly starting tone, even when there’s no actual click in the waveform, will dominate the perception, making it hard to perceive the tone itself.
This opto-isolator uses the natural properties of incandescent lamps to create a smoothly ramped onset and ending to each tone. Polistra is pushing the button at the bottom of the audiometer, turning current on and off abruptly. The squarely switched current causes the bulb to heat up and cool down each time, and the gradually changing light controls a selenium resistor. When the light is brightest, the pure tone comes through loudest, and when the light is off, the tone doesn’t get through to the earphones.
Modern digital systems do all the ramping mathematically, multiplying a digitally generated sine wave by a ramp pattern.
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After the test, the saleswoman decides on the combination of equipment. For the majority of noise-induced losses or elderly losses, the test isn’t crucially necessary; a proper hearing aid has amplitude and tone controls to amplify the range of frequencies that are deficient, and the patient could just fiddle with the controls to get it right. More serious losses need more complex aids.**
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Here’s the best known Zenith tube-type aid. It had two submini tubes inside, along with miniature batteries for filament and plate. The microphone is behind the grille on front, and the volume, tone, and phone switches are around the side.
The Phone Magnet was Zenith’s pet term, echoing the WaveMagnet antenna on the Trans-Oceanic receiver. It was a simple inductor that picked up the waving magnetic field from the ear end of a standard Bell phone handset. The patient had to hold the phone handset upside down, with the mic by her mouth and the ear end down by the pocket aid.
Newer all-in-ear aids are much easier to use, since the phone magnet (telecoil) is behind the ear along with the microphone.
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** This is the non-secret secret of audiology. The FTC finally recognized that most aids can be bought and used without medical-style testing, and broke the cartel.
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Epilogue: Since I wrote about the glorious unique start of Zenith, I should also write the boring predictable end. Zenith continued making radios and hearing aids through the 70s as usual, then mostly gave up manufacturing as usual. They bought Heath and rebranded Heathkits and Heath computers for a while, then faded out in a sequence of LBOs as usual. Their last real product was TV cable boxes, sold to Motorola in 2000 as usual.
Polistra's Mill 